Lap Shear Adhesion of Styrenic Triblock Copolymers to Polypropylene and Polystyrene

Abstract

Shear mode adhesive strength of several styrenic triblock copolymers to Polypropylene (PP) and Polystyrene (PS) was examined as a function of midblock structure, styrene content, and molecular weight. A modified lap shear test was designed to allow adhesive strength measurement in strongly adhering adhesive joints where, in the conventional lap shear test, tensile failure in one of the adherends occurred before the adhesive strength was attained. To both substrates, the adhesive strength was highest for poly(ethylenebutylene) midblock (S-EB-S), followed by polyisobutylene midblock (S-IB-S), polybutadiene midblock (S-B-S), and finally polyisoprene midblock (S-I-S). For the S-IB-S copolymers, as the styrene content and molecular weight increased the adhesive strength increased to the PS substrate but remained nearly constant to the PP substrate. Lap shear adhesive strength was related to the nanoscale block copolymer morphology. Adhesive strength increased as the adhering phase of the triblock, styrene when bonding to PS and the rubber midblock when bonding to PP, became more continuous.