ABSTRACT
A starting point for quantitative adhesion theories is the work o f adhesion WA, ex pressed as:
(1) The energy WA, o f the adhesion between a solid substrate S and a wetting liquid L, follows from the surface energies (in mJ/m2) ys and yL and from their interfa cial energy ysl-WA can be determined from contact angle measurements using groups o f standard liquids [1-2]. The idea behind the work of adhesion is that when the solid and the liquid are separated, two free surfaces (of the solid S and of the liquid L) are created out o f an equal area of contact which had, originally, the interfacial energy ySL·
Polymer molecules which are attached to the substrate surface are instrumental in the strength o f an adhesion bond. When forces are applied to separate the bond, there is a concentration o f stresses at the interface. The stored mechanical energy is released when a crack develops. Propagation o f the crack can be stopped, and the bond is saved, when the stresses are redistributed away from the tip o f the
The polymer molecules can only have such an effect if they are fixed strongly onto the surface. This fixation is the chemical secret in the formulation of adhe sives for different materials. Pre-treatment procedures are designed for the same purpose: to optimize the strength of the bond, by attaching the polymers to the bonding substrate. But the mechanism which determines the strength of the bond is not very well understood, as yet, from a theoretical point of view. New observa tions about the adhesion between polymers and solid surfaces [3] indicate why theories of adhesion which are based on the value of WA [4] have always had se rious trouble to adequately predict the strength of the bond between a polymer and the surface of a solid material. This is because the effective distance H be tween the polymer and the substrate has been overlooked as a variable. It can vary by an order of magnitude, depending on experimental conditions which affect polymer adsorption.
