Adhesion theory - Adsorption
The adsorption theory states that adhesion results from intimate intermolecular
contact between two materials, and involves surface forces that develop between
the atoms in the two surfaces.
This theory is the most important mechanism in achieving adhesion
[1]. The most
common surface forces that form at the adhesive-adherend interface are van der
Waals forces. In addition, acid-base interactions and hydrogen bonds, generally
considered a type of acid-base interaction, may also contribute to intrinsic
adhesion forces
[2-3].
Research [4-5]
has experimentally demonstrated that the
mechanism of adhesion in many adhesive joints only involves interfacial secondary
forces. The calculated attractive forces between two surfaces are considerably
higher than the experimentally measured strength of adhesive joints; this discrepancy
between theoretical and experimental strength values has been attributed to
voids, defects or other geometric irregularities which may cause stress concentrations
during loading [6].
To obtain good adsorption, intimate contact must be reached such that van der
Waals interaction or the acid-base interaction or both take place; hence good
wetting is essential. According to Young's equation, the surface tensions (liquid/vapor:
LV, solid/liquid: SL and solid/vapor: SV) at the three phase contacts are
related to the equilibrium contact angle 
through:
The one important factor that influences the adhesive
joint strength is the ability of the adhesive to spread spontaneously on the
substrate when the joint is initially formed
[7].
For spontaneous wetting to occur:
Angle of
contact of a drop of liquid with the surface of a solid object
We
can say that for good wetting: SV < LV
Generally speaking, the liquid surface
tension of the adhesive should be less than the critical wetting tension of the
solid surface of the substrate.
