Effect of CO2-philic surfactant on adhesion behavior and foamability of thermoplastic polyurethane (TPU) in water-free foaming process

Abstract

Carbon dioxide (CO₂) water suspension foaming can produce complex special-shaped components foamed products with microcellular and low-density bead foams. The traditional water suspension foaming process has wastewater, complexity, high energy consumption, and hydrolysable polymer degradation. Based on the above problems, the water-free bead foaming process in spouted bed is developed, which has simple process and high efficiency. However, due to the differences in molecular structure of different polymers and the different plasticization effect of CO₂ on polymers, there are differences in the adhesion behavior of polymer beads in the foaming process. In this work, thermoplastic polyurethane (TPU) beads were selected as raw materials, and the effects of different CO₂-philic surfactants on the adhesion and foaming behavior of TPU beads were investigated. Molecular dynamics (MD) simulations and experiments showed perfluorooctanoic acid (PFOA) can inhibit the adhesion of TPU foamed beads. Under 4 wt% PFOA, non-adhered TPU foamed beads with expansion ratio is 10.75, cell size is 49.83 μm and cell density is 4.30 × 10⁷ cells/cm³ can be prepared. Then, the shrinking TPU foamed beads are foamed twice by nitrogen (N₂), and expansion ratio is over 14 times, cell size is 60.06 μm, and cell density is 5.23 × 10⁷ cells/cm³. Through this method, achieving non-adhered TPU foamed beads characterized by high expansion ratio, high cell density, and small cell size. Finally, PFOA was efficiently recovered and reused via flash evaporation, achieving a recovery ratio exceeding 97.69 %. It’s worthy that minor losses may still present environmental and health concerns.