Handbook for Critical Cleaning

Citation preview

Handbook for

CRITICAL CLEANING Barbara Kanegsberg Associate Editor Edward Kanegsberg Editor-in-Chief

CRC Press Boca Raton London New York Washington, D.C. © 2001 by CRC Press LLC

Library of Congress Cataloging-in-Publication Data Handbook for critical cleaning / Barbara Kanegsberg, editor ; Edward Kanegsberg, associate editor. p. cm. Includes bibliographical references and index. ISBN 0-8493-1655-3 (alk. paper) 1. Integrated circuits--Cleaning. 2. Manufacturing processes. 3. Electronic packaging. 4. Surface preparation. 5. Surface chemistry. I. Kanegsberg, Barbara. II. Kanegsberg, Edward. TK7874 .H348 2000 621.381′046--dc21

00-048568 CIP

This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal use, or the personal or internal use of specific clients, may be granted by CRC Press LLC, provided that $.50 per page photocopied is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. The fee code for users of the Transactional Reporting Service is ISBN 0-8493-1655-3/01/$0.00+$.50. The fee is subject to change without notice. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe.

© 2001 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 0-8493-1655-3 Library of Congress Card Number 00-048568 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper

© 2001 by CRC Press LLC

The Editors Barbara Kanegsberg, President of BFK Solutions, is a consultant who works with components and parts manufacturers to resolve cleaning, contamination control, and environmental issues. She has over 25 years of involvement in process development. Her projects cover a range of areas of critical cleaning including metals, electronics, optics, semiconductors, motion picture film, and medical devices. She has conducted a number of product development projects for manufacturers of solvents, cleaning agents, and industrial equipment. Prior to establishing BFK Solutions, she managed the substitution of ozone-depleting chemicals at Litton Industries. Barbara has a background in biology, biochemistry, and clinical chemistry. She developed laboratory tests at BioScience Laboratories. Barbara is a recipient of the U.S. EPA Stratospheric Ozone Protection Award for her achievements in implementing effective, environmentally preferred processes. She is a member of the Editorial Advisory Board of Precision Cleaning Magazine and has been appointed to the University of Massachusetts Lowell Toxics Use Reduction Institute (TURI) Surface Cleaning Laboratory Advisory Committee. She has published extensively in surface preparation, contamination control, manufacturing process development, and regulatory issues. She has organized and participated in numerous seminars and conferences. She has a B.A. degree in Biology from Bryn Mawr College and an M.S. degree in Biochemistry from Rutgers University. She can be reached at (310) 459-3614 or by e-mail at [email protected].

Ed Kanegsberg, Ph.D., is a physicist with BFK Solutions and a member of the technical staff at Litton Guidance and Control (Woodland Hills, CA). He has over 30 years of experience in precision instrument development and technology transfer to the production facility. He addresses production yield issues through both process improvement and component modification. In addition to numerous papers and presentations, he has six patents in the area of optical instrumentation. He has a B.S. degree in Physics from MIT and a Ph.D. degree in Physics from Rutgers University. Ed can be reached at (310) 459-3614 or [email protected].

© 2001 by CRC Press LLC

Contributors John W. Agopovich Draper Labs

Ray A. Cull Dow Corning

David E. Albert North American Science Associates (NAmSA)

Phil Dale Layton Technologies, Ltd.

Stephen O. Andersen U.S. Environmental Protection Agency Sami B. Awad Crest Ultrasonics Mohan Balagopalan South Coast Air Quality Mgmt. District (SCAQMD) Matt Bartell Forward Technology Mark Beck Product Systems Inc. Michael Beeks Brulin Corp.

John Durkee Creative EnterpriZes Eric Eichinger Boeing Reusable Space Systems Division Max Friedheim PDQ Precision, Inc. F. John Fuchs Cleaning Technology Resources Christine Geosling Litton Guidance and Control Systems Arthur Gillman Unique Equipment

Rick Bockhorst Brulin Corp.

Don Gray University of of Rhode Island Department of Chemical Engineering

John Burke Oakland Museum Conservation Center

Ross Gustafson Florida Chemical Company, Inc.

Ahmed A. Busnaina Northeastern University

Steve R. Henly Layton PLC

Michael S. Callahan Jacobs Engineering

Barbara Kanegsberg BFK Solutions

Frank Cano Vatran Systems

David Keller Brulin Corp.

Mantosh K. Chawla Photo Emission Technologies, Inc.

Kenroh Kitamura Asahi Glass Co., Ltd.

John Chu SVC/Shipley

Jana Koran Litton Guidance and Control Systems

© 2001 by CRC Press LLC

Edward W. Lamm Branson Ultrasonics

Stephen P. Risotto Halogenated Solvents Industry Alliance (HSIA)

Carole LeBlanc Massachusetts Toxics Use Reduction Institite (TURI)

Reva Rubenstein U.S. Environmental Protection Agency

Joe McChesney Detrex Corp.

John F. Russo Separation Technologists

Abid Merchant DuPont Chemicals

Joe Scapelliti Detrex Corp.

Toshio Miki Asahi Glass Co., Ltd.

Ronald L. Shubkin Albemarle Corp.

William Moffat Yield Engineering Systems (YES)

P. Daniel Skelly Occidental Chemical Corp.

William M. Nelson Illinois Waste Management Research Center

Stephen P. Swanson Dow Corning

John G. Owens 3M Co. Michael Pedzy Zenith Ultrasonic, Inc. Richard Petrulio B/E Aerospace Galley Prods. Robert L. Polhamus RLP Associates Lou Rigali Ardency, Inc.

© 2001 by CRC Press LLC

Mahmood Toofan Semiconductor Analytical Services (SAS) Masaaki Tsuzaki Asahi Glass Co., Ltd. James L. Unmack Unmack Corp. Daniel J. VanderPyl Sonic Air Systems, Inc. Donald J. Wuebbles University of Illinois–Urbana

To our most valuable collaborative efforts: Deborah Joan Kanegsberg and David Jule Kanegsberg And to the memory and positive influence of: Israel Feinsilber Jule Kanegsberg Dr. Jacob J. Berman

© 2001 by CRC Press LLC

Contents Introduction What is Critical Cleaning? Barbara Kanegsberg SECTION 1: CLEANING AGENTS Chapter 1.1 Overview of Cleaning Agents Barbara Kanegsberg Chapter 1.2 Solvents and Solubility John Burke Chapter 1.3 Aqueous Cleaning Essentials Rick Bockhorst, Michael Beeks, and David Keller Chapter 1.4 Review of Solvents for Precision Cleaning John W. Agopovich Chapter 1.5 Hydrofluoroethers John G. Owens Chapter 1.6 Hydrofluorocarbons Abid Merchant Chapter 1.7 normal-Propyl Bromide Ronald L. Shubkin Chapter 1.8 Vapor Degreasing with Traditional Chlorinated Solvents Stephen P. Risotto Chapter 1.9 Volatile Methylsiloxanes: Unexpected New Solvent Technology Ray A. Cull and Stephen P. Swanson Chapter 1.10 Benzotrifluorides P. Daniel Skelly

© 2001 by CRC Press LLC

Chapter 1.11 HCFC-225: Alternative Precision and Electronics Cleaning Technology Toshio Miki, Masaaki Tsuzaki, and Kenroh Kitamura Chapter 1.12 d-Limonene: A Safe and Versatile Naturally Occurring Alternative Solvent Ross Gustafson SECTION 2: CLEANING SYSTEMS Chapter 2.1 Cleaning Equipment Overview Barbara Kanegsberg Chapter 2.2 The Fundamental Theory and Application of Ultrasonics for Cleaning F. John Fuchs Chapter 2.3 Ultrasonic Cleaning Mechanism Sami B. Awad Chapter 2.4 Higher-Frequency and Multiple-Frequency Ultrasonic Systems Michael Pedzy Chapter 2.5 Megasonic Cleaning Action Mark Beck Chapter 2.6 Equipment Design Edward W. Lamm Chapter 2.7 Cold and Heated Batch Solvent Cleaning Systems P. Daniel Skelly Chapter 2.8 Flushing Systems Richard Petrulio Chapter 2.9 Solvent Vapor Degreasing — Minimizing Waste Streams Joe McChesney and Joe Scapelliti Chapter 2.10 Vapor Degreaser Retrofitting Arthur Gillman

© 2001 by CRC Press LLC

Chapter 2.11 Enclosed Cleaning Systems Don Gray and John Durkee Chapter 2.12 Precision Cleaning and Drying Utilizing Low-Flash-Point Solvents Matt Bartell Chapter 2.13 Dense-Phase CO2 as a Cleaning Solvent: Liquid CO2 and Supercritical CO2 William M. Nelson Chapter 2.14 Carbon Dioxide Dry Ice Snow Cleaning Frank Cano Chapter 2.15 Gas Plasma: A Dry Process for Cleaning and Surface Treatment Lou Rigali and William Moffat Chapter 2.16 Super-Heated, High-Pressure Steam Vapor Cleaning Max Friedheim Chapter 2.17 Making Decisions about Water and Wastewater for Aqueous Operations John F. Russo Chapter 2.18 Overview of Drying: Drying after Solvent Cleaning and Fixturing Barbara Kanegsberg Chapter 2.19 Aqueous Parts Drying Daniel J. VanderPyl Chapter 2.20 Liquid Displacement Drying Techniques Robert L. Polhamus, Steve R. Henly, and Phil Dale SECTION 3: CONTAMINATION CONTROL; ANALYTICAL TECHNIQUES, COMPATIBILITY Chapter 3.1 How Clean Is Clean? Measuring Surface Cleanliness and Defining Acceptable Level of Cleanliness Mantosh K. Chawla

© 2001 by CRC Press LLC

Chapter 3.2 Contamination Control and Analytical Techniques Christine Geosling and Jana Koran Chapter 3.3 Material Compatibility Eric Eichinger SECTION 4: PROCESS SELECTION AND MAINTENANCE Chapter 4.1 Evaluating, Choosing, and Implementing the Process: How to Get Vendors to Work with You Barbara Kanegsberg Chapter 4.2 Optimizing and Maintaining the Process Michael S. Callahan SECTION 5: SPECIFIC AREAS OF CLEANING Chapter 5.1 Surface Cleaning, Particle Removal Ahmed A. Busnaina Chapter 5.2 Cleaning Metals: Strategies for the New Millennium Carole LeBlanc Chapter 5.3 Very High Performance, Complex Applications Barbara Kanegsberg Chapter 5.4 Cleaning Solutions in the Semiconductor Wafer Manufacturing Process Mahmood Toofan and John Chu Chapter 5.5 Biomedical Applications: Analytical Characterization for Biocompatibility David E. Albert SECTION 6: REGULATORY/SAFETY CONSIDERATIONS Chapter 6.1 Safety and the Environment — Some Editorial Thoughts Barbara Kanegsberg Chapter 6.2 Critical Cleaning — Working with Regulators — From a Regulator’s Viewpoint Mohan Balagopalan

© 2001 by CRC Press LLC

Chapter 6.3 Lessons Learned from the Phaseout of Ozone-Depleting Solvents Stephen O. Andersen Chapter 6.4 Screening Techniques for Environmental Impact of Cleaning Agents Donald J. Wuebbles and Reva Rubenstein Chapter 6.5 Health and Safety James L. Unmack SECTION 7 Glossary of Common Terms and Acronyms Contributors: Background and Contact Information

Color Figures

© 2001 by CRC Press LLC

Introduction What is Critical Cleaning? Barbara Kanegsberg

CONTENTS Soil Cleaning Identifying the Cleaning Operation Critical Cleaning Why Should One Be Concerned about Critical Cleaning? Performance, Reliability Costs Safety and Environmental Regulatory Requirements Overview of this book Philosophy Organization Section 1: Cleaning Agents Section 2: Cleaning Systems Section 3: Contamination Control; Analytical Techniques, Compatibility Section 4: Process Selection and Maintenance Section 5: Specific Areas of Cleaning Section 6: Regulatory/Safety Considerations Section 7: Glossary/Contributors’ Information Conclusions Acknowledgments References Critical cleaning is required for the physical manifestation of technology. We are in the information age, an age of thought, ideas, communication, but the underlying technology is based on physical objects, parts, or components. Many of these objects require precision cleaning or critical cleaning either because they are intrinsically valuable or because they become valuable in the overall system or process in which they are used. Some parts or components require critical cleaning not because of the inherent value of the part itself but instead because of their place in the overall system. For example, inadequate cleaning of a small inexpensive gasket can potentially lead to catastrophic failure in an aerospace system. © 2001 by CRC Press LLC

Nearly all companies that manufacture or fabricate high-value physical objects (components, parts, assemblies) perform critical cleaning at one or more stages. These range from the giants of the semiconductor, aerospace, and biomedical world to a host of small to medium to large companies producing a dizzying array of components. SOIL The concepts of contamination, cleaning, and efficacy of cleaning are open to debate and are intertwined with the overall manufacturing process and with the ultimate end use of the assembled product. Contamination or soil can be thought of as matter out of place.1 During manufacture, parts or components inevitably become contaminated. Contamination can come from the environment (dust, smog, skin particles, bacteria), from materials used as part of fabrication (oils, fluxes, polishing compounds), as a by-product of manufacturing, and as residue from a cleaning agent ostensibly meant to clean the component. CLEANING Cleaning processes are performed because some sort of soil must be removed. In a general sense, we can consider cleaning to be the removal of sufficient amounts of soil to allow adequate performance of the product, to obtain acceptable visual appearance as required, and to achieve the desired surface properties. You may notice that surface properties are included because most cleaning operations probably result in at least a subtle modification of the surface. If a change in the cleaning process removes additional soil and if as a result the surface acquires some undesirable characteristic (e.g., oxidation), then the cleaning process is not acceptable. Therefore, surface preparation and surface quality can be an inherent part of cleaning. IDENTIFYING THE CLEANING OPERATION Cleaning processes and the need for cleaning would seem to be trivial to identify. If you had a child who appeared in the doorway covered with mud, you would do a visual assessment of the need for cleaning and perform site-directed immersion or spray cleaning in an aqueous/saponifier mixture with hand drying. However, people perform critical cleaning operations without knowing it. This lack of understanding can be detrimental to process control and product improvement. Recognizing a cleaning step when it occurs is probably one of the major challenges in the components manufacturing community. Cleaning is often enmeshed as a step in the overall process rather than being recognized as a concept in itself. It may be considered as something that occurs before or after another process, but not as a process to be optimized on its own.2 A cleaning process is often not called a cleaning process. For example, optics deblocking (removing pitches and waxes), defluxing, degreasing, photoresist stripping, edge bead removal in wafer fabrication, and surface preparation prior to adhesion, coating, or heat treatment can all be thought of in terms of soil removal (cleaning). Sometimes the cleaning process is identified only by the name of the engineer who first introduced it. The sociological and psychological bases for this aversion to discussing cleaning are no doubt fascinating, but are beyond the scope of this book. The important thing is for you to recognize a cleaning process when you see it. © 2001 by CRC Press LLC

There are several reasons. One obvious reason is process control. A second is troubleshooting or failure analysis. If the product fails and you need to fix the process, it is crucial to identify not only where soil might be introduced but also what steps are currently being taken in soil removal. If the chemical being used comes under regulatory scrutiny, identifying cleaning is even more important. If a supplier provides the component and a problem arises, it is important to be able to recognize where the cleaning steps occur. Finally, identifying the cleaning steps allows you to apply technologies developed in other industries to your own process. CRITICAL CLEANING Defining critical cleaning or precision cleaning is a matter of ongoing debate among chemists, engineers, production managers, and those in the regulatory community. Certainly the perceived value or end use of the product is a factor, as are the consequences of remaining soil. The level of allowable soil remaining after cleaning is a consideration. Precision cleaning has been defined as the removal of soil from objects that appear to be clean in the first place.3 In some instances, however, high levels of adherent soil are involved in the processing of critical devices. Precision cleaning was once euphemistically said to be YOUR cleaning process for YOUR critical application, whereas everyone else’s process could be considered as general cleaning.4 In one sense, there is some truth that the manufacturer is often the best able to understand process criticality. At the same time, recognizing general cleaning and critical cleaning as parts of other operations can lead to overall industrial process improvement. WHY SHOULD ONE BE CONCERNED ABOUT CRITICAL CLEANING? Critical cleaning issues are becoming increasingly important. Competitive pressure is increasing. Higher demands are being placed on industry. A clean component produced efficiently and in an environmentally preferred manner (or at least in an environmentally acceptable manner) is a given in today’s economy. Performance, Reliability Products are becoming smaller, with tighter tolerances, and higher performance standards. Some products, such as implantable biomedical devices, are expected to perform for decades without a breakdown. Small amounts of soil and very tiny particles can irreparably damage the product. To remove the soils successfully, you have to understand the various cleaning chemistries and cleaning equipment, and how they are combined and meshed with the overall build process. Costs Pressure to keep costs down increases constantly. The costs of effective processes have tended to increase. Choosing the best option for the application can keep costs down.

© 2001 by CRC Press LLC

Safety and Environmental Regulatory Requirements The manufacturing community needs a wide selection of chemicals and processes to achieve better contamination control at lower costs. However, our understanding of health and the environment has led to restrictions on chemicals and processes. In order to foresee future trends, the manufacturer needs an understanding of atmospheric science and of the approaches used by regulatory agencies. OVERVIEW OF THIS BOOK Philosophy In setting out to put together this comprehensive book on critical cleaning, the editor sought inputs from the experts in the field. Frequently, these are people associated with vendors of cleaning equipment and/or cleaning agents. Naturally, each person’s viewpoint is somewhat colored by his or her own portion of the market. However, on the whole, the scope and fairness of the material submitted was impressive. An attempt has been made to minimize use of brand names. In some cases, there are several contributors in a similar area. In general, the editorial philosophy has been to include all but the most blatantly commercial material. By having a large number of contributors, a wide range of products and viewpoints are presented; the reader is expected to weigh the advantages and/or disadvantages of each approach for his or her own application. Organization This book is organized into seven sections: • • • • • • •

Cleaning Agents Cleaning Systems Contamination Control, Analytical Techniques, and Compatibility Process Selection and Maintenance Specific Areas of Cleaning Regulatory/Safety Considerations Glossary/Contributors’ Information/Index

Following is a capsule summary of each of the book chapters. Section 1: Cleaning Agents An overview of cleaning agents is presented by the editor, Barbara Kanegsberg. Included in this are agents that are discussed in detail by other authors as well as some that are not. John Burke, from the Oakland (California) Museum of Art, presents a scholarly discourse on solubility and the techniques used to classify solvents. It becomes clear why certain solvents are applicable to removing certain types of soil. Water, the most common cleaning agent, is discussed first. Rick Bockhorst, Michael Beeks, and David Keller from the Brulin Corporation, a producer of aqueous cleaning equipment, give a comprehensive review of aqueous cleaning essentials. Many of the subjects discussed are also applicable to nonaqueous solvent cleaning. John W. (Bill) Agopovich from the C. S. Draper Laboratories presents a detailed © 2001 by CRC Press LLC

D-LIMONENE: A SAFE AND VERSATILE NATURALLY OCCURRING ALTERNATIVE SOLVENT

xxi

overview of solvents, discussing requirements, methods, and environmental issues, as well as reviewing the different types of available solvents. John Owens from 3M thoroughly presents the hydrofluoroethers (HFEs), a class of solvents that has been introduced in the past few years as replacements for ozone-layerdepleting chemicals (ODCs). Abid Merchant from DuPont has a full discussion of the hydrofluorocarbons (HFCs) another class of recently introduced ODC replacements. Ron Shubkin of Albermarle Corporation gives particulars on normal-propyl bromide (nPB), a recently introduced substitute for the aggressive ODC solvent, 1-1-1-trichloroethane. Steve Risotto from the Halogenated Solvents Industry Association (HSIA) presents a detailed discussion of the chlorinated solvents, a group of traditional solvents that is seeing a resurgence of use in certain applications. Ray Cull from Schneller Inc. and Steve Swanson from Dow Corning have an informative presentation of volatile methylsiloxanes (VMSs), a group of silicon-based chemicals that have been found to have cleaning abilities in a number of areas. Dan Skelly from Occidental Chemical Corp. has a comprehensive chapter on the benzotrifluorides, a group of potentially useful solvents. Toshio Miki, Masaaki Tsuzaki, and Kenroh Kitamura, and all from Asahi Glass, give a good overview of HCFC-225, another solvent with utility for electronics and other precision cleaning applications. Ross Gustafson from Florida Chemical ends the cleaning agent section by specifying critical cleaning uses of the orange peel–derived d-limonene.

Section 2: Cleaning Systems The cleaning systems section is the largest, at least in terms of numbers of chapters, which reflects the wide range of process choices. Drying, an important aspect of cleaning processes, is covered in this section as are those systems, such as CO2 cleaning, where the cleaning agent and the cleaning equipment are inseparable. Barbara Kanegsberg gives an overview of cleaning systems. As with the overview for cleaning agents, this reviews processes that are treated in this book by other authors, as well as those for which there are not dedicated chapters. There are four chapters dealing with ultrasonics and the closely related megasonics technologies. John Fuchs of Cleaning Technology Resources and Sami Awad of Crest Ultrasonics each present an informative overview of ultrasonics. This is such a complex and widely used technology that it was felt that both authors had useful insight. Michael Pedzy of Zenith Ultrasonics expands on this by discussing higher-frequency and multiplefrequency ultrasonics. Mark Beck of Prosys introduces the interesting and efficacious technology of megasonics. Edward Lamm from Branson provides a practical chapter on optimizing the equipment design, covering solvent, aqueous, and semiaqueous cleaning equipment as well as rinsing, drying, automation, and other ancillary equipment. Dan Skelly of Occidental Chemical Corp. in addition to the chapter on benzotrifluorides, has written a useful chapter on equipment for cold and heated batch solvent cleaning, i.e., where cleaning agents are used below their boiling point. Richard Petrulio of B/E Aerospace contributes a very readable chapter on the design of flushing systems. This is one example where a company was able to design equipment for its own cleaning application. © 2001 by CRC Press LLC

Joe McChesney and Joe Scapelliti of Detrex Corporation discuss important techniques for minimizing waste streams in solvent vapor degreasers. Included are methods for calculating the size or capacity of the required equipment. Art Gillman of Unique Equipment talks of retrofitting vapor degreasers to allow use of different cleaning chemicals or to meet newer emission-control standards. This is an option that sometimes avoids major capital expense. Don Gray from the University of Rhode Island and John Durkee of Creative EnterpriZes present an informative chapter on contained airless and airtight solvent systems, one approach to using costly or heavily regulated solvents. Matt Bartell of Forward Technology lucidly writes about low-flash-point cleaning systems, an option that extends vapor degreasing to flammable chemicals. In some cases, the cleaning agent and the cleaning equipment are inseparable. Several examples are provided in the next four chapters. Bill Nelson from the Hazardous Waste Research and Information Center (HWRIC), University of Illinois explains the theory and application of supercritical and liquid CO2 cleaning. Frank Cano of Va-tran Systems clearly describes the use of carbon dioxide in the solid form, CO2 snow cleaning, a gentle approach for light soil loads. Lou Rigali of Ardency, Inc. and Bill Moffat of Yield Environmental Systems (YES) discuss an important solvent-free approach to removing organics, plasma cleaning. Max Friedheim of PDQ Mini-Max presents interesting uses of steam vapor cleaning to critical cleaning applications. After you have cleaned, there is the question of disposing of the cleaning agent. John Russo of Separation Technologies discusses selecting the best waste water treatment for aqueous operations. This comprehensive chapter also discusses pretreatment and water recycling techniques. Cleaning with liquids frequently means that drying is part of the process. The final three chapters in the Cleaning Systems section deal with this sometimes neglected process. Barbara Kanegsberg provides an overview of drying. Dan VanderPyl of Sonic Air Systems details physical methods of drying. Bob Polhamus of RLP Associates with Steve Henly and Phil Dale of Layton Engineering clearly present chemical displacement drying techniques. Section 3: Contamination Control, Analytical Techniques, and Compatibility As important as cleaning is knowing when to clean and knowing when the part is clean enough. Also important is knowing whether what you are using to clean with is compatible with your parts. The four chapters in this section address these topics. Mantosh Chawla of Photo Emission Technology (PET) discusses the important topic of measuring surface cleanliness and defining acceptable level of cleanliness. Christine Geosling and Jana Koran of Litton Guidance and Control Systems provide an informative explanation of contamination control, analytical techniques, and clean room standards and practices. Eric Eichinger of Boeing North America concludes with a thoughtful discussion of the critical issue of materials compatibility both for metals and nonmetals. Section 4: Process Selection and Maintenance This overview section contains two chapters. Barbara Kanegsberg discusses evaluating, choosing, and implementing the process, including productive interaction with vendors. This has previously been presented under the title “How to Survive a Trade Show.” Techniques for information gathering and communication with suppliers are emphasized. Mike Callahan from Jacobs Engineering provides valuable advice on optimizing and main© 2001 by CRC Press LLC

D-LIMONENE: A SAFE AND VERSATILE NATURALLY OCCURRING ALTERNATIVE SOLVENT

xxiii

taining the process. A number of topics such as fixturing, process monitoring, and process improvements are discussed. Section 5: Specific Areas of Cleaning A number of specific applications are presented in this section. Ahmed Busnaina from Northeastern University explains particle removal. This is an effective treatment of surface physics presented in a readable and understandable manner. Carole LeBlanc of the Toxics Use Reduction Institute (TURI) at the University of Massachusetts presents a study of aqueous cleaning of metals, which outlines the benefits of developing practical, industrially oriented studies in the academic community and provides valuable and logical guidelines for the individual manufacturer. Barbara Kanegsberg of BFK Solutions discusses very high performance and biomedical applications. Specific examples including aerospace and electronics are given. John Chu SVC and Mahmood Toofan of SAS provide a thoughtful presentation of challenges faced by the semiconducting wafer fabrication industry. David Albert of NAMSA concludes this section with a lucid explanation of analytical characterization for biocompatibility in biomedical applications. This is a must for anyone involved in biomedical-related products. Section 6: Regulatory and Safety Considerations Cleaning almost always is involved with using materials or processes with concerns about environmental or exposure effects. Dealing with regulations and regulators has become part of the overall picture. This final section treats a number of these issues. Barbara Kanegsberg provides an overview of the safety and environmental issues. Mohan Balagopalan of Southern California’s South Coast Air Quality Management District (SCAQMD) presents a thoughtful section on working with regulators—from a regulator’s viewpoint. Steve Andersen from the U.S. EPA rationally discusses how industry and government can work together. He presents lessons learned from the ozone-depleting chemical (ODC) phaseout. Don Wuebbles from the University of Illinois and Reva Rubenstein of the U.S. EPA teamed effectively to submit a readable, informative overview of screening techniques for the environmental impact of cleaning agents, with implications for industry. Last, Jim Unmack of Unmack Corporation Services outlines critical health and safety aspects associated with cleaning processes. Section 7: Glossary/Contributors’ Information/Index This supporting section contains a glossary of many of the commonly used terms and acronyms. An alphabetical list of contributing authors with their backgrounds and contact information is also included. CONCLUSIONS A diverse assortment of components and assemblies requires critical or precision cleaning. Some examples include: Accelerometers Automotive parts Biomedical/surgical/dental devices (e.g., pacemakers) © 2001 by CRC Press LLC

Bearings Computer hardware (metal, plastic, other composites—the insides of your computer and printer) Consumer hardware (telephones) Digital cameras Disk drives Electronics components Flat panel displays Gaskets Gyroscopes Motion picture film Optics Space exploration hardware Wafers/semiconductors/microelectronics Weapons, defense systems (missiles) While each application is very site specific, contamination control problems cut across industry lines. At the same time, each industry still tends to work in a separate world. It is hoped that this book will provide a synthesis of cleaning approaches.

ACKNOWLEDGMENTS This book is the result of a phenomenal level of effort by those involved in the worlds of critical cleaning, surface preparation, and environmental issues. The information, expertise, and guidance provided by the contributing authors is invaluable. Dr. Ed Kanegsberg, business associate and spouse, provided support, encouragement, and invaluable participation in the editing process. He also provided the viewpoint and experiences of a physicist and practicing engineer. Bob Stern and the staff at CRC Press provided excellent guidance throughout the process. I would also like to thank family members Deborah Kanegsberg, David Kanegsberg, Ruth Feinsilber, and Mimi and Murray Steigman for their patience and encouragement. Finally, I would like to thank Dr. Shelley Ventura-Cohen, a wise colleague and adviser She tells the story of her grandmother, who, on observing Shelley staring blankly at a cookbook while an inert, raw chicken sat on the counter, exclaimed: Look at the chicken, not the book. Dear reader, critical cleaning, surface preparation, and contamination control are complex subjects, but they are also intensely practical subjects that relate to a product— your product. My suggestion, therefore, is to look at this book, and at the same time look at the chicken. REFERENCES 1. Petrulio, R. and B. Kanegsberg, Back to basics: the care and feeding of a vapor degreaser with new solvents, in Proc. Nepcon West ‘98, Anaheim, CA, 1998. 2. Rosa, D., A2C2 Magazine, personal communication. 3. LeBlanc, C., Toxics Use Reduction Institute, personal communication. 4. Kanegsberg, B., Options in the high-precision cleaning industry: overview of contamination control working group XIII, presented at International CFC & Halon Alternatives Conference, Washington, D.C., October, 1993.

© 2001 by CRC Press LLC

SECTION 1

Cleaning Agents

© 2001 by CRC Press LLC

CHAPTER 1.1

Overview of Cleaning Agents Barbara Kanegsberg

CONTENTS Introduction Aqueous Cleaning Agents Simple Additives Commercial Blends, Aqueous Water-Soluble Organics Solvent-Based Cleaning Agents Classic Organic Solvents Restricted or Low Availability Ozone-Depleting Chemicals Solvents on the Horizon Oxygenated Solvents Esters Hydrocarbon Blend (Mineral Spirits), Soy-Derived Cleaning Agents d-Limonene Blends, α-Pinene Blends Cyclic Volatile Methyl Siloxanes Perfluorinated Compounds Solvent Blends, Azeotropes, Cosolvents Stabilization Extending the Solvency Range or Moderating the Solvency Cosolvents Surfactants Emulsions: Macroemulsions, Structured Solvents, Microemulsions Mystery Mixes Solvency and Physical Properties; Other Parameters Kauri-Butanol Number Wetting Index Other Physical Properties; Regulatory Features Costs Overall Considerations References

© 2001 by CRC Press LLC

INTRODUCTION Choosing the cleaning agent appropriate to the job at hand can be a traumatic experience, even for those with some background in chemistry. While the cleaning agent section of this book highlights a number of different types of cleaning agents, it is by no means exhaustive. This chapter provides an overview of cleaning agents and highlights a few additional cleaning agents not discussed in other chapters. Cleaning agents are generally divided into aqueous and solvent. When most people involved in cleaning refer to the term solvent, they actually mean organic solvent. Organic solvents are not those processed from, say, free-range, herbicide-free lemons. Instead, organic refers to materials that have the element carbon in them. However, when you think about it, both organic and aqueous-based cleaning agents can be thought of as solvents. If you dissolve sugar in tea, the water is acting as the solvent; the sugar is the solute. Because many oils are carbon based, organic solvents have been classically used for very heavy degreasing jobs. However, other mechanisms such as saponification (as described in the chapter covering aqueous cleaning agents) have been successfully used to lift heavy grease off parts. Certainly, water-based cleaning is widely used; most of us have successfully removed oils and greases from dishes using semiautomated aqueous cleaning, not a vapor degreaser. Although a segment of the industry has traditionally been carried out by aqueous cleaning, until the last 10 to 15 years, most cleaning was conducted using classic chlorinated solvents or with ozone-depleting compounds (ODCs). In fact, many products were designed specifically around the solvency properties of CFC-113 and 1,1,1-trichloroethane (TCA). The loss of ODCs has led to upheaval in the manufacturing world; and, because there are no true drop-in substitutes for ozone depleters, the market has fragmented.1 This fragmentation has continued for a number of reasons, including: • • • • • • • •

Lack of understanding of how to use the new methods Dissatisfaction with selected new approaches High cleaning costs Development of new solvent and aqueous blends Increasingly stringent and ever-changing regulatory conditions Differences in regulations in various geographic locations Increasingly stringent cleaning and performance requirements product miniaturization

To make matters even more complex, the line between cleaning agent and cleaning equipment or cleaning is often blurred. Sometimes the cleaning agent is generated in use, for example CO2 (solid, liquid, and supercritical) and plasma. Where solid or abrasive media2 are used, in a sense, they can also be considered as the cleaning agent. No one product or class of products is likely to satisfy all cleaning requirements. The cleaning agent must be matched to the soil, the substrate (the component or part to be cleaned), the cleaning requirements, drying requirements, and other performance and environmental constraints. Inorganic soils are often referred to as hydrophilic; they dissolve effectively in water. Organic-based soils, often referred to as hydrophobic, tend to dissolve more effectively in organic solvents. The choice of cleaning agent should, ideally, be based on technical considerations. Unfortunately, chemists, engineers, production people, and those involved in regulatory agencies may themselves become hydrophilic or hydrophobic. Although we all have cleaning agent prejudices, irrationally ruling out one class of cleaning agents can result in inadequate cleaning and can be economically and ecologically detrimental. © 2001 by CRC Press LLC

Another reason to keep an open mind and to try to understand both approaches is that the line between aqueous and organic cleaning tends to blur, so even if you and your firm are unalterably devoted to organic solvents, it will be helpful to learn about aqueous cleaning, and vice versa. Some inert organic cleaners are blended with small amounts of surfactants to improve removal of soils. Many aqueous cleaners contain significant amounts of organic additives. Some may be basically blends of water-soluble organic compounds. In fact, certain similar organic solvent blends may be classified as semi-aqueous or cosolvent only in that very small formulation differences allow for rinsing in water (for semiaqueous) or another solvent (for cosolvent).

AQUEOUS CLEANING AGENTS Simple Additives Water removes some soils. With appropriate cleaning action and constant rinsing to remove soils, water alone can clean. However, additives improve performance. Some blends are relatively simple and are accomplished by in-house blending. Such blends can be particularly desirable where residue is an issue either for the product or for disposal of the spent cleaning agent. Small amounts of peroxide (0.5%) have been added to water to clean and remove bacteria. Dilute hypochlorite (bleach) is often used to prevent biological contamination. Ammonia is often used for simple cleaning where residue is of concern. Alcohol and acetone are sometimes added to boost cleaning power and promote rapid drying. Sodium bicarbonate may be added. Acid washing and acid etching with Piranha, chromic acid, and other strong acids can be thought of as selective cleaning. Although the solutions are simple, process control, process monitoring, employee safety, potential flammability, and environmental regulatory issues must all be considered.

Commercial Blends, Aqueous Commercially available aqueous cleaning agents contain additive blends, often consisting of a dizzying array of organic and inorganic compounds. Some additive packages are totally inorganic; most are a mixture. Although a few companies disclose the additive package, more typically, for competition-sensitive issues and other factors, the exact formulation is a closely held secret. A few examples of additives are provided in Table 1. This summary is provided for several reasons. Well-designed aqueous formulations are complex, sophisticated, and are specifically designed to remove certain soils. Also, be aware that even though aqueous cleaning agents are used in dilute form, they are not formulated from organic carrot juice. As with other cleaning agents, aqueous cleaning agents must be used with understanding and respect. For general metals cleaning, aqueous formulations with relatively broad range of acceptability for substrate and soil have been found. However, for most high-precision applications the aqueous cleaning agent must be specifically matched to the soil, the expected soil loading, the substrate, and the expected end use of the product. For example, in some applications, phosphate or silicate residue is not acceptable. In addition, in cleaning certain metals, notably aluminum, careful selection of the aqueous cleaning agent and process is required. © 2001 by CRC Press LLC

Table 1 Some Additives Used in Aqueous Formulations Additive

Function

Description, Examples

Surfactants

• Wettability • Soil displacement/dispersion • Solubilization

Defoamers

• Control excessive foam • Allow use in high-pressure spray applications etc.

Solvents, assorted

• Decrease surface tension • Adjust pH • Improve solubility range

Corrosion inhibitors passivating

• Prevent corrosion of metals

Corrosion inhibitors, nonpassivating

• Prevent corrosion of metals

Builders

• Promote efficacy of cleaning by surfactants • Sequester water hardness • Maintain pH (acidity, alkalinity) • Decrease metal, lead content of waste stream • Promote solubility of organics in presence of high levels of inorganic salts

• Single molecule with hydrophilic and hydrophobic portions • May have long chain organic portion, many carbons in a row • Example: alcohol ethoxylates • Poorly soluble in bath at operating temperature • Impart slight oil-like quality • Usually nonionic surfactants • Example: nonionic block copolymers • Typically soluble in water • May be VOCs • Examples: butyl cellosolve, pyrollidone,morpholine, glycol ethers, alcohols • React with metal surface to reduce reactivity • Typically oxidizing agents • Examples: chromates, nitrates, permanganates, chlorates • Some reducing agents, e.g., Na sulfite • Adsorption, formation of protective films • Examples: silicates (most common), pyrophosphates, carbonates, amines, gelatin, tannic acid, thiourea • Typically salts • Chelating agents • Examples: sodium tripolyphosphate, sodium hexametaphosphate, sodium citrate • Precipitating builders (e.g., carbonates) • Important with nonorganic surfactant packages • Examples: toluene sulfonates, short-chain alcohols, benzoate salts • Example: hydrogen peroxide

Hydrotropes

Oxidizers

• Corrosion inhibitors • Adsorption, dissolution in soils, oxygen release • Better soil removal

Source: Adapted from Cala, F.R. and A.E. Winston, Handbook of Aqueous Cleaning Technology for Electronic Assemblies, Electrochemical Publications, 1996.

Water-Soluble Organics Some cleaning agents, nominally referred to as aqueous cleaning agents are primarily or significantly high in organic solvent blends including long-chain nonlinear alcohols or d-limonene. They can be rinsed with water, or in some cases with either water or solvents. Providing both solvent and aqueous cleaning in the same process has advantages. However, recovery of the waste stream and carryover can become a problem. © 2001 by CRC Press LLC

SOLVENT-BASED CLEANING AGENTS As in aqueous cleaning, there are mystery blends. However, it is often easier to identify the components of solvent-based cleaning agents. With the proliferation of new organic solvents, a number of by-products and natural products have been or are now used in cleaning processes. Cleaning agents based on orange, pine, cantaloupe, and grapes have been developed—an entire fruit basket of possibilities. Some solvents have been discussed in detail; others are alluded to in discussions of cleaning equipment or of specific cleaning agents. A few additional solvents and solvent categories are worth noting. In general, one must consider that while many of these solvents have been used for years, long-term inhalation toxicity data may not be available. In addition, blends and azeotropes of both new and well-established solvents may have their own solvency, compatibility, or toxicity properties. This holds true for organic and water-based cleaning agents. The best advice remains to test the solvent or blend in the application under consideration and to be conservative— minimize employee exposure and minimize loss of cleaning agent to the environment. In addition, compounds with higher boiling points and fairly complex blends with certain additives may leave undesirable residue, so rinsing is often required. Classic Organic Solvents Examples of classic organic solvents include toluene, hexane, heptane, benzene, and xylene. Flammability, worker exposure, air toxics, and company liability issues reduced the use of these solvents when ODCs were available. They have remained popular for specialized uses or as blends, particularly for specific, high-value applications. With the decrease in availability of ODCs and an increase in low-flash-point and well-contained cleaning systems, these solvents have enjoyed a resurgence in popularity. They have a wide solvency range, but they are particularly oil-like. If they are to be used, appropriate environmental and safety controls must be employed. Restricted or Low-Availability Ozone-Depleting Chemicals Chlorofluorocarbon 113 (CFC 113) and 1,1,1-trichloroethane (TCA) set the standards for cleaning for decades. CFC 113 has low to moderate solvency, TCA is an aggressive solvent. Both can be used for liquid/vapor phase degreasing, but both have high ozone depletion potentials. In the United States, they have been phased out of production. It is still possible to obtain recycled material, but at a high cost. Hydrochlorofluorocarbon 141b (HCFC 141b) has moderate solvency and a fairly low boiling point. It was first considered as a replacement for other ODCs, but was found to have an ozone depletion potential similar to that of TCA. It will be phased out of production. In the United States, HCFC 141b is highly restricted and falls under a usage ban in cleaning applications.3 It should be noted in general that regulation and availability of various cleaning agents vary from country to country and often from city to city. Regulation of Class I ODCs and of HCFC 141b differs in various parts of the world. Solvents on the Horizon The landscape of available cleaning agents and processes varies as advances in technology, requirements of build processes, and regulatory drivers change the perceived appeal of © 2001 by CRC Press LLC

various options. Because the assortment of products will inevitably change, it is hoped that the reader will gain an appreciation not just of the specific cleaning agents, but of the underlying, commonsense approaches to successful cleaning and contamination control. Having said this, it is important to mention emerging solvents that are being used to develop new cleaning agents. For example, Nippon Zeon Co. Ltd. is marketing a hydrofluorocarbon (HFC) that may have favorable formulation qualities. Two additional HFCs, HFC-365mfc and HFC-245fa, will be discussed in greater detail. Both were initially introduced as foam-blowing agents, and most of the information must be extracted from literature geared to the foam industry. However, both may have uses, alone and in blends, as replacements for HCFC 141b, and for other solvents that may be heavily regulated. HCFC 141b is under a federal usage ban for most cleaning applications. There are some exceptions. For example, HCFC 141b could be used under some conditions as an aerosol wasp repellent. Suffice it to say that there is anecdotal evidence of noticeable continuing use of HCFC 141b and of a significant number of components manufacturers who, shall we say, must be having severe problems with wasps. Those manufacturers should be aware (1) of the general usage restrictions on HCFC-141b as well as (2) the imminent production phaseout of HCFC-141b. These newer solvents may provide viable alternatives to HCFC-141b and additional options for other applications. It is important to be aware that while both materials are HFCs, they behave differently from the HFCs you may be more familiar with (described elsewhere in this book by Merchant). One product is produced and imported to the United States by Solvay.4 It is marketed under the trade name Solkane® 365mfc5 and it will be sold to other cleaning agent suppliers for formulation. DuPont recently began introduction of several blends of HFC 365 with other HFCs and with trans-1,2,-dichloroethylene. Other companies and formulators are also developing products based on 365mfc. It should be noted that, unlike other HFCs commonly used as solvents (Merchant), HFC 365 has a very low flash point. In nonflammable blends, and even in azeotropes, there is the potential for flammable mixtures to develop in use. Therefore end users should work with advisors and with responsible cleaning agent suppliers to evaluate their own production situation carefully. HFC-245fa is produced by Honeywell6, 7; it was also developed primarily as a foam-blowing agent. There is interest in use as a solvent for aerosols, and it has potential applicability as a cleaning solvent in some applications. With a 15°C boiling point, many end users will find HFC-245fa to be rather evanescent; and it would not be suitable for use in classic, unmodified vapor degreasers. The material will be more suitable for use in newer vapor degreasers with subzero chilling and auxiliary cooling coils. The material is also expected to be usable in many airless and airtight systems and in flushing applications. The very low boiling point can be advantageous for deposition and other applications where rapid evaporation is desirable. HFC245fa appears to have reasonable hydrolytic stability, and good materials compatibility with metals and with many plastics. It is miscible with some hydrocarbons, and it shows partial (over 10%) to full miscibility with a number of other solvents, including methanol, isopropyl alcohol, trans-1,2-dichloroethylene, and perfluoropolyethers. It shows good to very good miscibility with polyolester oils, limited miscibility with fluorinated hydrocarbon oils, and poor miscibility with mineral oil and silicone oil. Both HFC-365mfc and HFC-245fa are very mild solvents, similar to the currently more familiar HFCs and hydrofluoroethers (HFEs), and costs are expected to be somewhat lower. Of the two, HFC-245fa should have better solubility for polyolester oils. With a boiling point of 40°C, HFC-365mfc has a significantly higher boiling point than HFC-245fa. These boiling points may be compared with the 32°C boiling point for HCFC-141b. The differences may not seem significant, but remember that as a rule of thumb chemical processes double in rate with every 10°C increase. For both materials, some plastics © 2001 by CRC Press LLC

compatibility studies have been conducted. Because synergistic behavior can occur, you would be well advised to confirm the compatibility of any proposed blend to the specific mix of materials and in the application at hand (time of exposure, temperature, force of cleaning action). A comparison of HFC-365mfc and HFC-245fa is presented in Table 2. Oxygenated Solvents Examples of oxygenated solvents include the short-chain alcohols (methyl alcohol, ethyl alcohol, isopropyl alcohol), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), and acetone. Compared with, for example, hexane or heptane, the addition of oxygen makes these compounds more polar, that is, more like water. They are more suited to polar or inorganic soils. Despite the low flashpoint, some oxygenated compounds, such as isopropyl alcohol (IPA) can be used in appropriately designed vapor degreasing systems. However, it should be noted that oxygenated solvents cannot systematically be used to replace chlorinated solvents. After it was delisted as a volatile organic compound (VOC), there has been increased interest in acetone. Although acetone has been adopted for some processes, the extremely rapid evaporation rate, aggressiveness to some plastics, and low flash point have limited the extent of substitution of acetone. Long-chain alcohols such as tetrahydrofurfuryl alcohol (THFA) are used alone and in blends. The longer the carbon chain, the more they become fatlike (able to dissolve oil). However, the alcohol portion confers some waterlike qualities. Often, these alcohols can be part of aqueous, semiaqueous (rinse with water), or cosolvent (rinse with solvent) blends. N-Methyl pyrrolidone (N-methyl-2-pyrrolidone, NMP) is a high boiling (295°C), highflash-point (91°C, 196°F) solvent that is used alone and in blends. Because it is water soluble, it can be blended for removal of both rosin and organic acid flux, and it is used in photoresist systems. Other pyrrolidones are being developed, notably n-octyl pyrollidone (NOP) and n-hydroxy ethyl pyrrolidone (HEP). Used alone and in blends, they may serve to extend the range of cleaning in degreasing. For example, NOP has a longer carbon chain Table 2 Comparison of Characteristics of HFC-365mfc and HFC-245fa Characteristic

HFC-365mfc

HFC-245fa

Structure Molecular weight Boiling point (°C) Flashpoint (°C) UEL/LEL (% by volume) ODP Atmospheric lifetime VOC status SNAP status, solvents

CF3CH2CF2CH3 148 40.2 Below 27 3.8 to 13.3 Zero 10.8 years Exempt Acceptable

Toxicology

90-day subacute inhalation study in progress

CF3CH2CF2H 134 15 None None Zero Low to moderate Exempt Not yet submitted for solvent applications 90-day sub-acute inhalation study complete; under evaluation by AIHA WEEL committee; suggested inhalation levels not yet published

UEL/LEL  upper/lower explosion limit; ODP  ozone depletion potential; VOC  volatile organic compound; SNAP  Significant New Alternatives Policy.

© 2001 by CRC Press LLC

and is therefore more oil-like, so it could be used in formulations for degreasing, paint stripping, and deinking of paper. HEP shows promise in photoresist removal.8 Relatively recently, dimethyl sulfoxide (DMSO) has also been used in some photoresist applications as well as in other processes requiring a fairly aggressive solvent. DMSObased products are in the process of being developed and tested.

Esters Various monobasic and dibasic esters and notably lactate esters are used alone and in blends in semiaqueous and cosolvent applications. They have proved particularly effective in removal of pitches, waxes, and other difficult, mixed soils. The esters have a fairly strong odor. They have high boiling points and must be rinsed in high-precision applications. Esters may also be used in coatings formulations. Two of the esters, t-butyl acetate (VOC status pending) and methyl acetate, have relatively low tropospheric reactivity and may therefore be particularly useful where VOCs are an issue. In precision cleaning of electronics assemblies, t-butyl acetate has been found to provide a good complement to acetone. t-Butyl acetate has a higher boiling point. Unlike acetone, it does not dissolve nitrile gloves, and in one test evaluation the assemblers found the odor to be acceptable.9

Hydrocarbon Blend (Mineral Spirits), Soy-Derived Cleaning Agents Hydrocarbon blends (mineral spirits or Stoddard solvent or kerosene) consisting of a petroleum cut of hydrocarbons with a range of molecular weights have been used in cleaning applications for many years. Given the high boiling point and the potential for contaminants in some formulations, care must be taken in use and removal, and lot-to-lot variability may impact process control. It is possible to obtain very pure mineral spirit blends with a narrow, defined range of hydrocarbon chain lengths. Such well-defined hydrocarbon blends are better suited to high-precision applications, both for cold cleaning and, in the appropriately designed system, even for vapor-phase cleaning. Methyl soyate is a soy-derived substitute for mineral spirits. It is being developed for cleaning applications, and may prove to a renewable-resource alternative to hydrocarbon blends. d-Limonene Blends, -Pinene Blends In addition to d-limonene (citrus derived), which is discussed in more detail elsewhere in this book, cleaning agents have been based on -pinene (pine tree derived). Both have noticeable odors; both can leave appreciable residue, depending the application, and must be rinsed completely. In practice, d-limonene-based cleaners have proved more useful. As with many of the esters and other specialty cleaning agents, long-term inhalation exposure studies have not been conducted. Cyclic Volatile Methyl Siloxanes Linear volatile methylsiloxanes (VMSs) have been developed for critical cleaning processes and are discussed in Chapter 1.9 by Cull and Swanson. Cyclic VMSs alone and © 2001 by CRC Press LLC

in blends, have also been used in cold-cleaning applications where higher-boiling solvents are preferred. The cyclic VMSs do not have as favorable a toxicity profile as do linear VMSs. One supplier of cyclic VMS blends (QSOL) indicates a 10-ppm recommended inhalation level. Therefore, they should be used in enclosed, well-vented cleaning systems. Perfluorinated Compounds Perfluorinated compounds (PFCs) contain fluorine and carbon, but no chlorine or bromine. They are exceedingly mild, inert cleaners that can be used for removal of fluorinated lubricants and as rinsing and drying agents. PFCs are very effective for particulate removal. They are not ODCs. They are still sold alone and in various formulations. However, because of the global warming potential associated with a long atmospheric lifetime, often ranging in the thousands of years, industry has been under what might be termed strong regulatory encouragement to find substitutes. HFCs and HFEs can replace PFCs in many if not most applications. SOLVENT BLENDS, AZEOTROPES, COSOLVENTS Aqueous additives are used for such purposes as improving wettability, improving solubililization and removal of soils, compensating for water quality, and preventing corrosion. Blends involving solvents are a much more diffuse concept. Solvents are blended for a number of reasons. Blends can blur the lines of demarcation among various categories of cleaning agents. Stabilization Water and acidicity are the enemy of many halogenated solvents. Stabilizer packages are added to many chlorinated solvents (including TCA) and to n-propyl bromide (nPB) to prevent acid formation, breakdown, and reactivity with metals. Effective stabilization is important in degreasing (liquid/vapor cleaning). Stabilization becomes even more challenging in airtight and airless systems because the solvent is reused without replenishment over a much longer period than in traditional open-top degreasers. Extending the Solvency Range or Moderating the Solvency Solvent blends can provide custom-made, fine-tuned cleaning options. Sometimes, blends provide surprises. Azeotropes are strongly preferred over blends for vapor degreasing applications. An azeotrope is a constant-boiling mixture of two or more compounds. An extreme example of a nonazeotrope blend would be sugar in water. On heating, the water is boiled away, and the sugar remains. In contrast, in specific proportions, IPA and cyclohexane form a constant- boiling azeotrope. This means that the vapors contain both components in a constant proportion that does not change over the life of the blend. Azeotropes have to be managed with care. Even azeotropes can vary in composition if they are used at a temperature not in the azeotropic range. Blends that are not azeotropes will lose various components to evaporation at different rates. This means that the relative concentrations in the liquid and remaining blend will vary with time. Cleaning capability, compatibility, and flash point can all change. Blends © 2001 by CRC Press LLC

that are not true azeotropes should be viewed cautiously, particularly in vapor degreasing applications. In addition, azeotropes have been known to behave synergistically (nonadditively) in terms of performance and compatibility. These properties are not necessarily predicted by solvency parameters. In other words, while two solvents may each separately show acceptable materials compatibility with a given component, the blend could produce component deformation. Therefore, even if one thinks the solvency and compatibility issues of each component in an azeotrope are understood, it is prudent to test the mixture. Solvent blends are also used to modify or extend the solvency range in cold cleaning applications. An aggressive solvent can be toned down and a mild solvent made more aggressive. For example, HFE has been blended with NPB to tone down the aggressiveness of NPB. VMS may be blended with an alcohol to boost solvency. Cosolvents The terminology of cosolvents is a bit confusing. Most often, cosolvents are thought of as two chemicals used sequentially. However, strictly speaking, any blended solvent could be thought of as a cosolvent system. Cosolvents can be blends that are primarily aqueous or primarily solvent. Supercritical or liquid CO2 cleaning can also be accomplished with cosolvents. Surfactants Surfactants are used in solvent blends to provide some qualities similar to aqueous cleaners, to change emulsifying qualities, and to allow the solvent to be readily rinsed in water (as in cosolvent processes). Some blended cleaning agents are offered as similar formulations, with or without the surfactant. Emulsions: Macroemulsions, Structured Solvents, Microemulsions Oil and water do not mix, except in emulsions when they may coexist in a transient or relatively permanent form. An oil-and-vinegar salad dressing is typically a transient macroemulsion. Mayonnaise is a more permanent emulsion. Emulsifying qualities are used in aqueous blends, solvent blends, or both. In aqueous formulations, organic chemicals may be part of the mix only under certain conditions, such as temperature. For example, the separation of the organic phase in an aqueous cleaner at the operating temperature may serve to defoam the blend, allowing for spray applications. In the same way, immiscible organic chemicals may be used as emulsions, transient or permanent. Transient macroemulsions can be used to transfer the soil from one chemical to the other; the part is then rinsed in more of the chemical with very low solubility for the soil in question. Macroemulsions are typically cloudy. Sometimes one of the phases is aqueous; in other processes both are solvent. Microemulsions, structured solvents, liquid crystals, or continuous-phase emulsions have been introduced as cleaning agents. Structured solvents are stable mixtures of organic solvent, water, and coupling agents. The continuous phase may be solvent or water. They appear clear and may be primarily water or primarily solvent. Structured solvents can be made to separate during the application process. Such products are useful for mixed soils where both solvent and waterlike characteristics are desirable.10 © 2001 by CRC Press LLC

Mystery Mixes Blended solvents, particularly the high-boiling blends involving hydrocarbons, esters, and nonlinear alcohols, greatly extend the specificity of cleaning that can be obtained. However, blended solvents can be particularly difficult for the components manufacturer to evaluate. As with aqueous cleaning agents, many manufacturers consider the formulations to be highly proprietary and competition sensitive. Many of the comments regarding mystery mixes apply to both aqueous- and nonaqueous-based formulations. Some areas of concern in using complex mystery mixes include unexpected compatibility issues, regulatory constraints on one or more component, and unscheduled formulation changes. In such cases, it may be desirable to set up confidentiality agreements so that the ingredients are understood in detail. At the very least, particularly where the product is used in a process requiring high levels of validation and testing, it is prudent to obtain an agreement with the vendor that the product will not be changed. In addition, supposed improvements in formulations can have unintended consequences. This author has observed many instances where a blended product was “improved” in such a manner as to impact the process adversely. SOLVENCY AND PHYSICAL PROPERTIES; OTHER PARAMETERS Kauri-Butanol Number A number of solvency systems have been described in Chapter 1.2 by J. Burke. In addition, other solvency systems are in use. One cloud-point test, the Kauri-butanol (KB) number, is often referred to. The KB number is determined by the volume of solvent required to produce a defined degree of turbidity when added to standard solutions of Kauri resin in n-butyl alcohol. As a general rule, the higher the value, the stronger the solvent. The system was developed to indicate the relative solvent power of hydrocarbons, and it is not valid for oxygenated solvents. The KB number should be considered along with the boiling point, because, if the solvent can be heated to higher temperatures, more entropy is introduced into the system and better solvency occurs. Estimating solvency by mixing a cleaning agent with t-butyl alcohol and tree sap is a rather unsophisticated approach. However, the KB number remains widely used, and it is somewhat predictive of solvency.11 Table 3 lists the KB number and boiling points of several representative cleaning agents. Wetting Index The wetting index has been used as a guideline to the ability of a cleaning agent to penetrate closely spaced components. The wetting index is directly proportional to the density and inversely proportional to the surface tension and viscosity. In general, many of the vapor degreasing solvents have a higher wetting index than water or hydrocarbon blends. As with other indications, however, wetting index alone does not determine efficacy of cleaning. The wetting index of a few common cleaning agents are provided in Table 4.12 Other Physical Properties; Regulatory Features Physical properties of solvents constrain the range of choices. Other physical properties such as boiling point, flash point, and evaporation rate must be considered in choosing © 2001 by CRC Press LLC

Table 3 KB Number and Boiling Point (BP), Representative Cleaning Agents Cleaning Agent

KB No.

BP,°C

CFC-113 1,1,1-Trichloroethane HCFC 141b Methylene chloride Trichloroethylene n-Propyl bromide d-Limonene Parachlorobenzotrifluoride (PCBTF) HCFC 225 HFC 43–10 HFC 43–10 blend including trans-1,2,-dichloroethylene HFE 569sf2 VMS OS-10

32 124 56 136 129 125 68 64 31 9 30 10 17

48 74 32 40 87 71 150 139 54 55 37 76 100

a solvent, and the solvent must be considered in the particular regulatory microclimate where the process is being carried out. Tables 5a and 5b list some physical properties of some commonly used solvents, and a few regulatory considerations.13 The concept of a VOC-exempt solvent refers to the U.S. federal regulatory designations at the time of writing. Solvents that are VOC exempt are judged to have negligible reactivity relative to ethane. Many solvents, including those that are VOC exempt, can be used in vapor-phase cleaning applications. Those with low flash points, however, must be used in specially designed equipment. Such equipment has a high initial capital cost. Many solvents do not have a flash point but do have an upper explosion level (UEL) and a lower explosion level (LEL); this must be considered in specialized operations and in selecting and maintaining emission control equipment. The boiling point must be high enough to allow efficient cleaning, but not so high as to damage materials of construction or slow the build cycle. A very high boiling point may preclude use of the solvent in a standard vapor-phase degreasing operation. The evaporation rate must be sufficiently rapid to allow rapid drying, but not so rapid that the solvent is immediately lost. These considerations are all relative to the operation in question.

Table 4 Examples, Wetting Index Cleaning Agent Generally desirable CFC 113 TCA IPA NPB HCFC 225 HFE 449 sl Hydrocarbon blend Water Saponifier solution, 6% aqueous

© 2001 by CRC Press LLC

Density, g/cm3 High 1.48 1.32 0.785 1.33 1.40 1.52 0.84 0.997 0.998

Surface Tension, dyne/cm Low 27.4 25.9 21.7 25.9 16.8 14 27 72.8 29.7

Viscosity, cp Low 0.70 0.79 2.4 0.49 0.61 0.6 2.8 1.00 1.08

Wetting Index High 121 65 15 105 145 181 11 14 31

Table 5a Physical Properties, VOC, ODC Status

Cleaning Agent 1,1,1-Trichloroethane (ODC) CFC-113 HCFC-141b Stoddard solvent, typical (hydrocarbon blend) (VOC) n-Propyl bromide (VOC) Methylene chloride, VOC-exempt hazardous air pollutant Perchloroethylene, VOC-exempt hazardous air pollutant HCFC 225, VOC exempt HFE 569sf2, VOC exempt HFE 449s1, VOC exempt HFC 43-10mee, VOC exempt Water

Boiling Point, °C (°F)

Flash Point

UEL/LEL, %

Comments, Evap. Rate (ref. for evap rate)

74 (165)

None

15/7.0

5 (buac  1)a

48 (118)

None (TOC)

NA

32 (90) 152 (305)

None 40.6 (106)

17.7/7.6 6.1/1/1

0.45 (buac  1)  1 (ether  1) Hydrocarbon blend, VOC

71 (160)

None

8/3

4.5 (buac  1)

40 (104)

NA

19/12

NA

121 (250)?

None (TCC)

None

2.1 (buac  1)

54 (130)

None

None

0.9 (ether  1)

76 (169)

NA

—

NA

—

55 (131)

None (TCC, TOC) None (TCC, TOC) None (TOC)

NA

—

100 (212)

None

None

—

61 (142)

Note: These data were obtained from various standard publicly available references, primarily MSDS from the Cornell University Program Design Construction Web site (http://msds.pdc.cornell.edu/ISSEARCH/ MSDSsrch.htm); University of Vermont Web site, with some confirmation by Lange’s Handbook of Chemistry, 13th ed. McGraw-Hill, New York, and Dangerous Properties of Industrial Materials, 3rd ed., N. Irving Sax, Reinhold Book Corp). They should be used as guidelines only—the evaporation rate data in particular is prone to inconsistency among references. Boiling points rounded to nearest integer. Please confirm all information with current MSDS. buac  butyl acetate; NA = not available.

a

Costs Costs are relative. Few solvents are inexpensive, particularly if total process costs14 are considered. The most important considerations are cleaning agent quality and product stewardship by the chemical supplier.15,16 In terms of organic solvents, pound per pound, traditional solvents such as IPA, acetone, and the chlorinated solvents are relatively inexpensive. nPB and the VMSs are moderately priced, and the engineered solvents (HCFC 225, HFEs, and HFCs) are the most costly. Blended high-boiling solvents can vary markedly in price. The costs may be perceived © 2001 by CRC Press LLC

Table 5b Physical Properties, VOC, ODC Status, Low-Flash-Point Solvents

Cleaning Agent

Boiling Point, °C (°F)

Methyl acetate, VOC exempt t-Butyl acetate, proposed, VOC exempt para-Chlorobenzo trifluoride, VOC exempt Di-siloxane VMS, VOC exempt Tri-siloxane, VOC exempt Cyclo-tetrasiloxane, VOC exempt Acetone, VOC exempt

Flash Point

UEL/LEL, %

55.8 –58.2 (132 –137)

13 (9)

16/3.1

98 (208)

15 (59)

NA/1.5

139 (282)

43 (109)

10.5/0.9

100 (212)

3 (27) TCC

18.6/1.25

152 (306)

34 (94) TCC

13.8/0.9

205 (401)

76 (170) TCC

56 (134)

20 ( 4)

13/2.5

Comments, Evap. Rate (ref. for evap rate) Recently exempt, 5.3 (buac  1) Recently proposed, VOC exemption

Dow VMS OS10, 3.8 Dow VMS-OS20, 0.7 Dow VMS-OS245, not calc. 6

Note: These data were obtained from various standard publicly available references, primarily MSDS from the Cornell University Program Design Construction Web site (http://msds.pdc.cornell.edu/ISSEARCH/ MSDSsrch.htm), University of Vermont Web site, with some confirmation by Lange’s Handbook of Chemistry, 13th ed., (McGraw-Hill, New York), and Dangerous Properties of Industrial Materials, 3rd ed., N. Irving Sax, Reinhold Book Corp. They should be used as guidelines only—the evaporation rate data in particular is prone to inconsistency among references. Boiling points rounded to nearest integer. Please confirm all information with current MSDS. a buac  butyl acetate; NA = not available

as high in applications where soil loading is a problem and frequent solvent change-out is required. With heavy soil loading, it may be more effective to perform initial cleaning in a relatively inexpensive product, and then to conduct subsequent steps in the more sophisticated cleaning agent. Aqueous cleaning agents must be compared against each other in the intended application. Assume that two concentrates are under consideration and that one is twice as costly as the other. If the inexpensive cleaning concentrate must be used at a 1:4 dilution while the other provides equivalent performance at a 1:20 dilution, the picture changes.15 –16 Filtration17 markedly influences bath life and therefore modifies the overall cost of the cleaning agent. OVERALL CONSIDERATIONS One of the problems in developing a manufacturing process is the rather daunting list of considerations and provisos. To cope with the problem, there is the tendency to think linearly and to attempt to find the perfect cleaning agent. There is no perfect cleaning agent. However, we persist in our search for unattainable perfection. All too often, when a cleaning process is being developed, a cleaning agent selection committee is established to screen out all undesirable applicants. The safety/environmental group is likely to rule out any environmentally challenged cleaning agent, even if it © 2001 by CRC Press LLC

could be used nonemissively. Company management and sometimes the customer may submit a series of “don’t” lists. Whole classes of cleaning agents may be ruled out as being unacceptable on general environmental principles. The materials and process chemists may insist that for any cleaning agent to be considered, it should be able to be in contact with all materials of construction at some elevated temperature for, say, 24 hours. The purchasing department may insist that only one or two cleaning agents be selected—period. The manufacturing engineers may insist on an extremely rapid process time, instant drying, and aqueous cleaning. What’s left? Sometimes nothing, sometimes a class of cleaning agents that is totally unsuited to the cleaning application at hand. Perfection aside, for nearly every cleaning application, there are several workable solutions. Some of the considerations in cleaning agent selection are indicated in Table 6. The cleaning agent has to be considered in the context of the cleaning process and, indeed, in the context of the overall manufacturing process. The factors indicated in Table 6 are meant to provide a starting point. It usually becomes very apparent which factors are the most important in a given manufacturing situation. It is more productive to proceed with a nonlinear approach that considers performance, costs, cleaning agent, cleaning equipment, suitability to the workforce, worker safety, and the local regulatory microclimate. Table 6 Overall Considerations, Choosing Cleaning Agents Factor

Process Consideration

Cleaning properties, cleaning performance

• • • • • • • • • •

Materials compatibility

Residue

Cycle time

Cleaning equipment

© 2001 by CRC Press LLC

• • • • • • • • • • • • • • • • • • • •

Cleaning requirements of your process (how clean is clean enough) Performance under actual process conditions Solubility characteristics relative to soil of interest Wetting ability Boiling point Evaporation rate Soil loading capacity Ability to be filtered Ability to be redistilled Compatibility under actual process conditions (temperature, time of exposure) Product deformation at cleaning, rinsing, drying temperatures Nonvolatile residue (NVR) level Rinsing requirement Process time impact Cleaning Rinsing Drying Product cool-down Component fixturing Loading and unloading equipment Product rework Suitability with current cleaning equipment Ability, costs of retrofit Costs of new cleaning equipment Auxiliary equipment required Maintenance, repair Automation, component handling Footprint (length, width, height) Equipment weight Component fixturing (continued )

Table 6 Overall Considerations, Choosing Cleaning Agents (continued ) Factor

Process Consideration

Flash point

• • • • • • • •

Toxicity

Worker acceptance

Cleaning agent management

Regulatory, air

Regulatory, water Company, customer, product performance requirements

Costs

© 2001 by CRC Press LLC

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Choice of cleaning equipment Process control Control of proximal processes, activities Choice of auxiliary equipment Choice of emissions control Acute Long term Anticipated exposure under process conditions (including sprays, mists) Employee monitoring Inhalation Skin adsorption Method of application Drying speed Similarity to current process Automation Computer skills Perceived loss of control of process Odor Water preparation In-process filtration (water, organic, or aqueous cleaning agent) Wastewater disposal On board redistillation Global, national, local (ODC, VOC, HAPs, GWP) Neighborhood concerns Environmental justice issues Production phaseout Usage bans Disposal of waste stream Global, national, local Disposal of waste streams Contractual requirements, restrictions Company policy Testing, acceptance qualification required In-house safety, environmental policy Insurance company issues Cleaning agent Cleaning agent preparation and disposal Costs as-used (dilution) Capital equipment Disposables Sample handling Total process time Rework Insurance Regulatory permitting Process qualification Employee education, training Process monitoring

Table 6 Overall Considerations, Choosing Cleaning Agents Factor

Process Consideration

Supplier stewardship, cleaning equipment supplier

• • • • • •

Responsive distributor, supplier Supportive technical staff Clear, understandable MSDS Provides required technical information Provides required regulatory information Supports process development

HAP  hazardous air pollutant; GWP  global-warming potential; MSDS  Material Safety Data Sheet.

REFERENCES 1. Kanegsberg, B., Precision cleaning without ozone depleting Chem. Ind., 20, 787 –791, 1996. 2. Kanegsberg E. and B. Kanegsberg, Cleaning by abrasive impact A2C2 Mag, May 2000. 3. Dibble, C., EPA SNAP program update: solvent cleaning, presented at Nepcon West 2000, February 29, 2000, Anaheim, CA. 4. Information provided by K. Neugebauer, V.P. Specialty Fluorides, Solvay Fluorides, Inc. 5. Zipfel, L. and P. Dournel, HFC-365mfc, the key for high performance rigid polyurethane foams, presented at UTECH 2000, The Hague, The Netherlands, April 2000. 6. Information provided by G. Knopeck, Manager, Fluorocarbon Technical Services, Honeywell International. 7. Knopeck, G., Pentafluoropropane: an HFC solvent for aerosols, presented at SATA (Southern Aerosol Technology Association) Spring Meeting, Atlanta, GA, April 2000. 8. Waldrop, M.W., BASF, communications and technical summaries, January 2000. 9. Elias, W.G., Real-life applications with environmentally compliant solvents for electronics, Proc., Nepcon West 2000, Anaheim, CA. 10. Shick, R.A., Formulating cleaners with structured solvents, in Proc., Precision Cleaning 96, Anaheim, C.A., 1996, 285 –289. 11. Kenyon, W.G. and B. Kanegsberg, Accelerating the Change to Environmentally Preferred, CostEffective Cleaning Processes, Tutorial with Precision Cleaning ‘95, Chicago, May 1995. 12. Kenyon, W.G., Wetting Index, personal communication. 13. Kanegsberg, B., Economic and environmental costs of process conversion, presented at Nepcon West 2000, February 29, Anaheim, CA, 2000. 14. Kanegsberg, B. and C. LeBlanc, The cost of process conversion, (in Report to Toxic Use Reduction Institute, B. Kanegsberg Proc., CleanTech‘99, Rosemont, IL, May 1999. 15. O’Neill, E., A. Miremadi, R. Romo, M. Shub, and B. Kanegsberg, Four steps to process conversion, Parts Cleaning Mag., May 2000. 16. O’Neill, E., A. Miremadi, A. Guzman, R. Romo, M. Shub, and B. Kanegsberg, Simplifying aqueous cleaning, the value of practical experience, Products Finishing Mag., August 2000. 17. Kanegsberg, E., Liquid filtration in critical cleaning, A2C2 Mag., April 2000.

© 2001 by CRC Press LLC

CHAPTER 1.2

Solvents and Solubility John Burke

CONTENTS Introduction Solutions Molecular Attractions Cohesive Energy Solubility Parameters Three Component Parameters The Teas Graph Visualizing Solubility Solvent Mixtures References INTRODUCTION Solvents are ubiquitous. Not a day goes by when we don’t rely on one solvent or another, to accomplish some essential task. Given such frequent experience with solubility, we might conclude that we all would have a pretty good idea of how solvents work. And yet, who among us hasn’t tried in vain to remove one substance from another, guided by rules of thumb such as “like dissolves like” or vague concepts of solvent “strength.” While this approach may often succeed, it also might be risky if, for example, we needed to dissolve one material selectively while leaving other materials completely unaffected. Or, it would be clearly inefficient if, at the same time, we were also trying to control evaporation rates, solution viscosity, material costs, or environmental and health effects. The selection of a solvent or solvent mixture in the face of complex criteria moves beyond trial and error, and of necessity must rely on a system that can organize and predict solubility behavior. While this could be accomplished empirically, by simply testing the effects of specific solvents on specific materials, a universal system that could encompass solubility behavior in general would be immensely useful. Although understanding such a system may seem dauntingly complex, the practical application of solubility theory is actually quite straightforward. In fact, many solubility interactions can be predicted on a simple triangular graph. But before we can accomplish this feat, a certain amount of both theoretical and historical background is in order. © 2001 by CRC Press LLC

SOLUTIONS When two liquids are mixed together, simply stated, they either stay together or they don’t. They may stay together because of a chemical reaction that changes the liquids into some other compound, usually by an exchange of electrons (in which case it may be difficult to retrieve the original materials). Fortunately, for the purposes of this discussion, we can ignore solutions of this kind, since ionic and even water-based solutions are beyond the scope of simple solubility. Another reason a mixture may stay together is because the two materials are mutually soluble, such as gin and tonic. Simple solubility implies that the individual materials remain essentially unchanged even while mixed, and can usually be separated with some ease, say, by distillation. Solutions of this kind usually include solvents and are easier to characterize and predict. But to say this kind of interaction is simple may be misleading. Not just any two or three solvents may be successfully combined. When asked why oil and water do not mix, most people will reply that the oil is “lighter” than the water. Yet their mutual repulsion is not at all due to gravity. And at the limits of solubility solutions may become clear, cloudy, or separate out again after the slightest change in concentration. How can these seemingly complex behaviors be accounted for? Actually, dissolving a thing is very similar to evaporating it—but to understand that remark we first need to know a bit of what happens on the surfaces of molecules.

MOLECULAR ATTRACTIONS Liquids and solids differ from gases basically because their molecules stick together, resisting the tendency to evaporate completely into space. This must mean that the molecules that make up liquids and solids are somehow attracted to each other, that there is some kind of intermolecular stickiness that prevents them from flying apart into a gas. These sticky forces between molecules were first described by Johannes van der Waals in 1873, and thus bear his name. Originally thought to be small gravitational attractions, van der Waals forces are actually caused by electromagnetic interactions between molecules. The outer shell of an atom is composed entirely of a cloud of negatively charged electrons, completely enclosing the positively charged nucleus within. Molecules, because they are composed of atoms, are also covered by a cloud of electrons. In both cases we can imagine a positively charged center surrounded by a negatively charged outer shell. These positive and negative charges essentially balance out, with the result that the atom or molecule as a whole is neutral. It’s easy to see that the negatively charged surfaces of adjacent molecules should repel each other like the negative poles of two magnets (and it’s a good thing too, or the universe would collapse), but we were speaking earlier of intermolecular stickiness. How can we account for this anomaly? In reality, the electron cloud is never evenly distributed around a molecule. A single molecule, because of its structure, can exhibit a variety of electromagnetic charges on its surface, some strong and some weak, some which cancel out, and some which reinforce each other. The resulting sum of all the charges is what is known as the dipole moment of the molecule. Molecules that have permanent dipole moments are said to be polar, while molecules in which all the dipoles cancel out (zero dipole moment) are said to be nonpolar. Some atomic elements attract electrons more vigorously than others, causing the electrons to be unequally shared between the individual atoms in a molecule. If the molecule © 2001 by CRC Press LLC

is symmetrical, these charges may cancel out. If, on the other hand, the electron density is permanently imbalanced, with some atoms in the molecule harboring a greater share of the negative charge distribution, the molecule itself will be polar. The polarity of a molecule is related to its atomic composition, its geometry, and its size. A particularly strong type of polarity, for example, occurs in molecules where a hydrogen atom is attached to an extremely electron-hungry atom such as oxygen, nitrogen, or fluorine. In these cases, the sole electron of hydrogen is drawn toward the electronegative atom, leaving the strongly charged hydrogen nucleus exposed. In this state the exposed positive nucleus can exert a considerable attraction on electrons in other molecules, forming what is called a protonic bridge that is substantially stronger than most other types of intermolecular attractions. This special kind of polar attraction between molecules is called hydrogen bonding, and plays a major role in solubility as we shall see. But what about nonpolar molecules, where electron clouds are evenly distributed? The source of electromagnetic attractions in this case stems from the random movement of the electron cloud. From instant to instant, random changes in electron distribution give rise to polar fluctuations that shift about the molecular surface. Since the distribution of the electron cloud is uneven (maybe thicker in one place and thinner in another), small local charge imbalances are created. The parts of the molecule with a greater electron density will be more negatively charged, and the electron-deficient parts will be more positively charged. Although no permanent polar configuration is formed and the molecule is essentially nonpolar, numerous temporary poles are created constantly, move about, and disappear. When two molecules are in proximity, the random polarities in each molecule tend to induce corresponding polarities in one another, causing the molecules to fluctuate together. This allows the electrons of one molecule to be temporarily attracted to the nucleus of the other, and vice versa, resulting in a play of attractions between the molecules. These induced attractions are called dispersion forces. The degree of dispersion forces that these temporary dipoles confer on a molecule is related to surface area: the larger the molecule, the greater the number of temporary dipoles, and the greater the intermolecular attractions. Molecules with straight chains have more surface area, and thus greater dispersion forces, than branched-chain molecules of the same molecular weight. Deviations in electron shell density, therefore, result in minute magnetic imbalances, so that each molecule as a whole becomes a small magnet, or dipole. These electron density deviations depend on the physical architecture of the molecule. Certain molecular geometries will be strongly polar. Other molecules may be nonpolar, but are still capable of dispersion forces. It is these electromagnetic effects that account for the intermolecular stickiness holding liquids and solids together. Now we can explore the intriguing relationship among vaporization, van der Waals forces, and solubility. COHESIVE ENERGY To bring a liquid to its boiling point, we usually add energy in the form of heat. This heating raises the temperature of the liquid until it finally begins to boil. Once the liquid reaches its boiling point, however, the temperature of the liquid will not continue to increase. Any subsequent heat added is used up in continuing the boiling and separating the molecules of the liquid into a gas. Only when all the liquid has been completely vaporized will the temperature again begin to rise. If we measure the amount of energy that we add between the moment that boiling starts and the point when all the liquid has boiled away, we will have a direct indication of the © 2001 by CRC Press LLC

amount of energy required to separate that amount of liquid into a gas. Interestingly, this is also a measure of the amount of van der Waals forces that held the molecules of that liquid together. It is important to note here that the temperature at which the liquid begins to boil is not important, but rather the amount of heat that has to be added to separate the molecules. A liquid with a low boiling point may require considerable energy to vaporize, while a liquid with a higher boiling point may vaporize quite readily, or vice versa. Regardless of the temperature at which boiling begins, the liquid that vaporizes readily has fewer intermolecular attractions than the liquid that requires considerable addition of heat to vaporize. The energy required to vaporize the liquid is called, not surprisingly, the heat of vaporization, and reflects the attractions that exist between its molecules. Here is the connection we have been looking for: vaporization and solubility are similar because the same intermolecular attractive forces have to be overcome to vaporize a liquid as to dissolve it. This can be understood by thinking about what happens when two liquids are mixed. The molecules of each liquid have to be physically separated by the molecules of the other liquid. For any solution to occur, solvent molecules must be separated from each other to penetrate between the molecules of the solute. At the same time, the solute molecules must also overcome their own intermolecular stickiness to allow solvent molecules between and around them. This is similar to the molecular separations that need to occur during vaporization. The same intermolecular van der Waals forces must be overcome in both cases. So it stands to reason that for two materials to be soluble in each other their internal energies must be similar. The molecules of a polar liquid (such as water) with strong intermolecular attractions just will not be separated by the molecules of a nonpolar liquid (such as oil) that are held together only by weak dispersion forces.

SOLUBILITY PARAMETERS If we wanted to give a number to these attractions independent of temperature (basically to put everything on an even playing field), we can derive a value called the cohesive energy density from the heat of vaporization by the following formula: RT   c  H V

(1)

m

where c  cohesive energy density H  heat of vaporization R  gas constant T  temperature Vm  molar volume In 1936, Joel H. Hildebrand, in his landmark book on the solubility of nonelectrolytes, proposed the square root of the cohesive energy density as a numerical value indicating the solvency behavior of a specific solvent.





RT   δ  c  H V m

1/2

(2)

It was not until the third edition in 1950 that the term solubility parameter was proposed for this value, represented by the symbol δ. Subsequent authors have proposed that the © 2001 by CRC Press LLC

Table 1 Hildebrand Solubility Parameters Solvent

Parameters

n-Pentane n-Hexane Freon ® TF n-Heptane Diethyl ether Cyclohexane Amyl acetate 1,1,1-Trichloroethane Carbon tetrachloride Xylene Toluene Ethyl acetate Benzene Chloroform Trichloroethylene Tetrahydrofuran Cellosolve acetate Acetone Ethylene dichloride Methylene chloride Diacetone alcohol Butyl Cellosolve Morpholine Pyridine Cellosolve n-Butyl alcohol Ethyl alcohol Dimethyl sulfoxide n-Propyl alcohol Dimethylformamide Methyl alcohol Propylene glycol Ethylene glycol Glycerol Water

(7.0) 7.24 7.25 (7.4) 7.62 8.18 (8.5) 8.57 8.65 8.85 8.91 9.10 9.15 9.21 9.28 9.52 9.60 9.77 9.76 9.93 10.18 10.24 10.52 10.61 11.88 11.30 12.92 12.93 11.97 12.14 14.28 14.80 16.30 21.10 23.5

Sources: Hildebrand values from Hansen.6 Values in parenthesis from Crowley et al.3

term hildebrands be adopted for these units, in recognition of Dr. Hildebrand’s contribution. Table 1 lists solvents arranged according to their solubility parameter. In looking over a table of Hildebrand solubility parameters, it becomes apparent that by ranking solvents according to solubility parameter a solvent spectrum is obtained, with solvents occupying positions in proximity to other solvents of comparable “strength.” For example, if acetone dissolves a particular material, it may likely be soluble in neighboring solvents, like diacetone alcohol or methyl ethyl ketone, since these solvents have similar internal energies. It may not be possible to achieve solutions in solvents farther from acetone on the chart, such as ethyl alcohol or cyclohexane—liquids with very different internal energies. Theoretically, there will be a contiguous group of solvents that will dissolve a particular material, while the rest of the solvents in the spectrum will not. Some materials © 2001 by CRC Press LLC

will dissolve in a large range of solvents, while others might be soluble in only a few. A material that cannot be dissolved at all, such as a thermosetting resin, might exhibit swelling behavior in precisely the same way. Another interesting aspect of the solvent spectrum is that the Hildebrand value of a mixture can be determined by averaging the values of the individual solvents by volume. For example, a mixture of two parts toluene and one part acetone will have a Hildebrand value of 18.7 (18.3  2/3  19.7  1/3), about the same as chloroform. Theoretically, such a 2:1 toluene/acetone mixture should behave similarly to chloroform. Thus, for example, if a resin was soluble in one, it would probably be soluble in the other. What’s attractive about this approach is that it attempts to predict the properties of a mixture using only the properties of its components. No empirical information about the mixture is required. But, as you might expect, this is not completely accurate. Figure 1 plots the swelling behavior of a dried linseed oil film in various solvents arranged according to Hildebrand number. Of the solvents listed, chloroform swells the film to the greatest degree, about six times as much as ethylene dichloride, and over ten times as much as toluene. Solvents with greater differences in Hildebrand value have less swelling effect, and the range of peak swelling occupies less than 2 Hildebrand units. Theoretically, we would expect any solvent or solvent mixture with a Hildebrand value between 19 and 20 to swell a linseed oil film severely. But careful examination of the graph reveals an anomaly. Two solvents with Hildebrand values right in the middle of the severe swelling range, methyl ethyl ketone (19.3) and acetone (19.7), actually cause very little swelling behavior. How can this be? According to the theory, liquids with similar cohesive energy densities should have similar solubility characteristics, and yet actual behavior in this instance does not bear this out. To understand the reason for this discrepancy we need to recall our previous discussion about van der Waals forces. Remember the point about different molecular architectures giving rise to different kinds of polarity? Some materials are made of molecules that

Figure 1

The swelling behavior of linseed oil films in solvents arranged according to solubility parameter. (Adapted from Feller et al.4)

© 2001 by CRC Press LLC

are strongly polar, especially molecules capable of hydrogen bonding. Other materials are made of molecules that are essentially nonpolar, with intermolecular attractions due entirely to dispersion forces. It is precisely these differences in polarity that also must be taken into account if we want to improve our solubility predictions. The inconsistencies in Figure 1 stem from a difference in hydrogen bonding between chlorinated solvents and ketones. The intermolecular forces in linseed oil are primarily due to dispersion forces, with practically no hydrogen bonding involved. These polar configurations are perfectly matched by the intermolecular forces between chloroform molecules, thus encouraging interpenetration and swelling of the linseed oil polymer. Acetone and methyl ethyl ketone, however, are more polar molecules with moderate hydrogen bonding capabilities. Even though the total cohesive energy densities, and therefore Hildebrand solubility parameters, are similar in all four solvents, the differences in polarity, primarily hydrogen bonding, lead to differences in solubility behavior. A scheme to overcome the inconsistencies caused by hydrogen bonding was proposed by Harry Burrell in 1955.2 His solution was to segregate the solvent spectrum into three separate lists depending on hydrogen bonding capability. This is briefly summarized as follows: 1. Weak hydrogen bonding: Hydrocarbons, including chlorinated and nitrocompounds; 2. Moderate hydrogen bonding: Ketones, esters, ethers, and glycol monoethers; 3. Strong hydrogen bonding: Alcohols, amines, acids, amides, and aldehydes. Accordingly, solvents with similar Hildebrand numbers and similar polarity (especially hydrogen bonding) should exhibit similar solubility. This system of classification does in fact improve the prediction of solvent behavior, and is still widely used in practical applications.

THREE COMPONENT PARAMETERS But it was only a matter of time before researchers needing even greater precision in quantifying solvent behavior began to look at the cumulative effects of several different kinds of van der Waals forces. It was found that by using separate values for dispersion forces, polarity, and hydrogen bonding even greater accuracy was achievable. But the addition of a third value created practical problems in portraying this information, since it could not be presented in a simple list. For this reason most three-component parameter systems were represented as slices through three-dimensional solubility models. Unfortunately, this made developing formulations using solvents outside the slice impractical. The most widely accepted three-component system was developed by Charles M. Hansen in 1966. Hansen took the existing three values farther by relating all three to the total Hildebrand value. This was done by first getting the dispersion parameter of a solvent from the Hildebrand value of the nonpolar molecule most closely resembling it in size and structure (as n-butane would be to n-butyl alcohol). Hansen then used trial-and-error experimentation on numerous solvents and polymers to separate the polar value into polar and hydrogen bonding component parameters best reflecting empirical evidence. Hansen also used three-dimensional models but also found that, by doubling the dispersion parameter axis, an approximately spherical volume of solubility could be formed. This volume, being spherical, could then be described in a simple manner: the coordinates at the center of the solubility sphere could be located by means of the three component © 2001 by CRC Press LLC

parameters, along with the radius of the sphere, which he called the interaction radius (R). The mathematics involved are inconvenient, however, especially where solvent blends are concerned. THE TEAS GRAPH Given the awkwardness of presenting three-component data, Jean P. Teas devised a triangular graph in 1968, on which polymer solubility areas could be drawn in their entirety. Because of its clarity and ease of use, the Teas graph has found increasing application in problem solving, documentation, and analysis, and is an excellent vehicle for understanding complex solubility behavior. To plot all three parameters on a single planar graph, however, a departure had to be taken from established theory. Teas constructed his graph on the fantastic hypothesis that all materials have the same Hildebrand value. According to this assumption, solubility behavior is determined, not by differences in the Hildebrand value, but by the relative amounts the three component forces (dispersion force, polar force, and hydrogen bonding force) contribute to that value. This allows us the convenience of related percentages rather than unrelated sums. But because Hildebrand values are not the same for all liquids, it should be remembered that the Teas graph is somewhat more empirical than theoretical. Fortunately, this does not prevent it from being an accurate and useful tool, and perhaps the most convenient method by which solubility information can be illustrated. Hansen derived his parameters from the Hildebrand value: when all three Hansen parameters for a solvent are added together, their sum will be the Hildebrand value for that solvent. Teas parameters, also called fractional parameters, are mathematically derived from Hansen values by calculating the relative amount that each Hansen parameter contributes to the whole. In other words, when all three of Teas’s fractional parameters are added together, their sum will always be 100. Once this step has been taken, the rest is easy. Now the intersection of these three values can be easily located on a triangular grid (Figure 2).

Figure 2

Any point on a triangular Teas graph is the intersection of three percentage values.

© 2001 by CRC Press LLC

If we examine a Teas graph containing the locations of many solvents we can see that the alkanes, whose only intermolecular bonding is due to dispersion forces, are located in the far lower right corner, the corner corresponding to 100% dispersion forces. Moving toward the lower left corner are solvents with increasing hydrogen bonding contribution. Moving from the bottom of the graph upward are solvents with polarity due less to hydrogen bonding than to an increasingly greater dipole moment of the molecule as a whole. Overall, the solvents are grouped closer to the lower right apex than the others. This is because the dispersion force is present in all molecules, polar or not, and determining the dispersion component is the first calculation in assigning Hansen parameters, from which fractional parameters are derived. Unfortunately, this greatly overemphasizes the dispersion force relative to polar forces, especially hydrogen bonding interactions. It can also be seen that increasing molecular weight within each class shifts the relative position of a solvent on the graph closer to the bottom right apex (Figure 3). This is because, as molecular weight increases, the polar part of the molecule that causes the specific character identifying it with its class, called the functional group, is increasingly “diluted” by progressively larger, nonpolar “aliphatic” molecular segments. This gives the molecule as a whole relatively more dispersion force and less of the polar character specific to its class. This trend toward less polarity with increasing molecular weight within a class also accounts for the observation that lower-molecular-weight solvent’s are often “stronger” than higher-molecular-weight solvents of the same class, although determinations of solvent strength must really be made in terms of the solvent’s position relative to the solubility area of the solute. (Another reason for low-molecular-weight solvents seeming more active is that smaller molecules can disperse throughout solid materials more rapidly than their bulkier relatives.) The only class in which increasing molecular weight places the solvent farther away from the lower right corner is the alkanes. As previously stated, the intermolecular attractions between alkanes are due entirely to dispersion forces, and, accordingly, Hansen parameter values for alkanes show zero polar contribution and zero hydrogen bonding contribution. Since fractional parameters are derived from Hansen parameters, we would expect all the alkanes to be placed together at the extreme right apex. Observed behavior indicates, however, that different alkanes do have different solubility characteristics, perhaps because of the tendency of larger dispersion forces to mimic

Figure 3

Within each class, solvents with higher molecular weight tend to be closer to the lower right axis.

© 2001 by CRC Press LLC

slightly polar interactions. For this reason, Teas adjusted the locations of the alkanes to correspond to empirical evidence. Several other solvent locations were also shifted slightly to reflect observed solubility characteristics properly. The position of water on the chart is uncertain, because of the ionic character of the water molecule, and the placement in this paper is according to Teas.12 The presence of water in a solvent blend, however, can alter dramatically the accuracy of solubility predictions. VISUALIZING SOLUBILITY Now that solvent positions are located on the Teas graph, we can discuss (and visualize) complex solubility behavior. For example, it is easy to describe the solubility of a material by testing solvents on it and coding the results. Active solvents could have their positions marked with a star, marginal solvents with a filled circle, and nonsolvents with hollow circles. Once this is done, a solid area of stars will be seen, possibly with a border of filled circles. This would define the solubility window of the material (Figure 4). Not every solvent would need to be tested in this way; only enough to circumscribe the area. The boundaries of this solubility window could be more accurately defined by using two liquids near the edge of the solubility window, one within the window and one outside. The material could then be tested in various mixtures of these two liquids, and the mixture just producing solubility noted on the graph. If this procedure is repeated in several locations around the edge of the window, its boundaries may be accurately determined. Interestingly, some composite materials (such as rubber/resin pressure-sensitive adhesives, or wax/resin mixtures) can exhibit two or more separate solubility windows, more or less overlapping, that reflect the degree of compatibility and the concentration of the original components. The solubility window of a material will have a specific size, shape, and placement on the Teas graph depending on its polarity and molecular weight, and the temperature and concentration at which the measurements are made. Most published solubility data are derived from 10% concentrations at room temperature. Heat has the effect of increasing the size of the solubility window, because of an increase in the disorder (entropy).

Figure 4

The solubility window of a hypothetical material.

© 2001 by CRC Press LLC

Concentration also has an effect on solubility. Most polymer solubility windows are determined at 10% concentration of polymer in solvent. Because an increase in concentration also causes an increase in disorder, solubility information can be considered accurate for solutions of higher concentration as well. Solvent evaporation as the solution dries serves to increase concentration, thus ensuring that the two materials stay mixed. Solutions of less than 10% may become immiscible, however, especially with solvent combinations at the edge of the solubility window. Solution viscosity of a polymer will also vary depending on where solvent is located in the solubility window of the polymer. We might expect viscosity to be at a minimum when a solvent near the center of a polymer solubility window is used. However, this is not the case. Solvents at the center of a polymer solubility window dissolve the polymer so effectively that the individual polymer molecules are free to uncoil and stretch out. In this condition molecular surface area is increased, with a corresponding increase in intermolecular attractions. The molecules thus tend to attract and tangle on each other, resulting in solutions of slightly higher than normal viscosity. When dissolved in solvents slightly off center in the solubility window, polymer molecules stay coiled and grouped together into microscopic clumps, which tend to slide over one another, resulting in solutions of lower viscosity. As solvents nearer and nearer the edge of the solubility window are used to dissolve the polymer, however, these clumps become progressively larger and more connected and viscosity again increases until ultimately separation occurs as the region of the solubility boundary is crossed. The position of a solvent in the solubility window of a polymer has a marked effect on the dried film characteristics of the polymer as well. Because of the uncoiling of the polymer molecule, films cast from solvent solutions near the center of the solubility window exhibit greater adhesion to compatible substrates. This is due to the increase in polymer surface area that comes in contact with the substrate. Many other properties of dried films, such as plastic crazing or gas permeability, are related to the relative position that the original solvent occupied in the solubility window of the polymer. The degree of both crazing and permeability is predictably less when solvents more central to the solubility window have been used.

SOLVENT MIXTURES The Teas graph is particularly useful for creating solvent mixtures for specific applications. Solvents can easily be blended to exhibit critical solubility behavior such as dissolving one material but not another. The use of the Teas graph can reduce trial-and-error experimentation to a minimum, by allowing the solubility behavior of mixtures to be predicted in advance. This is because the solubility parameters of a mixture can be simply calculated by averaging its components. We can then expect the mixture to behave more or less like the single solvent with that solubility parameter. Determining the solubility parameters of a mixture can be done either by calculating from the fractional parameters of the individual solvents, or in the case of a binary mixture, by simply drawing a line between its two solvents and measuring their ratio. To calculate the solubility parameters from the individual components, the fractional parameters for each liquid are multiplied by the fraction that the liquid occupies in the blend, and the results for each parameter added together. Or, graphically, a mixture can be located by drawing a line connecting the two solvents and determining the distance that represents their ratio. © 2001 by CRC Press LLC

Figure 5

A mixture (M) of two nonsolvents (A and B) may act as a true solvent if the parameters of the mixture fall within the solubility window.

What is interesting about visualizing solvent blends on a Teas graph is the control with which effective solvent mixtures can be formulated. For example, two liquids that are nonsolvents for a specific polymer can sometimes be blended in such a way that the mixture will act as a true solvent (Figure 5). This is possible if the graph position of the mixture lies inside the solubility window of the polymer, and is most effective if the distance of the nonsolvents from the edge of the solubility window is not too great. This phenomenon is also valuable when selective solvent action is required, such as in selectively dissolving one material while leaving other materials unaffected, particularly if the solubilities of the materials involved are very similar. In this case it is helpful first to plot the solubility windows of all the materials in question. Once this has been done, it is easy to see the overlap of solubilities, and any areas where solubilities are mutually exclusive. A solvent blend can then be formulated that actively dissolves the proper material, while positioned as far away from the solubility window of the other material as possible (Figure 6). It is important to remember that differences in evaporation rates can shift the solubility parameter of the blend as the solvents evaporate, and this must be taken into account. Additionally, while a material may not show immediate signs of solution in a solvent or solvent blend, the solvent may still adversely affect the material, for example, by softening it or leaching out partial components. A further advantage of blending solvents is the ability to design mixtures with similar solubility characteristics and lower toxicity. In such cases, it should be pointed out that the similarity between solvents and blends having the same numerical parameters decreases as the distance between the components of the blend increases. Where alternate blends are effective, however, the use of a less toxic replacement can be a sensible choice, and the Teas graph a useful tool (Figure 7).

© 2001 by CRC Press LLC

Figure 6

In situations where one material must be dissolved while another remains unaffected, a solvent blend that falls within the solubility window of the first and outside the window of the second may be effective.

Figure 7

The Teas graph (numbers indicate solvents listed in Table 2).

© 2001 by CRC Press LLC

Table 2 Fractional Solubility Parameters Position

Solvent

100d

100p

100h

Alkanes 1 1 1 1 2 3 4

n-Pentane n-Hexane n-Heptane n-Dodecane Cyclohexane VMP naphtha Mineral spirits

100 100 100 100 94 94 90

0 0 0 0 2 3 4

0 0 0 0 4 3 6

8 7 5 8 4 3 0

14 13 12 22 18 10 3

21 19 12 12 2 19 17 10

20 14 21 20 13 11 8 0

13 19 7 22 20 18 21 23 18

23 26 26 39 38 36 35 29 36

32 [30] 28 27 24 22 20

21 [17] 17 17 21 20 18

Aromatic Hydrocarbons 5 6 7 8 9 10 11

Benzene Toluene o-Xylene Naphthalene Styrene Ethylbenzene p-Diethylbenzene

78 80 83 70 78 87 97 Halogen Compounds

12 13 14 15 16 17 18 19

Methylene chloride Ethylene dichloride Chloroform Trichloroethylene Carbon tetrachloride 1,1,1-Trichloroethane Chlorobenzene Trichlorotrifluoroethane

59 67 67 68 85 70 65 90 Ethers

20 21 22 23 24 25 26 27 25

Diethyl ether Tetrahydrofuran Dioxane Methyl Cellosolve Cellosolve Butyl Cellosolve Methyl carbitol Carbitol Butyl carbitol

64 55 67 39 42 46 44 48 46 Ketones

28 29 30

31 32

Acetone Methyl ethyl ketone Cyclohexanone Diethyl ketone Mesityl oxide Methyl isobutyl ketone Methyl isoamyl ketone

© 2001 by CRC Press LLC

47 [53] 55 56 55 58 62

Table 2 Fractional Solubility Parameters (continued ) Position

Solvent

100d

100p

100h

33

Isophorone Di-isobutyl ketone

51 [67]

25 [16]

24 [17]

36 38 18 [37] 12 39 13 15 12 12 15 21 20

19 14 31 [24] 24 19 27 25 25 28 34 35 32

45 41 47 43 37 36 26 15 19 32 30 [12] 42 32

16 15 13 13 13 12 18 28 31 20 32 [24] 30 27

8 36

4 23

22 18 16 [16] 15 [16] 16 12 13

48 46 44 [43] 42 [40] 36 38 41

Esters 34 35 36

37 38 39 40

Methyl acetate Propylene carbonate Ethyl acetate Trimethyl phosphate Diethyl carbonate Diethyl sulfate n-Butyl acetate Isobutyl acetate Isobutyl isobutyrate Isoamyl acetate Cellosolve acetate Ethyl lactate Butyl lactate

45 48 51 [39] 64 42 60 60 63 60 51 44 40

Nitrogen Compounds 41 42 43 44 45 46 47 48 49 50 51 52

Acetonitrile Butyronitrile Nitromethane Nitroethane 2-Nitropropane Nitrobenzene Pyridine Morpholine Aniline N-Methyl-2-pyrrolidone Diethylenetriamine Cyclohexylamine Formamide N,N-Dimethylformamide

39 44 40 44 50 52 56 57 50 48 38 [64] 28 41

Sulfur Compounds 53 54

Carbon disulfide Dimethyl sulfoxide

88 41 Alcohols

55 56 57 58 59

60 61

Methanol Ethanol 1-Propanol 2-Propanol 1-Butanol 2-Butanol Benzyl alcohol Cyclohexanol n-amyl alcohol

© 2001 by CRC Press LLC

30 36 40 [41] 43 [44] 48 50 46

Table 2 Fractional Solubility Parameters (continued ) Position

Solvent

62

Diacetone alcohol 2-Ethyl-1-hexanol 2-Ethyl-1-butanol

100d 45 50 48

100p

100h

24 9 10

31 41 42

18 23 16 29 28

52 52 50 40 54

15 23 18 20 [28] [22] [14] [13] 17 15 14 14 11 0

39 16 5 5 [39] [38] [24] [22] 17 18 17 16 14 0

Polyhydric Alcohols 63 64 65 66 67

Ethylene glycol Glycerol Propylene glycol Diethylene glycol Water

30 25 34 31 18 Miscellaneous Liquids

68 69 70 71

1

Phenol Benzaldehyde Turpentine Dipentene Formic acid Acetic acid Oleic acid Stearic acid Linseed oil Cottonseed oil Neets foot oil Pine oil Sperm oil Mineral oil

46 61 77 75 [33] [40] [62] [65] 66 67 69 70 75 100

Note: Numbers in left column refer to solvent positions in Teas graph, Figure 7. Sources: Values from Gardon and Teas.5 Values in brackets derived from Hansen’s original 1971 parameters recalculated by the author.

REFERENCES 1. Burrell, H., Solubility parameters, Interchem. Rev., 14, 13–16 and 31–46, 1955. 2. Burrell, H., Solubility parameters for film formers, Off. Dig. Paint Varn. Prod. Clubs, 27, 726, 1955. 3. Crowley, J.D., G.S. Teague, Jr., and J.W. Lowe, Jr., A three dimensional approach to solubility, J. Paint Technol., 38 (496), 1966. 4. Feller, R.L., N. Stolow, and E.H. Jones, On Picture Varnishes and Their Solvents, The Press of Case Western Reserve University, Cleveland, 1971. 5. Gardon, J.L. and Teas, J.P., Solubility parameters, in Treatise on Coatings, Vol. 2, Characterization of Coatings: Physical Techniques, Part II, Myers, R.R. and J.S. Long, Eds., Marcel Dekker, New York, 1976. 6. Hansen, C.M., The three dimensional solubility parameter—key to paint component affinities. I. Solvents plasticizers, polymers, and resins, J. Paint Technol., 39(505), 1967. 7. Hansen, C.M., The three dimensional solubility parameter—key to paint component affinities. II. Dyes, emulsifiers, mutual solubility and compatibility, and pigments, J. Paint Technol., 39(511), 1967. 8. Hansen, C.M., The three dimensional solubility parameter—key to paint component affinities. III. Independent calculations of the parameter components, J. Paint Technol., 39(511), 1967. 9. Hansen, C.M., Solubility in the coatings industry, Skand. Tidskr. Faerg. Lack, 17, 69, 1971 (English). 10. Hildebrand, J.H., The Solubility of Non-Electrolytes, Reinhold, New York, 1936. 11. Teas, J.P., Graphic analysis of resin solubilities, J. Paint Technol., 40(516), 1968. 12. Teas, J.P., Predicting resin solubilities. Columbus, Ohio, Ashland Chemcial Technical Bulletin, No., 1206, 1971. © 2001 by CRC Press LLC

CHAPTER 1.3

Aqueous Cleaning Essentials Rick Bockhorst, Michael Beeks, and David Keller

CONTENTS Introduction Cleaning Overview Cleaning Parameters Temperature Agitation Concentration Time Required for Cleaning Rinsing Redeposition Protecting the Substrate Equipment Controlling the Cleaning Line Improving Bath Life Water Physical Properties of Water Impurities Water Pretreatment Softening General Principles of Water Softening Deionization Reverse Osmosis General RO Components Cleaning Formulations Cleaning Chemistry Acid Cleaners Alkaline and Neutral Cleaners Ingredients Alkaline Builders © 2001 by CRC Press LLC

Water Conditioners Surface-Active Agents Corrosion Inhibitors Additional Ingredients Hydrocarbon Solvents Rinsing Importance of Rinsing Part Cleanliness Required Production Levels and Dragin Incoming Water Quality Number of Rinse Tanks Rinse Tank Design and Placement Dragin and Final Rinse Quality Disposal Conclusion Acknowledgment References

INTRODUCTION Largely as a result of environmental pressures, parts cleaners have been forced to explore alternatives to the various solvent cleaning processes and especially vapor degreasing. These pressures have come in the form of international, federal, state and local regulations, but the most significant influence has probably been the Montreal Protocols. While aqueous cleaning is almost as old as humanity, parts cleaners have long held that certain soils could be cleaned adequately only by nonaqueous methods. The reality is that many cleaning applications that were once strictly the province of nonaqueous cleaning methods are now being done quite successfully with aqueous processes.

CLEANING OVERVIEW * With few exceptions there are certain principles, treated generally here, that apply to all types of cleaning. Cleaning processes combine mechanical, thermal, and chemical energy sources to remove a soil from a substrate. The total energy needed is the sum of these energy sources over a given period. Within these parameters the following general guidelines apply: 1. Cleaning efficacy and rate improve as temperature increases. 2. Agitation improves the rate and efficacy of soil removal. Agitation provides mechanical energy to remove soils physically and assures that fresh cleaner will continuously contact the soil. 3. Cleaner solutions generally have a performance vs. concentration curve. A minimum level of cleaner is generally necessary for effective cleaning. Cleaning improves with incremental increases of cleaner up to some point, where further increases result in little or no further improvement in performance. 4. Time is the controlling factor in total energy input. If cleaning requires X mechanical energy for T time and the energy level is then decreased, to compensate the time must be increased proportionally. * Reference 16, Chapter 3. © 2001 by CRC Press LLC

5. Rinsing is necessary to remove any cleaner or soil residue remaining on the parts after washing. • Rinse type and quality are dependent on the cleanliness requirements of the application. • Multiple small rinses are generally more efficient and cost-effective than one large rinse. • An agitated rinse is generally more effective than a still rinse. • Final part cleanliness or, conversely, residue on the part is limited by rinse quality. 6. Soil must be prevented from redepositing on parts. The most obvious answer is to remove the soils from contact with the substrate. This may be accomplished by various methods, including: • Emulsification; • Emulsification followed by demulsification and physical removal; • Flocculation; • Ultra- or microfiltration. Additionally, redeposition can be controlled by: • Choosing cleaners formulated with “antiredeposition” properties; • Using cleaning tanks of sufficient size to disperse the soil and slow the rate of increase of contamination concentration. 7. The cleaning method or solution should not harm the item (substrate) being cleaned. 8. Precleaning to remove bulk soils may be an economical and commonsense way to increase cleaner life. 9. Cleaning systems should be designed as a unit. That is, the cleaner and the cleaning equipment should be chosen to work together and address the particular cleaning application. Typical concerns that should be addressed include: • Cleaning temperature and its effect on the cleaner as well as the parts being cleaned. Part of this consideration includes method of heating, insulation, and evaporation. • Equipment design, which should include an evaluation of cleaner and part compatibility with regard to materials of construction, economy of operation, electrochemistry, OSHA and other regulatory guidelines, and ease of service. • The compatibility of the mechanical energy input, which must be addressed in terms of effectiveness of removing soil from the substrate, controlling foaming tendencies of the cleaner, avoiding mechanical damage to the parts, and avoiding degradation of the cleaner. Aqueous cleaners can generally be categorized as being acidic, alkaline, or pH-neutral. Alkaline cleaners are by far the predominant of the three used in all commercial/ industrial cleaning applications. Thus, this discussion will mainly cover alkaline cleaning, but the principles are applicable to all types of cleaners. Agitation techniques represent the greatest variation in cleaning methods. The most important factor is that it costs money in equipment and/or labor to provide high levels of agitation. The equipment must be designed to meet the objective of providing adequate-tosuperior agitation for soil removal at the lowest cost. The major limitations for providing adequate agitation are equipment costs, equipment size (i.e., how big of a “footprint” does the equipment have), excessive foam generation, excessive generation of mist/spray, toxic vapors, or flammable gases. © 2001 by CRC Press LLC

Now that we have taken a broad look at some cleaning principles, let us look a little more closely at each. CLEANING PARAMETERS Temperature The effect of temperature depends on the type of soil being removed and the cleaner. The first consideration is what type of soil needs to be removed. Temperature is very important in speeding the removal of fats, greases, oils, and waxes. Increased temperature reduces the viscosity of oils and greases, making them more mobile, and therefore easier to displace from the substrate. Fats and waxes are often solids at room temperature. It is critical to melt these fats and waxes to remove them by aqueous methods. If the melt range of the fat/wax is above the boiling point of the cleaner, aqueous cleaning may not be effective on this type of soil. There is a well-established principle that the rate of a chemical reaction is doubled for each 10°C (18°F) increase in temperature. If the cleaning process works by reaction between a fatty acid/oil and alkali, by a paint coating undergoing chemical decomposition, or by an acid chemically removing rust and scale, then this reaction rate relationship is applicable. It is possible to remove solid fats/waxes if they can chemically react with the cleaner without melting them; however, the rate of removal may not be adequate. On the other hand, excessively high temperatures could “set” proteinaceous soils or may cause an undesirable reaction between the soil and the substrate, making the soil more difficult to remove. Just as increasing temperature will increase the rate of cleaning, it will increase the rate of undesired reactions. Most corrosion inhibitors work by forming a loose barrier on the clean metal surface. Excessively high temperature can disrupt this barrier and result in chemical attack, usually seen as discoloration and etching. The majority of industrial cleaning is carried out at 140 to 180°F. Agitation As has been previously stated, agitation techniques represent the greatest variation in aqueous cleaning systems. It is usually possible to find an aqueous cleaner to remove a soil from a substrate. Thus, one of the biggest problems users run into is inadequate cleaning performance because of inadequate or improper choice of agitation. The method of agitation should be matched to the size and shape of the part. For example, while spray washing may be very effective for cleaning large relatively flat parts, it may not be suitable for parts with blind holes where direct impingement is problematic. Relatively flat objects and components that do not have hidden areas can be cleaned by immersion or spray wash. Typically, parts too small or too large cannot be cleaned by spray wash; an exception for small parts is possible when specialty mounting racks are built. Spray wash cleaning is also limited on the chemical side because the formula must not be foamy. Eliminating foam restricts the choices of raw materials a chemist can use in formulating a spray wash cleaner. Parts that can be damaged from spray impingement should only be cleaned by immersion. Virtually all parts can be cleaned by immersion. Ultrasonic cleaning is the most effective method of agitation for immersion cleaning, but is restricted by cost (very expensive equipment) and size (not as effective on tanks above
1000-gal capacity). Spray-under immersion and turbulation are the next most effective methods of immersion agitation. Spray-under immersion and turbulation can sometimes create excessive foam if care is not

© 2001 by CRC Press LLC

taken in choosing a cleaner. General agitation from pump circulation can be adequate for noncritical cleaning applications, but is not usually suitable for precision cleaning. Air sparging can very effectively agitate an aqueous cleaner, but it has several restrictions: 1. The cleaner must be a low-foaming to nonfoaming product. Air sparging may cause foam to overflow the tank. Basically, cleaners designed for spray washing should only be used with this form of agitation. 2. The cleaning tank should be covered to eliminate generation of mist and spray. The popping of the bubbles will generate mist and spray that will create an exposure problem for workers. The only cure for this is to cover the tank or place the tank within a cabinet to contain the mist/spray. It is usually impractical to put a cover on most immersion tanks because of the mechanics of moving the cover and parts in and out of the tank. Adding on a cabinet increases the cost of the cleaning system and will likely increase the “footprint” taken up in the facility. 3. Air sparging can shorten the life of alkaline cleaners, especially heavy-duty caustic cleaners, by neutralization with carbon dioxide (CO2), a weak acid. Acids and bases will neutralize each other. Even though it makes up only a fraction of a percent of the atmosphere (0.035% measured at Mauna Loa Observatory, 1990, as reported in Handbook of Chemistry and Physics, pp. 14–25, 199620), the large volume of air passed through the tank will expose the cleaner to a significant amount of CO2. The CO2 will neutralize the alkaline builders, especially sodium or potassium hydroxide. Heavy-duty caustic cleaners are especially prone to this problem, whereas mildly alkaline cleaners are much less sensitive. The CO2 reacts with free hydroxide ions (OH  , the cause of alkaline pH) to form bicarbonate ions: CO2  OH → HCO3 The bicarbonate then goes on to react with more hydroxide ions to form a carbonate ion and water: HCO3  OH → CO32  H2O Thus, as the hydroxide ions (OH ) are consumed, the pH drops in the cleaner. This can adversely affect alkaline cleaners, especially heavy-duty caustic cleaners used in descaling operations. Acidic and pH-neutral cleaners are not affected by this problem. Finally, soaking the substrate in a stagnant tank is unacceptable, even for crude cleaning applications.

Concentration Concentration, also called use dilution can affect multiple attributes of the cleaning process. In many cases minimum or maximum cleaner concentrations can control corrosion characteristics, chemical etching, or the deposition of protective barriers as well as cleaning efficacy. The necessary cleaner concentration will vary with the type of agitation and temperature. As an example, in the absence of foaming problems, it may be possible to obtain similar cleaning performance from the same alkaline cleaner at 5 to 10% by immersion, 3 to 5% by spray, 1 to 3% by steam cleaning or high-pressure hot spray, or 2 to 4% in

© 2001 by CRC Press LLC

a high-pressure, room-temperature spray. For each of these applications, increasing the cleaner concentration will give better cleaning performance up to a certain point and then level off. This leveling-off point will be dependent on the specific chemistry used in the cleaner, the soil being removed, the agitation technique, and temperature. Time Required for Cleaning The cleaning time is dependent on the concentration of the cleaner, the specific chemistry used in the cleaner, the soil being removed, the agitation technique, and the temperature. It is important to emphasize that cleaning is not instantaneous; some time is necessary for the detergent to perform its work on the soil. In a stagnant bath, cleaning may take from 5 min to over an hour to occur, if at all. Most immersion cleaning situations do not exceed 10 min, although numerous exceptions can be found. Spray washes typically take no more than 5 min. One general rule of thumb does exist for ultrasonic cleaning; if it takes more than 5 min to clean, either there is something deficient in the cleaner or the process itself is not suited for the application. One notable exception to the “5-min rule for ultrasonics” is the cleaning of used automobile cylinder heads and other major engine components. Many automobile repair facilities have adopted ultrasonic processes as an alternative to heavyduty caustic or solvent cleaning tanks. The heavy-duty caustic tanks have fallen out of favor because they cannot be used on aluminum parts; nearly all major engine parts are now made of aluminum. Corrosion hazards and waste disposal considerations also have contributed to the decline of the caustic stripping tank. Solvents have been on the decline mainly because of environmental regulations that govern volatile organic content (VOC), ozone-depleting substances (ODSs), and hazardous air pollutants (HAPs). The few solvents (cresylic acids) that are effective on baked-on soils also pose serious health risks. Ultrasonic cleaning with highly concentrated, moderately alkaline cleaners has been found to be effective at removing most of the baked-on soils. The drawbacks include long processing times, often 15 to 45 min, and some cavitational erosion. The cavitational erosion appears as a “star-” or “Y-shaped” pattern on the metal. It must be stressed that the time available for cleaning is very closely related to the economics of the cleaning operation. The increased cost in equipment, energy, and chemicals to reduce time must be weighed against the economic gains in increased production. Additionally, consideration must be given to effects on reject rates and customer satisfaction. Rinsing No matter what cleaning method has been employed, the surface of the freshly cleaned part will contain some amount of soil and cleaner residue. In some situations this residue may present no problem, but in many others that residue must be removed to yield acceptable parts. This is an important issue. Just as with solvent rinses, water rinses may contain impurities that can dry on the parts. Water quality issues will be discussed later. The value of pressure sprays and mechanical action in rinsing is often neglected. Experience shows that direct spraying is far more effective in flushing away the loosened soil than just soaking the piece in an immersion tank. The use of a short spray rinse followed by a soak or agitated rinse to reduce contamination level is most effective at reducing contamination. Static or slow-moving rinses in which there is improper/inadequate flow usually result in parts that need to be recleaned or discarded. © 2001 by CRC Press LLC

Another important consideration in rinsing is the number of rinses performed; two rinse steps are more effective than one, three are better than two, etc. Multiple rinses can be of shorter individual duration and still be more effective than single rinses because of the exponential dilution of contaminants as the parts proceed from one rinse to the next. Unfortunately, multiple rinses can lead to higher costs in equipment (i.e., number of tanks needed, footprint taken up on the plant floor). The cost can be offset by counterflowing the water from the last rinse back into each previous rinse. This significantly reduces water consumption; some of the overflow can be used as add-back into the cleaning tank to replace evaporated water. Studies have shown that a counterflowed, triple rinse has the optimum balance of reducing water consumption, obtaining clean parts, and capital costs in the rinse step; more than three rinses yields diminishing returns. It is possible in some situations to equip the multistage rinse with a set of deionizing resin beds and an activated carbon filter. Closing the rinsing loop by deionizing the overflow water can reduce water consumption, replacing only the water lost to evaporation. Users with significant wastewater disposal costs should consider this kind of setup. For further reading, see Peterson13 and Spring.16 Another important consideration is the quality of water used in the rinse step. The quality of rinsing can only be as good as the quality of water used in the last rinse. Unsoftened water obtained from a municipal source or well often contains varying levels of hard water ions, carbonates, phosphates, and organic by-products from treatment processes. The water hardness can often be extremely high, leading to hard water deposits and soap scum residue. Softening the water to remove hard water ions (calcium and magnesium) will eliminate those hard water salts but leaves other impurities and therefore may not be adequate. Using deionized, distilled, or reverse-osmosis purity water gives the best rinsing performance. As usual, as the quality of water increases, so does the cost. The level of performance must meet the requirements of the application. The application requirements must be evaluated for each system.

Redeposition The design of the tank and cleaner is an important factor in reducing/eliminating redeposition of soils. The tank must be of sufficient capacity to provide room for the soils to move away from the parts. The size is also important to moderate the rate at which soil loading increases; too small of a tank can result in the cleaner becoming saturated in a matter of hours or days. Bag filters can be used to remove gross particulate matter. The cleaner may incorporate phosphates, silicates, specialty surfactants, and synthetic polymers, which remove and suspend soils in solution. Use of aqueous cleaners that can “splitout” oils instead of emulsifying them in combination with an oil coalescer/skimmer will slow down, possibly even prevent, the soil from reaching a saturated condition in the cleaner. This oil splitting followed by physical removal will lead to longer tank life and prevent redeposition. Self-emulsifying oils are difficult to handle in preventing redeposition. These lubricants contain their own surfactants that form stable emulsions. In general, aqueous cleaners cannot break these emulsions without being consumed themselves. The only viable alternative is to use microfiltration in the 0.1 to 0.5
m pore size range to remove the emulsion. The filter will not remove all of the emulsion but will remove some or most. The cleaner must also be designed to be able to pass through the filter. Micro- or ultrafiltration has been successfully done but generally results in some loss of cleaner constituents. Thus, monitoring and adjusting the tank becomes difficult and may not be an

© 2001 by CRC Press LLC

economically viable option. A good rinse must always follow the cleaning step to prevent soil redeposition. Waxes typically require a hot rinse to prevent resolidification on the clean surface. Protecting the Substrate The cleaner must be compatible with the substrate. Unless these effects are necessary, the cleaner must not discolor, etch, cause hydrogen embrittlement, cause stress cracking, or otherwise damage the substrate. Thus, the cleaner must contain additives to protect the substrate from these effects. For example, aluminum cleaners may either contain silicates and have an elevated pH or have a pH of around 7 to 9 to prevent corrosion. Stainless steel cleaners must not contain chlorides because they will cause stress cracking, especially in acidic cleaners. Rusting on steel may be avoided with strongly alkaline cleaners, especially cleaners containing amine-based corrosion inhibitors. High-chrome steels are especially prone to rusting and often need to have a corrosion inhibitor added to the rinse tanks to prevent rusting during the rinse and dry cycles. Acid cleaners sometimes require inhibitors to minimize corrosion of the base metal without reducing the efficacy on hard water or metal oxide scales. Good rinsing, careful drying, or use of inhibitors may avoid tarnishing. Copper and copper-based alloys are notorious for tarnishing during the drying cycle. Transfer time between cleaning and rinsing tanks should be minimized to avoid drying of cleaner residue on the substrate. Equipment Some of the greatest advances in the art of cleaning in the past decade relate to improvements in equipment. Intelligent, appropriate use and care of equipment may be the key to proper cleaning in many instances. Controlling the Cleaning Line It is necessary to determine when the cleaner is nearly exhausted so that fresh cleaner can be prepared or the old cleaner can be rejuvenated. This is not always easy to determine. Measuring properties such as alkalinity, conductivity, and pH are useful in determining the state of the cleaner, but it is not uncommon for tanks to fail even when the above-mentioned test results are within specifications. The properties that should be measured are those that are critical to the specific process. For example, silicate-based aluminum cleaners should be monitored mainly for pH; silicate testing should be done if affordable. The silicates protect aluminum from corrosion and they participate in soil anti-redeposition. The problem with silicates is that their solubility in water decreases as the pH drops in the cleaner. At some point, the silicate level will drop below the minimum level necessary to protect aluminum. When this happens, spotting or etching may occur. Heavy-duty caustic cleaners are best monitored by active and total alkalinity titration methods. Acid cleaners are best monitored by total acidity titration methods. Cleaners that undergo microfiltration to remove emulsified oils present the greatest difficulty in monitoring. The pH may need to be monitored if the cleaner contains silicates. Alkalinity titration is useful, but does not detect many of the surfactants that are stripped out by the filtration process. The emulsified oils may have ingredients that will severely impact the pH and skew the alkalinity titration. Monitoring how much the solution bends light, called the refractive index, can help in tracking loss of cleaner to stripping by the filter.

© 2001 by CRC Press LLC

Unfortunately, this measurement requires that the solution be relatively clear; cloudy solutions are difficult to impossible to measure. Some tanks are so difficult to maintain that the user must seek out a chemical maintenance firm or change some of the manufacturing processes that precede the cleaning step to eliminate some of these problems. Improving Bath Life One way of improving bath life is to have two cleaning tanks in series. Most of the soil is removed in the first tank. The part going into the second tank is relatively clean. After the first tank becomes heavily contaminated it can be discarded and the cleaner in the second tank pumped over to the first tank. In this manner the cleaners can be used for long periods and, if handled intelligently, it is possible to operate with the first bath very heavily contaminated so that cleaner is seldom discarded. Contamination of the second tank can be reduced by employing a short rinse after the first cleaning operation, the rinsing water either going to sewer or back to the first tank. Besides better performance and economy, this procedure may be necessitated by governmental regulation of effluent that prohibits or limits the discarding of cleaning baths. The single biggest problem in obtaining long tank life is understanding that cleaning is part of the manufacturing process and must be evaluated as part of the whole. The design of the cleaning process must consider the soils and their effects not only on processing but on disposal. The choice of cleaning equipment and of cleaner should be made as a coordinated effort that takes into account soil removal from the part and from the bath. These choices must also consider the ultimate costs of operation, including the costs of disposal. It is in this area that chemical and equipment manufacturers and their customers most often fail. Great strides can be made that will allow aqueous baths with suitable replenishment to be useful for multiple years, but cooperation of chemical manufacturers, equipment manufacturers, and industrial users is critical.

WATER For most aqueous cleaners, water comprises 80 to 99% of the cleaning solution and is used in practically all rinsing steps. Although most people do not think of it in this way, water is actually a solvent in aqueous cleaners. A major key to understanding the efficacy of aqueous cleaners lies in the role played by water, its natural properties, and impurities. Water has been vital to life and nature from the beginning of time. The basic cycle by which water evaporates, condenses, and flows along the surface of the Earth governs all animal and plant life. Approximately 61.8% of the human body is water. Water covers almost 70% of the Earth’s surface, most of it in oceans, with the balance found in lakes, rivers, the atmosphere, and absorbed in soil and rocks. Water is never absolutely pure in nature and its impurities are the factors of concern in industrial applications. Humans have contributed to impurities found in water sources. One concern of aqueous cleaning is disposal of spent cleaning solutions. When an aqueous cleaner is used to remove contaminants from a surface, the water is basically the solvent in which the cleaning takes place. The importance of its function cannot be overstated. As the solvent, water is able to dissolve and disperse the soils being removed. Additives such as acids, alkalis, chelants, and detergents significantly augment the cleaning process. These additives are not nearly as effective by themselves unless they are dissolved in a solvent, i.e., water. The combination of these additives with water yields the powerful, synergistic effects that are exploited today. © 2001 by CRC Press LLC

Physical Properties of Water Pure water is colorless, odorless, and tasteless. Its chemical formula is H2O, which shows that it is made from the two elements, hydrogen and oxygen, in a ratio of 2:1. These two specific elements combined in a 2:1 ratio yield physical properties unmatched by any other molecule: 1. 2. 3. 4.

Very small size. Not flammable. Very high boiling point for its size. The two elements that make it up are so different that they impart a high polarity to the molecule. 5. The high polarity of water accounts for the high boiling point. It also accounts for: a. The high level of thermal energy that it can absorb per degree of temperature increase (called the heat capacity) and to get it to boil (called the heat of vaporization). The high heat of vaporization is what makes water so effective in steam boiler heat exchanging systems. b. Ability to dissolve numerous substances, especially minerals and other polar substances. c. Inability to dissolve nonpolar substances like fats, greases, and oils. The very high boiling point gives aqueous cleaning the flexibility of using a wide temperature range. The temperature of choice can be fine-tuned to the properties of the soil and substrate. This property also minimizes solvent loss due to evaporation. Water loss due to evaporation may become a concern especially at temperatures above 150°F. The very high boiling point, 212°F, is beneficial in that most aqueous cleaning operations do not exceed 180°F so outright boiling is not a problem. Many substrates cannot tolerate the extreme heat of boiling water without suffering from discoloration, etching, or mechanical deformation. The high heat capacity of water makes it very effective in heating metal parts up to the cleaning temperature of the bath while having minimal impact on the bath temperature. Because metals have a low heat capacity, very little energy, relatively speaking, is expended in raising them to the bath temperature. Traditional organic solvents have low heat capacities like metals so they are more prone to temperature fluctuations when used as heated immersion cleaners. The high polarity of water can be viewed as a double-edged sword. The high polarity makes it possible for water to dissolve many inorganic compounds, such as caustic soda, caustic potash, borates, carbonates, phosphates, and silicates. Water is also an effective solvent for many surfactants used in formulating aqueous cleaners. Unfortunately, this polarity results in water also being contaminated by numerous impurities both from the Earth’s crust and from anthropogenic pollution. Some of the impurities of the starting water are identical to cleaning ingredients, i.e., carbonates and phosphates. The key is which impurities are beneficial and which are detrimental. Contaminant levels in water used to make aqueous cleaners are usually low so one must focus on which impurities are detrimental. Elimination/suppression of these impurities is essential in preventing problems including reduced cleaner performance, longevity, corrosion, contaminated surfaces, and water spotting. Chemical manufacturers and industrial users spend millions of dollars annually on water-conditioning equipment to reduce/remove the impurities as part of preventive maintenance. Many users remain uninformed about their water quality needs. Many of these users suffer from increased cost of recleaning and rejects as a result of the impact that poor water quality has on the whole cleaning process. A discussion of water treatment options will follow. © 2001 by CRC Press LLC

An often forgotten property of water is its ability to dissolve oxygen gas. Oxygen gas in water can be corrosive and will attack metals. High chrome steels are exceptionally prone to rusting in these situations. When these parts are damp and left exposed to the air, flash rusting will occur. The boiler water treatment industry knows all too well how detrimental dissolved oxygen is in boiler systems. They have to treat these systems with what are known as “oxygen scavengers” to remove the oxygen gas from water chemically. Oxygen scavengers are not normally used in aqueous cleaning but corrosion inhibitors may have to be used to combat the corrosive effects of oxygen and other ingredients dissolved in water. Impurities If water were H2O and nothing else, or if all waters carried the same impurities, the use of water for industrial applications would be simple and straightforward. But natural waters, even rain, snow, sleet, and hail, as well as all treated municipal supplies contain some impurities. The type and amount of contaminants in natural waters depend largely on the source. Well and spring waters are classed as groundwaters, rivers and lakes as surface waters. Groundwater picks up impurities as it seeps through the rock strata, dissolving some part of almost everything it contacts. But the natural filtering effect of rock and sand usually keeps the water free and clear of suspended matter. Surface waters often contain organic matter, such as leaf mold, and insoluble matter, such as sand and silt. Pollution from industrial waste and sewage is frequently present. Stream velocity, amount of rainfall, and where this rain occurs on the watershed can rapidly change the character of the water. Below is a list of the more common and troublesome impurities: Turbidity—Suspended insoluble matter, including coarse particles (sediment) that settle rapidly on standing. Amounts range from zero, in most groundwaters, to some surface supplies of over 6% or 60,000 parts per million (ppm) in muddy and turbulent river waters. Hardness—Water content of soluble calcium and magnesium salts equals hardness expressed as calcium carbonate equivalents in gpg (grains per gallon) or ppm. 1 gpg  17.1 ppm. These salts, in order of their relative average abundance in water, are (1) bicarbonates, (2) sulfates, (3) chlorides, and (4) nitrates. Calcium salts are about twice the concentration of magnesium salts. Hardness is undesirable because the salts become less soluble and drop out of solution as the water is heated and upon drying, producing a hard, stony water spot that can be difficult to remove. Iron—The most common soluble iron in groundwater is ferrous bicarbonate (black iron). Although some water is clear and colorless when drawn, on exposure to air ferrous bicarbonate can cloud up and deposit a yellowish or reddish-brown sediment that stains everything it contacts. Iron can also shorten the life of a water softener, contaminating the resin. Although the majority of iron-bearing waters have less than 5 ppm, as little as 0.3 ppm can cause trouble. Manganese—Although rarer than iron in water, manganese occurs in similar forms and can form deposits in pipelines and tanks very rapidly with as little as 0.2 ppm. Silica—Most natural waters contain silica ranging from 1 to over 100 ppm. When silica spotting occurs, it can be very difficult, if not economically impossible, to remove. © 2001 by CRC Press LLC

Mineral acidity—Surface waters contaminated with mine drainage or trade wastes will contain sulfuric acid, plus ferrous, aluminum, and manganous sulfates. These contaminants are corrosive and therefore waters contaminated with mineral acidity are unfit to use without a pretreatment system. Carbon dioxide—Free carbon dioxide is found in most natural supplies. Surface waters have the least, although some rivers contain up to 50 ppm. In groundwaters (wells), it varies from zero to concentrations so high that carbon dioxide bubbles out when pressure is released (as in “sparkling” or seltzer water). Most well waters contain from 2 to 50 ppm. Carbon dioxide is also formed when bicarbonates are destroyed by acids, coagulants, or heating the water. This can reduce the pH of an alkaline cleaning solution. Carbon dioxide is corrosive and accelerates the corrosion properties of oxygen. Oxygen—Found in surface and aerated waters. Deep wells contain very little oxygen. The oxygen content of water is inversely proportional to the temperature, meaning that the hotter the water, the less oxygen is present. (Note that in elevated temperatures, the water contains less oxygen; however, what oxygen remains is much more aggressive and corrosive.) Oxygen is very corrosive to iron, zinc, brass, and other metals. Flash rusting of metals can be a problem when hot parts are rinsed in cold water that contains higher amounts of oxygen.

WATER PRETREATMENT Softening General The impurities that cause the most trouble in aqueous cleaning processes are the inorganic salts. Salts are ionic compounds, which are chemicals that have a positively charged species called a cation and a negatively charged species called an anion. It must be pointed out that the term salt has commonly meant sodium chloride, i.e., table salt. The chemical definition is “salts are ionic compounds that contain any negative ion except the hydroxide ion and any positive ion except the hydrogen ion.” An ionic compound that contains the hydrogen ion is called an acid and an ionic compound that contains the hydroxide ion is a base. Specific examples of common ionic impurities that are encountered as impurities in water are given in Table 1. It must be pointed out that silicates are usually discussed/represented as silicon dioxide, SiO2, when they are actually present in natural waters as the ions listed in Table 1. There are more complex forms of phosphate and silicate ions and numerous other trace impurities that can be present in natural, untreated water but the above examples represent the bulk of the impurities that the cleaning industry must be concerned with.

Principles of Water Softening The most problematic impurities are the cations. The most economical method for removing them is by passing the water through an ion-exchange column, better known as a water softener. A standard water softener contains polystyrene beads that have been modified such that the surface of each bead has numerous negatively charged sites. Nature © 2001 by CRC Press LLC

Table 1 Dissolved Impurities in Water Ion Type Cations

Anions

Impurity 2

Ca Mg2 Fe2 and Fe3 Mn2 and Mn4 Na K CO32 HCO3 PO43 SiO44 and SiO32 Cl NO32

Property Hardness Hardness Iron stains Manganese stains and scales Too much sodium in rinse water can cause spotting Too much potassium in rinse water can cause spotting Alkalinity, carbonates form hard water deposits with calcium, magnesium, iron, and manganese Alkalinity, bicarbonates form hard water deposits with calcium, magnesium, iron, and manganese Alkalinity, ortho-phosphates form hard water deposits with calcium, magnesium, iron, and manganese Silicates can form the most tenacious of deposits, especially in the presence of calcium, magnesium, iron, and manganese Chlorides promote corrosion on aluminum, iron, and steel, too much chloride in rinse water can cause spotting Too much nitrate in rinse water can cause spotting

requires that charge must be balanced; which is accomplished by pairing each negatively charged site with a sodium cation. As the impure water passes through the water softener, the hard water ions become attached to the resin beads and displace the sodium cations. The number of displaced sodium cations equals the charge of the hard water ion trapped in the softener. The hard water ions are bound more tightly to the resin because higher positively charged cations bind more strongly to negatively charged surfaces. Basically, the water softener removes highly charged cations and replaces them with sufficient sodium cations to maintain the balance of charge. Thus, sodium contamination increases but it does not cause nearly as much trouble as calcium, magnesium, iron, and manganese. There are only a finite number of resin beads in a water softener so there is a point where the softener becomes saturated. At this point, the softener must be recharged. This is accomplished by passing a saturated salt solution, usually sodium chloride, through the softener. The overwhelming quantity of sodium cations slowly replaces the calcium and magnesium ions, which returns the softener back to working order. Iron and manganese are more difficult to remove from a water softener. Iron and manganese can have very high positive charges, 3 and 4, respectively, which make them bind so tightly to the resin bead that the mass action of the regenerating salt solution cannot knock them off. This is typically called “iron poisoning.” These cations can be washed out of the softener if their charge can be lowered first. Reducing agents can be used to lower the charge of the iron and manganese ions, typical “reducing agents” are sodium sulfite, sodium hydrosulfite, and sodium thiosulfate. Deionization Basic water softening removes only the undesirable cations by replacing them with less problematic cations. It is also possible to replace anions by the same technique, only this time the charge on the surface of the resin bead is positive and the charge is balanced by pairing up with an anion. The chloride anion is an economical choice for balancing charge

© 2001 by CRC Press LLC

in an anion exchanger. However, excessive chloride content in water can lead to stress cracking of certain stainless steels and promote corrosion on aluminum and mild steels. When industry has to be concerned with both cation and anion contaminants, it is easier to perform a process called “deionization” than it is to replace undesirable cations with sodium and the anions with chloride. Deionization involves passing impure water through a series of cation and anion exhange resins where the cations are replaced by hydrogen ions (H  acid) and the anions are replaced by hydroxide ions (OH base). The liberated acid and base then neutralize each other to form water: H  OH → H2O When deionization is performed, theoretically an equal amount of acid and base is liberated. The neutralization reaction should then result in pH-neutral water. This is usually not the case. Deionized water typically is slightly acidic; pH of 4.5 to 5.5. The mild acidity is caused by the carbonate impurities initially present in the water. As the carbonate passes through the cation exhange column, carbonic acid is formed. The examples below will assume that the carbonates are passing through as sodium salts: Na2CO3  2 HResin → H2CO3  2 NaResin NaHCO3  HResin → H2CO3  NaResin Carbonic acid is not stable in water so it self-destructs to form carbon dioxide, CO2, and water: H2CO3 → CO2  H2O Carbon dioxide has some solubility in water so unless the water is boiled to drive off the CO2 after deionization, the reverse reaction can occur, which liberates some acid. This causes the low pH. CO2  H2O → H2CO3 H2CO3 → H  HCO3 Both water softening and deionizing systems are very effective at removing impurities, but they are not perfect. The exchange of ions is really an equilibrium process so some material can work its way through a column before the saturation state is reached. Imperfectly sealed control valves and incorrect flow rates can result in some impurities never coming in contact with the resin so that exhange never occurs. The ion exchange columns will not effectively remove nonionic impurities. Incorporation of an activated carbon filter will remove the nonionic impurities.

Reverse Osmosis General Reverse osmosis (RO) involves separating water from a solution of dissolved solids by forcing water through a semipermeable membrane. As pressure is applied to the solution, water and other molecules with low molecular weight pass through micropores in the membrane. The membrane retains larger molecules, such as organic dyes, cleaners, oils, © 2001 by CRC Press LLC

metal complexes, and other contaminants. RO membrane systems feature cross-flow filtration to allow the concentrate stream to sweep away retained molecules and prevent the membrane surface from clogging or fouling. In the past, RO applications for industrial operations were mostly limited to final treatment of combined wastewater streams. Such applications typically involved discharging permeate to a publicly owned treatment works (POTW) and returning the concentrate to the head of the wastewater treatment system. Because of the high flow rates associated with treating combined wastewater streams, large, costly RO units were required. More recent applications in cleaning involved installing RO units in specific process operations (such as wash tank or rinse water maintenance), allowing return of the concentrate to the process bath, and reuse of permeate as fresh rinse water. By closing the loop, process contaminations are removed and fresh water is recycled. Furthermore, a waste stream is eliminated that would otherwise be discharged to the POTW. RO systems have been successfully applied to a variety of industrial operations, reducing the cost of waste treatment and disposal. RO Components The essential components of an RO unit include strainer, pressure booster pump, cartridge filter, and the RO membrane modules. The strainer removes large, suspended solids from the feed solution to protect the pump. The booster pump increases the pressure of the feed solution. Typical operating pressures range from 150 to 800 psi. Commercially available cartridge filters are used to remove particulates from the feed solution that would otherwise foul the units. Cartridge filter pore sizes are typically between 1 and 5 m. Cleaning Formulations Water has been used as a cleaner for centuries. The first water-soluble soaps were a blend of lye and animal fat. The chemical reaction of this mixture is a process defined as saponification. The addition of heat made the soap work better at removing the oils and greases of the day, which were also made from animal fat. As industry advanced and metal processing became more sophisticated, various organic and inorganic salts were found to enhance detergency by combining with metal ions to prevent them from reacting with the soap. These salts are termed “builders” and include phosphates, carbonates, silicates, and gluconates, just to name a few. The development of synthetic detergents as a substitute for soap in wartime has completely changed the cleaning industry. Today, true soaps make up only a small portion of the surfactants used in either industrial or consumer cleaning applications. By the mid-1970s, government regulations were starting to be felt at the job site. OHSA and Material Safety Data Sheets (MSDS) became common terms within the industrial arena. Chemicals came under increasing scrutiny for worker safety. During the following decades, environmental issues have played an increasingly important role in chemical evaluation. The terms EPA, chlorofluorocarbons (CFCs), the Montreal Protocol, global warming, SARA Reportables, and air quality boards are all commonplace. No cleaning process can completely escape the concerns of health, safety, and the environment. CLEANING CHEMISTRY 21 Aqueous cleaners are acid, neutral, or alkaline. Acid products, which have a pH of less than 6, are used for removal of inorganic soils and to pickle or passivate a metallic surface. Neutral and alkaline cleaners have a pH range from 6 to above 13. These products are very © 2001 by CRC Press LLC

effective on organic oils and greases. Additional ingredients are frequently added for increased effectiveness on inorganic soils as well. When defined as the removal of soil or unwanted matter from a surface to which it clings, cleaning can be accomplished by one or more of the following methods: Wetting: Through the use of surface active agents, the cleaning penetrates and loosens the substrate–soil bond by lowering surface and interfacial tension. Emulsification: Once wetting occurs, two mutually immiscible liquids are dispersed. Oil droplets are coated with a thin film of surfactant, thus preventing them from recombining and floating to the surface. Solubilization: The process by which the solubility of a substance is increased in a certain medium. The soil is dissolved in the cleaner bath. Saponification: The reaction between any organic oil containing reactive fatty acids with free alkalies to form soaps. Insoluble Fatty Acid
Alkali  Water Soluble Soap Deflocculation: The process of breaking the soil into very fine particles and dispersing them in the cleaning media. The soil is then maintained as a dispersion and prevented from agglomerating. Displacement: Soil is displaced by mechanical action. Movement of the workpiece or fluid enhances the speed and efficiency of soil removal. Sequestration: Undesirable ions such as calcium, magnesium, or heavy metals are deactivated, thus preventing them from reacting with material that would form insoluble products (i.e., hard water soap scum).* Water-based cleaners are generally divided into five major pH groups as follows: Caustic, pH 12 to 14 High alkaline, pH 10 to 13 Low alkaline, pH 8 to 10 Neutral, pH 6 to 8 Acid, pH 1 to 6 Acid Cleaners Acid cleaners are generally not used for the removal of organic oily soils. A typical acidic solution with a pH of 4.5 could include citric acid and nonylphenol ethoxylate. It would be effective for removing metal oxides or scale prior to pretreatment or painting. Systems using acid cleaners generally require constant maintenance because the aggressive chemistry attacks tank walls, pump components, and other system parts, as well as the materials to be cleaned. Inhibitors can be used to reduce this attack. Acid cleaners often suffer from rapid soil loading, particularly metal loading. This loading leads to frequent decanting and dumping of the cleaner solution. Both of these disadvantages lead to relatively high operating costs compared with alkaline cleaners. Alkaline and Neutral Cleaners Ingredients Ingredients frequently contained in alkaline cleaners include alkaline builders, water conditioners, surface-active agents, corrosion inhibitors, fragrances and/or dyes, defoamers or foam stabilizers, and water. Occasionally, hydrocarbon solvents are also added to a formulation. * See Acknowledgment at end of chapter. © 2001 by CRC Press LLC

Alkaline Builders Alkaline builders are selected based on the pH, detergency, corrosion inhibition, and/or cost limitations required or desired for a specific formulation. Environmental or process restrictions must also be considered. These builders may include one or more of the items listed in Table 2. Neutral-pH cleaners contain little or no alkalinity builder(s) or the alkalinity reserve is neutralized with an organic or mineral acid. Water Conditioners Sequestrants or chelators are frequently used to deactivate undesirable ions such as calcium, magnesium, or heavy metals. These ions or heavy metals are then no longer free to react with bath substances that would subsequently form undesirable compounds, such as hard water soap scum. Some of the more commonly used sequestrants include: EDTA: Ethylenediamine tetraacetic acid NTA: Nitrilotriacetic acid HEEDTA: Hydroxyethylenediamine triacetic acid STPP: Sodium tripolyphosphate ATMP: Amino tri(methylene) phosphoric acid HEDP: 1-Hydroxyethylidine-1, 1-diphosphonic acid Sodium gluconate Sodium glucoheptanate Low-molecular-weight polyacrylates EDTA has maximum effectiveness in tying up calcium and magnesium, thereby softening the water used to dilute the cleaner bath. Sodium gluconate or glucoheptanate has maximum effectiveness in tying up heavy metals. Complex phosphates are extremely costeffective, but have come under environmental pressure since the late 1960s. Lowmolecular-weight polyacrylates have only found a limited market to date. Table 2 Alkaline Builders 22 Component

Advantage

Disadvantage Caustic Cleaners (pH 12 to 14)

Hydroxides

Cost-effective

Corrosive

High Alkaline Cleaners (pH 10 to 13) Amines Carbonates Hydroxides Phosphates Silicates

Detergency corrosion inhibition Detergency, soil holding, low cost Cost-effective Detergency, sequestration, corrosion inhibition Detergency, corrosion inhibition

More costly Consumable Corrosive Environmental restrictions Residues, restricted use

Low Alkaline Cleaners (pH 8 to 10) Amines Borates Sulfates

Detergency, corrosion inhibition, sequestration Corrosion inhibition Filler, carrier

© 2001 by CRC Press LLC

More costly Limited effect Restricted use

Surface-Active Agents Surface-active agents, also known as surfactants, are used to reduce the surface or interfacial tension on aqueous solution. Selection of the surfactant package used in cleaner formulations depends on the performance characteristics desired. Surface active ingredients frequently used in water-based cleaners include fatty acid soaps or organic surfactants. These surfactants are classified into four basic types: Anionic: Negatively charged ions that migrate to the anode Cationic: Positively charged ions that migrate to the cathode Nonionic: Electronically neutral ions Amphoteric: Ions charged either negatively or positively, depending on the pH Physical properties affected by surfactants include the cloud point, the foaming characteristics, and the detergency, emulsifying, or wetting mechanisms used to facilitate the cleaning process. Often a combination of ingredients is used to obtain the specific properties desired. Corrosion Inhibitors Corrosion inhibitors are also contained in some alkaline cleaners, depending on the application involved. If a wide variety of substrates is involved, a combination of inhibitors may be used. These inhibitors are water soluble and therefore are removed with a thorough rinse if desired. Inhibitors frequently added to aqueous cleaner formulations include, but are not limited to, aldehydes, amine benzoates, borates, carboxylates, molybdates, nitrites, thiols, triazoles, and urea. Phosphates and silicates could also be added to this list. The intended cleaner application dictates the type of inhibitor package selected. As the alloying of metals and composites becomes more complex, there is a greater need for sophisticated inhibitor packages, which provide protection on a broad spectrum of substrates. The synergism of chemicals allows the formulator to obtain the inhibiting properties desired and may be limited only to the imagination of the formulator and the cost restrictions of the chemicals selected for this use.

Additional Ingredients Aqueous cleaner formulations may also contain a broad spectrum of ingredients designed to affect the appearance, odor, or physical properties of the composition. These include dyes, fragrances, thickeners, defoamers, foam stabilizers, or fillers for cost reduction. Again, the intended cleaner application will dictate final composition of a formula. Hydrocarbon Solvents A variety of hydrocarbon solvents have been blended with surfactants to make emulsion or semiaqueous cleaners. Glycol ethers have been added to stabilize formulations or to increase the cleaning efficiency of a composition. Environmental regulations have identified these ingredients as VOCs, which are regulated by air quality boards. In addition, certain glycol ethers, including 2-butoxyethanol, have been identified as health hazards. © 2001 by CRC Press LLC

RINSING Rinsing is often the most overlooked aspect of cleaning. While a process may employ the best cleaning equipment money can buy and an optimized cleaning solution, without adequate rinsing the overall result may be unsatisfactory. Rinsing is no more than a reduction in contamination by dilution. Adding mechanical or thermal energy may enhance rinsing. Although it is important to use adequate water in rinsing, the key to economy is to use no more than is necessary for acceptable parts. Importance of Rinsing Rinsing is a science in itself. Many factors enter into the design of a rinse system. Let us start with the end in mind. Below is a partial list of considerations: • • • • • • •

Part cleanliness required Production levels required Type of contamination to be removed (amount and type of dragin and residue) Incoming water quality Treatment capabilities Number of rinse tanks, size, layout Water usage and disposal

Each concern will be discussed further. Part Cleanliness Required This consideration is the controlling factor in the whole process of rinsing. If the manufacturer is only concerned with gross contamination, then perhaps rinsing is not even necessary. At the other extreme, if the slightest residue leads to failure, then perhaps the final rinse may need a conductivity of 50 microsiemens/cm (S/cm) or less. Obviously, final quality must be determined before other decisions can be made.

Production Levels and Dragin Production levels and dragin will determine what measures are needed to meet the above determined quality requirements. Dragin will depend on many factors including part configuration, orientation, temperature, drain time, etc. Production levels will determine the amount of dragin per time. Rinsing equations must deal with these factors to predict final rinse quality. Incoming Water Quality Incoming water quality can vary from naturally soft water with few contaminants to water that contains many hundreds of ppm hardness. Water hardness is generally measured either in ppm of equivalent calcium carbonate, CaCO3, or in grains of hardness. Values greater than 120 ppm are considered to be hard water, with values greater than 180 ppm considered to be very hard. Hardness can have many deleterious effects in the cleaning process. Hardness can react with the surfactants to deactivate them, cause corrosion to © 2001 by CRC Press LLC

increase on steel surfaces, and leave deposits on the cleaned surfaces. These residues may cause paint adhesion problems, plating problems, or aesthetic problems, just to name a few. Fortunately, as has been discussed, there are alternatives. Whether softened water, deionized, distilled, or RO water is chosen will depend on the application needs. Number of Rinse Tanks Generally speaking, one large rinse is less effective than multiple small rinses. As an intermediate step, a single rinse may be adequate, but, as a final rinse, that is seldom true. Experience has shown that past a certain point, generally three rinses, increasing the number of rinses is of little value. Rinse Tank Design and Placement Rinse tank design and placement can greatly affect rinsing efficiency. Rather than an afterthought, rinsing needs should be addressed early in the planning of the cleaning line. Considerations should include tank geometry and composition, rinse flow, rinse temperature, and necessary final quality of the parts. It should be noted that rinsing can only be effective if it reaches the parts. Part orientation, loading, and rinse flow dynamics are important and often overlooked considerations. The rinsing needs are quite different for the manufacturer of circuit board components and the rebuilder of motorcycle engines. In the first case the choice might be a heated cascaded triple rinse with a deionized water source and, for the latter, perhaps a quick dip in a cold tap water tank is sufficient. A whole chapter could be easily devoted just to basic rinsing concepts. Certain basic principles should be considered in any case. • Multiple rinses are more efficient than single rinses with the optimum balance being about three rinses. • Water usage can be minimized by cascading the final rinse overflow into the previous rinse and that rinse into the one before. • The final rinse quality is the determining factor in final residue on the part. • Rinsing will not be effective if it does not reach the parts. For the interested reader, further reading should include Peterson’s Practical Guide to Industrial Metal Cleaning.13 Dragin and Final Rinse Quality Knowing and understanding how much and what kind of dragin occurs from the previous step is crucial to predicting rinsing needs. Quite simply dragin over some time equals the quantity that must be diluted to some lower specified level. Solve for the quantity of diluent. In a dynamic situation at equilibrium the flow rate of overflow rinse water must equal the dragin times the process chemical concentration divided by rinse chemical concentration. If this equation is extended to multiple rinse tanks, the dragin is obviously reduced in each successive tank. If the overflow for each rinse tank is cascaded to the previous tank, for all practical purposes, the equation takes the form Overflow  Dragin (process chemical concentration/rinse chemical concentration)1/number of rinse tanks. © 2001 by CRC Press LLC

This cascading obviously reduces water use dramatically over either single rinsing or multiple rinses that are not cascaded to achieve the same final rinse quality. The final rinse quality should be maintained just as carefully as the processing tanks. After all, the rinse is the last liquid the parts will see. It does not make sense to go to great lengths to clean them only to recontaminate them with a contaminated rinse. Acceptable rinse quality will depend on the needs of the application. As such, the steps to measure rinse quality will vary with the needs. Generally speaking, cleaning needs only be adequate to eliminate subsequent problems. While rinse quality is relative, some general guidelines may be helpful. Some sources would classify applications into general cleaning, critical, and very critical cleaning. Although different standards may be proposed, one suggestion is to use conductivity as a guide and divide these applications as follows: • General rinsing operations having a residual level of 1000 S/cm • Critical rinsing with a residual of 500 S/cm • Very critical at less than 50 S/cm Most plating operations call for a high-quality final rinse of less than 50 S/cm as a minimum rinse quality. It can be seen that at these higher-quality rinses, higher-quality water must be used to achieve the desired cleanliness. These guidelines are suggested as a place to start when determining final rinse quality. DISPOSAL Oftentimes local regulations limit or prohibit the discharge of any process water to sewers. Thus, water conservation becomes an increasingly important issue. Bath life and water conservation become very important issues. The result is that overall planning and coordination includes a cradle-to-grave approach for planning and setting up a cleaning line including the rinse and its disposal. For that reason, closed cycle systems for water treatment may actually be economical long-term alternatives. At the very least water use minimization is an important consideration.

CONCLUSION Aqueous cleaning is an increasingly important segment of the cleaning industry. That importance will probably increase with time and the development of improved cleaning systems. The emphasis must be on cleaning systems, as the interactions between parts, soils, equipment, cleaner, and water are much more complex than they appear on the surface. Environmental, economic, and other business concerns demand that industry obtain acceptable parts with minimal impact on the environment and at the least possible cost. The challenge to the cleaner manufacturer and the equipment manufacturer is to develop effective cleaners and equipment that meet those criteria and are compatible with each other. The newer generation of aqueous cleaners is designed to reject contaminants rather than emulsify soils. This feature allows the cleaner to be filtered routinely without significant adverse effect on the cleaner chemistry. These newer formulations can be replenished with routine chemical additions of the cleaner concentrate, according to the maintenance procedures recommended by the chemical supplier. Extension of cleaner bath life obtained with regular bath maintenance results in © 2001 by CRC Press LLC

reduced chemical consumption, reduced waste generation, reduced waste liability, and reduced cleaning costs. Very often the newer cleaning processes also yield cleaner parts as well. Aqueous cleaning is changing to meet the economic and environmental needs of the times. Clearly the future progress of aqueous cleaning will require close cooperation between the chemical and equipment industries. It is also apparent that water quality is a make-or-break issue in critical cleaning, especially as it applies to rinsing. This area is consistently the most neglected aspect of aqueous cleaning. ACKNOWLEDGMENT Portions of the text were reprinted from Metal Finishing, September 1995, J.A. Quitmeyer, The Evolution of Aqueous Cleaner Technology, pp. 34–39, copyright 1995, with permission from Elsevier Science. REFERENCES 1. Archer, W., Reactions and inhibition of aluminum in chlorinated solvent systems, in Corrosion 1978 Conference Proceedings, 1978. 2. Betz, Handbook of Industrial Water Conditioning, 8th ed., Betz Laboratories, Inc., Trevose, 1980. 3. Durkee, J.B., The Parts Cleaning Handbook, Gardner Publications, Cincinnati, 1994. 4. Farrell, R. and Horner, E., Metal cleaning, Metal Finishing, 96, (1), 1998. 5. Gruss, B., Cleaning and surface preparation, Metal Finishing, 96, (5A), 1998. 6. Hanson, N. and Zabban, W., Plating, 46, 1959. 7. Hirsch, S., Deionization for electroplating, Metal Finishing, January, 145 –149, 1997. 8. Kanegsberg, B.F., Aqueous cleaning for high-value processes, A2C2 Mag., 2, (8), 1999. 9. Metals Eng. Q., November 1967. 10. Mohler, J.B., The rinsing ratio applied to practical problems, Part 1, Metal Finishing, May 1972. 11. Nelson, W., The key to successful aqueous cleaning is water, Precision Cleaning, Flemington, April 1996. 12. Permutit, Water and Waste Treatment Data Book, 18th printing, U.S. Filter/Permutit, 1993. 13. Peterson, D.S., Practical Guide to Industrial Metal Cleaning, Hanser Gardner, Cincinnati, 1997. 14. PPG Handbook, Vapor Degreasing, 1986. 15. Schrantz, J., Rinsing: a key part of pretreatment, Ind. Finishing, June 1990. 16. Spring, S., Industrial Cleaning, Prism Press, Melbourne, 1974. 17. Vapor zone solvent cleaning systems, Precision Cleaning, Flemington, December 1996. 18. Wolf, K. and Morris, M., Ozone depleting solvent alternatives: have you converted yet?” Finishers’ Manage., June/July 1996. 19. Zavadjancik, J., Aerospace manufacturer’s program focuses on replacing vapor degreasers, Plating Surf. Finishing, April 1992. 20. CRC Handbook of Chemistry and Physics, 77th ed., CRC Press, Boca Raton, FL, 1996. 21. Quitmeyer, J., The Evolution of Aqueous Cleaner Technology, Metal Finishing, Tarrytown, September 1995. 22. Quitmeyer, J., All Mixed Up: Qualities of Aqueous Degreasers, Precision Cleaning, Fleminton, September 1997.

© 2001 by CRC Press LLC

CHAPTER 1.4

Review of Solvents for Precision Cleaning John W. Agopovich

CONTENTS Review of Solvents for Precision Cleaning Background General Requirements of a Precision Cleaning Solvent Cleaning Methods for Solvents Environmental Issues with Solvents Volatile Organic Compounds Global Warmers Flammability SNAP Approval Discussion of the Different Solvent Classes Flammable Solvents Hydrocarbons (Aliphatic) Hydrocarbons (Aromatic) Ketones Alcohols Halogenated Solvents Perfluorocarbons Hydrofluoroethers HFEs HFE Azeotropes and Blend Hydrofluorocarbons HFCs HFC Azeotropes and Blends Chlorinated Solvents Nonozone Depleters Ozone Depleters Other Solvents References

© 2001 by CRC Press LLC

REVIEW OF SOLVENTS FOR PRECISION CLEANING Background Since the early 1990s, organizations have been actively searching for alternatives for ozone-depleting chemicals (ODCs). CFC-113 and 1,1,1-trichloroethane (also called methyl chloroform) were the major ODCs utilized for precision cleaning processes. Before the Montreal Protocol mandates, these two materials were excellent for cleaning a wide range of contaminants from many different devices. CFC-113 was a good general cleaner for a wide range of contaminants, including hydrocarbon- and halogenated-based oils and particles. 1,1,1-Trichloroethane was required for degreasing of parts contaminated with heavier oils and residual greases, along with solder flux. Both CFC-113 and 1,1,1-trichloroethane were available in high-purity grades required for precision cleaning operations and readily evaporated after use. Neither material is highly toxic and neither has a flash point. 1,1,1Trichloroethane does have flammable limits in air, however. This chapter discusses the several classes of solvents that have been implemented as replacements for these two materials in precision cleaning. For purposes of definition in this chapter, a solvent is defined as a cleaning agent that readily evaporates after use, when cleaning a component. No follow-up cleaning is required after the use of this class of material. These solvents will evaporate even after cold-cleaning. This will generally mean that the solvent has a vapor pressure of greater than 25 torr at ambient temperature. Another better quantifier of volatility is the evaporation rate compared to n-butyl acetate. n-Butyl acetate is set at unity (ether and trichloroethylene are also used as reference materials). The majority of solvents discussed in this chapter all have an evaporation rate  n-butyl acetate or 1. Another parameter related to volatility is the heat of vaporization, the heat required when a solvent transforms to the vapor phase from the liquid phase. When possible, it is best to choose solvents that have a low heat of vaporization, as these materials will evaporate without absorbing heat (the part cools). Solvents with high heats of vaporization cool a part as more heat is absorbed from the environment. Often water condenses on a device that is cleaned, if the environment is humid. This is not desirable in precision cleaning. Volatility and ease of vaporization have drawbacks. These include issues of containment, flammability, toxicity, local regulations on emissions, and cost. These issues are discussed later in the chapter. Cleaners, such as any aqueous based terpenes and hydrocarbons in the combustible range (flash point 100°F), do not fall under the definition of a solvent and will not be discussed in this chapter. Critical precision cleaning areas that use solvents under this definition include but are not limited to medical devices, directional devices (gyroscopes, accelerometers, and components within), computer components (disk drives), precision ball bearings, oxygen transport systems, and circuit boards. General Requirements of a Precision Cleaning Solvent In addition to the need for volatility discussed previously, a precision cleaning solvent must meet other critical requirements. Obviously, the contaminant requiring removal must have a finite solubility in the cleaning solvent. This definition varies as factors such as time, temperature, and agitation can be altered. Generally speaking, increasing the temperature of a solvent will improve cleaning effectiveness, as will increasing the exposure time. As an example, 3M defines “soluble” when a material dissolves in another in the range of 5 to 25 g/100 g of solvent at room temperature.1 In another article, solubility has been defined as © 2001 by CRC Press LLC

50 g/100 ml of solvent.2 There are many other ranges of definitions. The solubility and cleaning effectiveness required will vary depending on how contaminated the part is, as well as the user’s final requirements. Other solubility parameters include a Kauri-butanol (KB) number. The KB numbers for CFC-113 and 1,1,1-trichloroethane are 31 and 124, respectively. Generally speaking, the highly chlorinated compounds have the higher reported KB values. The KB number reflects the ability of a solvent to dissolve heavy hydrocarbon greases. Specifically, it is a measure of ability to dissolve a solution of butanol and Kauri resin. ASTM D 1133-90 describes the standard test method for determining the KB value of hydrocarbon solvents. This procedure has been extended to evaluate ODC replacements discussed in this chapter. It is not applicable to oxygen-containing solvents. However, the author believes that as a first approximation, the “like dissolves like” concept is very useful. This means that polar solvents dissolve polar contamination and nonpolar solvents dissolve nonpolar contaminants. Hydrocarbon solvents will dissolve hydrocarbon oils and fluorocarbon-based solvents dissolve fluorocarbon oils and greases. To clean solder flux residues from printed circuit boards, polar oxygen-containing solvents like alcohols or chlorinated solvents are required. In precision cleaning, it is beneficial to have a solvent with a low viscosity and low surface tension. This property will allow solvents to enter very narrow gaps in a complicated device, to clean a contaminant. Particle removal is often a critical part of precision cleaning operations. A solvent with a low viscosity and low surface tension also facilitates particle removal. A high-density solvent provides additional momentum to remove particles from surfaces. Another critical requirement is chemical stability during use and also a solvent having a long or infinite shelf life. This was been a problem with the use of 1,1,1-trichloroethane without stabilizers. Some azeotropes and mixtures discussed later require stabilizers. Of even more importance is compatibility with the component one is cleaning. There cannot be a chemical reaction or a physical change such as irreversible swelling or extraction of the materials of construction of the component being cleaned. CFC-113 and 1,1,1trichloroethane were compatible with most materials. ODC alternative solvent manufacturers are very cognizant of the concerns of customers about solvent compatibility. Extensive solvent/materials compatibility tests are performed on a wide range of materials when a new solvent is introduced to the public. If one still has a question of the compatibility of material and solvent, it is best to have the solvent user perform the compatibility test in one’s particular application. Another critical solvent property is the nonvolatile residue (NVR). It is critical that, when a solvent evaporates from a surface after cleaning, no residue is left behind. Solvent manufacturers typically have NVR specifications in the range of 1 to 10 parts per million (ppm) for precision cleaning. Very often the reported NVR of a given batch of solvent is well under the company-set specification. The issue of NVR is also important when expensive solvents such as the perfluorocarbons (PFCs), hydrofluoroethers (HFEs), and hydrofluorocarbons (HFCs) are reclaimed and recycled. Any recycling process (such as distillation) must produce a product with the NVR meeting the original manufacturer (OEM) specifications. Recycling is desirable for cost savings when using expensive solvents. The halogenated-containing solvents (PFCs, HFCs, and HCFCs) are particularly expensive. Recycling of used solvents is possible when solvents are used in cleaning and subsequently contaminated with particles or high-boiling oils and greases. A simple strip or low theoretical plate distillation can be performed to purify the reclaimed solvent, to obtain NVR levels equal or to better than the OEM specifications. © 2001 by CRC Press LLC

Table 1 Physical Properties of CFC-113 and 1,1,1-Trichloroethane

Solvent

Vapor Boiling Pressure, Density Point mm Hg Flash TWAa (g/cc), (°C) at 25°C Point (ppm)b 25°C

CFC 113 48 1,1,1-Trichloroethane 74

334 121

None None

1000 350

1.56 1.32

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 25°C

0.68 0.80

17.3 25.0

Note: Physical properties compiled from published information. a Time-weighted average. b American Conference of Governmental Industrial Hygienists (ACGIH) 8-h TWA (1998).

Later in this chapter, the topic of azeotropes is discussed. The use of azeotropes is beneficial if the user requires expanding the capabilities of an existing single-component cleaning solvent. In the days when CFC-113 was used, many azeotropes of this material were available and popular for a wide range of applications. In many cases, an azeotrope will allow the ability of a precision cleaning agent to remove contaminants not possible when using the pure component. An example is using a PFC/hydrocarbon azeotrope that can clean light hydrocarbon oils. Many existing PFC alternatives form azeotropes with alcohols, chlorinated solvents (nonozone depleters), and hydrocarbons. The alcohol azeotropes are useful for removing ionic contamination from solder flux residuals. Listed in Table 1 are typical physical properties of CFC-113 and 1,1,1-trichloroethane. This table is presented for comparison purposes to the upcoming discussion of the recent generation of ODC replacement solvents. Cleaning Methods for Solvents These solvents can be used in vapor degreasers (both single component and as a cosolvent), ultrasonic cleaners, power spray (low- and high-pressure), dip, and wipe cleaning. The issues with the flammable solvents and toxicity in equipment must be addressed in light of local regulation requirements and standards set by each manufacturer. Environmental Issues with Solvents Unfortunately, even though most of the solvents discussed in this chapter do not deplete the ozone layer, there are other environmental issues that must be dealt with. Some ODC alternatives are classified as volatile organic compounds (VOCs), hazardous air pollutants, (HAPs), global warmers, or fall under National Emission Standards for Hazardous Air Pollutant (NESHAP) regulations. Volatile Organic Compounds Any compound that is released in the troposphere can undergo chemical reactions via hydroxyl radical extraction. According to the Environmental Protection Agency (EPA), all solvents are under the definition of VOC unless specifically exempted. VOCs can lead to the formation of ground-level, tropospheric ozone. The formation of ozone can in turn lead to the formation of smog via a series of complex free radical and photochemical reactions. Many compounds are exempted from the VOC restrictions, when it is determined that the release of this material in the troposphere will not lead to ozone formation. © 2001 by CRC Press LLC

Global Warmers There is an environmental concern about materials such as PFCs that have no mode of breakdown in the atmosphere after release. These materials do not react with hydroxyl radicals or ultraviolet (UV) radiation and break down. They simply accumulate and have atmospheric lifetimes on the order of hundreds or thousands of years. These materials can trap heat (infrared radiation) from escaping the troposphere. Alternatives to PFCs have been developed and have much shorter lifetimes. Another measurement of atmospheric lifetime is global warming potential (GWP). Typical materials are compared with carbon dioxide. The carbon dioxide GWP is set at 1.0 for a 100 year integration time horizon (ITH), per the Intergovernmental Panel on Climate Change (IPCC).3 Flammability CFC-113 and 1,1,1-trichloroethane had the advantage of not being flammable materials. Many of the alternatives discussed in this chapter possess flash points. Others do not flash, but have flammable limits in air, like 1,1,1-trichloroethane. Some azeotropes and azeotrope-like mixtures that will be discussed contain a high percentage of a nonflammable solvent (inerting agent). SNAP Approval The EPA has required that the Significant New Alternatives Policy (SNAP) program must approve all alternatives to ODCs. This program was formalized in 1994. Unless specifically mentioned in this chapter, all solvents discussed are SNAP approved. Some materials are SNAP approved with conditions. An example is the PFCs. The PFC materials can only be used if no other solvents can be used for technical or safety concerns. Other materials are conditionally SNAP approved for safety reasons, such as requiring that personal exposure limits be met. This is especially the case with chlorinated solvents. The safety issues with solvents, such as flammability and exposure limits, should be verified by a user by reviewing the Material Safety Data Sheets (MSDS) and product information literature from the solvent manufacturer.

DISCUSSION OF THE DIFFERENT SOLVENT CLASSES Flammable Solvents Hydrocarbons (Aliphatic) Aliphatic hydrocarbons such as n-hexane, n-heptane, and isooctane have utility in cleaning a wide range of hydrocarbon-based contaminants. These include hydrocarbonbased oils and greases. These materials can be used as wipe solvents to remove residual hydrocarbons and fingerprints. A list of physical properties is shown in Table 2. All these are exceedingly flammable with flash points well below room temperature. Hydrocarbon solvents are SNAP approved. These materials have KB values in the range of 30, from comparison to mineral spirits. Cyclohexane is added to this group as a saturated cyclic hydrocarbon.

© 2001 by CRC Press LLC

Table 2 Physical Properties of Aliphatic Hydrocarbons

Solvent

Boiling Point (°C)

n-Hexane n-Heptane Isooctane Cyclohexane

69 98 99 81

Vapor Pressure (torr) at 20°C 124 36 41 78

Flash Point (TCC), °F 15 25 10 17

TWA (ppm)a 50 400 300b 300c

Density (g/cc), 20°C

Viscosity (cP), 20°C

Surface Tension (dyn/cm), 20°C

0.66 0.68 0.69 0.78

0.31 0.41 0.50 1

18 (25°C) 20.3 18.8 25

Note: Physical properties compiled from published information. TCC  tag closed cup. a ACGIH 8-h TWA (1998). b TWA for gasoline. c Planned to be reduced to 200 ppm per the ACGIH, 1998 TLVs ® and BEIs ® Threshold Limit Values for Chemical Substances and Physical Agents, “Notice of intended changes for 1998,” p. 74.

Hydrocarbon solvents are not terribly aggressive cleaning solvents. However, they are compatible with a wide range of materials. Flammable hydrocarbons must be used per VOC and HAP regulations, as required. All should be high-performance liquid chromatography (HPLC)-grade solvents or equivalent. Hydrocarbons (Aromatic) Aromatic hydrocarbons such as toluene often display increased cleaning effectiveness as compared with aliphatic hydrocarbons. Toluene is used instead of benzene for the obvious toxicity reasons. The physical properties of toluene are shown in Table 3. Aromatic hydrocarbons have KB numbers greater than the aliphatic hydrocarbons and mineral spirits. The KB number of toluene is 105. Aromatic solvents are not as popular as the aliphatic materials because of toxicity reasons. Toluene is classified as a VOC and HAP. Toluene is SNAP approved. The HPLC grade or equivalent should also be used. Ketones Acetone and methyl ethyl ketone (MEK) are popular cleaning solvents where aggressive cleaning is required. These materials are used for removal of polar contaminants and also solder flux and conformal coatings. These materials are sometimes too aggressive for general cleaning operations so care must be taken if these are used. Compatibility verification with organic polymers (plastics, elastomers, etc.) is required for each application. Physical Table 3 Physical Properties of Toluene

Solvent

Boiling Point (°C)

Vapor Pressure (torr) at 20°C

Flash Point (TCC), °F

Toluene

111

29

35

TWA (ppm)a

Density (g/cc), 20°C

Viscosity (cP), 20°C

Surface Tension (dyn/cm), 20°C

50

0.87

0.59

28.5

Note: Physical properties compiled from published information. TCC = tag closed cup. a ACGIH 8-h TWA (1998).

© 2001 by CRC Press LLC

Table 4 Physical Properties of Acetone and MEK

Solvent

Boiling Point (°C)

Vapor Pressure (torr) at 20°C

Acetone MEK

56 80

185 74

Flash Point (TCC), °F 0 30

TWA (ppm)a

Density (g/cc), 20°C

Viscosity (cP), 20°C

Surface Tension (dyn/cm), 20°C

500 200

0.79 0.80

0.36 0.43

23.3 24 (25°C)

Note: Physical properties compiled from published information. a ACGIH 8-h TWA (1998).

properties of acetone and MEK are shown in Table 4. Acetone and MEK are SNAP approved. HPLC grades are adequate. Acetone has recently been exempted as a VOC and is not a HAP. KB values are not applicable for acetone and MEK as they are oxygen-containing solvents. Alcohols Short-chain alcohols such as methanol (MeOH), ethanol (EtOH), and isopropyl alcohol (IPA) are also more polar than hydrocarbons and can therefore clean a wider range of contamination. Many water-soluble materials are miscible in alcohols. Also, alcohols can be used to follow up an aqueous cleaning process to remove traces of water left behind. Alcohols are popular in mixtures and azeotropes for cleaning solder flux and ionic residues. These mixtures are discussed later. KB values are not applicable for alcohols. Physical properties are shown in Table 5. Alcohols are SNAP approved and must be used per VOC and HAP regulations, as required. Halogenated Solvents This section discusses a popular class of solvents that are heavily halogenated containing fluorine and/or chlorine. These solvents are all expensive cleaning materials and range from $10 to $20/lb.

Table 5 Physical Properties of Short-Chain Alcohols

Solvent Methyl Ethyl (200 proof) Isopropyl

Boiling Point (°C)

Vapor Pressure (torr) at 20°C

Flash Point (TCC), °F

TLV/ TWA (ppm)a

Density (g/cc), 25°C

Viscosity (cP), 20°C

Surface Tension (dyn/cm), 20°C

65 78

97 45

54 58

200 1000

0.79 0.79

0.55 1.10

22.6 22 (25°C)

82

32

53

400

0.78

2.40

21.8 (15°C)

Note: Physical properties compiled from published information. TCC
tag closed cup. a ACGIH 8-h TWA (1998).

© 2001 by CRC Press LLC

Figure 1

Perfluoro-N-methyl morpholine.

Perfluorocarbons (PFCs) 3M has had a range of PFCs under the name Fluorinert™ for several years. As the replacements of ODCs became an issue, 3M introduced a line of solvents referred to as Performance Fluids (PFs). Some of these were from the Fluorinert line (FC) of fluids that did not meet the tight requirements for these FC applications, but were acceptable for cleaning requirements. Examples are FC-84 and PF-5070. The predominant material in both is perfluoroheptane, but the PF-5070 is acceptable for cleaning. PFCs have a high liquid density, a low viscosity, and low surface tension. These physical properties make these materials excellent replacements for ODCs in critical particle removal applications. Because these are fluorinated, they are not miscible with hydrocarbon oils. Therefore, they are not candidates for general cleaning operations (fingerprints, oils, etc.). Properties of PFCs have been discussed elsewhere.4 Other popular PFCs for cleaning are PF-5060 (predominant component is perfluorohexane) and perfluoro N-methyl morpholine (PF-5052). The structure of PF.-5052 is shown in Figure 1. The ring carbons are completely fluorinated. PFCs are excellent solvents for PFPEs (perfluoropolyethers such as Krytox™ fluids), which are used as lubricants in computer disk drives and precision ball bearings. PFCs are used as carriers to deposit PFPEs on a computer disk drive and also clean them from surfaces. PFCs are also excellent solvents for cleaning the Halocarbon Corporation product line of chlorotrifluoroethylene (CTFE)-based fluids, as these materials are heavily fluorinated. Additional details of the use of PFCs in precision cleaning have been discussed previously.4 When PFCs are used for critical particle removal applications, the solvent may have to be filtered before use. This will depend on the application. PFCs are relatively inert, nontoxic, and compatible with essentially all materials. They are not classified as VOCs. PFCs have a KB number of 0 as they have no miscibility with the Kauri resin. PFCs are SNAP approved with conditions when they are the only materials available for technical and safety reasons. The physical properties of PFCs are shown in Table 6. Hydrofluoroethers HFEs The HFEs are the 3M second generation of replacements for ODCs. HFEs are ethers with an n-butyl/isobutyl fluorocarbon group and a shorter hydrogen-containing alkyl

© 2001 by CRC Press LLC

Table 6 Physical Properties of PFCs

Solvent

Boiling Point (°C)

Vapor Pressure, mmHg at 25°C

Flash Point

PF-5060 PF-5070 PF-5052

56 80 50

232 79 274

None None None

TWA (ppm)

Density (g/cc), 25°C

Viscosity (cS), 25°C

Surface Tension (dyn/cm), 20°C

N/A N/A N/A

1.68 1.73 1.70

0.4 0.6 0.4

12.0 13.0 13.0

Note: Physical properties taken from recent 3M product information. N/A — not applicable.

group. The HFE-7100 (3M™ Novec™ HFE-7100) has a methyl group and HFE-7200 (3M™ Novec™ HFE-7200) has an ethyl group. The structures are shown below. HFE-7100

HFE-7200

CH3–O–C4F9 CH3CH2–O–C4F9

The atmospheric lifetime, although still significant, is reduced drastically from that of the PFCs because of the hydrogen-containing moiety (carbon–hydrogen bonds). This becomes a site in the molecule for hydroxyl radical attack and breakdown in the troposphere. These materials are SNAP approved by the EPA without conditions. HFEs are therefore the logical replacements for PFCs. The HFEs have cleaning characteristics similar to PFCs as they are solvents for PFPEs and CTFE fluids. The hydrocarbon side chain allows for increased cleaning effectiveness for light hydrocarbon oils, especially in the case of HFE-7200, and at elevated temperatures. Because of these solubilities, these materials have KB numbers greater than zero but still very low, on the order of 10. Physical properties of the HFE-7100 and HFE-7200 are summarized in Table 7. These materials still have the low viscosity, high density, and low surface tension (like PFCs) to assist particle removal from surfaces. For some critical particle removal applications, the HFEs may require filtering before use. 3M has recently introduced a high-purity HFE-7100DL (disk lubricant) material, which has controls on particles, ions, metals, etc. HFEs are compatible with most materials2 and are not classified as VOCs. Both materials have no flash points. HFE-7200 has flammable limits in air. Other details about HFEs are also discussed in Reference 2, 3M product literature, and in Chapter 1.5 by Owens. Table 7 Physical Properties of HFE-7100 and HFE-7200

Solvent

Boiling Point (°C)

Vapor Pressure, mmHg at 25°C

Flash Point

HFE-7100 HFE-7200

61 76

202 109

None None

TWA (ppm)a

Density (g/cc), 25°C

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 20°C

750 200

1.52 1.43

0.61 0.61

13.6 13.6

Note: Physical properties taken from recent 3M product information. a HFE-7100 exposure guideline from the American Industrial Hygiene Association. 3M exposure guideline for HFE-7200.

© 2001 by CRC Press LLC

Table 8 Physical Properties of HFE Azeotropes and Blend

Solvent

Boiling Point (°C)

Vapor Pressure, mmHg at 25°C

HFE-71DE HFE-71DA HFE-71IPA

41 40 55

383 381 207

Flash Pointa

TLV/ TWA (ppm)b

Density (g/cc), 25°C

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 25°C

None None None

See below See below See below

1.37 1.33 1.48

0.45 0.45 nlc

16.6 16.4 14.5

Note: Physical properties taken from recent 3M product information. a The HFE-71IPA material has no closed-cup or open-cup flash point. See 3M literature about details of the flash points of these materials. b

3M reports separately the level of HFE-7100 (750 ppm) and the other components (components were mentioned earlier in chapter).

c

nl — not listed in literature reviewed.

HFE Azeotropes and Blend Azeotropes are defined as mixtures that maintain an essentially constant relative proportion in the vapor and liquid phase as the mixture components boil. 3M has two commercially available azeotropes and one azeotrope-like blend to improve cleaning effectiveness of hydrocarbon-based materials. These include HFE-71DE, HFE-71IPA, and HFE-71DA, whose physical properties are summarized in Table 8. HFE-71DE is a 50/50 mixture by weight of HFE-7100 and trans-1,2-dichloroethylene. This material can be used to clean a wider range of hydrocarbon-based oils. The KB value of this azeotrope is 27, which approaches the KB number of mineral spirits. In general, HFE-71DE is compatible with most materials. However, the high percentage of trans-1,2dichloroethylene warrants users to verify the compatibility of this azeotrope-like mixture with certain organic materials. Another product is a ternary azeotrope, HFE-71DA. This is similar to HFE-71DE, but has a low percentage of ethyl alcohol to remove ionic materials in flux removal applications. HFE-71IPA is an azeotrope-like mixture of HFE-7100 and approximately 5% IPA. The small percentage of IPA increases solubility of light oils and hydrocarbon oils. The HFE azeotropes have no flash points. The HFE-71DA and HFE-71IPA have a flammability range in air. The HFE-71DE does not have a flammability range in air. Hydrofluorocarbons HFCs HFCs are PFCs where selected fluorines are replaced in the molecule by hydrogen. In actuality, it is usually not chemically possible to replace fluorines in a PFC molecule. The HFC molecule is synthesized via a different route than a fluorocarbon, allowing a selected few hydrogens in the molecule. As with the HFEs, the HFC have a weak area in the molecule where hydroxyl radical attack can occur to break these molecules down in the troposphere. The atmospheric lifetime (global warming potential) of this molecule is much less than a PFC and they are SNAP approved without conditions. DuPont introduced an HFC material with the trade name Vertrel® XF. This solvent is perfluoropentane, with fluorine replaced by hydrogen at the second and third carbons:

© 2001 by CRC Press LLC

CF3CF2CHFCHFCF3 This material is still very similar to a PFC for cleaning effectiveness. PFPEs and CTFE fluids are exceedingly soluble in this material. However, XF has limited utility for dissolving hydrocarbon materials, as noted by a KB value of 9. This material can be used for particle removal also, as it still has a relatively high density, low viscosity, and low surface tension. HFC Azeotropes and Blends There are several azeotropes of XF available, expanding the cleaning capability of these products. These include XM, XE, SMT, and MCA. MCA Plus and XMS Plus are blends. The XM and XE azeotropes have a small amount of methyl alcohol and ethyl alcohol, respectively. These azeotropes can be used for light oil cleaning and particle removal. The remaining four cleaners contain trans-1,2-dichloroethylene, analogous in concept to the HFE azeotropes. MCA is a binary azeotrope of XF and trans-1,2-dichloroethylene. The SMT is a ternary azeotrope containing a small amount of methyl alcohol in addition to the trans-1,2-dichloroethylene, making it useful for cleaning ionic and solder flux contamination from circuit boards. MCA Plus is a ternary blend useful for heavy oil and grease cleaning. It contains trans-1,2-dichloroethylene and cyclopentane. XMS Plus is a quaternary blend and is similar to MCA Plus, but contains a small amount of methyl alcohol. It therefore has potential cleaning effectiveness of both the SMT and MCA Plus. DuPont has the details of the percentages of each component of the above-mentioned mixtures. Physical property and toxicity information is shown in Table 9. All details of XF formulations (compatibility, environmental issues) can be found in the latest DuPont product specification literature. Discussions of some DuPont HFC materials are also presented in Reference 2 and in Chapter 1.6 by Merchant. Another HFC material, benzotrifluoride (BTF), is toluene except the methyl group is completely fluorinated. The fluorinated methyl group results in this material having a lower KB number than toluene (49 compared with 105). However, it evaporates more Table 9 Physical Properties of HFC Azeotropes and Blends

Solvent

Boiling Point (°C)

Vapor Pressure, mmHg at 25°C

Flash Pointa PMCC, °F

AEL Limit, ppm

Density (g/cc), 25°C

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 25°C

XF XM XE MCA SMT MCA plus XMS plus

55 48 52 39 37 38 38

226 298 250 464 471 461 470

None None None None None None None

200b 200c 235d 200c 192d,e 214d 197d,e

1.58 1.49 1.52 1.41 1.37 1.33 1.34

0.67 0.63 0.73 0.49 0.47 0.49 0.46

14.1 14.1 14.1 15.2 15.5 16.1 14.9

Note: Physical properties taken from recent DuPont product information. a Pensky-Martens closed cup except for XE and XM, which are tag-open-cup tested. b Dupont AEL/8- and 12-h TWA. c trans-1,2-Dichloroethylene and methyl alcohol have a 200-ppm TLV/ 8-h TWA per ACGIH. d ACGIH calculation for TLV of mixtures. e Small amount of stabilizer required.

© 2001 by CRC Press LLC

Table 10 Physical Properties of BTFa

Solvent

Boiling Point (°C)

Vapor Pressure (mmHg) @ 20°C

Flash Point TCC, °F

BTF

102

30

54

CEL (ppm)

Density (g/cc), 20°C

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 20°C

100

1.19

nl

23

Note: Physical properties taken from OxyChem product literature. TCC
tag closed cup, nl
not listed in literature reviewed. a BTF is SNAP approved per the condition of the 100-ppm exposure limit.

quickly after use, is less toxic, and has a higher density and lower surface tension than toluene. This KB value is higher than any other pure, single-component HFC, likely because of the presence of aromatic hydrogens. This material forms azeotropes with a wide range of materials and has a flash point in the flammable range6. Other details can be found by review of literature from OxyChem and in Chapter 1.10 on benzotrifluorides by Skelly. Properties are shown in Table 10. BTF is also known as Oxsol® 2000. Chlorinated Solvents Nonozone Depleters Chlorinated solvents are popular because of their excellent solvency for cleaning a wide range of contaminants. These materials do not have flash points but all have flammable limits in air, similar to 1,1,1-trichloroethane. When 1,1,1-trichloroethane was available for use, it was the preferred degreasing chlorinated solvent. All of these have high KB numbers, ranging from 90 for perchloroethylene to 136 for methylene chloride. Methylene chloride Trichloroethylene Perchloroethylene (PCE) trans-1,2-Dichloroethylene (trans)

CH2Cl2 CCl2
CHCl CCl2
CCl2 CHCl
CHCl

In a recent issue of Parts Cleaning, an article details the toxicity and carcinogenicity of these three solvents: trichloroethylene, methylene chloride, and perchloroethylene.5 All these materials are suspected of being carcinogenic. Even though these materials contain carbon–chlorine bonds, none is an ozone depleter because of their short atmospheric lifetimes. However, chlorinated ethylenes are VOCs. trans is not a HAP. These materials are SNAP approved with the condition of meeting personal exposure limits. The chlorinated ethylene materials are not completely stable, need to be protected from moisture, and some require the presence of stabilizers. A listing of physical properties is shown in Table 11. They are also discussed in Chapter 1.8 by Risotto. Ozone Depleters AK-225 is a product offered by AGA chemicals. Of all the solvents discussed in this chapter, this material is most similar to CFC-113 in physical properties and therefore cleaning effectiveness. Some of these details are discussed in Reference 2. They have identical KB numbers of 31. AK-225 is a solvent for hydrocarbon oils but because it contains fluorine and chlorine, it is also a solvent for heavily fluorinated materials such as the PFPEs and CTFE © 2001 by CRC Press LLC

Table 11 Physical Properties of Non-Ozone Depleting Chorinated Solvents

Solvent Methylene chloride Trichloroethylene Tetrachloroethylene trans-1,2Dichloroethylenec

Vapor Pressure (torr) at 20°C

Flash Point TCC, °F

40

350

None

50

87 121 48

47b 18b 324b

None None 36d

50 25 200

Boiling Point (°C)

Viscosity (cP), 20°C

Surface Tension (dyn/cm), 20°C

1.33

0.44

28.1

1.46 1.62 1.26

0.57 nl nl

29.5 nl nl

TLV/ Density TWA (g/cc), (ppm)a 20°C

Note: Physical properties compiled from published information; nl  not listed in literature reviewed. a ACGIH TWA (1998). b At 25°C. c This material is generally used as a part of a mixture or azeotrope with HFEs and HFCs. d Closed-cup flash point from 7/6/94 Dupont MSDS.

fluids. This material is a mixture of the ca and cb isomers. The chemical structures of the two isomers are as follows: HCFC-225ca isomer HCFC-225cb isomer

CF3CF2CHCl2 CF2ClCF2CFHCl

This material is a Class II ozone depleter, with an ODP of 0.03 (CFC-11 is 1.0 on this scale). It is SNAP approved and is not classified as a VOC. These materials are not scheduled for start of phaseout of production until the year 2015, with complete phaseout by 2030. Interestingly, the ca isomer is more toxic than the cb isomer, necessitating the conditions placed upon the SNAP approval. The SNAP conditions that a company-set exposure limit of 25 ppm for the ca isomer is required (the level for the cb isomer is 250 ppm). The exposure limit for the mixture was set at 50 ppm by the manufacturer. Physical properties of AK225 are shown in Table 12. Note at the writing of this chapter that AGA chemicals has mentioned that a material designated as AK-225 G will possibly be made available for critical government applications. This material is the purified cb isomer, which is less toxic. Several azeotropes or mixtures with HCFC-255ca/cb exist and are discussed in product literature. These can be used for heavier oil and grease removal or solder flux removal. Some are discussed in Reference 2.

Table 12 Physical Properties of AK-225

Solvent

Boiling Point, °C

Vapor Pressure, kg/cm2 at 25°C

AK-225 (HCFC-255ca/cb)

54

283

Flash Point

TLV/ Density TWA (g/cc), (ppm) 25°C

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 25°C

None

50

0.59

16.2

1.55

Note: These physical properties are taken from recent AK-225 product information and are further discussed in Chapter 1.11 by Miki et al.

© 2001 by CRC Press LLC

Other Class II ozone depleters include HCFC-141b and HCFC-123. The SNAP program deemed HCFC-141b unacceptable for precision cleaning because of its high ODP, which is similar to 1,1,1-trichloroethane. HCFC-123 has a similar ODP to AK-225, but has a boiling point near room temperature, making it difficult to handle. It can have utility in properly designed vapor degreasing equipment. It is SNAP approved with the condition of meeting the 30 ppm exposure limit. Other Solvents This group includes: n-Propyl bromide (nPB) CH3CH2CH2Br Volatile methyl siloxane (VMS) (CH3)3–Si–O–Si–(CH3)3 para-Chlorobenzotrifluoride (PCBTF) There are three other solvent-type precision cleaners that merit discussion. n-Propyl bromide is a replacement for 1,1,1-trichloroethane in degreasing operations. As of this writing, this material has not yet been SNAP approved but is listed as SNAP pending and therefore can be legally sold and used. The EPA has petitioned inputs from users of this material to assist in the SNAP-approval process. This material has a low ODP. The exact value is being reexamined because of uncertainties in the model when the material has a short atmospheric lifetime, such as n-propyl bromide. It likely will range from 0.006 to 0.027. The EPA also has questions about the toxicity of this material. It will likely be SNAP approved with conditions. n-Propyl bromide is discussed in Chapter 1.7 by Shubkin. The n-propyl bromide material has a KB value of 129, which is similar to 1,1,1trichloroethane. It is therefore an aggressive solvent. Physical properties of this material are shown in Table 13. Dow Corning has a series of VMSs (volatile methyl siloxanes) under the trade name Ozone Safe (OS) fluids. The OS-10 material has a vapor pressure in the solvent range. OS10 is SNAP approved, compatible with many materials, and not classified as a VOC. It is flammable as noted in Table 13. The KB values of these materials are low, with OS-10 at 16.6. It is suitable for precision cleaning of materials with light to medium hydrocarbon and silicone oil contamination. VMS is further discussed in Chapter 1.9 by Cull and Swanson. Table 13 Physical Properties of Other Solvents

Solvent n-Propyl (nPB) bromide OS-10 para-Chlorobenzotrifluoride (PCTBF)

Boiling Point, °C

Vapor Pressure, mmHg at 25°C

Flash Point CC, °F

TWA (ppm)

Density (g/cc), 25°C

Viscosity 25°C

Surface Tension (dyn/cm), 25°C

70

110

None

TBDa

1.33

0.49 cP

26 (20°C)

100

42

27

200b

0.76

0.65cS

15.2

109

25c

1.34

0.79cP

25

139

7.9



Note: Physical properties compiled from published information. nPB data from EnSolv ; OS-10 properties from Dow Corning; PCTBF data from OxyChem. a To be determined, EPA estimates 50 to 100 ppm b Dow Corning limit. c The OxyChem Corporate exposure limit for 8-h TWA is also 25 ppm (May 5, 1998 MSDS).

© 2001 by CRC Press LLC

OxyChem produced para-chlorobenzotrifluoride (PCBTF), which is a replacement for 1,1,1-trichloroethane. (Oxsol® 100) has a KB value of 64, intermediate for a wide range of cleaning operations. It is compatible with a wide range of organic polymers. It was SNAP approved with the condition of a 25-ppm exposure limit. It is exempt from VOC restrictions, is not an ODC, and is not an HAP. The vapor pressure of PCTBF is lower than solvents discussed previously and has a flash point in the combustible range, but does have utility as a cleaner if used safely to comply with the low exposure limit. Other details can be found by review of OxyChem literature or in Chapter 1.10 on benzotrifluorides by Skelly. [Editor’s note: As of press time, OxyChem is exiting the market and is ceasing production of benzotrifluorides. Therefore, the future availability of PCBTF is uncertain. — B.K.] REFERENCES 1. 3M Fluorinert™ Liquids Product Manual, 1991. 2. Agopovich, J.W., PFC alternatives analyses, Precision Cleaning, March 1997 and references cited therein. 3. IPCC, Radiative Forcing of Climate Change, The 1994 Report of the Scientific Assessment Working Group of the IPCC. 4. Agopovich, J.W., Fluorocarbons and supercritical carbon dioxide serve niche needs, Precision Cleaning, February 1995, and references cited therein. 5. Reynolds, R., TCE and cancer fact and fiction, Parts Cleaning, March 1999. 6. Ostrowski, P., Benzotrifluoride: a new HFC for cleaning, Precision Cleaning, April 1997.

© 2001 by CRC Press LLC

CHAPTER 1.5

Hydrofluoroethers John G. Owens

CONTENTS Introduction Properties of Segregated HFEs and Their Impact on Cleaning Processes General Properties Physical Properties Solvency and Mixtures Safety Considerations Environmental Considerations Materials Compatibility Cleaning Systems and Equipment Neat Cleaning System Azeotrope Cleaning System Cosolvent Cleaning System Parts Cleaning in HFE Cleaning Systems Drying/Water Removal Processes Operation Practices to Maximize Solvent Containment References INTRODUCTION The identification of suitable long-term alternatives to ozone-depleting substances (ODSs) is challenging because of the complex combination of performance, safety, and environmental properties required. Researchers investigating alternatives have evaluated hundreds if not thousands of compounds. This effort has significantly increased the understanding of the structure–property relationship for a number of compound classes. Much of the initial research has focused on various halogenated alkanes (i.e., organic molecules with a carbon–carbon backbone substituted with fluorine, chlorine, or bromine). However, some researchers believe that the requirement that a long-term alternative be both nonflammable and nonozone depleting effectively eliminates compounds containing chlorine or bromine.1 Recently, a number of researchers have investigated the new class of

© 2001 by CRC Press LLC

hydrofluoroethers (HFEs).2 –7 This class of compounds is nonozone depleting since the compounds contain neither chlorine nor bromine.8 A variety of HFE structures have been investigated including hydrofluoropolyethers.9 The insertion of an ether oxygen atom into the backbone of the molecule was often done to modify the thermophysical properties of a compound for specific end uses. However, one of the principal advantages of the HFE class is that certain HFE structures lead to significantly improved environmental properties. Results on numerous segregated HFE compounds demonstrated that they could have significantly shorter atmospheric lifetimes and, as a result, lower global warming potentials (GWPs) when compared with alkanes such as hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).10 Segregated HFEs are those in which all of the hydrogen atoms reside on carbons with no fluorine substitution and are separated from the fluorinated carbons by the ether oxygen, i.e., CxFyOCmHn. This segregated structure maximizes the effect of the ether oxygen in reducing the atmospheric lifetime of the compound. The research also showed that segregated HFEs combined these environmental attributes with the performance and safety properties required in a suitable alternative solvent.7 The HFE solvents commercialized to date have all been of the segregated HFE class. These materials have been shown to be low in toxicity.7 They also have physical properties similar to the solvents they replace. Although their inherent solvent power is relatively low, HFEs are readily mixed with other components to produce nonflammable, high-strength solvents. As a result, HFEs have been identified as a class of compounds capable of replacing ODSs, high-GWP solvents, and chlorinated solvents for a number of new as well as conventional industrial cleaning processes. PROPERTIES OF SEGREGATED HFEs AND THEIR IMPACT ON CLEANING PROCESSES General Properties The novel structure of the segregated HFEs results in a unique combination of properties. HFEs are clear, colorless liquids that have very little odor. This class of compounds offers a wide range of boiling points from 34°C to well over 100°C. The materials have very low freezing points, often well below 100°C. The liquid HFEs are low in toxicity, nonflammable, noncorrosive, thermally stable, and electrically nonconductive. Like the solvents they replace, these fluids have high densities, but low viscosity and surface tension. To date, two HFEs compounds have been commercialized. Each consists of two inseparable isomers with essentially identical properties. The structures and several identifying names and numbers are listed in Table 1. Physical Properties The physical properties that determine the performance of solvents in typical end-uses are listed in Table 2 in comparison with CFC-113. The higher boiling points of the HFEs mean that they have lower vapor pressures, resulting in easier containment of the fluid in cleaning systems. These fluids have extremely low freezing points, which allows the use of low-temperature cooling coils to enhance fluid containment further. The wide liquid range of the HFEs also enables their use in other applications such as heat transfer fluids. Like many of the materials they are intended to replace, HFEs are nonflammable, having no open or closed cup flash point. © 2001 by CRC Press LLC

Table 1 Structures and Names of Commercially Available HFEs Chemical Structures Common name CAS numbers CAS names

Halocarbon numbers Simplified halocarbon numbers Commercial names

C4F9OCH3

C4F9OC2H5

Methyl perfluorobutylether 163702-07-6 163702-08-7 2-(Difluoromethoxymethyl)1,1,1,2,3,3,3-heptafluoropropane 1,1,1,2,2,3,3,4,4-Nonfluoro-4methoxybutane HFE-449sccc1 HFE-449scym1 HFE-449s1

Ethyl perfluorobutylether 163702-05-4 163702-06-5 1-Ethoxy-1,1,2,2,3,3,4,4,4nonfluorobutane 2-(Ethoxydifluoromethyl)1,1,1,2,3,3,3heptafluoropropane HFE-569sfccc2 HFE-569sfcym2 HFE-569sf2

3M™ Novec™ HFE-7100a

3M™ Novec™ HFE-7200

a

Registered trademark of 3M Company, St. Paul, Minnesota, USA.

The solubility of water in the HFEs is very low, which limits their ability to absorb humidity from the air. And their solubility in water is extremely low, minimizing the potential to contaminate contacting water streams. The high density of the HFEs and their very low viscosity and surface tension are important properties in cleaning applications, particularly for penetrating and cleaning components having complex geometry. Some researchers in the cleaning industry have suggested that a useful parameter for assessing the potential performance of a precision cleaning solvent is the “wetting index,” which is defined as the ratio of the density of the solvent to its viscosity and surface tension.11 The higher the wetting index, the better the ability of a solvent to wet component surfaces and penetrate into tight spaces, especially for the removal of particulate contamination. The HFEs’ solvent properties indicate that they are well suited for precision cleaning applications with wetting indices exceeding that of CFC-113 (see Table 2). Another advantage of superior wetting capability is that it provides good drainage of the solvent from components at the end of the cleaning cycle. This property, combined with Table 2 Physical Properties of HFEs Compared with CFC-113

Structure Boiling point, °C Freezing point, °C Flash point, °C, open or closed cup Solubility for water, ppmw Solubility in water, ppmw Density, ρ, g/ml Viscosity, µ, cp Surface tension, γ, dyn/cm Heat of vaporization, cal/g Wetting index (1000* / * )

© 2001 by CRC Press LLC

CFC-113

HFE-449s1

HFE-569sf2

CCl2FCClF2 48 31 None

C4F9OCH3 61 135 None

C4F9OC2H5 76 138 None

110 170 1.56 0.7 17 35 131

95 12 1.52 0.6 14 30 181

92 3 1.43 0.6 14 30 170

the low heat of vaporization, allows the HFEs to dry rapidly from part surfaces and minimizes fluid dragout from the cleaning process. Measurements of solvent dragout have confirmed the effectiveness of a solvent with a high wetting index. If a part is subjected to a typical cleaning cycle with a dwell in the vapor zone, it will carry residual solvent when it enters the freeboard zone (the area between the top of the vapor zone and the lip of the machine). Thus, it is usually necessary to hold parts in the freeboard for a period of time prior to removal from the cleaning system to minimize solvent dragout. The amount of liquid remaining on a representative, high-surface-area part is shown in Figure 1* for several solvents as a function of residence time in the freeboard. Solvents with high wetting indices, such as the HFEs, drain more completely from the part surface and dry faster. HFEs exhibit lower dragout compared with the ozonedepleting solvents they are intended to replace. The difference is even greater when compared with conventional chlorinated solvents such as trichloroethylene (TCE) and methylene chloride (MeCl). Solvency and Mixtures The segregation of the hydrogen and fluorine atoms on the HFE molecule leads to higher solvency than if the hydrogen atoms had been placed on a simple fluorochemical backbone. However, the pure HFEs are relatively mild solvents as indicated by the solvent parameters listed in Table 3. They do not have the hydrocarbon solvency to match the ODS solvents, but since they can be readily mixed with other materials to form nonflammable azeotropes or cosolvent systems, they can be just as effective in cleaning. Azeotropes are mixtures that are inseparable by boiling. As a result, azeotropes have found significant use in cleaning processes. For example, the binary azeotropic mixture of C4F9OCH3 with trans-1,2-dichloroethylene (t CClH  CClH) as well as the ternary azeotrope of C4F9OCH3 with trans-1,2-dichloroethylene and ethanol are nonflammable and have Kauri-butanol values similar to CFC-113. trans-1,2-Dichloroethylene is used in a number of azeotropic solvent mixtures since it has a wide margin of safety with an exposure Table 3 Solvency Properties of HFEs Compared with CFC-113 CFC-113 Structure Hildebrand solubility parameter Kauri-butanol (KB) value Solubility for mineral oil (weight %) Solubility for silicone oil (weight %) Solubility for fluorinated oil (weight %)

HFE-449s1

HFE-569sf2

Azeotrope 1

CCl2FCClF2 7.3

C4F9OCH3 6.5

C4F9OC2H5 6.3

7.7

7.8

31

10

10

27

33

Mc

1

1

20

20

M

1

1

M

M

M

M

M

M

M

a

Azeotropic mixture of 50% by weight of trans-1,2-dichoroethylene in C4F9OCH3, commercially available as HFE-71DE from 3M. b Azeotropic mixture of 44.6% by weight of trans-1,2-dichoroethylene and 2.7% by weight of ethanol in C4F9OCH3, commercially available as HFE-71DA from 3M. c M  miscible in all proportions. * Chapter 1.5 Color Figure 1 follows page 104. © 2001 by CRC Press LLC

Azeotrope 2

a

b

guideline of 200 ppm, is nonozone depleting, and its flammability is easily inerted by mixing with a nonflammable solvent such as an HFE.12 The HFEs are miscible with a wide range of organic solvents and form homogeneous, azeotropic mixtures with numerous compounds.13 Since they are highly fluorinated, HFEs have very high solubility for fluorocarbons and other halogenated compounds. Fluorinated oils and greases are typically completely miscible in an HFE solvent. Safety Considerations A wide range of safety considerations is necessary for proper design of cleaning solvents. As indicated in Table 2, the commercially available HFE compounds are nonflammable. These products as well as the commercially available azeotropes have been assigned NFPA (National Fire Protection Association) flammability indices14 of zero to one similar to the ODS solvents. These products do not become flammable under any conditions of normal use.12 Beyond flammability, the next most important safety consideration is the toxicity of the solvent. Under normal conditions, workers can be exposed to a small amount of a solvent during its use. The risk of greater exposure also exists in the event of a solvent spill, leak, or equipment failure. Extensive toxicological tests should be completed to determine if a solvent is safe in its intended applications. A number of the key findings for the HFEs15 are listed in Table 4 in comparison to CFC-113. The HFE solvents have very high acute lethal concentrations. That is, workers can be exposed to the solvent in very high doses for short periods of time without adverse effects. The HFEs were found to be nonmutagenic, are not cardiac sensitizers, and are dermally and ocularly nonirritating. The high exposure guidelines and lack of exposure ceiling indicate that the HFEs have a wide margin of safety. Another important safety consideration is the thermal stability of the solvent. The HFEs are capable of being continuously refluxed without degradation. Even in the presence of air and metals there has been no evidence of peroxide formation, which is common to many hydrocarbon ethers. All solvents can degrade if severely overheated. This degradation can produce byproducts that are more hazardous than the original solvent. For example, it was known that Table 4 Toxicological Properties of HFEs15 Compared with CFC-113

Structure Acute lethal conc., 4 h LC50, ppmv Mutagenicity Cardiac sensitization threshold, ppmv Ocular irritant Dermal irritant Exposure guideline 8-h TWA, ppmv Exposure ceiling, ppmv

© 2001 by CRC Press LLC

CFC-113

HFE-449s1

HFE-569sf2

CCl2FCClF2 55,000

C4F9OCH3  100,000

C4F9OC2H5  92,000

Negative 10,000

Negative  100,000

Negative  18,900

No No 1000

No No 750

No No 200

None

None

None

Table 5 Environmental Properties of HFEs Compared with CFC-113

Structure Atmospheric lifetime, years1 Ozone depletion potential,1 [CFC-11  1] Global warming potential,1 [CO2  1] Photochemical smog precursor18

CFC-113

HFE-449s1

HFE-569sf2

CCl2FCClF2 85 0.8

C4F9OCH3 4.1 (19) 0

C4F9OC2H5 0.77 0

6000

320 (20)

55

No

No

No

CFC-113 could produce decomposition products such as HCl and HF if overheated.16 Similarly, the HFEs can generate hazardous decomposition products such as HF if severely overheated (e.g., exposure to temperatures 150°C or higher).17 Conventional cleaning equipment has safety interlocks incorporated into its design to prevent overheating the solvents. In addition, the high temperatures required to decompose an HFE solvent (90°C or more above its boiling point) provide a wide margin for safe use. Solvent manufacturer’s recommendations should be followed when recycling and recovering solvent for reuse. The HFEs also have a high degree of chemical stability. The materials are hydrolytically and oxidatively stable under normal use conditions. The pure HFEs are stable when refluxed in the presence of water or strong aqueous base. Contact with many other relatively strong acids and bases produces little, if any, reaction. Exceptions are reactions with amines such as piperidine. As with all halogenated solvents, the HFEs should not be contacted with finely divided active metals, alkali, or alkaline earth metals (i.e., groups IA and IIA of the periodic table). Environmental Considerations An increasing number of environmental properties need to be considered when selecting cleaning solvents, such as those listed in Table 5. These properties affect both local environmental issues, such as smog formation, and global issues, such as ozone depletion and global warming. The atmospheric lifetimes of the HFE solvents are in a very desirable range. They are long enough not to contribute to formation of photochemical smog (i.e., volatile organic compound, VOC, exempt18) yet short enough to preclude concerns of accumulation in the atmosphere.1,19 These short atmospheric lifetimes, compared with PFCs, lead to lower global warming potentials.1,20 Since the HFEs contain no chlorine or bromine, they have no ozone depletion potential. The HFEs are not considered hazardous air pollutants since they are low in toxicity. The stability of the HFEs allows their recovery and recycling for repeated use in cleaning processes. Used HFE solvents are not classified as hazardous waste; however, the soils present in them may change that classification (e.g., metals from a defluxing operation would typically be considered hazardous waste). For this reason, manufacturers have established return programs to assist users in disposal of used solvents. Materials Compatibility The pure HFE solvents are compatible with essentially all common metals, most plastics and a number of elastomers. Table 6 lists a number of the specific materials that have been tested with the HFEs. Test coupons of the materials listed in Table 6 were exposed to © 2001 by CRC Press LLC

Table 6 Materials Compatibility with HFE Solvents

Aluminum Copper Carbon steel 302 stainless steel Brass Zinc Molybdenum Tantalum Titanium Tungsten Acrylic Polyethylene Polypropylene Polycarbonate Polyester Nylon Epoxy PVC PET ABS PTFE Butyl rubber Natural rubber Nitrile rubber EPDM Fluoroelastomer Polychloroprene

HFE-449s1

HFE-569sf2

C4F9OCH3

C4F9OC2H5

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2

Azeotrope 1 C4F9OCH3t-CClH  CClH 1 1 1 1 1 1 1 1 1 1 3 2 2 3 2 1 2 3 3 3 1 2 3 3 2 2 2

1. Compatible over extended exposures to the solvent with less than 5% changes in weight or volume. 2. Compatible with limited exposure to the solvent (i.e., short time exposures such as a cleaning cycle). 3. Typically incompatible.

the HFE solvents for 7 days at the boiling point of the solvent and subsequently examined for weight, volume, and appearance changes. The HFE azeotropes containing trans-1,2dichloroethylene exhibit compatibility similar to the pure HFEs with all metals but due to their higher solvency have limited compatibility with most polymeric materials. Compatibility of the components to be cleaned as well as materials of construction for the cleaning equipment should be evaluated with the various cleaning fluids during process selection.

© 2001 by CRC Press LLC

CLEANING SYSTEMS AND EQUIPMENT Several different cleaning processes exist that employ HFE solvents. These include: 1. Neat cleaning systems using pure HFEs 2. Azeotrope cleaning systems using azeotropic mixtures 3. Cosolvent cleaning systems using zeotropic mixtures The appropriate cleaning system is selected based upon the soil to be removed, the materials of construction of the part to be cleaned, and a number of other factors as indicated in Table 7. All of the HFE cleaning processes can be conducted in conventional cleaning equipment such as a vapor degreaser (Figure 2) as well as in-line cleaning systems. Neat Cleaning System The neat cleaning process, employing pure HFE solvents, is used when a single-component, mild solvency cleaning fluid is required. This process is conducted in a conventional vapor degreaser (Figure 2) and can effectively clean light hydrocarbon and silicone oils, particulate contamination, and halogenated lubricants, oils, and greases from parts. The HFE solvent can be used in a single or multiple sump system. Parts cleaning with this process is conducted in a manner similar to that used with conventional vapor degreasing solvents. Both vapor-phase and liquid-phase cleaning is possible depending upon the parts and soils to be removed. The mechanism of cleaning with neat HFE systems can be by dissolving or displacement of the soil.

Table 7 HFE Cleaning Process Selection Guidelines Process KB value of cleaning solvent Soils

Neat Cleaning Process

Azeotrope Cleaning Process

Cosolvent Cleaning

10

27 –33

20 to  150

Light hydrocarbon and silicone oils, halogenated oils, particulate

Medium oils, lubricants, release agents, some waxes and fluxes

Heavy oils, greases, buffing compounds, heavy flux

Substrates Plastic partsa Metal parts Circuit boards Coils Ball or roller bearings Cathetersa Elastomer partsa Glass parts a

OK OK Not appropriate OK OK OK OK OK

Not appropriate OK OK OK OK May be applicable May be applicable OK

OK OK OK OK OK Not appropriate OK OK

Careful evaluation of the compatibility of the parts with the various cleaning fluids should be considered during process selection.

© 2001 by CRC Press LLC

Freeboard

Freeboard Secondary Coils Primary Coils Cleaning Agent Vapor

Cleaning Agent

Single Sump Figure 2

Water Separator

Cleaning Agent Vapor

Cleaning Agent

Cleaning Agent

Multiple Sump

HFE cleaning equipment.

Azeotrope Cleaning System The azeotrope cleaning process uses an HFE azeotropic mixture, such as those listed in Table 3, as the cleaning fluid. This process is used in applications requiring a stronger solvent mixture. Since the solvents used in this process are true azeotropes, the compositions of the cleaning and rinse sumps remain essentially constant throughout use. The mechanism of cleaning with azeotrope systems is almost exclusively via dissolving the soil. The equipment required is similar to the conventional degreasers used with CFCs and their azeotropes. The HFE azeotropic mixtures can be used in single or multiple sump equipment in the same manner as the neat cleaning system described above. This process effectively cleans many oils, waxes, greases, and fluxes depending upon the azeotropic solvent used. Cosolvent Cleaning System A cosolvent process combines two different fluids to conduct the cleaning process. One is a low-volatility, high-solvency organic solvent that is used to dissolve the soil from a part’s surface. The second component, the HFE, functions as a rinsing agent since it is used to rinse the solvating agent from the component. This process typically uses solvating agents that are miscible with the HFE. These mixtures are not azeotropes (i.e., they are zeotropes) since the components separate when boiled, resulting in very different compositions in the cleaning sump and rinse sump. The process operates analogously to a twosump vapor degreaser, with a mixture of the solvating agent and rinsing agent in the boil sump and pure HFE in the rinse sump (see Figure 3). Because of the large difference in boiling points between the two components, very little of the solvating agent is distilled into the rinse sump. The rinse sump contains essentially pure HFE throughout the process. The mechanism of cleaning in this process is most often by dissolving the soil into the mixture of solvating and rinsing agents in the first sump. A wide variety of high-boiling, combustible solvents can be used as solvating agents in this process, provided that their flash points are well above the operating temperature of the system. Use of the HFE rinse agent renders the solvating agent nonflammable during

© 2001 by CRC Press LLC

. . . . . .

. . . . . .

. . . . . .

. . . . . . . . . . . . . . . . . . Rinsing . . . . . Agent . . . Vapor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Solvating Agent + Rinsing Agent Mix

Figure 3

Rinsing Agent

HFE cosolvent cleaning equipment.

use. Higher-solvency mixtures are created by increasing the ratio of solvating agent to rinsing agent in the boil sump. The process is controlled by monitoring the boiling temperature of this mixture since the operating temperature is determined by composition in the boil sump as shown in Figure 4. Temperature and composition control can be accomplished manually through periodic measurements of the boiling temperature followed by solvent additions when necessary or via an automated system. Since this process requires immersion into the solvent mixture in the cleaning sump, vapor cleaning is not possible. The HFE cosolvent process is capable of effectively cleaning a very wide variety of soils including heavy oils, greases, fingerprints, waxes, and flux. Cosolvent processes offer a great deal of flexibility by selecting a solvating agent and rinsing agent combination that best meets the needs of a particular cleaning application. The process can provide sufficient solvency for a given soil while maintaining compatibility with the part’s materials of construction. The higher solvency mixtures of the solvating and rinsing agents can accommodate higher soil loading levels than the neat or azeotrope systems.

Figure 4

Operating temperature of HFE cosolvent cleaning process. Example using C4F9OCH3 and methyl decanoate as a solvating agent.

© 2001 by CRC Press LLC

Parts Cleaning in HFE Cleaning Systems HFE solvents can be used with a variety of cleaning equipment designs such as conventional vapor degreasers, in-line systems, enclosed systems, and vacuum systems. The most common equipment employed with HFE solvents is a vapor degreaser. Both single and multiple sump systems are available (see Figure 2). Single sump systems are generally limited to vapor-phase cleaning and may require some form of spray rinsing. Multiple sump systems typically require immersion in a cleaning sump followed by rinsing in one or more rinse sumps. Efficient fluid containment requires holding the cleaned parts in the vapor zone followed by a dwell in the freeboard zone (the space between the vapor zone and the top of the machine) prior to exiting the equipment. This allows the maximum amount of solvent to drain and evaporate from the part while it is in the equipment, minimizing fluid dragout. The dwell times during the various steps of the process (cleaning sump, rinse sump, vapor zone, freeboard zone) are typically only a few minutes each, resulting in a relatively short cleaning cycle. Several basic steps are followed in most cleaning processes using HFE solvents to ensure the parts are thoroughly cleaned, rinsed, and dried in the specified cycle time. An example of a liquid immersion cleaning process would include: 1. Maintain cleaning system at operating temperature (cleaning sump, fluids boiling, full vapor zone established, condensate flow into the rinse sump, ultrasonic generators operating, etc.). 2. Arrange parts to maximize the flow of solvent around them. Avoid shadowing the parts from direct spray. Prevent the nesting of parts and the cupping of fluids. 3. Slowly lower the basket of parts into the cleaning sump. The parts must be completely covered by the cleaning fluid. The parts must remain covered for the entire immersion time. Ultrasonics or recirculating pumps may enhance cleaning in some applications. 4. Raise the basket of parts above the boil sump and allow excess cleaning solvent to drain back into the sump. 5. Move the basket over the rinse sump and completely immerse it into the rinsing fluid. 6. Let parts dwell in the rinse sump for a specified time. 7. Slowly raise the basket into the vapor zone, dwelling in the vapor until fluid drainage has stopped. Rapid ascent of the basket can result in increased vapor losses. 8. Slowly raise the basket into the freeboard zone. Dwell in the freeboard should last until the solvent has visibly evaporated from basket and parts. 9. Ensure that soil loading in the cleaning solvent is maintained below the level determined for process efficacy. Soil loading is typically monitored via the boiling temperature in the cleaning sump (the boiling temperature increasing with soil loading). Additional factors to consider when selecting the appropriate combination of cleaning solvent and equipment are listed in Table 8.

© 2001 by CRC Press LLC

Table 8 Equipment Comparisons for HFE Cleaning Processes

Vapor-phase cleaning Single sump Boil-up ratea Ultrasonics Distillation equipment

Water separator Pump seals Heaters

Filtrationd

System piping

HFE Neat Cleaning Process

HFE Azeotrope Cleaning Process

HFE Cosolvent Cleaning Process

Possiblea

Possiblea

Not possible

Possibleb 1–3 times rinse sump volume/h Suggested for use where possible Recommended (external)

Possibleb 1 –3 times rinse sump volume/h Suggested for use where possible Recommended (external)

Decanter PTFE or seal-less Sized to field voltage with 15% safety margin Recommended with immersion sumps Welded or compression fittings recommended

Desiccant PTFE or seal-less Sized to field voltage with 15% safety margin Recommended with immersion sumps Welded or compression fittings recommended

Not possible 1 –3 times rinse sump volume/h Suggested for use where possiblec Recommended with control features (internal) Decanter PTFE or seal-less Sized to field voltage with 15% safety margin Recommended with all sumps Welded or compression fittings recommended

a

Depends on part geometry, soil, soil loading, throughput, and part type. Single sump systems should be limited to vapor-phase cleaning and may require spray rinsing. c May be necessary in both clean and rinse sumps. d As required by part cleaning specifications. b

Drying/Water Removal Processes HFE solvents are sometimes used as a drying fluid following processes such as aqueous cleaning or metal plating. The drying processes can be conducted in equipment similar to vapor degreasers used for cleaning applications. Water can be absorbed from the surface of a component using HFE/alcohol solutions such as the azeotrope that forms between C4F9OCH3 and isopropanol (6.7% isopropanol by weight). These mixtures are typically useful for applications with relatively low water removal needs since the solutions can become saturated with water, reducing their efficacy. The solutions can be distilled to remove the water but care must be taken to avoid extracting the alcohol into the water and removing it from the drying solution. Applications demanding larger water removal capability typically use HFE/surfactant mixtures to displace the water from the component’s surface. Parts to be dried are immersed into a dilute solution of a silicone- or fluorochemical-based surfactant in the HFE solvent. The surfactant allows the solvent to wet the surface preferentially. This wetting action causes the water to bead up and displace from the surface, floating to the top of the solution. The part is then immersed in one or more sumps of pure HFE solvent to rinse the surfactant from its surface. This process is capable of efficiently removing water from complex components in short cycle times.

© 2001 by CRC Press LLC

Operation Practices to Maximize Solvent Containment In nearly all applications, the HFE solvent emissions are significantly lower than those of the ODS it replaces.21 However, to ensure economical operation of the cleaning system as well as reduce emissions of solvent to the environment, the following practices are recommended to maximize containment of the solvent: 1. Eliminate drafts near the cleaning equipment. Drafts can increase vapor losses by causing disturbances in the vapor–air interface. 2. A sliding cover is recommended to reduce losses while the system is idling (i.e., the system is operating but parts are not being processed) and during downtime. Hinged or plug-type lids are typically not as effective since their operation can cause disturbances in the vapor–air interface. 3. Cleaning equipment should incorporate extended freeboard ( 120%) and low temperature (20°C) secondary cooling coils to minimize diffusive losses of solvent. 4. Do not begin cleaning operation until the equipment is at the operating temperature. This will ensure the parts are adequately heated and that the solvent will dry in the freeboard zone prior to exiting the equipment. 5. The cycle times established for specific parts should be strictly followed. Inadequate dwell times in any of the zones (cleaning sump, rinse sump, vapor zone, or freeboard zone) can have a deleterious effect on process performance. Suitable residence times should be established to ensure that the parts are completely cleaned, rinsed, and dried. 6. Use of spray wands should be limited to only those parts where absolutely necessary and then only below the cooling coils since misdirected spray can increase losses. 7. Parts should be arranged so that solvent can efficiently drain and trapping or cupping of fluid is avoided. Tumbling of parts before removing them from the vapor zone can help remove excess solvent and reduce dragout. 8. Parts should not be raised above the vapor zone until the end of the cleaning cycle. 9. Use automated hoists and transfer equipment where possible. 10. Perform periodic checks for fluid leaks and follow equipment maintenance procedures. 11. Soil-loaded solvent should be recovered and recycled to every extent possible. Solvents used in the neat, azeotrope, or cosolvent cleaning processes can be distilled to be efficiently recovered for reuse following the manufacturer’s directions. Often this distillation can take place within the cleaning equipment. REFERENCES 1. WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion: 1998, Global Ozone Research and Monitoring Project, Rep. 44, WMO, Geneva, 1999. 2. Cooper, D. L., Cunningham, T. P., Allan, N. L., and McCulloch, A., Tropospheric lifetimes of potential CFC replacements: rate coefficients for reaction with hydroxyl radical, Atmos. Environ., 26A, 1331, 1992. 3. Cooper, D. L., Cunningham, T. P., Allan, N. L. and McCulloch, A., Potential CFC replacements: tropospheric lifetimes of C3 hydrofluorocarbons and hydrofluoroethers, Atmos. Environ., 27A, 117, 1993.

© 2001 by CRC Press LLC

4. Bartolotti, L. J. and Edney, E. O., Investigation of the correlation between the energy of the highest occupied molecular orbital (HOMO) and the logarithm of the OH rate constant of hydrofluorocarbons and hydrofluoroethers, Int. J. Chem. Kinetics, 26, 913, 1994. 5. Zhang, Z., Saini, R. D., Kurylo, M. J., and Huie, R. E., Rate constants for the reactions of hydroxyl radical with several partially fluorinated ethers, J. Phys. Chem., 96, 9301, 1992. 6. Owens, J. G. and Minday, R. M., Update on hydrofluoroethers alternatives to ozone-depleting substances, presented at the International CFC and Halon Alternatives Conference, Washington, D.C., October, 1995. 7. Koenig, T. A. and Owens, J. G., The role of hydrofluoroethers in stratospheric ozone protection, presented at the International Conference on Ozone Protection Technologies, Washington, D.C., October, 1996. 8.Wallington, T. J., Schneider, W. F., Sehested, J., Bilde, M., Platz, J., Nielsen, O. J., Christensen, L. K., Molina, M. J., Molina, L. T., and Wooldridge, P. W., Atmospheric chemistry of HFE-7100 (C4F9OCH3): reaction with OH radicals, UV spectra and kinetic data for C4F9OCH2 and C4F9OCH2O2 radicals, and the atmospheric fate of C4F9OCH2O radicals, J. Phys. Chem. A, 101, 8264, 1997. 9. Marchionni, G., Silvani, R., Fontana, G., Malinverno, G., and Visca, M., Hydrofluoropolyethers, J. Fluorine Chem., 95, 41, 1999. 10. Owens, J. G., Segregated hydrofluoroethers: low GWP alternatives to HFCs and PFCs, presented at Joint IPCC/TEAP Expert Meeting on Options for the Limitation of Emissions of HFCs and PFCs, Petten, the Netherlands, May 26–28, 1999. 11. Kenyon, W. G., New ways to select and use defluxing solvents, presented at Nepcon West, Anaheim, 1979. 12. Owens, J. G., Hydrofluoroethers: a growing family of alternatives to ozone-depleting compounds, presented at the International Conference on Ozone Protection Technologies, Baltimore, November, 1997. 13. Flynn, R. M., Milbrath, D. S., Owens, J. G., Vitcak, D. R., and Yanome, H., U.S. patent 5,827,812, Minnesota Mining and Manufacturing Company, 1998. 14. NFPA (National Fire Protection Association) 49, Hazardous Chemicals Data, 1994 ed., NFPA, Quincy, MA. 15. Product Toxicity Summary Sheets: HFE-7100 and HFE-7200, 3M Company, 1998. 16. Ellis, B. N., Cleaning and Contamination of Electronics Components and Assemblies, Electrochemical Publications Limited, Ayr, Scotland, 1986, 175. 17. Material Safety Data Sheet: HFE-7100, 3M Company, 1998. 18. U.S. Fed. Regist., 62, August 25, 44900, 1997. 19. Wallington et al. (1997) reported that the n- and i-isomers of HFE-7100 were expected to have similar reactivity with OH based upon the observation that there was no difference in isomer reactivity toward Cl and F radicals. The n-C4F9OCH3 was reported in Wallington et al. (1997) as  5 years and was requoted in WMO Report No. 44 as 5.0 years. The value calculated to two significant figures is 4.7 years. Later measurements by Molina et al. at MIT on the pure i-C4F9OCH3 determined the lifetime to be 3.7 years. The commercial HFE-7100 is an approximate 60/40 mixture by weight of the iso and normal isomers, which results in an average lifetime of 4.1 years. 20. WMO Report 44 reports the GWP of HFE-7100 as 390 over a 100-year integration time horizon using the lifetime of 5.0 years. Calculation using the 4.1-year lifetime of the commercial product yields a GWP of 320 (100 year ITH). 21. Warren, K. J., Use of hydrofluoroethers in electronics cleaning applications, presented at the International Conference on Ozone Protection Technologies, Baltimore, November, 1997.

© 2001 by CRC Press LLC

CHAPTER 1.6

Hydrofluorocarbons Abid Merchant

CONTENTS Introduction Structure and Properties of HFC-43-10mee Chemical Structure and Nomenclature Properties Electrical Properties Replacement of Ozone-Depleting Substance (ODS) and High Global Warming Gases— Opportunities and Alternatives Ozone-Depleting Substances PFC Alternatives Alternatives to ODS Toxicity Environmental Regulatory Considerations Compatibility of Materials of Construction Plastics and Elastomers Metals Compatibility Chemical and Thermal Stability Selective Solvent Power Applications of HFC-43-10mee, Neat Carrier Fluid Particulate Removal Rinsing Agent Fingerprint Solvent Displacement Drying Application HFC-43-10mee Formulations Binary Alcohol Azeotropes Binary Heptane Azeotrope Binary 1,2-trans-Dichloroethylene Azeotropes Multicomponent Azeotrope Azeotrope-Like Formulations Nonazeotropic Blends

© 2001 by CRC Press LLC

Equipment Design Considerations for Low Emission Emission Measurement Data Summary References INTRODUCTION Hydrofluorocarbons (HFCs) are a family of compounds containing carbon, fluorine, and hydrogen. These compounds were developed and commercialized as a result of the Montreal Protocol to replace ozone-depleting substances (ODS), such as chlorofluorocarbons (CFCs) for refrigeration, foams, propellants, and solvent applications. The absence of chlorine in the HFC molecules makes them nonozone-depleting substances. The presence of hydrogen atom(s) reduces atmospheric life, and therefore these compounds have significantly lower GWP than the similar fully fluorinated or CFC compounds. The presence of a large number of fluorine atoms tends to make these compounds nonflammable, low in toxicity, stable to heat, low in reactivity, and compatible with most materials of construction. In mid-1990s, an HFC containing 5 carbon, 2 hydrogen, and 10 fluorine atoms, CF3CFHCFHCF2CF3 was introduced to replace CFC-113, perfluorocarbons (PFC) (C6F14, C7F16), and hydrochlorofluorocarbons (HCFC-141b and HCFC-225) in some solvent applications. This new HFC, called HFC-43-10mee in ASHRE (American Society of Heating, Refrigeration and Air Conditioning/Engineers) nomenclature, has physical properties similar or better than CFC-113 and C6F14. Compared with CFC-113, this HFC has very low surface tension, higher boiling point, and lower heat of vaporization so that it dries rapidly without leaving any residue. These properties combined with nonflammability, chemical and thermal stability, low toxicity, and ease of recovery by distillation make this HFC ideal for a broad range of applications. Solvency lies between CFC-113 and PFC and can be enhanced significantly by use of appropriate azeotropes and blends with alcohol, hydrocarbons, esters, and hydrochlorocarbons. STRUCTURE AND PROPERTIES OF HFC-43-10MEE Chemical Structure and Nomenclature HFC-43-10mee is a straight-chain HFC. Its molecular structure and that of CFC-113 are shown in Figure 1. The absence of chlorine in the HFC molecule requires a large number of carbon (5), fluorine (10), and some hydrogen (2) atoms to achieve the desired boiling point, good environmental properties, and nonflammability similar to CFC-113. In the IUPAC (International Union of Pure and Applied Chemistry) nomenclature this compound is 1,1,1,2,3,4,4,5,5,5-decafluorpentane or 2,3-dihdrodecafluorpentane. It is marketed under a trade name of Vertrel XF. Because of the position of hydrogens on the second and third carbons, it consists of an identical pair of diesteriomers with very similar properties.

F

F

F

F

F

F

C

C

C

C

C

F

H

H

F

F

F F

HFC-43-10mee

Figure 1

Structures of HFC-43-10mee and CFC-113.

© 2001 by CRC Press LLC

F

C1

C

C

C1

F

CFC-113

F

Properties HFC-43-10mee is a clear, dense, colorless liquid with a faint ethereal solvent odor like CFC-113. Table 1 gives a list of physical, transport, and thermodynamic properties of the HFC along with those of CFC-113. The HFC has a boiling point of 55°C, which is higher than that of CFC-113. The freezing point is 80°C which is lower than the freezing point of CFC-113. Thus, it can be used as a liquid over a broader temperature range than CFC-113. The higher boiling point also reduces emissions in the existing degreasing equipment, making it environmentally more friendly. The high liquid density helps to displace soils and particulate from the surfaces of parts being cleaned and to float these soils to the surfaces of the solvent. One of the clear advantages of CFC-113 was its low surface tension compared with other solvents such as chlorocarbons, hydrocarbons, alcohols, and water. The surface tension of the HFC is lower than that of CFC-113, which makes it easier to wet surfaces and thus assist in removal of soils through small crevices and openings found in Table 1 Physical Properties Propertya

HFC-43-10mee

CFC-113

Molecular weight Boiling point, °C (°F) Vapor pressure, mmHg (psia) Freezing point, °C (°F) Liquid density, g/cc (lb/gal) Surface tension, dyn/cm Viscosity, cPs Solubility in water, ppm Solubility of water, ppm Critical temperature, °C (°F) Critical pressure, psia (atm) Critical volume, cc/mol Heat of vaporization (at boiling point), cal/g (Btu/lb) Specific heat at 20°C (68°F), cal/g°C (Btu/lb·°F) Diffusivity, cm2/s Thermal conductivity Btu/h ft °F Vapor Liquid Refractive index Flash point Closed cupb Open cupc Flammable range in air Autoignition point in air

252 55 (130) 226 (4.4) 80 (112) 1.58 (13.2) 14.1 0.67 140 490 181 (357) 331.9 (22.6) 433 31.0 (55.7)

187 47.6 (117.6) 334 (6.46) 35 (31) 1.56 (13.1) 17.3 0.68 170 110 214 (417) 495 (33.7) 325 35.1 (63.1)

0.27 (0.27)

0.21 (0.21)

0.066

0.068

0.0057 0.036 1.24

0.0043 0.043 1.35

None None None Noned

None None None None

a

At 25°C (77°F) except where indicated. Pensky –Martens closed cup tester (ASTM D 93). c Tag open cup tester (ASTM D 1310). d None detected up to 540°C. b

© 2001 by CRC Press LLC

Table 2 HFC-43-10mee Density and Vapor Pressure Change with Temperature Temperature, °C (°F) 20 (4) 10 (14) 0 (32) 10 (50) 20 (68) 30 (86) 40 (104) 50 (122) 60 (140) 70 (158) 80 (176) 90 (194) 100 (212) 110 (230) 120 (248) 130 (266)

Density, g/cc (lb/g)

Vapor Pressure, mmHg (psia)

1.70 (14.2) 1.68 (14.0) 1.66 (13.8) 1.62 (13.5) 1.60 (13.3) 1.57 (13.1) 1.55 (12.9) 1.51 (12.6) 1.49 (12.4) 1.46 (12.2) 1.43 (11.9) 1.40 (11.7) 1.38 (11.5) 1.34 (11.2) 1.32 (11.0) 1.30 (10.8)

16 (0.3) 36 (0.7) 62 (1.2) 109 (2.1) 176 (3.4) 284 (5.5) 434 (8.4) 641 (12.4) 921 (17.8) 1288 (24.9) 1753 (33.9) 2343 (45.3) 3072 (59.4) 3961 (76.6) 5032 (97.3) 6309 (122.0)

surface mount printed wiring boards and close tolerance precision inertial guidance components. The energy consumption of a recirculating solvent system in a degreaser is a direct function of the heat of vaporization of the solvent. The heat of vaporization is lower than that of CFC-113, and much lower than that of alcohol, hydrocarbons, chlorocarbons, and water, making it more energy efficient in use. It has no flash point by both open and close cup methods and no flammable limits in air. Also, it does not have autoignition temperature in air measured up to 540°C. Table 2 gives both vapor pressure and specific gravity data as a function of temperature. From Antoine’s equation for vapor pressure of HFC-43-10mee,

where a  7.03668 b  1093.094 c  208.3936 P  millimeter of mercury T  °C

b Log10 P  a   (c  T)

Electrical Properties Electrical properties are given in Table 3. The dielectric constant is slightly higher than that of CFC-113 and the breakdown voltage is lower than that of CFC-113. Thus, the dielectric strength of the HFC is lower but still considered acceptable. The volume resistivity is in the most desirable range to minimize electrostatic discharge (ESD). ESD has become very important in the computer and electronic industry. The high-density disk drives use new technology consisting of giant magneto resistive (GMR) heads, which are extremely sensitive to small changes in current flow through the devices. ESD can result in a momen-

© 2001 by CRC Press LLC

Table 3 Electrical Properties of HFC-43-10mee Resistivity Dielectric constant Dissipation factor Breakdown voltage

2.9  109 ohms-cm 7 0.14 17.8 kV

tary current surge, which could as a minimum cause severe damage to the magnetic properties or, at most, a complete failure by the fusion of the GMR head. Therefore, it is desirable for all materials used in the manufacturing process to have low propensity to generate ESD. The ESD properties of a solvent can be measured by measuring its volume resistivity. The resistivity can be classified per EIA-541 and other specifications as: Conductive Less than 1  105 ohm/sq. Dissipative Between 1  105 and 1 x 1010 ohm/sq. Insulative Above 1  1012 ohm/sq. Lower value of the resistivity means lower propensity to generate ESD. As can be seen, the HFC has a resistivity number in the range called the “dissipative,” which is considered most desirable to minimize ESD generation and at the same time have adequate insulative properties. REPLACEMENT OF OZONE-DEPLETING SUBSTANCE (ODS) AND HIGH GLOBAL WARMING GASES—OPPORTUNITIES AND ALTERNATIVES Ozone-Depleting Substances HFC-43-10mee was primarily developed to replace ODS’s CFC-113 and methyl chloroform in applications where not-in-kind alternatives and other alternative solvents were not acceptable because of safety, process, or material incompatibility problems. To aid understanding of solvent markets and applications in 1989, before ODSs were restricted, solvent uses of CFC-113 and methyl chloroform are provided in Figures 2 and 3, respectively. With the implementation of the Montreal Protocol, the manufacture of both of these ODSs for solvent applications ceased in January 1996 (except in a few Article 5 countries). It is estimated that the current production of these ODS substances in these countries is less than 5% of the peak production of 1989. PFC Alternatives PFCs such as C5F12, C6F14, C7F16 and C8F18 were introduced as substitutes for the ODS CFC-113 in the late-1980s to early 1990s, and their uses grew rapidly as the phaseout of CFC113 began. Prior to this, PFCs were used in small quantities in niche applications. PFCs were heavily promoted as a substitute for CFC-113 in such applications as carrier fluid for fluorinated lubricants for the computer hard disk drives; flush fluid for particulate removal in precision cleaning; drying fluid in displacement drying application; coolants in other electronic components; and rinsing agents in a cosolvent process for cleaning printed circuit boards and mechanical components containing oil, grease, and other soils. The PFC uses for 1998 are estimated to be around 2500 MT of C6F14 (PFC-5060) and 500 to 1000 MT for other PFCs.1

© 2001 by CRC Press LLC

Medical Uses Miscellaneous 3% 9% Dielectric/ Coolant 2% Carrier 4%

Electronic 40%

Particulate Removal 4%

Drying 8%

Metal Cleaning 30%

Figure 2

CFC-113 uses in 1989, 251,000 metric tons (MT). (Breakdown by DuPont internal data, AFEAS Report, 1998.)

Alternatives to ODS The use of CFC-113 and methyl chloroform in electronic applications, especially defluxing, has been primarily replaced by three not-in-kind technologies: aqueous, semiaqueous, and “no-clean.” However, there are a few instances where these not-in-kind alternatives have not performed satisfactorily or were rejected because of compatibility, safety, flexibility, or reliability reasons and other solvent alternatives have been considered. HFC solvent (neat and in formulations) is used in very small niche specialty applications. It is estimated that total HFC-43-10mee use will be less than 1000 to 2000 MT and is equal to less than 1% of the CFC-113 market in 1989. The vast majority of the CFC-113 uses have been replaced by not-in-kind technology such as aqueous, no clean, and hydrocarbon.

Aerosols 13%

Coatings 10%

CPI 7% Adhesives 13%

Electronics 4%

Vapor Degreasing/ Cold Cleaning 53%

Figure 3

Methyl chloroform uses in 1989, 706,000 MT. (From ECSA, 1996.)

© 2001 by CRC Press LLC

Fluorinated Solvents in 2004 (Estimated 5% of 1989)

HFC projected use in 2004 (