RHEOLOGY OF SHORT GLASS-FIBER REINFORCED POLYPROPYLENE

An anisotropic rheological constitutive equation for short fiber reinforced polypropylene is presented. The bulk stress is the sum of a contribution of the Newtonian suspending fluid and of the fiber contribution. This contribution depends on the orientation which is described by an orientation tensor. The rate of change in orientation is calculated using semidilute suspension theories. After simplifications the model contains only one adjustable parameter and the transient relative viscosity depends only on the strain and not on the shear rate or time separately. Transient and steady-shear viscosities were determined for two reinforced polypropylenes using a constant stress and a rotational parallel plate rheometer. An overshoot in the viscosity versus time curve was observed. It is attributed to changes in fiber orientation, and it depends on the fiber content but not on the shear rate. The model predictions for 20 and 30 wt. % glass fiber reinforced polypropylene are fairly good.