Double-arrowhead castor-oil biobased polyurethane foam metamaterials

Environmental concerns surrounding petrochemical-based materials are constantly increasing and they are pushing the industry to find renewable sources and sustainable solutions. Recent years have also witnessed a focus on the development of multifunctional and metamaterials such as auxetic structures. This work describes the manufacturing of castor-oil-biobased polyurethane foams and their use as platforms to make double-arrowhead auxetic (DAH) metamaterials via laser ablation. A microstructural analysis is conducted on the biobased foam, revealing a micro-cellular structure formed by rounded and closed cells. The proposed DAH structures are innovative because they possess two different micro and macro cellular scales. The effects of the geometric parameters (angles) of the macro-cells of the DAH structures on the equivalent density, elastic modulus, yield stress, ultimate stress, modulus of toughness, and Poisson ratio are investigated. Configurations with a lower unit cell α angle of 45° exhibited improved properties, including the maximum tensile performance (yield stress of 33.2 kPa, ultimate stress of 38.9 kPa, and modulus of toughness of 10.1 μJ/mm3) and the lower negative value for the Poisson ratio (-0.16), although with a higher density (70 kg/m3). In contrast, DAH structures made with a 70° α angle resulted in a reduced equivalent density of 48 kg/m3 and increased specific strength (0.61 kPa.m3/kg). In terms of specific stiffness, the 45° α DAH structures outperform the other configurations, with a specific elastic modulus close to 10 kPa.m3/kg.