Auxetic polypropylene foams as high mechanical performance materials

Abstract

This study focuses on converting polypropylene (PP) foams into auxetic metamaterials using vacuum and mechanical compression (VMC). The PP foams (initial density of 45 kg m⁻³) underwent treatments involving heat, vacuum and mechanical pressure to create a re-entrant cellular structure essential to generate a negative Poisson's ratio (NPR). The resulting foams exhibit significantly enhanced stiffness, making them suitable for applications in sports and military protection. The effect of vacuum and mechanical compression are optimized to achieve the best auxetic properties. The treated foams are characterized for density, porosity, open cell content (OCC), cell morphology, Poisson's ratio (ν) and mechanical properties. The optimized auxetic foams have high density (100–133 kg m⁻³) and show NPR under both tensile (−0.23) and compressive (−0.08) deformation. More importantly, the toughness is improved by up to 438% compared to the original foam with a 200% increase of the elastic limit. Furthermore, the compression stress is improved by 416% at 50% compressive strain. Finally, thermo-mechanical data show that auxetic PP foams have improved properties as temperature increases.