Shear and extensional properties of short glass fiber reinforced polypropylene

Shear and extensional properties of a commercial short glass fiber reinforced polypropylene were carefully investigated using commercial rheometers and a novel on-line rheometer. This on-line slit rheometer, installed on an injection molding press, has been designed to measure the steady shear viscosity, the first normal stress difference, and the apparent extensional viscosity of polymer melts and composites for high strain rates up to 10(5) s(-1) in shear and 200 s(-1) in extension. Our results show that the steady-state viscosity measurements using the on-line rheometer are in excellent agreement with those obtained using commercial rheometers. The steady-state and the complex viscosities of the composites were found to be fairly close to that of the matrix, but the Cox-Merz rule was not verified for the composites at high rates. The elasticity of the composites was found to be equal to that of the polypropylene matrix. The apparent extensional viscosity was obtained from the pressure drop in the planar converging die of the slit rheometer using the analyses proposed by Cogswell [1] and Binding [2]. The extensional viscosity of the polypropylene was found to be much larger than the shear viscosity at low strain rates with a Trouton ratio of about 40 that decreased rapidly with increasing strain rate down to the value of 4 at 200 s(-1). The extensional viscosity of the composites was also found to be close to that of the matrix, with values 35 and 5% larger for the 30 and 10 wt% reinforced polypropylenes, respectively. These results are compared with the predictions of the Goddard model [3], which are shown to overpredict our experimental results. POLYM. COMPOS., 26:247-264, 2005. (c) 2005 Society of Plastics Engineers