Solubility and diffusivity of CO2 and N2 in TPU and their effects on cell nucleation in batch foaming

Abstract Solubility and diffusivity of CO2 and N2 in thermoplastic polyurethane (TPU) were measured to distinguish the foaming behavior of TPU with different blowing agents. Measurements were conducted by magnetic suspension balance (MSB) and a high-pressure visualization chamber at temperatures from 190 °C to 210 °C and pressures up to 19.3 MPa. CO2 solubility was one magnitude higher than that of N2, and their solubilities had an inverse dependence on temperature. The foaming behavior of TPU with CO2 and N2 was observed in situ by a visualization batch foaming system, and was simulated by cell growth model. The cell density of foamed TPU increased with gas concentration, but it was higher with N2 than it was with CO2, despite the lower solubility of N2. This is different to the results for polypropylene (PP) and polystyrene (PS) foaming because of the low diffusivity and the low plasticization effect of N2 in TPU.