Thermoplastic-Guayule Rubber Blends-Rheological Properties

Abstract

Abstract An experimental study of the flow characteristics of blends of natural Guayule rubber with high-density polyethylene was carried out, and the effect of crosslinking and carbon black loading of the rubber phase was examined. It was observed that (1) an increase in shear rate decreases the viscosity and increases the extrudate swell of the blends; (2) an increase in temperature decreases the viscosity of the blends; (3) the viscosity and extrudate swell of the blends appear to be nonadditive functions of the viscosity and extrudate swell of the homopolymers and are lower than predicted by simple additivity; (4) the viscosity increases with increasing degree of crosslinking of the rubber phase; (5) the viscosity increases with increasing carbon black content in the rubber phase, the increase being more pronounced when the rubber-black compound is the continuous phase; (6) when crosslinking was effected, extrudate swell varied in a very peculiar manner, which requires further study. Based on the data presented, it was assumed that in these blends, both components present the same tendency to form the continuous phase at about 60% GR. The formation of a sheath-type configuration, with the component of lower viscosity forming the sheath, was suggested as one of the reasons for the low viscosity of the blends. Finally, it is concluded that as these blends present a relatively low viscosity (the melt flow index at 190°C, with a 5 kg weight, of a 50% GR is about 11 ), they would be suitable for processing in extrusion and/or injection molding. These properties, combined with the mechanical properties mentioned in the accompanying paper, make GR-HDPE blends useful as thermoplastic rubbers and, therefore, offer a promising alternative in the commercialization of natural guayule rubber.