Effect of steam conversion on the cellular structure, Young's modulus and negative Poisson's ratio of closed-cell foam

Open-cell auxetic foams have shown potential for use in sporting and other protective equipment. Previous conversion methods for auxetic closed-cell foam used a bespoke pressure vessel. This work expands upon a recent method, using steam absorption followed by cooling and condensation, to change the cell shape of closed-cell foam to impart a negative Poisson's ratio. Aiming to present a simple, repeatable conversion method, foam samples (20 × 10 × 100 mm) floating in water-filled (~20 °C), covered ceramic containers (300 × 200 × 100 mm) were heated in an oven at 105 °C for between 1 and 6.5 hrs. Based on the results for these smaller samples, a larger sample of foam (30 × 100 × 100 mm) was also converted, with a steaming time of 8 hrs. The final volume ratio (original/final volume) increased with heating time, up to a maximum of 4.6. The amount and angle of re-entrant (inward-folding) cell walls increased with the final volume ratio as cells contracted further, as evidenced by micro-computed tomography. Poisson's ratios, measured using digital image correlation, were as low as −0.3 in tension and −1.1 in compression. The tensile Young's modulus increased from ~2 up to ~6 MPa with the final volume ratio and the compressive Young's modulus reduced from ~1.5 MPa for low values of the final volume ratio between one and three and then remained close to 1.5 MPa.