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

Abstract This paper describes a rheological investigation of specific structural features of carbon black-filled rubber compounds, known as ‘grain’ in rubber technology. The grain reflects a process-induced anisotropy of certain properties, for instance tensile strength, orientation (or anisotropy) of the material resulting from some stages of the whole processing steps that involve strong flows, such as milling. In the experiments reported, rubber samples sheeted off from an open mill were submitted to shear measurements in the sliding cylinder rheometer (SCR). By design, this device allows the shearing direction to be either parallel or orthogonal to the grain orientation of the test sample, and an analysis of the overall anisotropy of the material is feasible since viscous properties are expected to vary according to the direction of the grain. Roll-milling conditions were found to be either favourable or detrimental towards the structural anisotropy of the compounds, depending on the cylinder rotation speed and the friction factor. Complementary tests with a dynamic torsional rotational rheometer helped characterising the limits for de-structuring of the material. The interpretation of results relies on the present knowledge of rubber compounds structure, which includes a polymer matrix and a soft 3D rubber-filler network. Orientation mechanisms and their consequences on flow properties are addressed and illustrated in this study, which demonstrates the interest of the rheological approach in investigating material microstructure. Further applications of the method could be easily found with other filled polymer systems.