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 so that 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, or monomers, 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 diffusion are briefly discussed in Section 2.3.

To these four theories should be added a theory of non-adhesion, due to the existence of regions of low cohesive strength in the interfacial region. Bikerman (a.1) first suggested that adhesion problems may be due to weak boundary layers. He suggested that molecules of low molecular weight create a region of low strength on the surface.

The weak boundary theory received considerable support from other research work and some of this is outlined next. It is certainly easy to envisage...