Page 91 - Handbook of Adhesives and Sealants

Theories of Adhesion 59

not result in an increase in the mechanical strength of the adhesive
joint.

2.3 Theories of Adhesion
The actual mechanism of adhesive attachment is not explicitly de-
fined. Several theories attempt to describe adhesion. No single theory
explains adhesion in a general, comprehensive way. Some theories are
more applicable for certain substrates and applications; other theories
are more appropriate for different circumstances. Each theory has
been subjected to much study, question, and controversy. However,
each contains certain concepts and information that are useful in un-
derstanding the basic requirements for a good bond.
The most common theories of adhesion are based on adsorption,
simple mechanical interlocking, diffusion, electrostatic interaction,
and weak-boundary layers.

2.3.1 Adsorption theory
The adsorption theory states that adhesion results from molecular
contact between two materials and the surface forces that develop.
Adhesion results from the adsorption of adhesive molecules onto the
substrate and the resulting attractive forces, usually designated as
secondary or van der Waals forces. For these forces to develop, the
respective surfaces must not be separated more than five angstroms
in distance. Therefore, the adhesive must make intimate, molecular
contact with the substrate surface.
The process of establishing continuous contact between an adhesive
and the adherend is known as ‘‘wetting.’’ Figure 2.6 illustrates good
and poor wetting of an adhesive spreading over a surface. Good wet-
ting results when the adhesive flows into the valleys and crevices on
the substrate surface; poor wetting results when the adhesive bridges
over the valleys formed by these crevices. Obtaining intimate contact
of the adhesive with the surface essentially ensures that interfacial
flaws are minimized or eliminated. Poor wetting causes less actual
area of contact between the adhesive and adherend, and stress regions
develop at the small air pockets along the interface. This results in
lower overall joint strength.
Wetting can be determined by contact angle measurements. It is
governed by the Young equation which relates the equilibrium contact
angle, , made by the wetting component on the substrate to the ap-
propriate interfacial tensions:
LV cos SV SL