some reflections on contemporary views of theories of adhesion
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Some reflections on contemporary views of theories of adhesion
(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
1 Ibf in -2 = 6 .895 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