True adhesion

Although the importance of Buonocore s discovery cannot be overemphasized, micromechanical attachment cannot be regarded as true adhesion. True adhesion must be on the molecular level and must involve chemical or physicochemical bonds. 

The tensile adhesion values show no correlation with the extent of corrosion the bisphenol A epoxy cured with a polyamide amine showed blistering, which represents a complete loss of adhesion. The polyester showed cohesive failure at less than 1000 hours of exposure, so a true adhesion value could not be determined. The other epoxies and the vinyl ester all had values in the 150-200 psi range, with no apparent relationship to the amount of corrosion. 

Borroff and Wake48, and later Meardon49, developed a direct tension method which was claimed to more nearly measure the true adhesion between fabric and rubber. It is particularly useful for discriminating between adhesive systems, when the peel tests can be misleading. The main objection to the method is practical in that the preparation of test pieces is rather difficult. 

Evidently, desorption and coil-stretch transition are two interfacial molecular processes, both of which may produce massive interfacial slip between a highly entangled melt and a solid surface. The former can be viewed as a true adhesive failure whereas the latter may not be regarded as such and can be viewed as cohesive. However, it is misleading and inappropriate to link the coil-stretch transition of adsorbed or entrapped chains to any bulk behavior in shear because these chains are either confined or trapped at the surface and should be distinguished... 

Pure solvents such as methylene chloride are not true adhesives , but certain... 

Adhesive Strengths. The strengths of the adhesive bonds between the inclusion materials and the epoxy were determined with an experimental technique described in detail elsewhere (24). The technique involves embedding a sphere of the candidate material in the contoured-neck portion of a cylindrical epow bar. The decohesion of the sample from the epoxy is then observed as the bar is strained in tension. The radial stress at decohesion is then detennined by means of a numerical solution of the deformation problem. Because no stress singularities exist at the interface prior to debonding, as is often the case with the usual popular tests, the technique used here provides a measure of the "true adhesive strength for the bimaterial systems employed. 

In addition to the already discussed influence of the actual micro topography at the coordination point, other conditions may influence the true adhesion forces that act between the solid partners. In the case of van-der-Waals forces the average distance a as shown in Section 5.1.1, Fig. 5.11, may be changed by the presence of adsorption layers (Fig. 5.23). 

The condition for the true adhesion is not sufficient for the two interacting bodjes to fuse and become one. As long as the force of adhesion of aqueous molecules onto the surfaces of the bodies is stronger than the attractive forces between the interior phases of the two hydrocarbon bodies at a certain distance or the nature of the surface hydrophilic layers, which have a certain thickness, is similar to aqueous phase, the two bodies would be separated apart like two different phases and stay side by side. 

Recently, Ohki proposed a physical principle underlying membrane fusion processes in terms of molecular interaction. For simplicity, two interacting membranes are considered as two flat hydrocarbon bodies having hydrophilic layers on their surfaces, separated by an aqueous solution at a certain distance R. The thickness of the hydrophilic layer is h which may be different from those of the flat body as well as the aqueous phases in their molecular nature and molecular density. Since the electrostatic interaction gives a repulsive force and is not a main factor for membrane fusion, we assume the electrostatic energy term to be a constant contribution to the total interaction energy at the true adhesion, and the van der Waals interaction energy would contribute mainly to membrane fusion. Then, the van der Waals interaction between the two bodies will be expressed as a function of the separation distance R, the thickness h, and the Hamaker constant A in each phase ... 

FIGURE 19.3. In what is generally classified as adhesive failure, breakage may occur at various locations including entrapped bubbles (a), at the interface ( true adhesive failure) (b), in the substrate (cohesive failure) (c), or in the adhesive (also cohesive failure) d). 

For a system with a sharp interface and weak intermolecular interaction, as in the nonpolar-polar polymer system mentioned above, failure exactly at the interface is a distinct possibility (or even probability). Thus, interfacial separation may be expected when interfadal strength is weaker than the bulk strength of the bonded materials. As we have seen, if the intermolecular interactions across the interface are more specific (including chemical bond formation) or if significant interpenetration of polymer chains occurs, rupture at the interface becomes less likely. In some cases, the locus of failure may depend on the rate at which stress is applied rapid application leads to cohesive failure and very slow apphcation, tending more toward true adhesive failure (since slow application of stress gives more time for the entangled molecules to slide past one another). 

Fibre-matrix adhesive strength is unimportant for the modulus of compounds, which is determined by the volume fraction of fibres, the moduli of fibres and matrix, the fibre aspect ratio and the fibre orientation. The modulus is a small-strain property and the matrix shrinkage is usually sufficiently high to ensure the modulus increase desired without any true adhesion [4]. 

Fundamental adhesion In one use, adhesion refers to the forces between atoms at the interface. This is sometimes called true adhesion or fundamental adhesion. Here, of course, the concept is necessarily tied to one of the Theories of adhesion and to a particular model for the interface concerned. 

Figure 2.16 A comparison of the degree of retention displayed by mechanical techniques, anaerobic adhesives - which essentially function by jamming components -and several true adhesives.

Figure 2.21 Typical torque patterns demonstrated during removal of a nut locked with an anaerobic adhesive. A The anaerobic composition has a high level of true adhesion as well as compressive strength.

The use of true adhesives - materials other than anaerobic compositions... 

As pointed out in Section 1.1, most anaerobic compositions are not true adhesives and, although very stable, they are not currently as robust or durable as the heat-cured, toughened, single-part epoxies. These epoxies are therefore used to replace solder on brass components when only a few threads are available for engagement. The softness of the alloy coupled with the low engagement area militate against anaerobics, especially when it is intended to use these at high temperatures. 

A distinction needs to be made here with the observations of Section 2.3.1.3 (p.25), recommending use of the higher-strength anaerobic compositions -preferably those with true adhesive characteristics - to ensure retention of fitted non-threaded parts fabricated from soft alloys and plastics. 

The accommodation of surface contamination shown by some adhesives depends upon two distinct factors. First, as the anaerobic adhesives rely almost exclusively on their jamming action, any further reduction in their levels of true adhesion is unimportant provided that the contamination is not so gross that the hardened film of adhesive slides on the oil film. Situations as bad as this are rare and, when likely, must be either prevented or countered by cleaning. 

Essentially limited as a class to co-axial mechanical assembly, retention and sealing, they also make good general purpose gasketting media. The cure rate depends upon surface activity and may require a supplementary catalyst. The family copes with the gaps of normal engineering practice. As clearances increase, the anaerobics capacity to cope well falls rapidly. The majority of materials in the family are only suitable for use in lap joints as gasketting media or to seal a gap. Only the special anaerobic materials can be considered to be true adhesives and suitable for use on unsupported lap joints. 

---