Adhesion and Bonding to Polyolefins

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Report 143

Adhesion and Bonding to Polyolefins D.M. Brewis and I. Mathieson

Volume 12, Number 11, 2002

RAPRA REVIEW REPORTS A Rapra Review Report comprises three sections, as follows: 1. A commissioned expert review, discussing a key topic of current interest, and referring to the References and Abstracts section. Reference numbers in brackets refer to item numbers from the References and Abstracts section. Where it has been necessary for completeness to cite sources outside the scope of the Rapra Abstracts database, these are listed at the end of the review, and cited in the text as a.1, a.2, etc. 2. A comprehensive References and Abstracts section, resulting from a search of the Rapra Abstracts database. The format of the abstracts is outlined in the sample record below. 3. An index to the References and Abstracts section, derived from the indexing terms which are added to the abstracts records on the database to aid retrieval.

Source of original article Title

Item 1 Macromolecules

33, No.6, 21st March 2000, p.2171-83 EFFECT OF THERMAL HISTORY ON THE RHEOLOGICAL BEHAVIOR OF THERMOPLASTIC POLYURETHANES Pil Joong Yoon; Chang Dae Han Akron,University The effect of thermal history on the rheological behaviour of ester- and ether-based commercial thermoplastic PUs (Estane 5701, 5707 and 5714 from B.F.Goodrich) was investigated. It was found that the injection moulding temp. used for specimen preparation had a marked effect on the variations of dynamic storage and loss moduli of specimens with time observed during isothermal annealing. Analysis of FTIR spectra indicated that variations in hydrogen bonding with time during isothermal annealing very much resembled variations of dynamic storage modulus with time during isothermal annealing. Isochronal dynamic temp. sweep experiments indicated that the thermoplastic PUs exhibited a hysteresis effect in the heating and cooling processes. It was concluded that the microphase separation transition or order-disorder transition in thermoplastic PUs could not be determined from the isochronal dynamic temp. sweep experiment. The plots of log dynamic storage modulus versus log loss modulus varied with temp. over the entire range of temps. (110-190C) investigated. 57 refs.

Location

GOODRICH B.F. USA

Authors and affiliation

Abstract

Companies or organisations mentioned

Accession no.771897

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Adhesion and Bonding to Polyolefins D.M. Brewis and I. Mathieson (Institute of Surface Science & Technology, Loughborough University)

ISBN 1-85957-323-1

Adhesion and Bonding to Polyolefins

Contents Abstract ............................................................................................................................................................ 3 1.

Introduction .............................................................................................................................................. 3

2.

Principles .................................................................................................................................................. 4

3.

4.

5.

2.1

Theories of Adhesion ...................................................................................................................... 4

2.2

Wettability ....................................................................................................................................... 4

2.3

Diffusion ......................................................................................................................................... 5

Methods Used to Study Surfaces ............................................................................................................ 6 3.1

Introduction ..................................................................................................................................... 6

3.2

X-Ray Photoelectron Spectroscopy XPS ....................................................................................... 6

3.3

Static Secondary Ion Mass Spectrometry ....................................................................................... 9

3.4

Reflection IR ................................................................................................................................... 9

Pretreatments and Primers for Polyolefin Plastics ............................................................................ 10 4.1

Introduction ................................................................................................................................... 10

4.2

Flame Treatment ............................................................................................................................11

4.3

Corona Treatment ......................................................................................................................... 13

4.4

Low Pressure Plasma Treatment ................................................................................................... 13

4.5

Chromic Acid Treatment ............................................................................................................... 14

Pretreatments and Primers for Polyolefin Elastomers ...................................................................... 15 5.1

Introduction ................................................................................................................................... 15

5.2

Ethylene-Propylene Copolymers .................................................................................................. 15

5.3

Butyl Rubber ................................................................................................................................. 16

5.4

Unsaturated Hydrocarbon Elastomers .......................................................................................... 16 5.4.1 Natural Rubber ................................................................................................................. 16 5.4.2 Styrene-Butadiene Copolymers ........................................................................................ 18

6.

Discussion ............................................................................................................................................... 19

7.

Conclusions ............................................................................................................................................. 23

References ...................................................................................................................................................... 23 Abbreviations and Acronyms ....................................................................................................................... 25 Abstracts from the Polymer Library Database .......................................................................................... 27 Subject Index ............................................................................................................................................... 121

1

Adhesion and Bonding to Polyolefins

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2

Adhesion and Bonding to Polyolefins

Abstract

1 Introduction

Polyolefins have many applications. Polyethylene is the most widely used plastic and in tonnage terms olefinic elastomers such as styrene-butadiene copolymers, dominate applications such as tyres.

Polyolefins are among the most widely used of all polymers finding many applications in packaging, building, transport and other industries. It is often necessary to join polyolefins to other substrates, including metals and other polymers in order to combine the properties of the two materials. An example is a laminate of polyethylene and aluminium foil used in the food and drinks industries; the polyethylene provides heat-sealing properties while the aluminium provides excellent barrier properties. While some applications require joining polyolefins to other substrates, other applications require the polyolefins to be painted, printed upon or metallised. The polyolefins that must be bonded may be in the form of plastics, elastomers (EPDM, styrene-butadiene-styrene (SBS), etc.) and fibres (especially polypropylene). Important examples where it is necessary to paint polyolefins include car bumpers and other automotive components. Good print adhesion to film, bottles and jars is clearly essential. Likewise, good adhesion of metal, deposited via solution or vacuum, to polyolefins is often required.

It is often necessary to join a polyolefinic substrate to another substrate. A pretreatment will normally be required unless a diffusion mechanism operates. Diffusion mechanisms operate during welding together of two pieces of polyolefin, bonding a polyethylene with a hot melt adhesive based on ethylene-vinyl acetate copolymers, and using an ethylene-acrylic acid copolymer as a tie layer when extruding two incompatible polymers. A wide range of pretreatments for polymers exists. However, there are usually preferred methods for plastics and elastomers. The most common methods for pretreating plastics are corona discharge, flame, and low pressure plasma treatments whereas the most common method for olefinic elastomers is the use of an organic chlorine donor. It is now possible to determine the surface chemical changes caused by pretreatments. X-ray photoelectron spectroscopy is especially useful in this respect providing quantitative information on the first few nanometres on the surface of a material. Much research has been carried out on pretreatments for plastics where changes in surface chemistry have been determined and in some cases related to adhesion performance. However, few such studies have been carried out with elastomers partly because of the complexity of their formulations. Several compounding ingredients migrate to the surfaces of elastomers making it difficult to differentiate between changes to these additives and changes to the underlying polymer chains. There has been considerable controversy on whether pretreatments for polyolefins are effective by removing potential weak boundary layers or by introducing functionality in the form of carbonyl and other groups. The present authors believe that if a diffusion mechanism is not operating then such groups are necessary to improve wetting and increase interaction across interfaces. That is not to say that ‘small molecules’ do not sometimes affect adhesion. If these small molecules are not absorbed, for example by the adhesive, then a weak boundary layer will exist with resulting poor adhesion. Also, additives on polyolefins will affect pretreatments such as the corona discharge method as there will be a tendency to chemically modify the additive rather than the underlying polymer chains.

Adhesion failures occurring in service can be very expensive to rectify. For example poor paint adhesion to polypropylene bumpers could involve recall of many thousands of cars. Even when identified at the production stage, poor adhesion can be very expensive to rectify. It is not uncommon for several days’ production to be lost while an adhesion problem is being solved. When it is necessary to join two pieces of the same polyolefin, heat sealing can be used; several welding options exist, including electrofusion, ultrasonic, hot gas, hot plate and infrared techniques (a.1). If it is necessary to join a polyolefin to another substrate, options include coextrusion, adhesive bonding and mechanical fastening. To achieve satisfactory adhesion when bonding with an adhesive, printing, painting or metallising it is usually necessary to pretreat the polyolefin. Pretreatments of polyolefins have been the subject of a great amount of research and development (377) and these treatments form a major part of this review. Exceptions where a pretreatment is not required are noted in Section 2.3. This review explores the joining of polyolefins to a variety of substrates especially by means of adhesive bonding and the coating of polyolefins with printing inks, paints and metals. The use of mechanical fasteners is not covered and the use of thermal methods is only briefly discussed (see Section 2.3). The various types

3

Adhesion and Bonding to Polyolefins

of polyethylene (PE), polypropylene (PP) and ethylenepropylene (EP) polymers, together with elastomers based on dienes, will be examined. Our understanding of adhesion to polyolefins has been greatly helped by the development of techniques which provide information on surface chemistry. These include X-ray photoelectron spectroscopy (XPS), static secondary ion mass spectrometry (SSIMS) and reflection infrared (IR) analysis. The principles of these techniques are outlined in Section 3 and examples of their use are given in Sections 4 and 5 which cover plastics and elastomers respectively.

2 Principles 2.1 Theories of Adhesion It is first necessary to consider why materials adhere to each other. There are four main theories of adhesion, namely adsorption, electrostatic, diffusion and mechanical keying. According to the adsorption theory, macromolecules of the mobile phase (adhesive, printing ink, etc.) are adsorbed onto the substrate and held there by forces ranging from weak dispersion forces to chemical bonds; an interface exists. In the electrostatic theory there is a transfer of charge between the mobile phase and the substrate such that they are held together by electrostatic forces. The third theory requires the diffusion of macromolecules of the mobile phase into the substrate, thereby eliminating an interface. With the mechanical keying theory, the mobile phase flows into the irregularities (pits and troughs) of the substrate surface and after hardening, a keying action occurs. These theories will each be important with particular systems, but the adsorption theory is likely to be the most generally applicable. Aspects of joining polyolefins by a diffusion mechanism are briefly discussed in Section 2.3. To these four theories should be added a theory of nonadhesion, due to the existence of regions of low cohesive strength in the interfacial region. Bikerman (a.2) first suggested that adhesion problems associated with polyethylene are due to weak boundary layers. He suggested that molecules of low molecular weight, that are normally present in commercial polyethylenes separate from the melt and create a region of low strength on the surface.

4

The weak boundary theory received considerable support from other research work and some of this is outlined below. It is certainly easy to envisage various possible sources of weak boundary layers on polyolefin surfaces, namely: •

impurities arising from the polymerisation process;

•

the low molecular weight tail of a polymer;

•

additives, e.g., antioxidants and slip agents;

•

external processing aids, e.g., mould release agents;

•

contamination after processing.

Schonhorn and co-workers have put forward evidence in favour of the weak boundary concept using polyethylene as the substrate. In 1966, Hansen and Schonhorn (a.3) reported work in which they bombarded polyethylene and certain other polymers with activated inert gases and found that the adhesion of an epoxide adhesive to the polymers greatly increased, although the critical surface tensions of the polymers were unchanged. Also, using reflection infrared analysis they were unable to detect any chemical changes in the surface. They therefore proposed that regions of low molecular weight on the surface had been crosslinked to the long polymer chains thereby eliminating weak boundary layers. In fact, Hansen and Schonhorn suggested that surface treatments in general act primarily by eliminating weak boundary layers. In some later work Schonhorn and Ryan (401) exposed polyethylene to ultraviolet (UV) radiation. They found joint strengths with an epoxide adhesive much increased, but there was no evidence of oxidation using reflection IR and contact angle measurements. They concluded that crosslinking at the surface occurred thereby eliminating a potential weak boundary layer. The work of Schonhorn and co-workers will be discussed further in Section 6. Before any adhesion mechanism can operate, good contact between the two materials is necessary. The question of wettability is therefore of crucial importance and this topic will now be briefly considered.

2.2 Wettability A satisfactory level of contact between the mobile phase, for example an adhesive, and the substrate is essential for good adhesion.

Adhesion and Bonding to Polyolefins

A direct measure of wettability may be carried out via a contact angle measurement (θ).

γs is the SFE of the solid γsp is the polar component of the solid SFE

This is the tangent a drop of liquid makes when placed on a substrate surface. Viz:

γsd is the dispersion component of the solid SFE This is the equation of a straight line where (1+Cos θ) /2γl (γld)1/2 is plotted against (γlp/γld)1/2. Hence the square of the gradient is γs p , i.e., the polar component of the surface free energy of the solid and the square of the γ intercept is γsd, i.e., the dispersion component of the surface free energy of the solid.

When there is a strong attraction between the liquid and the solid, θ will be small (or zero for perfect wetting). Conversely when the attraction between liquid and solid is poor a large contact angle is obtained, possibly greater than 90° as illustrated below.

Although the greatest contribution to the surface free energy of a polymer comes from the dispersion component, the polarity of the surface is more easily altered with a pretreatment. For example, an untreated low density polyethylene (LDPE) sample with a zero polar component and a 31.9 mJ m -2 dispersion component, shows an increase in the polar component to 8.0 mJ m-2 after a flame treatment. The dispersion component stayed fairly constant at 30.7 mJ m-2 (a.4). Typically a ten-fold increase in adhesion may be observed from such a treatment. Adhesion improvement comes from better wetting and stronger interfacial attraction due to the new functionality. Hence surface free energy estimation can be a useful tool to assess adhesion performance.

Such is the case with polyolefins; the water contact angle on high density polyethylene (HDPE) is around 104°. The poor wetting is due to relatively low attractive forces between water, which is a very polar molecule, and PE that has no polarity within its molecule. Contact angle values from various pure liquids may be used to estimate the surface free energy of a substrate via various thermodynamic theories; a review of which is given elsewhere (378). One example is the Owens, Wendt and Kaelble approach which enables the polar and dispersion components of the surface free energy (SFE) to be evaluated from a knowledge of the contact angle of various liquids of known polar and dispersion values. The equation employed is as follows: (1+ cos θ) γl/2(γld)1/2 = (γsp)1/2 (γlp/γld)1/2 + (γsd)1/2 where: γl

is the SFE of the liquid

γlp is the polar component of the liquid SFE γld is the dispersion component of the liquid SFE

2.3 Diffusion If two pieces of the same polymer are heated to a sufficiently high temperature and brought together under pressure, then chain segments from the two pieces will interpenetrate and the interface will be eliminated. This process is often termed autohesion. There is no dispute that this process will occur when two pieces of the same polymer are involved as in heat sealing, although ethylene-propylene copolymers are known to exhibit relatively poor autohesion. Whether diffusion between two different polymers occurs will depend on a number of factors including temperature, time available and chemical compatibility. It is often desirable to combine the properties of two or more polymers and this can be achieved by coextrusion. If two polymers A and B are insufficiently compatible to achieve good adhesion, then a tie-layer, often a copolymer, may be used. This is a polymer that is compatible with both A and B. A tie-layer may also be used to join a polyolefin to a metal; for example,

5

Adhesion and Bonding to Polyolefins

ethylene-carboxylic acid copolymers have been used to bond LDPE to aluminium (391). There is evidence that chlorinated polypropylene (CPP) primers diffuse into polypropylene; for example Tomasetti and co-workers (91) used radioactive labelling to demonstrate the diffusion of CPP into PP and PP/EP blends. Other primers used in conjunction with cyanoacrylates also diffuse into polyolefins (see for example (384)). Hot melt adhesives may be used to bond untreated polyolefins apparently by a diffusion mechanism. The most common type of hot melt is based on ethylenevinyl acetate copolymers and these may be used to bond polyethylenes (for an example see (325)). Such hot melts have been commercially available for nearly forty years. More recently a number of reaction-setting adhesives which can bond untreated polyolefins have been developed. For example, Fields and co-workers (375) using an acrylic resin with an anaerobic curing system achieved good adhesion to untreated LDPE. The joining of PE, PP and other plastics in pipelines has been described (127); both fusion and electrofusion welding are included. Details of fusion methods to join polyolefins and other plastics are given in reference (a.1).

3 Methods Used to Study Surfaces

3.2 X-Ray Photoelectron Spectroscopy XPS In this technique a solid, e.g., a plastic film, is bombarded with X-rays of known energy under high vacuum. Photoelectrons from different core levels are ejected. Photoelectrons from the first few atomic layers have a characteristic kinetic energy depending on the elements present in the surface regions. The binding energy of a photoelectron from a particular core level is given by the following equation. EK = hν – EB – φ where: EK is the kinetic energy of the photoelectron EB is the binding energy of the photoelectron hν is the X-ray energy φ is the constant for a given instrument A schematic diagram of the equipment used is shown in Figure 1. By scanning different energies, a spectrum is obtained; an example is given in Figure 2. The elements present in the first few atomic layers can be readily identified from their characteristic binding energies. The percentage concentration of each element can be calculated from the following equation.

3.1 Introduction A knowledge of the surface chemistry of substrates is important in our understanding of adhesion. There are a number of techniques which provide information on the surface chemistry of plastics of which the following are the most important: X-ray photoelectron spectroscopy (or electron spectroscopy for chemical analysis)

Ix Cx =

∑

Sx × 100 ⎛ In ⎞ ⎝ Sn ⎠

where: Cx = percentage concentration of element X

XPS

Ix = quantity of photoelectrons from element X

(ESCA)

Sx = sensitivity factor for element X

Static secondary ion mass spectrometry SSIMS

Σ(In/Sn) = summation of I/S for all elements

reflection IR

It is thus a routine matter to obtain a quantitative elemental analysis of the surface regions of a solid.

As far as polyolefins are concerned these techniques are especially useful for determining the chemical changes that have occurred after a pretreatment, revealing the true locus of failure of a structure and identifying small molecules, including contaminants, on surfaces. The three techniques listed above are now outlined.

For a given element, there are small differences in binding energies depending on the chemical environment of the element. Thus for carbon the bonding energy for C1s varies depending on the neighbouring atoms (see Table 1).

Reflection infrared analysis

6

Adhesion and Bonding to Polyolefins

Figure 1 Schematic diagram of an XPS instrument

Figure 2 Broad scan XPS spectrum of treated polypropylene

Table 1 Binding energies for the C1s peak in various chemical environments Structure

Binding energy eV

C-C, C-H

285.0

C-O

286.5

C-O-O

287.1

C=O

288.1

COOH

288.9

It is therefore possible, by obtaining a high resolution spectrum, to determine which chemical groups are present in the surface regions of a polymer. In Figure 3, the C1s peak for corona treated polypropylene is shown (a.5). The concentrations of the various chemical groups present are given in Table 2. The concentration of different chemical groups in the surface of a polymer may also be determined using derivitisation reactions. These are reactions specific

7

Adhesion and Bonding to Polyolefins

Figure 3 High resolution C1s spectrum of corona-treated PP showing separation of the ‘shoulder’ into component contributions

Table 2 Assignment of peaks for corona-treated polypropylene (a.5) Position (eV)

Area (%)

Assignment

285.0

91.7

C-C, C-H

286.5

1.2

C-O

287.1

2.3

288.1

Table 3 Concentration of different chemical groups after CD treatment of polyethylene (393) Concentration* Initial

Water-washed

Peroxide

1.2

0.9

C-O-O

Hydroxyl

1.7

1.1

2.3

C=O

Carbonyl

1.8

0.9

288.9

1.2

COOH

Epoxide

2.3

1.1

289.5

1.3

O=C-O-C=O ?

Carboxylic acid

1.6

0.8

-NO3

0.8

0.4

to a particular group and which introduce a new element to the surface. Such reactions are illustrated in Figure 4. Some results for corona treated polyethylene (393) are given in Table 3. In an earlier study (395), Briggs and Kendall used derivitisation reactions to study the discharge treatment of LDPE. They converted keto groups in corona treated polyethylene to the corresponding pentafluorophenylhydrazone. They found that the

8

Group

* Moles of functional species per initial unreacted carbon atom (x102) Reprinted from L.J. Gerenser, J.F. Elman, M.G. Mason and J.M. Pochan, E.s.c.a. studies of corona-discharge-treated polyethylene surfaces by use of gas-phase derivatization, Polymer, 1985, 26, 1162, with permission from Elsevier Science.

autoadhesion between two pieces of the treated PE was very low, thus confirming the mechanism for autoadhesion in terms of keto-enol interactions.

Adhesion and Bonding to Polyolefins

Figure 4 Derivatisation reactions used to quantify the groups introduced into a polyolefin by a corona discharge (CD) treatment (Reprinted from L.J. Gerenser, J.F. Elman, M.G. Mason and J.M. Pochan, E.s.c.a. studies of corona-discharge-treated polyethylene surfaces by use of gas-phase derivatization, Polymer, 1985, 26, 1162, with permission from Elsevier Science.)

3.3 Static Secondary Ion Mass Spectrometry XPS is limited in the molecular information it provides and its surface sensitivity. Static SIMS is especially good in these respects. However, it does not normally give quantitative information. The basic principles of SSIMS are as follows. The sample is bombarded under high vacuum by a beam of ions, causing the ejection of ions and neutrals from the surface. The ions may be due to elemental species (e.g., Cu+, O-) or to ‘cluster’ ions (e.g., MgO+, C6H5O-). The positive and negative ions may be separated and two mass spectra obtained. By carefully limiting the ion beam current density, the information obtained can be limited to the first 2-3 atomic layers; this is the basis of static SIMS. From the fragment pattern of ions it is often possible to determine the chemical nature of the surface. For example, polydimethylsiloxane (dimethysilicone) is readily identified from the fragment pattern (73, 133, 147, 207, 221) from the positive SSIMS spectrum.

3.4 Reflection IR Infrared spectroscopy is a method of detecting chemical groups. It depends on the fact that different chemical groups absorb radiation at characteristic wavelengths. For examples, a C=O group in a saturated aliphatic

ketone, absorbs in the region 5.80-5.86 μm. The location of the absorption is usually expressed as a wave number, i.e., waves per centimetre, in this case 1725-1715 cm-1. Most modern instruments use Fourier transform analysis and are termed FTIR. Information on plastic surfaces can be obtained by using reflection techniques. The sample, typically in the form of a plastic film, is placed in close contact with a prism of either germanium or KRS-5 (a mixed TlBr-TlI crystal). The sampling depth is given by the Harrick equation. dp =

λo 2 πn1[sin θ − ( 2

n2

2 n1 ) ]

1

2

where: dp is the distance below the surface at which the amplitude of the electric field is 1/e of its initial value

θ is the angle of incidence between the IR beam and the surface normal n1 and n2 are the refractive indices of the reflection element and sample λo is the wavelength of the radiation Some typical values of dp are given in Table 4.

9

Adhesion and Bonding to Polyolefins

Table 4 Multiple reflection infrared (MIR) conditions and depths of penetration (a.6) Reflection element

n2/n1a

Angle of incidence

dp/λo

dp at 1723 cm-1 (μm)

Ge

0.378

45

0.067

0.39

KRS-5

0.631

60

0.122

0.65

KRS-5

0.631

45

0.208

1.21

a Ratio

of refractive index of PE (1.515) to that of reflection element (Ge=4.0, KRS-5=2.4)

D. Briggs, V.J.I. Zichy, D.M. Brewis, J. Comyn, R.H. Dahm, M.A. Green and M.B. Konieczko, Surface and Interface Analysis, 1980, 2, 3, 107. ©John Wiley and Sons Limited. Reproduced with permission.

Figure 5 The reflection IR spectra of low density polyethylene treated with chromic acid at 70 °C for various times (a.7)

The use of reflection IR is illustrated by the results shown in Figure 5. The spectra show that various functional groups are introduced into polyethylene on treatment for the different times with chromic acid.

4 Pretreatments and Primers for Polyolefin Plastics 4.1 Introduction The pretreatments used for plastics and elastomers are generally different and the two groups of materials

10

will be examined separately; plastics will be examined in this section and elastomers in Section 5. Over the last 50 years a number of pretreatments have been developed to improve adhesion to polyolefins. Much of the early work was directed towards improving the printability of polyethylene. Around 1950, the methods that were developed included treatment with corona discharge, flame, chromic acid immersion and chlorine gas activated by UV. The first three methods became firmly established for the treatment of polyethylene and later polypropylene. Corona discharge, which involves breaking down air into active species including oxygen atoms, ozone and ions, by the application of a high voltage, is still the

Adhesion and Bonding to Polyolefins

preferred method for treating film. Flame treatment, which involves exposing the plastic to a flame for less than one second, is still favoured for treating cylindrical objects such as bottles. Chromic acid is very effective for treating three-dimensional objects but is being phased out for environmental reasons. Around 1960, a number of workers studied the effect of low pressure plasmas on polyolefins and other plastics. Workers at Bell Laboratories found that large improvements in bondability were achieved if polyethylene and other plastics were exposed to a radiofrequency (RF) plasma, they termed this process CASING (Crosslinking by Activated Species of INert Gases) as they believed the process was effective by crosslinking low molecular weight material to long polymer chains rather than introducing new functional groups. This work has led to considerable controversy (see Section 6). In the 1980s interest was renewed in the use of halogen gases to pretreat polyolefins and there is significant commercial interest in this approach; treatment of polyolefins for a few seconds with mixtures of fluorine and inert gases gives large improvements in bondability. Chlorinated polypropylene primers are sometimes used commercially instead of a pretreatment, e.g., they are applied to polypropylene bumpers prior to painting. A number of other pretreatments have been examined although they have not found widespread industrial use.

These include organic peroxides (a.8), ammonium peroxydisulphate (a.9), sodium hypochlorite (388) and an electrochemical method (74). Some of the important pretreatments are now described in more detail.

4.2 Flame Treatment The object to be treated, e.g., a bottle, is passed over one or more burners, each of which possesses a large number of closely-spaced jets. The burners are fed with an air-hydrocarbon gas mixture, whose proportions are controlled within definite limits. The treatment time is usually in the region of 0.04-0.1 seconds (Figure 6). The ratio of air-to-hydrocarbon for complete combustion is known as the stoichiometric ratio. For example, the complete combustion of one volume of methane requires 9.55 volumes of air, so the stoichiometric ratio for an air: methane flame is 9.55:1. In one of the first detailed studies of the flame treatment Ayers and Shofner (402) examined most of the key variables namely: the nature of the gas, the air-to-gas ratio, the effect of contact time and the distance of the polymer from the flame. The nature of the polyolefin used was not stated. The effectiveness of the treatments was assessed using a tape peel-test and the adsorption of a dye. The authors found that higher levels of treatment were achieved

Figure 6 Schematic representation of flame treatment

11

Adhesion and Bonding to Polyolefins

Sutherland and co-workers carried out a detailed study of the pretreatment of various PP types (see for example a.10). Improvements in adhesion were assessed by means of a butt test. In some cases the adhesion to an epoxide adhesive was assessed, but in many cases the PPs were first coated with a polyurethane paint so that improvements in paint adhesion could be determined. Changes in adhesion to a PP homopolymer after flame treatment under a variety of conditions are given in Table 5. It can be seen that good adhesion is obtained under a broad range of conditions.

with an excess of air over the stoichiometric requirements to burn all the propane; this was also true with methane and butane. The authors concluded that the optimum treatment time was about 0.02 s and that the optimum distance between the polymer and the top of the inner cone was about 10 mm. The study of Ayres and Shofner (402) was carried out without access to X-ray photoelectron spectroscopy XPS, a technique which, as stated earlier, enables a quantitative elemental analysis of the surface regions of a polymer to be carried out.

For comparison the tensile strength of the control involving PP wiped with trichloroethane was 2 MPa.

Garbassi and co-workers (390) found that the flame treatment of polypropylene resulted in large increases in the adhesion of polyurethane and acrylic paints to the polymer. Curve fitting of the C1s spectra indicated that hydroxyl and carbonyl were the predominant groups formed, although some carboxyl groups were formed after repeated flame treatment.

A study of the flame treatment of low-density polyethylene showed very high levels of oxidation (Table 6), although the oxidised layer was less than 10 nm thick.

Table 5 Adhesion tests of polyurethane-painted flame-treated polypropylene (a.10)

Air-to-gas ratioa

Total flow

rateb

(l

min-1)

Distance of polymer surface from inner cone tipc (mm)

I interfacial between paint and polymer C complex failure including cohesive within PP a Total flow rate 24 l min-1; distance 10 mm b Air-to-gas ratio 11:l; distance 10 mm c Air-to-gas ratio 11:l; total flow rate 24 l min-1

12

Tensile strength (MPa)

Locus of failure

8:1

24.7

C

9:1

25.4

C

11:1

26.4

C

13:1

26.7

C

14:1

25.8

C

12

26.0

I

18

25.6

C

24

26.4

C

36

27.2

C

48

24.0

C

2.5

22.8

C

10

26.4

C

20

22.1

C

40

6.5

I

60

4.2

I

Adhesion and Bonding to Polyolefins

Table 6 XPS and joint strength data for flame treated low density polyethylene (397) Polymera

Time (s)b

O:C (atom %)

Lap shear strengthc (MPa)

X

0

0.25

0.6

X

0.1

16.9

6.6

X

0.4

31.0

7.2

Y

0