Adhesion modification by shear-enhanced localized interfacial segregation in model reaction injection moulded (RIM) polyurethanes

—Polyurethane (PU) adhesives are generally multiphase systems and thus naturally consist of materials or phases of intrinsically different chemical and mechanical properties. We describe here a rheological technique (annular RIM system) which has been used to alter the surface chemistry and mechanical properties, and hence the adhesion of the materials, by imposing a shear strain generated during their fabrication. The results of ATR-IR, WAXS, DMTA, HPLC, friction and adhesion measurements are described to assess the degree of phase segregation developed as a function of the shear strain. In the case of simple PU systems, both the degree of phase segregation and the adhesion were found to increase progressively with the introduction of extended shear strains. Additive [multiphase solid and liquid internal mould release (IMR) systems] doped PUs, on the other hand, exhibited a substantial attenuation in the adhesion following nominal shear strain. It is suggested that the observed effect arises from the viscosity difference between the phases and the stress inhomogeneity generated in the processing channel. The augmentation in the adhesion of simple PU systems is thought to occur because of a pressure-induced transverse diffusion of the more viscous soft segment, resulting in an accentuated viscoelastic loss character at the PU/metal interface. The diminution in the adhesion of the IMR doped PU systems is thought to occur due to a synergistic effect of an increased interfacial concentration of the additive species and, for multiphase solid systems, the ability of the particles to modify the mechanical properties of the polymer in the interface regions.