Approach to fabricating thermoplastic polyurethane blends and foams with tunable properties by twin-screw extrusion and microcellular injection molding

Thermoplastic polyurethane (TPU) has been widely used in many applications because of its broad range of properties. However, synthesis of TPU with desirable properties is time consuming and only produces limited quantities. In this paper, twin-screw extrusion was used to produce TPU blends of varying soft and hard segments with tunable properties. Foamed samples of TPU blends were produced by microcellular injection molding. Multiple characterization tests were performed to confirm the miscibility and tunability of the blends. It was confirmed that soft and hard TPUs were compatible at ratios of 2:1 and 1:2, but partially miscible at a ratio of 1:1. Rigidity increased while the shape-recovery rate decreased with increasing the hard TPU content. The blend ratio influenced the foaming morphology, solid skin-layer thickness, as well as mechanical properties. It was found that microcellular injection molding could reduce the injection-shot volume without sacrificing too much mechanical property and, compared with their solid counterparts, the foamed blends could retain mechanical properties better than foamed pure TPUs. Decomposition and degradation behaviors were also studied. The biocompatibility of soft and hard TPU-foamed samples, which could be potentially used as tissue engineering scaffolds, was confirmed by fibroblast cytotoxicity test.