Rheological behavior of coir‐fiber‐filled polypropylene composites at constant shear stress

The rheological behavior of coir‐fiber‐filled polypropylene (PP) composite has been studied at constant shear stress. The shear stress versus shear rate relationship for the composite follows power law model of viscous flow. Unlike similar studies in the literature, the viscosity is treated as a stress‐independent parameter, which increases with the increase of fiber loading; but decreases with the rise of temperature. The SEM reveals that the fibers are loosely bound to the polymer matrix and the outer surface of the composite is rough and irregular, making it susceptible to high friction with the wall of the flow channel. With analogy to nth order chemical reaction, new formula has been derived for the activation energy of viscous flow, which is found to increase with the increase in the fiber content. The die‐swell ratio decreases with the increase of fiber loading, but increases with the rise of temperature. The elastics parameters of the composite such as the recoverable shear strain, the first normal stress difference, and the elastic strain induced by the stored energy in the capillary reservoir have been estimated based on the die‐swell data. POLYM. COMPOS., 36:51–61, 2015. © 2014 Society of Plastics Engineers