The effect of pressure on the wall slip of linear polyethylene

Experiments were performed on a slit die rheometer to determine the effect of pressure on the wall slip of linear polyethylene. A novel shear stress transducer was developed to measure local wall shear stress at several locations along the die. It was found that above the critical shear stress for the onset of slip, the shear stress distribution along the die was nonuniform. Furthermore, it was discovered that the local wall shear stress near the exit of the die was greater than the nominal, or length-averaged, shear stress within the die as calculated from the pressure drop. Teflon$ sp circler$ wall coatings were used to promote slip by lowering the critical shear stress for the onset of slip. Relative local slip velocities were calculated and found to be a function of pressure and wall shear stress.A new semi-empirical model for the pressure dependence of slip was developed based on the effect of pressure on the work of adhesion and the work needed for flow. The new model indicates that pressure can both suppress and promote slip depending on the level of stress involved. At low pressures, and for a given shear stress, slip is markedly suppressed due to the increase in the work of adhesion. As pressure increases, however, the work needed for flow overcomes the work of adhesion, and slip dramatically increases. However, at higher pressure, the effect of pressure on slip becomes weaker. Numerical simulation results with the new model predict the existence of a local maximum in the shear stress distribution along the die for flow with slip