Effect of nucleating agent and two-stage expansion on the morphology and mechanical properties of microcellular injection molded thermoplastic polyurethane

Improved rebound resilience while lowering bulk density is desirable in several sport-wear applications. While rebound resilience properties generally deteriorate with reduction of the bulk density of the foam, a method of increasing rebound resilience by improving the control on microstructure is explored in this study. Low density thermoplastic polyurethane (TPU)/clay nanocomposite foamed parts were prepared using twin-screw extrusion compounding followed by microcellular injection molding. Samples with two densities were created by microcellular injection molding and an optional cavity expansion at a preset time during cooling. Scanning electron microscopy, rheological analysis, uniaxial compression tests, and rebound resilience tests were conducted on both of the non-expanded and expanded samples. Presence of well dispersed nanoclay in the TPU matrix acting as a nucleating and melt strengthening agent, coupled with cell growth at lower temperature helped achieve better microstructures, especially at high density reductions. The expansion of TPU at lower temperatures with directional second-stage expansion also helped to increase the rebound resilience, while achieving softer foams and lower hysteresis loss ratios at lower densities.