Recent developments in shear rheometry of uncured rubber compounds - II. Use of the sliding cylinder rheometer to probe sample anisotropy

This paper describes a rheological investigation of specific structural fea tures of carbon black-filled rubber compounds, known as 'grain' in rubber t echnology. The grain reflects a process-induced anisotropy of certain prope rties, for instance tensile strength, orientation (or anisotropy) of the ma terial resulting from some stages of the whole processing steps that involv e strong flows, such as milling. In the experiments reported, rubber sample s sheeted off from an open mill were submitted to shear measurements in the sliding cylinder rheometer (SCR). By design, this device allows the sheari ng direction to be either parallel or orthogonal to the grain orientation o f the test sample, and an analysis of the overall anisotropy of the materia l is feasible since viscous properties are expected to vary according to th e direction of the grain. Roll-milling conditions were found to be either favourable or detrimental t owards the structural anisotropy of the compounds, depending on the cylinde r rotation speed and the friction factor. Complementary tests with a dynami c torsional rotational rheometer helped characterising the limits for de-st ructuring of the material. The interpretation of results relies on the present knowledge of rubber com pounds structure, which includes a polymer matrix and a soft 3D rubber-fill er network. Orientation mechanisms and their consequences on flow propertie s are addressed and illustrated in this study, which demonstrates the inter est of the rheological approach in investigating material microstructure. F urther applications of the method could be easily found with other filled p olymer systems. (C) 2001 Elsevier Science Ltd. All rights reserved.