Some reflections on contemporary views of theories of adhesion

K.W. Allen

(The City University, UK)

This paper reviews the literature on mechanical interlocking and adsorption interactions, the two main mechanisms postulated to account for adhesive bonding. It is shown that interpenetration of adhesive and adherend is important in obtaining optimum adhesion in a wide range of applications. While for fibrous materials this interpenetration is fairly obvious on a macro scale, for metal/metal oxide surfaces it is on a micro scale and revealed by sophisticated surface examination techniques. Wetting and the thermodynamics of solid/liquid interfaces are reviewed and the development of theories relating to secondary interaction forces is discussed. Recent evidence for the existence and significance of primary chemical bonding in adhesion is then outlined.

Key words: adhesion; mechanical interlocking; adsorption interactions; review

For a very considerable period when reviewing theories or postulates of the fundamental basis of adhesive bonding it has been usual to distinguish four distinct mechanisms: mechanical interlocking, adsorption interactions, electrostatic forces and diffusion mechanisms 1. Of these, the last two have always been recognized as being of significance in only a few, rather restricted instances; although important within these limitations. For the main areas of industrial application, where adhesive bonding is required to resist significant stress and to play a serious part in the integrity of structures, then it is the first two which are relevant. It is the deeper understanding of these which has underpinned the confidence that has enabled the recent steady growth of the use of structural adhesives in engineering. Accordingly it is these two -- mechanical interlocking and adsorption interactions -- which will be considered in this present paper.

Mechanical interlocking In essence, the idea of mechanical interlocking of the adhesive within irregularities of the adherend surface is the oldest and simplest explanation of the whole phenomenon of adhesive bonding. It has an intuitive appeal and immediately led to the idea of roughening surfaces to increase adhesion. While the main uses of adhesives were in joining papyrus, paper, leather or

wood, this was largely true. This was because all these substrates were fibrous materials and successful bonding generally did involve penetration of the adhesive between the fibres and thus embedding in the cured adhesive. However, there were oddities and apparent exceptions as illustrated in Table 1, where the bond strength between wood specimens decreases with increasing roughness.

This idea was justified by the classical work of Borroff and Wake 3 on the adhesion of rubber to textiles in the production of motor tyres. The early tyre cords were made from spun staple of natural origin, mainly cotton. This work demonstrated conclusively that the bond strength depended upon the penetration of the fibre ends from the spun yarn into the rubber (and not significantly upon the penetration of the rubber into the weave). If the spun staple natural fibre was replace by a synthetic monofilament (e.g. nylon), which might give enhanced mechanical properties, the bond strength to the rubber was very considerably reduced. A variety of special treatments and finishes had to be devised and used to provide specific interaction between the surface of these monofilaments and the rubber to achieve satisfactory bonding when they were used.

Another instance in which a mechanical interlocking was clearly demonstrated to be important was the electroless plating of certain plastics with

0143-7496 /93 /020067-06 © 1993 Butterworth-Heinemann Ltd

INT,J.ADHESION AND ADHESIVES VOL. 13 NO, 2 APRIL 1993 67

Table 1. Maple wood samples, bonded side grain with urea-formaldehyde resin under 5 Ibf in -2 pressure, tested in shear 2

Surface Shear strength (Ibf in -2)

Planed 3120 Sanded 2360 Sawn 2690 Combed 2400

Roughness

1 Ibf in -2 = 6 . 8 9 5 kPa

metal. The plastics involved, either ABS (acrylonitrile/ butadiene/styrene) or high impact polystyrene, consist of a matrix of glassy polymer with the elastomer dispersed within it. The first step in the process is etching with an oxidizing agent; this oxidizes and removes the rubbery material, leaving a highly porous and reticulate surface upon which the metal is then deposited. Electron micrographs of sections 4" 5 show quite clearly the penetration and interlocking of the metal within the plastic to a depth of up to 10pm. A very careful study by Perrins and Pettett 6 demonstrated that there were two mechanisms involved and reinforcing each other -- mechanical interlocking and a specific interaction depending upon the surface chemistry of the plastic. It has been authoritatively suggested that the synergy of these two mechanisms should be represented by a multiplicative rather than an additive mathematical function.

However, these, which all involve roughness on a relatively macro scale in the micrometre range, are largely irrelevant to the mainstream of structural bonding. In more recent times, particularly with more sophisticated techniques available, attention has focused on roughness on a much smaller scale, at the AngstrOm level rather than the micrometre level.

Probably the first clear example was due to Packham 7-9, who was investigating the adhesion of molten polyethylene to metals and especially aluminium. It had already been established that for good adhesion the polyethylene needed to be oxidized and that if this was prevented then only poor adhesion could be achieved. It was also well established that anodic oxidation of aluminium in acid electrolytes gave a barrier layer of oxide immediately on the metal and beyond this a structure of hexagonal oxide cells each with a pore, approximately circular in cross- section, at its centre. The dimensions of the cells was governed by the anodizing voltage but the diameter of the pores was not. However, the nature and concentration of the electrolyte controlled the diameter of the pores.

Packham found that, while for aluminium oxide surfaces without pores it was necessary for polyethylene to be oxidized if good bonds were to be produced, for oxide surfaces with pores goods bonds were produced with polyethylene which was carefully protected (in various ways) from oxidation. Indeed, the bond strength was directly related to the depth of the pores and the overall porosity of the surface. Electron micrographs were taken of surfaces of polyethylene which had been sintered on to aluminium anodized in 4% phosphoric acid and on to steel surfaces and from

which the metal had been dissolved away. The polyethylene which had been in contact with steel showed tne surface roughness of the metal but no other features. The polyethylene which had been in contact with the aluminium, at a magnification of 15 000×, showed regular projections normal to the surface which were fibrous in appearance. The tufts were of the order of I pm (1 X 10",~) in diameter. The best explanation, and the one which certainly corresponds with the general appearance, is that each projection is a tuft of a number of individual fibres, each one of which was originally in a single pore. Estimates of the diameter of the fibres gives a value of approximately 550A, which is not far removed from the diameter of the pores of 330,h, given by Keller et al. Io

Clearly here the mechanical interlocking of the polyethylene (regarded as the adhesive) with the surface is an important factor in achieving good adhesion.

A little later, in studies of the bonding of titanium alloys II with epoxide adhesives, a significant effect of surface roughness was noted. A variety of chemical treatments all gave a surface layer of the rutile form of titanium oxide. The one which resulted in the greatest bond strength also gave the roughest surface, with needles of ruffle strongly adhering to the substrate and with the a-phase of the alloy preferentially removed.

More recently Venables and colleagues 12 13, using a variety of surface analytical techniques but especially scanning transmission electron microscopy (STEM) at 30A resolution, studied the surfaces of aluminium and titanium which had been subjected to the standard treatment used in preparation for adhesive bonding in aerospace applications. The chromic acid etching of aluminium, usually known as the Forest Products Laboratories (FPL) process, produces a shallow hexagonal cell structure with a high concentration of protruding oxide 'whiskers' (~50A diameter and ~400,~ high). If this etching is followed by anodizing in phosphoric acid (PAA), the whole oxide structure is considerably deeper with much more pronounced cellular formation and longer (~ 1000A) oxide whiskers. Further evidence leads to the conclusion that, in the absence of any degradation due to the environment and especially water, the bond only fails when the polymer itself fails by viscoelastic deformation. When the metal oxide surface does not have this particular pore/whisker structure, then failure occurs at considerably lower stress levels.

Closely similar results were produced for titanium alloys.

Venables et al. also demonstrated that moisture caused a considerable change in the surface morphology quite quickly. The oxide is converted to hydroxide which has a totally different structure. The pores and whiskers are replaced by a lamellar 'cornflake' structure which adheres only weakly to the underlying material, and the bond strength is considerably reduced. There is, however, some controversy about whether this hydration degradation occurs before the bond is parted or afterwards.

Nevertheless, all this very elegant work undoubtedly demonstrates that mechanical interlocking on a micro scale is an important factor (even if not the dominant one) in achieving high adhesive bond strength.

In conclusion, it is now clear that. in a wide range

68 INT.J.ADHESION AND ADHESIVES APRIL 1993

of applications, the interpenetration of adhesive and adherend is important in securing the optimum adhesion. For fibrous materials this will be on a fairly obvious and macro scale, while for metal/metal oxide surfaces it is on a micro scale and only demonstrable by surface examination of a high degree of sophistication.

Adsorption interactions The cohesive strength of any solid depends upon the various forces of attraction that exist between the ultimate particles of which it is composed. These forces are of various types depending upon the particular nature of the solid but, in general, will include those regarded as of chemical origin -- covalent, ionic and metallic bonding, hydrogen bonding -- as well as those considered as of physical origin -- the secondary or van der Waals' forces including dipole interactions and the dispersion forces. In any particular instance, which of these are present and their relative significance will depend upon the chemical nature of the material, but the dispersion forces will always be present and effective to a greater or less extent. A quantitative consideration of these forces will enable an ideal strength of the material to be calculated. However, this strength is never normally achieved. This is because of irregularities, flaws and defects both throughout the structure and more particularly in the surface layer, as was demonstrated originally by Griffith t4.

This is evident from simple calculations 15, which indicated a theoretical strength for steel of about 3 × 104 MPa compared with the experimental values of 4 X 102 MPa for ordinary commercial material and 3 × l03 MPa for the strongest steel wires. Moreover, steel is rather exceptional in achieving a real strength which is only one order of magnitude less than the theoretical value. For most materials the difference is two or three orders of magnitude.

It is only under very special conditions such as when 'whiskers' are grown or very fine fibres are drawn that these ideal strengths can be approached.

It is these same forces of cohesion within a single material that are responsible for adhesion when it occurs across the interface between two different materials. Ultimately there are no other forces available. One important feature which is common to all these forces is that they are only significant over very short distances. Most of them are quite negligible beyond a few AngstrOms, and those which have the greatest range are only effective over distances less than l0 times as great. This means that if they are to have any effect to hold two surfaces together, these must be in very close contact indeed.

It is for this reason that all normal adhesives are, at one stage in their use, mobile liquids which can wet and flow across the adherend, penetrating all the irregularities and roughness of the surface so that the very closest and most intimate contact is achieved. It is from this requirement that all the concern with, and study of, the phenomenon of wetting and the thermodynamics of liquid/solid interfaces arises.

The first quantitative ideas about the wetting of a solid surface by a liquid are due to Thomas Young 16, who conceived the relationship between surface tensions and contact angle that is now very familiar:

~'SV = VSL + YLV cOs0 (Young equation)

The theoretical consideration of the situations involved in adhesion begins from the expression for the thermodynamic work of adhesion first enunciated by Dupr617:

WA = 71 + 72 - 712 (Dupr6 equation)

That is, the thermodynamic work of adhesion of a liquid to a solid is equal to the sum of the surface free energies of the two separate phases minus the interfacial free energy. It is important to recognize immediately that the two surface free energies relate to clean surfaces, each in equilibrium with its own vapour. They are not the values which may commonly be measured or considered, where the surface of the solid is in equilibrium with the vapour resulting from the liquid. If this equation is to be combined with that due to Young (which involves equilibrium with the vapour from the liquid), then the term n, the spreading pressure, must be introduced in order to relate }'s and }'sv to give

]IS = }'SL + 7LV c O s O + n (Young-Dupr6 equation)

where n, the equilibrium spreading pressure, is given by Is

~ o tr = Ys - }'sv = R T Fd(lnp)

and F is the surface concentration of adsorbed vapour, p is the vapour pressure and P0 is the equilibrium vapour pressure.

But going back to the Dupr6 equation, the }'12 term and how it might be expressed in useful terms remain important questions. Two separate and distinct paths of development have emerged.

Good and Girifalco I,' 20 gave their attention to this interfacial free energy and how this might be expressed in terms of the properties of the two separated surfaces. They began with Berthelot's relation 21 for the attractive constants between like molecules Aaa, Abb and that between unlike molecules Aab which is

Aab (Aaa .Aab)l /2 - 1

Then, by analogy, they related the surface and interfacial free energies (F) and introduced the constant * for generality:

F12

(Fll" F22) I/2

but

F12 = Y1 + }'2 -- }'12

Ftj = 2}'1, F22 = 272

SO

Yl + Y2 - Y I2

o r

T12 = }'1 + Y2 - - 2~(y I " 72) 1/2 (Good & Girifalco equation)

INT.J.ADHESION AND ADHESIVES APRIL 1993 69

It may be interpolated here that a similar, geometric mean method had been used successfully by Hildebrand and Scott 22 in considering the intermolecular interactions and energies of solutions.

Good 23 has shown that • can be calculated from values for each of the two components of dipole moment (p), polarizability (a), ionization energy (I) and molecular volume (V) (or radius oc VU3). Equally it may be determined more directly experimentally if the two phases are both liquid, when the values of the interfacial tension and of the two separate surface tensions may be measured.

This term • is of greater importance than is sometimes recognized. When the forces involved at both sides of the interface are solely London dispersion forces, then it is justified to equate • to one; and this can be demonstrated to be correct to within about 2% experimentally. However, when the forces within the two phases are of dissimilar types, then • will deviate considerably from unity. For water/mercury it is as low as 0.31, while for water/isobutyl alcohol it is 1.15.

Thus any discussion which is based, explicitly or implicitly, upon the assumption that • = 1 is fundamentally flawed in its logic. While in some cases it may give answers which correspond with the truth, it can only do this within strict limits.

However, proceeding with the development of these concepts, we find that this has gone in several ways.

Immediately following from the Good and Girifalco equation, this may be combined with the Dupr6 equation to give:

WA = 71 + Y2 -- {71 + 72 -- 2~(71 • '}"2) 1/2}

= 24)(71 • 72) 1/2

Alternatively, it may be combined with the Young- Dupr~ equation to give:

[ r2( l + COS(I~) + ]1"] 2

Yl = 4(]D272

If one considers a situation where (1) lr is negligible -- for example, with a relatively high energy liquid (Y2) and a relatively low energy solid (Y0, and (2) the liquid wets and spreads on the solid, so that 0 = 0 ° and cos0 = 1. then

Y2 Yt = ~

But in this situation 72 < Yc where 7c is the critical surface energy of the solid. Noting that the inequality sign (rather than the equal sign) is appropriate because condition (2) above is fulfilled whenever Y2 < Yc, hence

(1)2 =Yc YJ

Fowkes24, 25 recognized that the total surface free energy comprises a number of contributions from different types of force components and went on to suggest that the total could be represented by the simple sum of these components. Thus he reached the expression:

y = yd + yh + yx + yi + yab

where the superscripts d, h, x, i and ab represent London dispersion forces, hydrogen bonding, dipole-

dipole interactions, induced dipole interactions and acid-base interactions, respectively. This was often contracted to

Y = gd + Tp

o r

WA = +

where p represents all the non-dispersion forces arising from various polar interactions.

It should be noted here that, following Good and Girifalco, it is correct to put

wda = 2(yd. yd),/2

but not to put

W~ = 2(r~'r~) In

because these both assume • = 1 which is generally correct in the first relationship but not necessarily true in the second.

This work of Fowkes recognizes that in any real situation there will inevitably be dispersion forces acting. Further, there may or may not be other forces in addition. So far this interpretation is generally accepted. Much of the recent discussions have centred around the nature of the non-dispersion, polar forces, about which of these forces are significant and the most advantageous ways in which they may be represented, investigated and considered.

The earliest of this generation of discussions was due to Bolger and Michaels 26 who, in discussing polar polymers interacting with metal oxides, suggested that the only significant polar forces which need be considered were hydrogen bonds. These they treated using the classical Bronstead proton definition and theory of acid/base relationships and obtained satisfactory results for the adhesion of these particular materials.

Then it was recognized, particularly by Fowkes 27, that the use of the Lewis acid/base definitions and theory would be more general and helpful. Further, he demonstrated that the dipole interactions were not significant in adhesive bonding and that the only factor of importance in addition to dispersion forces was hydrogen bonding 28.

This appeared to contradict the results of Keesom 29 and of Debye 3° on dipole interactions but it was realized that all their work had been concerned with molecules in the gas phase. Under those conditions the individual molecules were far enough apart to be considered as behaving independently. In the solid the situation was different and the interaction of molecules with near neighbours was significant. It was claimed that, as a result, the overall dipole forces were negligible if not zero.

In his development of these ideas, Fowkes followed the work of Drago et aL 31" 32 on the heats of reaction (AH) of acid/base pairs. He used an equation of the form

/XH = E A • E B + C A • C B

where the E parameters are said to represent the electrostatic contributions and the C parameters similarly to represent the covalent contributions of the acid and the base respectively. Recently it has been suggested 33 that, while this four-parameter equation is

70 INT.J.ADHESION AND ADHESIVES APRIL 1 9 9 3

valuable, the theoretical basis for this representation is uncertain.

Fowkes and Maruci 34 prepared a series of polymers of controlled acidity or basicity by copolymerizing ethylene with either acrylic and or vinyl acetate. For each series of solid polymers they measured the contact angles of a series of liquids of known characteristics. From this data they calculated the total work of adhesion and, taken with results for the dispersion force contribution, the acid/base contribution. In each case a set of smooth curves was obtained relating the acid/base contribution to the work of adhesion to the degree of acidity or basicity of the polymer. Further, for an acidic liquid with the acidic polymers there was only a dispersion forces interaction, no acid/base interaction; and similarly for a basic liquid with the basic polymers.

Following this, Fowkes and Mostafa 3s investigated the adsorption by inorganic solids (particularly silica and calcium carbonate) of polymers from solution and showed this to be a triangular competition. It involved simultaneous dispersion force and acid/base interactions between: (1) polymer and inorganic solid, (2) polymer and solvent and (3) solvent and inorganic solid. The differences in dispersion forces cancel out and it is the acid/base competition which controls the overall adsorption behaviour.

Direct effects on adhesion have been illustrated by comparison of the adhesion of films of basic poly(methyl methacrylate) cast from solution on to an acidic glass (less than 0.1% alkali metal oxides) and on to a basic glass (20% alkali metal oxides). It was difficult to remove the film from the acidic glass but very easy to peel it away from the basic glass 2s.

Very recently, Lee 36 has drawn attention to molecular interactions -- intermediate in nature between the van der Waals' secondary forces and the primary valence forces. This is an extension of the concept of electron sharing and the interaction of molecular orbitals and has some relevance to our current concerns.

So far all this discussion has involved only secondary interaction forces (van der Waals's forces), particularly dispersion forces, and acid/base interactions, especially hydrogen bonding. While it is clear that these are very significant in adhesive bonding, and are often more than adequate to explain observed bond strengths, yet there are instances where covalent bonding is undoubtedly involved.

The use of primers or coupling agents is now well established practice to enhance the durability of adhesive bonds, especially where glass is one of the components. Indeed, it was the development of suitable silane compounds that enabled extensive use to be made of glass fibre-reinforced composites, especially in boat construction. These silane layers are undoubtedly themselves polymerized, and laser Raman spectroscopy 37 has demonstrated the existence of Si-O-Si covalent bonding across a polysiloxane/glass interface. Similarly, Gettings and Kinloch 38 have shown by secondary ion mass spectroscopy (SIMS) the presence of FeOSi + radicals on the surface of primer- coated mild steel and both FeOSi + and CrOSi + radicals on the surface of primer-coated stainless steel, thus giving firm evidence for the formation of primary bonding between the primer and the metal substrate.

Studies by inelastic electron tunnelling spectroscopy (lETS) of titanium coupling agents on aluminium by Allen et aL 39 have also demonstrated the presence of covalent bonding between the substrate and the coupling agent.

Wu~" has reviewed a series of investigations of the effects of introducing quite small amounts (0.001-0.1 mole fraction) of reactive functional groups into an adhesive which often increases adhesive bond strengths considerably.

There is also indirect support for the contribution of primary bonding in a thermodynamic study by Allen et al. 41 which demonstrated that the secondary interactions were insufficient to account for the observed properties of strength and durability.

Thus there is a steadily growing body of evidence for the existence and significance of primary chemical bonding in addition to the secondary bonding in adhesion.

R e f e r e n c e s

1 Allen, K.W. "A review of contemporary views of theories of adhesion' J Adhesion 21 Nos 2-3 (1987) pp 261-277

2 Maxwell, J.W. Trens Amer Soc Mech Engrs 67 (1945) p 104 3 Boroff, E.M. end Wake, W.C. Trans/nst Rubber Industry 25

(1949) pp 39-50, 199-209 4 James, D.I. in "Aspects of Adhesion 7" edited by D.J. Alner and

K.W. Allen (Transcripta Books, London, 1973) pp 64-71 5 Wake, W.C. "Adhesion and the Formulation of Adhesives" 2nd

edition (Applied Science Publishers, London, 1982) pp 68-71 6 Perrins, L.E. and Pettett, K. Plastics and Polymers 39 (1971) p

391 7 Packham, D.E., Bright, IL end Malpass, B.W. "Mechanical factors

in the adhesion of polyethylene to aluminium' J Appl Polym Sci 18 (1974) pp 3237-3247

8 Malpsss, B.W., Packham, D,E. and Bright, K. "A study of the adhesion of polyethylene to porous aluminium films using the scanning electron microscope' J Appl Polym Sci 18 (1974) pp 3249-3258

9 Packhem, D.E. in "Aspects of Adhesion 7" edited by D.J. Alner and K.W, Allen (Transcripta Books, London, 1973) pp 51-63

10 Keller, F., Hunter, M.S. end Robinson, D.L. J Electrochem Soc 100 (1953) p 411

11 Allen, K.W., Alsslim, H.S. and wake, W.C. 'Bonding of titanium alloys' J Adhesion 6 (1974) pp 153-164

12 Venebles, J.D. in "Adhesion 7" edited by K.W. Allen (Applied Science Publishers, London, 1983) pp 87-93

13 Venebles, J.D. 'Adhesion and durability of metal-polymer bonds' J Mater Sci 19 (1984) pp 2431-2453

14 Griffith, A.A. PhilTransRoySocA221 (1921) p 163 15 Gordon, J.E. The New Science of Strong Materials', 2nd edition

(Penguin Books, London, 1976) pp 70-72 16 Young, T. Phil Trans Roy Soc 95 (1805) p 64 17 Dupr~, A. Theorie Mechanique de la Chaleur" (Paris, 1869) 18 Bangham, D. and Rezouk, R.I. Trans Faraday Soc 33 (1937) p

1459 19 Girifelco, L.AL sod Good, R.J. 'A theory for the estimation of

surface and interfacial energies. I. Duration and application to interfacial tension' J Phys Chem 61 (1957) pp 904-909

20 Good, R.J. "The role of wetting and spreading in adhesion" in "Aspects of Adhesion 7" edited by D.J. Alner and K.W. Allen (Transcripts Books London, 1973) pp 182-201

21 Berthelot, D. Compt Rend 126 (1898) p 1703 22 Hildebrand, J.H. and Scott, R.L. "Regular Solutions" (Prentice-Hall,

Englewood Cliffs, N J, 1962) 23 Good, R.J. in "Contact Angle - - Wettability and Adhesion;

Advances in Chemistry Series 43 (American Chemical Society, 1964) pp 74-87

24 Fowkes, F.M. J Phys Chem 66 (1962) p 382 25 Fowkes, F.M. Ind Eng Chem 12 (1964) p 40 26 Bolger, J.C. and Michsels, A.S. 'Molecular structure and

electrostatic interactions at polymer/solid interfaces' in "Interface Conversion for Polymer Coatings" edited by P. Weiss and G.D. Cheever (Elsevier Publishing Co, New York, 1968) pp 3-60

INT.J.ADHESION AND A D H E S I V E S A P R I L 1 9 9 3 71

27 Fowkes, F.M. 'Donor-acceptor interactions at interfaces' J Adhesion 4 (1972) pp 155-159

28 Fowkes, F.M. "Acid-base interactions in polymer adhesion' in "Physico-Chemical Aspects of Polymer Surfaces" edited by K.L. Mittal (Plenum Press, New York, 1983) pp 583-603

29 Keesom, W.H. PhysZeit 22 (1921) p 129, 643 30 Debye, P. PhysZeit21 (1920) p 1 7 8 ; 2 2 ( 1 9 2 1 ) p 3 0 2 31 Drago, R.S., Vogel, G.C. and Needham, T,E. J Amer Chem Sac 93

(1971) p6014 32 Dingo, R.S., Parr, L.B. and Chamberlain, C.S. J Amer Chem Soc

99 (1977) p 3203 33 Jensen, W.B. in "Acid-Base Interactions" edited by K.L. Mittal and

H.R. Anderson Jr (WSP, Utrecht, 1991) p 10 34 r-owkes, F.M. and Maruci, S. in Organic Coatings and Plastics

Chemistry Preprints 37 (Amer Chem Soc, 1977) p 65 35 Fowkes, F.M. and Mostafe, M.A. Ind Eng Chem Prod Res & Dev

17 (1978) p3 36 Lee, L.-H. 'Molecular bonding mechanisms for solid adhesion' J

Adhesion 37 (1992)pp 187-204

37 Koenig, J.L. and Shih, P,T.K. JColloidlnterfacia/Sci 36 (1971) p 247

38 Gettil,gs, M. and Kinloch, A.J. J Mater Sci 12 (1977) p 2511 39 Allen, K.W., Spencer, J,E.D. and Field, B.D. in "Adhesion 13"

edited by K.W. Allen (Elsevier Applied Science London, 1989) pp 278-293

40 Wu, S. "Polymer Interface and Adhesion" (Marcel Dekker New York, 1986) pp 420-426

41 Allen, K.W., Greenwood, L. and Wake, W.C. 'The stability of adhesive bonding between silicone rubber and aluminium for neural prostheses' J Adhesives 16 (1983) pp 61-76

Author

K.W. Allen is with the Adhesion Science Group at The City University, London ECIV 0HB, UK.

72 INT .J .ADHESION A N D A D H E S I V E S APRIL 1 9 9 3