Investigating the role and impact of N2 as a blowing agent in the fabrication of thermoplastic polyurethane foams

Abstract

Fabricating low-density elastomeric foams with a homogenous cell structure has been challenging because of foam shrinkage. The low modulus of the elastomers and the high diffusivity of blowing agents lead to foam shrinkage and poor surface quality. Using N2 as a physical blowing agent (PBA) is an eco-friendly foaming method to overcome foam shrinkage. In this study, PTMEG-MDI/BD-based TPU with a hardness of 85A, 90A, and 95A were foamed using N2. Nitrogen as a blowing agent created a homogeneous cell structure with an expansion ratio of more than five times, an average cell size of less than 25 μm, and a cell density of more than 109 cells/cm3. The shrinkage ratio and cellular morphology of CO2- and N2-blown TPU foam were compared at their maximum expansion ratio. At the maximum expansion ratio, the N2-blown foams exhibited a fine cell structure with a shrinkage ratio of less than 4%. However, the CO2-blown TPU foam showed a shrinkage ratio of 12.7% with significant cracks in the sample. The results demonstrate that using N2 as a blowing agent can achieve an expansion ratio similar to that of CO2-blown foam, and the shrinkage problems of elastomeric foams can be significantly eliminated, which may help to maintain the physical properties of the foam.