Fundamentals of Adhesion

How PSAs work from Scientific American (http://www.sciam.com/askexpert_question.cfm?articleID=000E47BD-6690-1C71-9EB7809EC588F2D7&catID=3):
" There is no chemical bonding or reaction between a pressure-sensitive adhesive and the substrate, the surface to which it bonds. A pressure-sensitive adhesive is a sticky, viscous, liquidlike material that adheres to a surface using only pressure. To function well, it should have good adhesion to a surface and good cohesion, or internal strength. For good adhesion, it is important that the adhesive can readily flow out on the surface. The degree, or freeness, of the flow often determines the intensity of adhesion. Degree of flow depends on the difference between the surface energy of the material and the surface energy of the adhesive. Pressure-sensitive adhesives tend to flow out on materials having a high surface energy--for instance, metals, glass and plastics such as acrylic, polycarbonate and nylon. Pressure-sensitive adhesives tend not to flow out as easily on low surface-energy materials such as polyethylene, polypropylene and Teflon.
Mechanisms for adhesion include both mechanical adhesion and specific adhesion. Mechanical adhesion occurs when the adhesive flows into the texture of the substrate. Specific adhesion includes electrostatic forces, van der Waals forces and acid-base interactions that take place between the adhesive and the substrate. In the case of acrylic pressure-sensitive adhesives, the intensity of the bond typically increases over time. This growing degree of 'stickiness' occurs because of the long time it takes for the adhesive to flow onto and into the surface texture, and for the adhesive polymer to align with the surface to create the interactions. "

Mechanisms for Adhesion from http://www.specialchem4adhesives.com/resources/adhesionguide/index.aspx?id=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. Generally speaking, the liquid surface tension of the adhesive should be less than the critical wetting tension of the solid surface of the substrate.

Chemisorption:

Mechanical Interlocking:

Diffusion:

Electrostatic: