Auxetic foams for sports applications

This thesis assesses whether current auxetic foams can improve the performance of sporting protective equipment, and lays out steps to realise their commercial potential. A wide range of conversion temperatures (120 °C to 200 °C) and times (20 to 180 minutes) for ~3 x 3 x 9 cm conversions of polyurethane foam with volumetric compression ratios (VCRs) of 2 or 3 changed polymeric bonding, fixed imposed compression, and changed their mechanical properties. Effects of conversion time and temperature were approximately interchangeable, and are summarised as heat exposure. As predominantly hydrogen bonding between urea segments increased with heat exposure, shape fixing (final volume ratio, FVR) also increased. Shape fixing of imposed compression (i.e. to an FVR of ~2 in samples with a VCR of 3) caused anisotropic foams to become re-entrant and isotropic, initially reducing Young's modulus and Poisson's ratio from ~50 kPa to ~30 kPa and ~0.3 to ~-0.2, respectively. Further heating increased hydrogen bonding, did not change isotropy, continuously increased Young's modulus to ~120 kPa and Poisson's ratio increased to an approximate plateau at zero. The foams described above, and the conventional parent foam, were indented by two cylinders (10 and 50 mm diameters) and a stud (12 mm diameter). A value (x) connecting elastic properties to indentation resistance in Hertzian indentation theory was calculated for each indenter. Integrating force vs displacement, giving energy absorption, mitigated non-linearity. During cylindrical indentations, x was higher (0.6 to 0.8) than the expected 0.33. During studded indentations x was 0.91, close to its expected value of 1 after outlying unconverted samples were excluded. Digital image correlation showed densification was significantly higher for auxetic samples (α > 0.95), which deformed with a flatter surface for the 10 mm cylinder and stud (α > 0.95). Densification and flatter deformation could have increased x during cylindrical indentations and caused unconverted samples to be outliers in studded indentations. To utilise improvements in often large area PPE, sheets (30 x 30 x 2 cm) of auxetic foam were produced with internal compression controlled and varied using through thickness rods. The sheets fabricated with graded compression levels displayed clearly defined quadrants of differing cell structure and mechanical properties, shown through analytical modelling to be fully consistent with expectations from honeycomb theory. Isotropic sheets and quadrants had a maximum magnitude of NPR of ~-0.1, and Young's modulus of ~50 kPa. Anisotropic quadrants had direction dependant Poisson's ratios as low as ~-0.4 and yet Young's moduli similar to open cell foam (~30 kPa in tension and up to ~5% compression, ~0 kPa beyond ~10% compression). Finally, steam processing produced closed cell foams with a Poisson's ratio of ~-0.1 and Young's modulus (~1 MPa) similar to closed cell foam in sporting PPE.