Characterization methods to predict extrusion performance in thermoplastic polyurethane batches

Abstract

Thermoplastic polyurethanes (TPUs) are a class of urethane elastomers which flow at elevated temperatures, enabling a diverse set of processing techniques including industrial-scale extrusion and injection molding. Due to variations in the relative content of hard and soft segments within the polymer chains, as well as block lengths and chemistries, this material exhibits a wide range of viscoelastic properties depending on the synthetic process and thermal history. Numerous manufacturers produce their own proprietary brands of TPU, thus creating a diverse market for a product whose processability varies depending on the supplier. Lot dependent undesirable characteristics may be observed during melt extrusion, including degradation at process-relevant temperatures, increases in viscosity over time, and buildup of solid-like particulates during extrusion. This manuscript implements a combination of chemorheological, spectroscopic, solubility, light scattering, and calorimetric techniques to understand the formation of this solid particulate phase during extrusion. The presence of endothermic transitions, large degrees of branching, rapidly changing viscoelastic properties, and poor dissolution capabilities were correlated with the worst performing lots of TPU in terms of extrudability. These findings suggest that solidification is caused by physical crosslinking through hard phase aggregation, which is attributed to differences in polymer branching architecture, high chain mobility, and strain under extrusion conditions. The strong association between polymer branching and solidification during extrusion highlights the capability for these characterization techniques to rapidly screen for extrusion-related performance of TPUs prior to processing.