Manipulation of pressure release histories to control foaming dynamics and shrinkage in thermoplastic polyurethanes

Post-foaming shrinkage is one of the main challenges in the production of thermoplastic elastomeric foams. This phenomenon is particularly pronounced in polyurethanes, where foam volume reductions exceeding 80 % have been reported. Shrinkage is generally attributed to both uneven gas exchange between the cells and the surrounding environment, as well as the elastic recovery of the polymer after the strain imposed during bubble growth. In this work, we focus on the latter mechanism by controlling the foam growth dynamics and, consequently, the elastic component of the deformation. To this end, a batch foaming apparatus was modified to enable systematic variations in the blowing agent release history, resulting in different foam growth durations. Experiments conducted over three orders of magnitude in pressure release time reveal notable correlations between foaming dynamics and post-foaming shrinkage. Although partial improvements were achieved, the overall reduction in shrinkage remained limited, suggesting that further strategies are required to effectively address this issue.