Rheological properties of short fiber filled polypropylene in transient shear flow
Résumé
The behavior of a commercial short glass fiber filled polypropylene has been studied. It is well known that a thermoplastic polymer filled with short fibers with an initial isotropic orientation state exhibits a viscosity overshoot when sheared in the parallel plate geometry of a rheometer in the molten state. In this work, following a first deformation, the sample has been sheared in the reverse direction and a small viscosity overshoot has been measured. Similarly under stress growth experiments a large normal stress overshoot was observed. When the flow was reversed the normal stresses were initially negative and then exhibited a small positive overshoot. A model has been used to simulate these viscosity and normal stress overshoots. It is based on the Folgar–Tucker equation for fiber motion and the Lipscomb constitutive equation. This equation has been empirically modified to correctly predict the overshoots.