Page 96 - Handbook of Adhesives and Sealants

64 Chapter Two

1. Mechanical interlocking
2. Formation of a clean surface
3. Formation of a highly reactive surface
4. Formation of a larger surface

It is widely believed that although the surface becomes rougher be-
cause of abrasion, it is a change in both physical and chemical prop-
erties of the surface layer that produces an increase in adhesive
strength. While some adhesive applications can be explained by me-
chanical interlocking, it has been shown that mechanical effects are
not always of prime importance.

2.3.3 Electrostatic and diffusion theories
The electrostatic and diffusion theories of adhesion are generally not
regarded as highly as the adsorption theory or mechanical theories.
However, there are certain applications where each is very appropri-
ate.
The electrostatic theory states that electrostatic forces in the form
of an electrical double layer are formed at the adhesive-adherend in-
terface. These forces account for resistance to separation. This theory
gathers support from the fact that electrical discharges have been no-
ticed when an adhesive is peeled from a substrate. Electrostatic ad-
hesion theory is regarded as an accepted theory for biological cell ad-
hesion. A simple form of adhesion can also arise from direct contact
electrification. This has been demonstrated for thin films of metal
sputtered onto polymeric surfaces.
The fundamental concept of the diffusion theory is that adhesion
arises through the inter-diffusion of molecules in the adhesive and
adherend. The diffusion theory is primarily applicable when both the
adhesive and adherend are polymeric, having compatible long-chain
molecules capable of movement. The key is that the adhesive and the
adherend must be chemically compatible in terms of diffusion and mis-
cibility. Solvent or heat welding of thermoplastic substrates is consid-
ered to be due to diffusion of molecules. Other than certain thermo-
plastics, situations in which the adherend and adhesive are soluble in
one another are relatively rare. Therefore, the diffusion theory of ad-
hesion can be applied in only a limited number of cases.

2.3.4 Weak-boundary-layer theory
According to the weak-boundary-layer theory, as first described by
Bikerman, 12 when bond failure seems to be at the interface, usually a