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 θ 0 , for various rubber-to-polymer substrate joints. For some joints, θ 0 was equal to the thermodynamic work of adhesion W A , but for others θ 0 ≫ W A . By the use of a variety of microscopical and spectroscopical techniques it is shown in the present paper that when θ 0 ≈ W A , joint failure is wholly interfacial, but that θ 0 ≫ W A 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 θ 0 = iI + rJ 0 + 8 F , where i , r and s are the area fractions of interfacial failure, rubber cohesive failure and substrate cohesive failure respectively and I , J 0 and F are the corresponding failure energies (per area). For purely interfacial failure, i = 1 and I = W A . For strong joints, however, about 70 to 80 % of the value of θ 0 is provided by the term rJ 0 . 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.