Wall slip of polyisobutylene: Interfacial and pressure effects

A high-pressure sliding plate rheometer was used to investigate the flow be havior of polyisobutylene in simple shear. Experiments carried out using sm ooth steel surfaces revealed that wall slip is a dominant feature of the fl ow at the pressures, temperatures, and rates of deformation typically found in the processing of elastomers. A set of grooved plates made it possible to obtain viscosity data at stresses up to 20 kPa, but at higher stresses, the sample "slipped" even on the grooved plates. These data were fitted to a Cross viscosity model and extrapolated in order to estimate the viscosity at higher stresses. Flow curves (shear stress versus nominal shear rate) g enerated using fluoroelastomer-steel and steel-steel pairs of plates exhibi ted four distinct flow regimes: no-slip, adhesive slip, mixed adhesive, and cohesive slip, and primarily cohesive slip. Slip velocities were calculate d by comparing the no-slip stresses estimated using the Cross model with me asured values. The effect of pressure on cohesive wall slip was found to sc ale with pressure in the same way as viscosity, while the effect of pressur e on adhesive wall slip did not. (C) 1999 The Society of Rheology. [S0148-6 055(99)00405-8].