The effects of nanoclay and deformation conditions on the inelastic behavior of thermoplastic polyurethane foams

Under cyclical loading–unloading compression conditions, flexible thermoplastic polyurethane (TPU)/nanoclay composite foams exhibit inelastic behaviors consisting of stress softening, hysteresis loss, and residual strain. In this research, the inelastic behavior of TPU and TPU/nanoclay composite foams was investigated by incorporating different amounts of nanoclay and maximizing strain deformation conditions. As the maximum compression strain was increased from 20% to 40%–60%, the inelastic behavior of TPU neat resin foams (TPUNR) with the same microcellular structures became more profound. Transmission electron microscopy (TEM) and compression testing results indicated that the nanoclay fillers were dispersed in the cell walls (struts). Fully exfoliated nanoclay increased the material's compression strength from 0.13 MPa for TPUNR foam to 0.16 MPa for TPU/5 wt% nanoclay foam (TPUNC5) and 0.36 MPa for TPU/10 wt% nanoclay foam (TPUNC10). With increasing nanoclay content, the relative hysteresis loss, relative residual strain, and relative stress softening of the TPU/nanoclay composite foams decreased markedly. Cell deformation mechanism analyses revealed that the dispersed nanoclay in the cell walls effectively reinforced the foams and retarded the permanent deformation of the cell walls, thereby reducing the inelastic behavior of the TPU/nanoclay foams. That is, the addition of organically modified nanoclay provided an effective way to tune the inelastic behavior of TPU and TPU/nanoclay composite foams.