Surface treatment and analysis for adhesive bonding

Surfacetreatment and analysis for adhesivebonding J. Comyn (Leicester Polytechnic, UK)

Improper or inadequate surface treatment is one of the commonest causes of failure in adhesive bonding. On the other hand, the selection of a good surface treatment can bring marked improvements in the wet durability of adhesive bonds to metals and glasses, and can permit the bonding of otherwise difficult-to-bond materials such as polytetrafluoroethylene (PTFE) and the polyolefrns. The reasons why untreated surfaces may be unsatisfactory for adhesive bonding are that they may be contaminated, they may lack polar chemical groups or the interface they make with an adhesive may be susceptible to hydrolysis. Key words:

Theories

surface treatment; coupling agents; surface analysis techniques

of adhesion

To understand the nature of surface treatment we must first consider the theories of adhesion. There are six in current use and these are the diffusion, mechanical interlocking. electrostatic, physical adsorption, chemical bonding and weak bounda 7 layer theories. They have been reviewed by Kinloch and by Wake2e3. Weak boundary layers certainly occur on contaminated substrates, rust on steel and lubricating oils being well known examples. However, weak boundary layers may be more common on polymers because of their tendency to reject foreign substances and additives to their surfaces by the process of diffusion. An important source of weak boundary layers can be low molar mass polymer. The methods of surface treatment that are often successful in removing weak boundary layers are abrasion and solvent cleaning. The mechanical interlocking theory places importance on the penetration of the adhesive into surface irregularities on the substrate. The fact that good adhesion can be obtained with flat surfaces rules out the general applicability of the theory, but there are many instances where the contribution of interlocking may be important. One case is the adhesion of polymers to textiles where the former penetrates the yam. A second is the electroless plating of modified polystyrene plastics, where the surface can be roughened by chemical etching. Later in this paper reference will be made to the etching and anodizing of aluminium alloys. These produce a honeycomb structure with deep pores. The application of a structural adhesive to such a surface at moderate 0143-7496/90/03016

temperatures (e.g., 150°C) and under slight pressure might not seem the ideal conditions for the adhesive to penetrate the honeycomb, but this has been shown to occur by Knock and Locke4. Charge transfer across the interface which gives rise to an electrical double layer is the basis of the electrostatic theory of adhesion, which was proposed by Deryaguin. It is well known’ that when some adhesive tapes (e.g., masking tape) are quickly stripped from the roll in the dark, light emisions occur which might be due to electrical discharge. It was upon evidence of this kind that the theory was first proposed, but controversy still exists as to whether such discharges are the cause of adhesion or simply the result of cleavage. The theory seems to have no important messages about practical surface treatments, and this can also be said about the diffusion theory. The chief advocate of the diffusion theory is Voyutsk? and its basis is that diffusion of polymer chains occurs across the interface, leading to mixing at the interface. For this to occur it is necessary for the level of molecular motion in the adhering phases to be high, and for the compatibility of the phases to be such that some mixing may take place. Because of these restrictions the theory is most appropriate to the self adhesion of unvulcanized elastomers, and the solvent welding of plastics. The physical adsorption theory proposes that for materials in intimate contact, van der Waals’ forces are sufficiently strong to give good adhesion. Whilst van der Waals’ interactions are amongst the weakest of intermolecular forces it has been demonstrated that their combined strengths may greatly exceed measured joint strengths. However, it is a general property of

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materials that strengths are much less than the sum of the component interatomic or intermolecular forces; the presence of flaws or cracks is the reason for this. Van der Waals' forces include dispersion forces which are due to the forces between fluctuating dipoles and will thus occur between all materials, and also the stronger polar forces which are due to permanent dipoles. This theory can thus be exploited by introducing polar chemical groups to the surfaces of non-polar materials such as grvE and the polyolefins. This finally leads us to the chemical bonding theory. Convalent and ionic bonds are much stronger than van der Waals' forces and their presence across an interface should lead to strong and durable adhesive joints. They are postulated to occur with silane primers, which are discussed below. They may also be a factor in the superior durability of phenolic adhesives. Covalent bonds might be formed by the coordination of phenolic oxygens to aluminium ions, as has been observed by the formation of complexes between aluminium and some phenols 7. Further support for this view comes from inelastic electron tunnelling spectra of phenol and hydroquinone on aluminium oxide 8.

M e t h o d s of surface t r e a t m e n t

Surface treatment may have one or a mixture of the following effects. • Removing contamination or weak boundary layers • Changing the surface geometry or morphology • Changing the surface chemistry. Abrasion and solvent cleaning

Substrates may be abraded by sandblasting or abrasive paper, and a general effect is to remove contamination and roughen the surface. Solvent degreasing can involve a vapour degreasing bath or a cloth soaked in solvent, and in the latter case it is important to remember that the aim is to remove oils, greases, etc, and not just to redistribute them on the surface. Adhesion is a phenomenon which occurs within a few molecular layers close to the interface, and a monolayer of oil or grease can constitute an effective weak boundary layer. The use of solvents immediately brings problems of flammability and toxicity. An alternative method of degreasing could be to use an aqueous solution of detergent, but this always seems less popular than the use of solvents. Is this because soap molecules may adsorb on polar surfaces by their hydrophilic heads, so leaving the non-polar tails to face the adhesive?

many steps and require precise control. This is illustrated by the following example. 1) Remove grease with butanone or trichloroethylene. 2) Immerse in a mild alkaline degreasing agent, e.g. Stripalene 532 (37 g 1- I) at 64"C for 5 min. 3) Rinse in hot tap water. 4) Immerse in a solution of 50 g 1- I chromium trioxide at 38-40°C. Over 10 min raise the potential to 40 V in 4 V steps. Hold at 40 V for 40 rain, with a current density of 0.1-0.6 A dm -2. Increase to 50 V over the next 5 min in 2 V steps, and maintain for 5 rain. The cathode is of steel. 5) Wash immediately in cold running tap water for 20 min. Tap water is better than distilled water for the wet durability of joints. 6) Dry in warm air for 20 min at below 60°C. 7) Bond or prime within 3 h. The effect of these treatments on aluminium alloys is to produce a honeycomb structure of durable oxide. Fig. 1 shows some proposed honeycomb structures taken from the literature. The possible reasons for the improvements in durability are • Mechanical interlocking into the honeycomb. • The oxide produced has improved resistance to hydrolysis. • Exchangeable ions are produced which may lead to the formation of interfacial ion-pairs. Whatever the interpretation, these methods of surface treatment have a significant effect in improving the durability of bond in wet air, as is illustrated by some data of Butt and Cotter II in Fig. 2. Other surface treatments for metals include an alkaline peroxide bath for titanium and the sulphuric acid etching of stainless steel. Surface treatment procedures for a variety of substrates are given in Ref. 12. Etching in chromic or chromic-sulphuric acids can also be used to surface treat polyolefin plastics. PTFE is widely known as a non-stick material, but when the need arises to adhere to it, it can be pretreated by etching in a solution of sodium naphthalenide in tetrahydrofuran. This causes some discolouration of the surface due to the removal of fluorine atoms and the formation of a carbonaceous surface layer. Sodium napthalenide is easily prepared by adding pieces of sodium metal to a solution of naphthalene in tetrahydrofuran, whereupon a dark green solution is formed, but clearly this is a hazardous process. An alternative method 13 is to place a platinum wire cathode in contact with grFE, whilst it is immersed in a solution of an electrolyte in dimethylformamide. In this case a discoloured zone grows from the point of contact of the cathode.

Wet channel treatments

Aluminium is the most widely studied metal in adhesive bonding because of its use in aircraft building, and here the methods of surface treatment are etching and anodizing in chromic or phosphoric acids. It is the practice in the N o a h American aircraft industry to use phosphoric acid anodizing whilst anodizing in chromic acid is favoured in Europe. However, there is another variable in the system, namely the use of different aluminium alloys. A serious disadvantage with such methods is that they involve

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INT.J.ADHESION AND ADHESIVES JULY 1990

The use of coupling agents

The most important situation in the adhesive bonding of glass is in fibre-reinforced composites. Water is capable of displacing polyester and epoxide matrices from glass fibres, and under such circumstances water 'wicks' along the interface with catastrophic consequences. A solution to this problem is to use silane coupling agents. These have the general structure R-Si (OR')3 where R' is methyl or ethyl and R contains a group which can chemically react with the

2O 32 nm

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40 nm

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1000 Time (h)

2000

Fig. 2 Effect of warm moist air (43°C and 9 7 % relative humidity) on the strengths of aluminium single-lap joints witlh an epoxide-polyamide (FM 1000) adhesive, after Butt and Cotter 1 . Surface treatments are o chromic-sulphuric etch, I-1 alkaline etch,==solvent degreese and • phosphoric acid etch

200 nm

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Fig. 3

I " ~ Aluminium Fig. 1 ~,ide morphologies on anodized al~ninium as proposed by Bathune (a) and (b), and by Venables et al (c) for anodizing in phosphoric acid

plastic matrix, e.g. epoxide or amine groups where the matrix is an epoxide, or a vinyl group where it is an unsaturated polyester. Silanes can be applied from dilute aqueous solutions, and their attachment to the surface involves hydrolysis of the OR' units, followed by condensation of SiOH groups both on the coupling agent and on the surface. The product of this is a siloxane network which is covalenfly bonded to the surface. The reaction sequence is illustrated in Fig. 3. In treating glass with a silane coupling agent we are using two silicon compounds, and exploiting the basic chemical principle that like on like is a favourable situation. However, it does appear that silane coupling agents can be very effective on metals. This has been demonstrated in a series of papers by Walker 14"Is

OH I

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The hydrolysis and condensation of a silane coupling agent

where epoxide and polyurethane paints were applied to mild steel or aluminium surfaces which had been brushed with solutions of some silanes in wet acetone. Cadmium, copper and zinc are generally regarded as difficult metals to bond and here Walker demonstrated that silanes can improve initial adhesion and water durability. There is some spectroscopic evidence that covalent bonds are formed between silane coupling agents and metals. Gettings and Kinloch 19 treated mild steel with the epoxysilane 3-glycidoxypropyltrimethoxysilane and examined the surface by secondary ion mass spectrometry (SZMS).An ion of mass 100 was assigned to FeSiO +. Naviroj, Koenig and Ishida 2° detected weak peaks in the 950-960 c m - l region in Fourier transform infra-red spectra of a silane on alumina and titania surfaces, and assigned these to Si-O-Al and Si-O-Ti. Although silanes are pre-eminent amongst coupling agents, they are not exclusive. Zirconates and titanates operate in a similar manner to silanes in that they condense with surface hydroxyl groups, and zirconium propionate is used as an adhesion promoter for inks on pretreated polyolefins2k DeNicola and Bell 22 used some ~-diketones as palmers for steel, and Yoshida and Ishida 23 have reported some imidazole compounds as coupling agents for copper. Davis and Venables 24 have used some phosphonic acids as hydration-restricting primers for aluminium. For some time reactive dyes for cellulose fibres have been available which become

INT.J.ADHESION AND ADHESIVES JULY 1990

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covalently bound by the presence of a chlorotriazine group. This principle has been extended to the bonding of cellulose fibres by Zadorecki and Flodin 25. The use of coupling agents in adhesive bonding has recently been reviewed by the author 26. Corona and flame treatment Corona discharge and flame treatment are widely used for preparing polyolefin surfaces to accept printing inks, and can also be used as surface treatments for adhesive bonding. Corona treatment is used for treating films, and it involves passing the film over an earthed roller whilst an electrode placed just above the film generates a corona discharge in air, which is evident as a purple glow. A high frequency, high voltage bias is applied to the electrode, typically 15 kV and 20 kHz. Thicker sections of polyolefins (e.g. plastic squeezy bottles) can be treated by quickly rotating them in the oxidizing region of a natural gas/air flame (ie just above the blue cone) for a fraction of a second. The effect of both these treatments on polyolefins has been reported in a series of papers by Brewis and Briggs 27. In both corona and flames excited species are produced which attack the surface to substitute polar chemical groups such as -COOH, C = O , - O H , -NO2, -NO3 and -NH2. These produce an increase in polar forces between the adhesive and substrate. Plasma treatment Plasmas can be generated by pumping air down to a moderate pressure and applying high frequency (usually 13.56 MHz) and high voltage. The coupling can be capacitive (e.g. parallel plate electrodes in the chamber) or inductive (a coil wound round the vessel). The great advantage of the method is that any gas or vapour can be then introduced into the plasma. Excited species are formed in the plasma which can have the following effects on the substrate. • • • • •

Surface cleaning Ablation Deposition of a layer of polymer Substitution of chemical groups on the surface Crosslinking

There has been much activity in the scientific literature over the past three years in plasma polymerization, and to a lesser extent in the plasma modification of surfaces. The treatment of plastics for adhesive bonding with plasmas of argon, air, ammonia, nitrogen or oxygen has been reported by Rose and Liston 28, and Matsuda and Yasuda 29 have prepared titanium plates for painting in plasmas of some silanes. The analysis of treated surfaces A number of surface analysis techniques are available which are of use in assessing surfaces for adhesive bonding. Wetting and contact angle A useful and simple way of assessing polymer surfaces is to measure the contact angle made by a small drop of liquid. Water purified to a level such that its surface tension is 72.8 mN m -I is commonly used, and what

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INT.J.ADHESIONAND ADHESIVES JULY 1990

can be observed is that the contact angle decreases as the polarity of a polyolefin surface is increased by surface treatment. A variant on this theme is to write on a surface with a felt-tip pen filled with a dyed liquid of known surface tension (these are commercially available). As the polarity of the surface increases it will cause liquids of higher surface tension to spread, so that the written pattern will persist and not separate into droplets. Surface infra-red spectroscopy Attenuated total reflectance (ATR)and multiple internal reflectance (MIR) spectroscopies in the infrared 0R) have been available for quite a long time, and both techniques depend on the same principle. This is that a surface is placed in contact with a medium of high refractive index (germanium or thallium bromoiodide), and an IR beam is reflected at the interface. This is carded out in a conventional IR spectrophotometer, so that an IR spectrum of the surface is obtained. It only usually works with relatively soft surfaces that will make good contact with the refracting medium, but really the major drawback is the penetration depth, which is about one [R wavelength (--- 500 nm), that is, about 1000 molecular layers. In other words the effect of a surface treatment will only be detected by this method if it penetrates fairly deeply. A technique that has come to the fore over recent years is Fourier transform IR spectroscopy (FXm) and in principle it could remove the difficulty of penetration depth. The strength of FTIR lies in the computer which obtains a spectrum from the primary data, which is an interferogram. This can be used firstly to improve weak spectra by multiple scanning, and secondly to obtain difference spectra. The difference of interest in the present context would be between a treated and an untreated surface. X-ray photoelectron spectroscopy (xPs) In this technique a monochromatic beam of soft Xrays from an Al or Mg target is directed at a surface in an ultrahigh vacuum chamber. Electrons are ejected from the core levels of atoms in the surface, and their kinetic energies (KE), which are recorded, are characteristic of the atoms, and are related to binding energies Eb by the following equation. KE = b y -

Eb--¢

Here hv is the X-ray quantum energy and ~ is a calibration constant for the instrument. An xas spectrum can have a number of peaks arising from electrons emitted from different core levels, and when the relative atomic sensitivities are taken into account, this can give an atomic analysis of the surface. Hydrogen is not detected by x],s because it has no core electrons, and it is neglected in the atomic analysis. Although the X-rays penetrate deep into the sample, only electrons from near the surface are able to escape and be detected; the sampling depth is about 5-10 nm. This technique was used by Brewis and Briggs 24 to demonstrate which chemical groups are introduced to polyolefin surfaces by flame and corona treatment (Fig. 4). It is particularly useful at detecting contamination involving a heteroatom, such as might

= CI7H3y-CO-NH + + CI7H35-CO-NH-CH2-CH2 + 282D

310D

Acknowledgements

This paper, presented at ASE '88, the Third Adhesives, Surface Coatings & Encapsulants Exhibition & Conference, Brighton, October 1988, appeared in Construction and Building Materials 2 No 4 (December 1988) p 210.

01s

References

o-C-i

,

i

b

I 291

I

I 287

I

I 283

I

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537

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Kinloch, A.J. Adhesion and Adhesives (Chapman and Hall, 1987) Wake, W.C. Polymer 19 (1978) p 291

3

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4

Knock, K.K, and Locke, M.C. Adhesion Aspects of Polymer Coatings (ed. K.L. Mittal) (Plenum, 1983) p 301

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Lewis, B.F., Bowser, W.M., Hum, J.L., Luu, T. and Wainberg, W.H. J Vac Sci Techno/11 (1974) p 262

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Venables, J.D., McNamara, D.IL, Chen, J.M., Sun, T.S. and Hopping, R. Nat SAMPE Tech Conf 10 (1978) p 362

11

Butt, R.I. andCotter, J . L J A d h e s 8 ( 1 9 7 6 ) p11

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Barker, D.J., Brawis, D.M., Dahm, R.H., Gribbin, J.D. and Hoy, L.R.J. JAdhes 13 (1981) p67

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533

(eV) Fig. 4 High resolution carbon ls and oxygen ls xPS spectra of (a} untreated and (b) and (c) corona discl]~rge-treeted low density polyethylene, after Briggs sod Brewis . It can be seen that treatment introduces oxygen to the surface in the form of C=O, COOH and C-O groups

be the case with silicone and fluorocarbon mouldrelease agents. Static secondary ion mass spectrometry

1 2

(SSIMS)

This technique 3° places the sample in an ultrahigh vacuum chamber and bombards it with ions (At +, Xe + or Ga + < 4keV). The surface then emits secondary ions which are analysed in a mass spectrometer, and in separate experiments both the positive and negative ion spectra can be obtained. In static, as opposed to dynamic, SXMSthe primary ion current is so low (= 1 nA c m - 2) that the surface is not significantly ablated. Data are obtained from the top few molecular layers. A major problem with insulators is the build up of surface charge. This can be avoided by flooding the surface with low energy (= 700 eV) electrons. SSIMS spectra are complicated and it is difficult to assign all the mass peaks. However, there have been a number of instances where it has been very successful in identifying surface contamination. One is the detection of ethylene bisstearamide on the surface of a polyurethane, from the presence of peaks at 282D and

310D, which are due to the following cleavage reaction 31. CI7H35-CO-NH-CH2-CH2-NH-CO-CI7H35

15

Walker, P. JOCCA 65 (1982) p 415

16

Walker, P. JOCCA 66 (1983) p 188

17

Walker, P. JOCCA 67 (1984) p 108

18

Walker, P. JOCCA 67 (1984) p 126

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Gettings, M. and Kinloch, A.J. J Mater Sci 12 (1977) p 2511

20

Naviroj, S., Koanig, J.L and Ishida, H. J Adhesion 18 (1985) p93

21

Moles, P.J. Polym Paint Co/our J 173 (1983) p 391

22

DeNicola, A.J. and Bell, J.P. Org Coat Plast Chem 45 (1981 ) p120

23

Yoshida. S. and Ishida, H. J Adhes 16 (1984) p 217

24

Davis, G.D. and Venables, J.D. Ch 2 in Durability of Structural Adhesives (ed. A.J. Kinloch) (Applied Science, 1983)

25

Zadorecki, P. and Flodin, P. J A p p l Polym Sci 31 (1986) p 1699

26

Comyn, J. Ch 8 in Structural Adhesives: Developments in Resins and Primers (ed. A.J. Kinloch) (Elsevier, 1986)

27

Brawls, D.M. and 8riggs, D. Polymer 22 (1981) p 7

28

Rose, P.W. and Liston. E. ANTEC "85 (1985) p 685

29

Matsuda, Y. end Yasuda, H. Thin Solid Films 118 (1984) p211

30

Vickerman, J.C. Chem Brit (1987) p 969

31

Briggs, D. Surface lnterface Ana19 (1986) p 391

Author

The author is in the School of Chemistry, Leicester Polytechnic, PO Box 143, Leicester, LEI 9BH, UK.

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