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Effect of a cross-linking agent on the foamability of microcellular injection molded thermoplastic polyurethane

Thermoplastic polyurethane is one of the most versatile thermoplastic materials being used in a myriad of industrial and commercial applications. Thermoplastic polyurethane foams are finding new applications in various industries including furniture, automotive, sportswear, and packaging because of their easy processability and desirable, customizable properties. Low bulk density and a good foam microstructure are important properties that affect the mechanical properties, economics, and performance of the final product. In this study, the effect of a cross-linking agent on the foamability of microcellular injection molded thermoplastic polyurethane was studied with an aim to reduce the bulk density while achieving a consistent microstructure. Gel permeation chromatography showed an increase in the weight average molecular weight by 5.0% with the addition of a cross-linking agent. Rheological studies on the materials showed that the addition of a cross-linking agent increased the storage modulus and viscosity, while reducing the tan δ value. Using microcellular injection molding, cross-linked thermoplastic polyurethane could be foamed to a minimum density of 0.159 g/cc at the higher end of the processing window, as compared with a minimum density of 0.193 g/cc for pure thermoplastic polyurethane foam. Scanning electron microscope analyses of the foamed parts showed a bimodal foam structure for thermoplastic polyurethane with a cross-linking agent and a more integral foam structure with less cell coalescence even at higher density reductions.