Mechanics of adhesive failure. II

Abstract

In the preceding Part I of this investigation a relation was derived among the measured failure energy, θ, the energy dissipated viscoelastically during joint separation, and an intrinsic failure energy θ0, for various rubber-to-polymer substrate joints. For some joints, θ0 was equal to the thermodynamic work of adhesion WA, but for others θ0 ≫ WA. By the use of a variety of microscopical and spectroscopical techniques it is shown in the present paper that when θ0 ≈ WA, joint failure is wholly interfacial, but that θ0 ≫ WA when substantial cohesive failure occurs during joint separation. It is shown that the intrinsic failure energy (which controls the total failure energy under given conditions) may be expressed as θ0 = iI + rJ0 + 8F, where i, r and s are the area fractions of interfacial failure, rubber cohesive failure and substrate cohesive failure respectively and I, J0 and F are the corresponding failure energies (per area). For purely interfacial failure, i = 1 and I = WA. For strong joints, however, about 70 to 80 % of the value of θ0 is provided by the term rJ0. The departures from interfacial failure, which occur only with etch-treated substrate films, can be attributed to covalent bonding across the interface during cure of the elastomer. The reactive groups in the substrate are C=C double bonds produced by the etching treatment.