Development of Elastomeric Auxetic Structures for Impact Energy Absorption

Abstract

Elastomeric sponge and auxetic materials have unique characteristics which distinguish them from other solid materials like metals and ceramics. Such engineering materials with unique performance are a continuous requisite so as to assist the evolution for advance engineering applications. Auxetic materials (materials with negative Poisson’s ratio) have a unique feature of getting fatter when pulled and contract transversely when compressed, longitudinally. They are mainly used for their improved indentation resistance, higher fracture toughness, better thermal shock resistance, and good acoustic damping. Twenty diverse compositions were developed with various reinforcements’ incorporation such as carbon black nano particles etc., varying blending ratios of natural rubber (NR), styrene butadiene rubber (SBR), and nitrile butadiene rubber (NBR) in ethylene propylene diene monomer (EPDM). Sponge and auxetic structures were developed during vulcanization a hot isostatic biaxial hydraulic press by volumetric compression and cooling under pressure. The reinforcements were impregnated into the elastomeric matrices using internal dispersion kneader and two-rolls mixing mill. Seven types of mold geometries were designed and used as per ASTM standards to fabricate nanocomposites using a hot press in order to evaluate the elastomeric composites for rheological, mechanical, structural, vulcanization, compressive strain, dynamic mechanical thermal analysis and impact energy absorption applications. Mechanical characteristics were executed using Universal Testing Machine (UTM), Dynamic Mechanical Thermal Analyzer (DMTA) and rubber hardness tester. Scanning electron microscopy coupled with EDS were used to evaluate sponge and auxetic structures. The synthesized auxetic materials verified by scanning electron images and Poisson’s ratio determined by processing images using ‘Matlab’ software. With the advances in the fabrication and synthesis of a wider range of these thrilling materials, there is enormous potential for applications in industrial and commercial sectors. Among the various rubber systems investigated EPDM based elastomeric composites proved to be the best for development of auxetic structures. Hardness of the EPDM sponge composite with 20% incorporation of carbon black nano fillers enhanced up to 112%. EPDM with the addition of 20% carbon black attained tensile strength 142%, compared to EPDM without reinforcement; this nanocomposite also showed a 20% reduction in impact energy absorption. EPDM-30%NR reinforced with 20% carbon black showed a minimum reduction of 33% in storage modulus indicating good retention of elasticity. Maximum enhancement of 400% loss modulus was obtained in case of EPDM-30%SBR reinforced with 20% carbon black. Rheological bahaviour such as loss tangent during vulcanization enhanced up to 130%, 300%, 85%, 11% by incorporation of 30% carbon black, 30% NBR, 30% NR, and 30 % SBR.