Conversion of polystyrene foams into auxetic metamaterials

Abstract

AbstractAuxetic foams have gained popularity within the research community because of their enhanced properties, such as low density combined with high relative stiffness, toughness, and damping properties. Low density polystyrene (PS) foams are commonly used in the packaging industry, but have a short service life and generate a high volume of waste (white pollution). This is why their recycling and valorizing is necessary and imperative. The objective of this work is to present a simple and environmentally friendly process combining low pressure (vacuum) and mechanical compression to convert recycled PS foams (15 kg/m3) into low density foams (50–63 kg/m3) having negative tensile and compressive Poisson's ratios (NPR). The effect of processing conditions (vacuum level, temperature, mechanical pressure, and time) were studied. Based on the optimized conditions, the tensile Poisson's ratio of the resulting auxetic foams reached −0.65 for the Y direction (width) and −0.74 for the Z direction (thickness) when stretching in the X direction (length). On the other hand, the minimum compressive Poisson's ratios were −0.32 for the Y direction and −0.28 for the Z direction. The foam structure was characterized via morphological analysis to determine the changes after treatment. Finally, tensile and compressive properties (Young's modulus, strain energy, energy dissipation, and damping capacity) are also discussed. It was observed that the mechanical properties of the resulting auxetic foams were improved compared to the original PS foam (PS‐O) in terms of resilience and strength. For example, the elongation at break of auxetic foams (31%–62%) were much higher compared to PS‐O (7%). These auxetic foams can be used in several applications, such as sports and military protective equipment.