Flow Properties and Melt Distortion in Molten Rubber Compounds under Capillary Extrusion: Effects of Vulcanizing Systems and Fillers

Abstract

This work aimed to make use of a rate-controlled capillary rheometer for investigating the effects of vulcanizing system using various fillers on the apparent viscosity and extrudate swelling of natural rubber (NR) compounds. The results suggested that the rubber compounds exhibited a pseudoplastic non-Newtonian behavior. At any given shear rates, the viscosities of rubber compound utilizing conventional (CV) and efficient vulcanizing (EV) systems were lower than that of non-sulfur (NS) system. This was due to the occurrence of premature crosslinking at the skin layer and subsequently led to the wall slip of rubber compound during the flow in capillary die. The irregular surface and the onset of smooth surface of rubber extrudate were evidently seen, especially for CV and EV systems. This could be associated with the amount of required energy to obtain the steady state flow in the die. The results also suggested that the swelling ratio of rubber extrudate ranged from 1.2 to 2.2 and the effect of filler type was more pronounced at high shear rates above 400 s−1. In the case of silica filler (SiO2) system, the severe irregularity of rubber extrudate was observed. The lower shear rate employed to obtain the smooth surface for rubber extrudate containing 30 phr of SiO2 was possibly caused by high amount of PEG acting as an external lubricant which promoted the uniform slippage during the flow in capillary die.