Hot‐rolling and forging of mica‐filled polyolefins

As in the case of fibers, the mechanical properties of plastic composites containing mica flakes are extremely sensitive to flake orientation in the direction of an applied stress, so that even a small angular displacement can cause major reductions in the strength, modulus, and fracture toughness. In order to encourage parallel alignment of mica flakes in a thermoplastic composite, two methods of flow orientation were examined. In the first series, rectangular billets of mica‐filled, high‐density polyethylene were hot‐pressed in order to cause longitudinal melt flow in a narrow channel. A parallel series of experiments was also carried out with mica‐filled polypropylene in which the composite was extruded and calendered into a thin, continuous strip. In both processing techniques, the resulting extensional flow produced large increases in the tensile and flexural properties. The performance of the mica‐filled poly‐propylene was limited by its tendency to fibrillate during rolling. Drop‐impact measurements recorded a four‐fold increase in fracture toughness. The increased tensile and flexural properties were attributed to both the greater degree of parallel alignment of the mica flakes and the increased molecular orientation in the direction of flow. Such flow orientation methods appear necessary if the full benefit of mica‐flake reinforcement is to be achieved.