Auxetic polyurethane foam: Manufacturing and processing analysi

Materials with negative Poisson's ratio are referred to as auxetic materials. They are different from conventional materials in their deformation behavior when responding to external stresses. The cross-section of the materials widens in the lateral direction when being stretched in the longitudinal direction and becomes narrower when being compressed longitudinally. While a number of natural auxetic materials exist, most auxetic materials are synthetic. They show interesting properties and have potential in several important applications. Auxetic materials exhibit better mechanical properties than conventional materials such as enhanced indentation resistance, shear resistance, toughness, damping and energy absorption capacity, sound absorption, variable permeability and capability of producing complex curvature. These properties are beneficial in a wide range of applications including personal protective equipments, sound absorbers, packaging, smart filtration, drug delivery, tissue scaffolding, seat cushioning, etc.;A wide range of auxetic materials has been synthesized. They include different polymers, metals, composites and ceramics. Among these, auxetic polyurethane (PU) foam is one of the most widely studied types of auxetic materials. Auxetic PU foams are usually fabricated by altering the microstructure of conventional foams and the unusual mechanical properties originate from the deformation characteristics of the microstructures. Three most important processing parameters in fabricating auxetic PU foam that dictate auxetic behavior are processing temperature, heating time and volumetric compression ratio.;This study addresses several important issues in the manufacturing and characterization of auxetic PU foam. First, an improved automatic measuring technique has been developed to determine Poisson's ratio of auxetic PU foam. The technique involves development of a Matlab based image processing program. The second part of the study includes an experimental design approach to identify significant processing parameters followed by optimization of those processing parameters in fabrication of auxetic PU foam. A split-plot factorial design has been selected for screening purpose. Response Surface Methodology (RSM) has been utilized to optimize the processing parameters in fabrication of auxetic PU foam. Two different designs named Box-Behnken and I-optimal designs have been employed for this analysis. The results obtained by those designs exhibit that I-optimal design provides more accurate and realistic results than Box-Behnken design when experiments are performed in split-plot manner. Finally, a near stationary ridge system is obtained by optimization analysis. As a result a set of operating conditions are obtained that produces similar minimum Poisson's ratio in auxetic PU foam.