Effect of hardness on the foaming behavior, surface quality, and mechanical properties of thermoplastic polyurethane foams by chemical foaming injection molding

Although the influence of hard segment content on the foaming behavior of thermoplastic polyurethane (TPU) has been reported in the literature, most of the studies have been limited to batch foaming, and the relationship between hardness on the foaming quality and the surface quality of TPU is still unclear in injection molding foaming. In this study, three hardnesses of TPU foams were prepared by core‐back injection foaming technique to investigate the relationship between hardness and TPU foaming behavior. Firstly, the relationship between hardness and viscosity of TPU was investigated using rotational rheometer. The results showed that the complex viscosity increased with the increase of hardness, especially the viscosity of high hardness TPU responded more significantly at low frequencies. In addition, a scanning electron microscope was used to analyze the cell morphology of TPU foams with different hardness. It was revealed that the cell morphology of 1098A foam with low hardness was deteriorated and deformed severely, with a cell size and cell density of 172.02 μm and 0.59 × 105 cells/cm3, respectively. With the increase of hardness, the cell morphology of high hardness 1071D foam was regular and rounded, its cell size decreased to 95.98 μm and cell density increased 4.43 × 105 cells/cm3. Surface roughness and mechanical experiments showed that the medium hardness 1065D foam had better surface quality and tensile toughness. The elongation at break of the TPU foams decreased with increasing hardness, while the high hardness 1071D foam had better tensile and flexural strength. This study provides understanding for the selection of a suitable hardness to prepare TPU foams with favorable foaming quality and corresponding mechanical properties.Highlights TPU foams of different hardness were prepared using core‐back chemical foaming injection molding technology. High hardness TPU foam had excellent tensile strength and flexural strength.