A novel semi-continuous preparation mode of ultra-low density thermoplastic polyurethane foam

The production of elastomer foam commonly faces challenges like low production efficiency, limited controllability, uneven foaming, and inherent shrinkage characteristics, especially in manufacturing thick foam sheets. Addressing these, this study introduces innovative "semi-continuous foaming with ultra-low-pressure" (SFU) strategy that integrates low-temperature gas-locking, conduction-microwave heating, and N-2-assisted gas-exchange, aimed at optimizing the entire process of TPU foam production, from manufacturing and storage to post-processing. The low-temperature gas-locking scheme realizes semi-continuous preparation of elastomer foam; the conduction-microwave heating scheme significantly improves the heating speed and uniformity; and the N-2-assisted gas exchange scheme prevents elastomer foam shrinkage, facilitating the creation of high expansion ratio foam. This process offers the flexibility to adjust initial cell sizes (5-30 mu m after shrinkage) and expansion ratios (1-24 times) by manipulating parameters such as microwave radiation time, power, and saturation time. Furthermore, the foam was prepared under ultra-low pressure (<7 MPa), resulting in a final expansion exceeding 14.2 times TPU thick sheet (5 mm thickness before foaming) foam with excellent resilience (energy loss coefficient similar to 10 % and ball resilience similar to 68.8 %). This study explores the mechanism by which TPU can self-absorb during microwave heating and extends this process to TPU materials with varying thicknesses (16.5 mm thickness before foaming) and chain compositions, as well as different types of elastomeric materials or materials with elastomer-like properties (e.g., PBAT, etc.). The SFU strategy not only significantly reduces energy and gas consumption but also steers the technological advancement of foam production towards a greener and more sustainable future.