Theories of Fundamental Adhesion

Abstract

The historical development and current status of the four classical theories of adhesion are first reviewed. The adsorption theory emphasizes the point that once adhesive and substrate come into contact forces of attraction will act between them. As long as the extent of wetting is good, these forces, whether primary bonds, such as covalent, or secondary van der Waals forces, are generally considered sufficient to give a high bond strength. Primary bonding may be necessary to achieve bond durability in a hostile environment.

The mechanical theory focuses on interlocking between adhesive and a rough substrate surface. Again good wetting is required, or surface roughening is likely to lead to poor bond strength. It has been shown to apply to some surfaces rough on a macroscale as well as to microfibrous and microporous surfaces, such as anodized aluminum. The enhanced adhesion is associated with increasing plastic energy dissipation during fracture in the bulk adhesive.

The electrostatic theory points to electrical phenomena such as sparking, which may be observed during the destruction of an adhesive bond, and consider the transfer of electrostatic charge between the adhesive and substrate. It regards the adhesive-substrate system as analogous to a parallel plate condenser. Estimates of the energy associated with this process are generally small compared with adhesion fracture energies, and the theory is much less vigorously supported than was the case some 50 years ago.

The diffusion theory has attracted increasing interest since the development of reptation theory of polymer chain dynamics. It provides a model for polymer-to-polymer adhesion, and gives an explanation of the time and molecular weight dependence of adhesion to polymers of various compatibilities.

The role of weak boundary layers is discussed with emphasis on the importance of careful investigation of the locus of failure of an adhesive bond.

In conclusion it is argued that the classical theories are best regarded as emphasizing a different aspect of a more comprehensive model which, in principle, relates molecular dispositions in the region of the interface to macroscopic properties of an adhesive joint.