The Rheology and Processing of Olefin-Based Thermoplastic Vulcanizates

Abstract

Abstract Olefinic thermoplastic vulcanizates (OTV's) comprise rubber in the form of discrete, completely crosslinked particles uniformly dispersed in an olefinic thermoplastic matrix. By their nature, they flow like filled polymer melts: die swell is low, and shows evidence of a particle association at low shear rate and high temperature; viscosity obeys a power law relationship with shear rate over a wide range. In place of a Newtonian plateau at low shear rate, the viscosity of OTV's begins to rise precipitiously in building toward a yield stress. As with other olefinic thermoplastic elastomers, their viscosity is more highly sensitive to shear rate than to temperature. Both shear viscosity and die swell were measured for evidence of morphological character and definition of fabricability. Due to their high shear sensitivity, these OTV's can be easily processed by injection molding and extrusion. Processing temperatures are not critical, but lower ranges are preferred to optimize appearance and promote melt stability. The thixotropic nature of the melt minimizes drooling and allows easy cooldown without distortion of the formed part. Melt viscosity is little affected by moisture or repeated processing. Although melt extensibility is only moderate, it is sufficient to qualify these materials for most extrusion and blow molding applications. Low shear processing, such as the compression molding of prototype parts or calendering, is equally feasible. Thus, olefinic thermoplastic vulcanizates can be fabricated under a wide range of conditions by using conventional plastics processing tooling and methodology. They represent a versatile class of new elastomers that can be economically processed for across-the-board applications in industrial rubber products.