Auxetic Polyurethane Foam (Fabrication, Properties and Applications)

Modern technology requires new materials of special properties. For the last two decades there has been a great interest in a class of materials known as auxetic materials. An auxetic material is a material that has a negative Poisson's ratio which means that this material expands laterally when they subjected to a tensile force unlike most of the other traditional materials. This material has superior properties over the traditional material such as high shear modulus and high impact resistance, which makes this material a good candidate for many engineering applications. In the present research work, auxetic flexible polyurethane polymeric foams having different densities were fabricated from conventional flexible polyurethane polymeric foam at different compression ratios. The microstructure of conventional and processed foams was examined by optical microscope to compare between the two structures. The microstructure of processed foam was compared with the one presented in the literature and it has shown the auxetic structure configuration. This is the first time to produce auxetic foam in Egypt. Conventional and auxetic foam samples having cylindrical and square cross-sections were produced from foams having different densities (25 kg/m³ and 30 kg/m³). The compression ratios used to produce the auxetic samples are (5.56, 6.94 and 9.26). Four mechanical tests were carried out to get the mechanical properties for both conventional and auxetic foams. Two quasi-static mechanical tests ^tension and compression^and two dynamic mechanical tests ^Hysteresis and resilience^were carried out to compare between the conventional and auxetic foams. The quasi-static tensile test was carried out at speed was adjusted to be position control rate of 0.2 mm/s. The compression and hysteresis tests were carried out at strain control rate of 0.3 S^-1. The data recorded from the machine were stress and strain. The modulus of elasticity and Poisson's ratio of the test samples were obtained from tensile and compression tests. Poisson's ratio of the test samples was measured using video measurements using a dedicated Matlab and Get Data Graph Digitizer programs. Generally, the auxetic behaviour was observed for most of the processed foam. It has been observed for all compression ratios and the yellow and the grey foam only. The obtained values of Poisson's ratios was between -0.27 and 0.74. The value of the modulus of elasticity for auxetic foam was lower than the conventional foam. For example the grey auxetic foam (B) with a compression ratio of 50% has a modulus of elasticity of 30.02 kPa which is lower than the conventional foam sample (A) by 77.3 %.