Wetting Kinetics and the Strength of Adhesive Joints

The work of adhesion, depending solely on the contact angle and surface tension, is unreliable as a guide to the strength of a joint. The kinetics of joint formation, exemplified by the rate of wetting, is also important and can be measured by rate of change of contact angle, a process for which an empirical equation has previously been suggested. A proposed mechanism introduces a viscosity term into the differential form of this equation.A series of metal lacquers was prepared from epoxy, U/F and M/F resins in various ratios and also epoxy/P/F and epoxy/acrylic mixtures. Stainless steel substrates, coated with these lacquers were then cemented together using polyethylene as an adhesive. It is shown that joint strength correlates better with a high wetting constant (γ/ηL) than with a low contact angle except where this is very low.