The effects of processing and microstructure on the tensile behavior of microcellular foams

An experimental investigation of the processing, microstructure and tensile properties of microcellular foams is presented, offering novel insights into several microcellular foam systems, and a framework for further evaluation of the properties of microcellular foams.; The polycarbonate-CO{dollar}sb2{dollar} steady state process space was characterized, establishing processing conditions where a wide range of microcellular microstructures can be produced. Foam density decreased linearly as a function of foaming temperature for a wide range of saturation pressures, providing excellent control over this important microstructural variable. Cell nucleation densities ranged from 10 million to 100 billion cells in each cubic centimeter of the original polymer. Cell nucleation was found to be relatively insensitive to foaming temperature, but was strongly affected by saturation pressure and temperature. The average cell size varied between 2 {dollar}mu{dollar}m and 50 {dollar}mu{dollar}m depending upon the processing conditions.; Novel insights regarding the formation of an integral, unfoamed skin on microcellular foams were made. It was determined that the skin thickness is affected by the foaming temperature and desorption time. A model for the skin thickness was proposed, based on the diffusion of gas from the polymer, and the hypothesis that the minimum gas concentration for foaming is a function of foaming temperature. It was determined that the minimum gas concentration for foaming can be determined directly from the steady-state process space. The model showed good agreement with experiments for a wide range of desorption times and several foaming temperatures.; Interestingly, it was determined that microcellular foams can be produced that have the same density and different average cell size by employing different processing conditions. This control over the foam microstructure opened the possibility of exploring the effect of cell size on the properties of these unique materials. It was found that the average cell size does not affect any of the tensile properties of the typical microcellular structures produced for this study, leaving the volume fraction of the voids, or equivalently, the relative density, as the primary microstructural variable of concern.; A comprehensive investigation of the tensile behavior of microcellular polycarbonate was conducted. In addition, the tensile properties of microcellular ABS and PET systems were explored to a lesser extent. It was found that many of the engineering properties of micro-cellular PC are strain rate independent for nominal strain rates spanning four decades. It was also found that all of the tensile properties, including stiffness, yield stress, stress at break, and toughness, decrease as the foam density decreases. The relative tensile properties were all found to lie in the region below the rule of mixtures (i.e. the line: relative property = relative density). Furthermore, despite their relatively small thicknesses, typically less than 2 mm, the strength and stiffness of microcellular PC, PET, and ABS foams were found to be comparable to those of structural foams, in marked contrast to the excessive loss of strength observed in thin foams produced by the traditional processes.; The tensile modulus and yield stress of microcellular foams was modeled using Mori-Tanaka's mean field theory. This model is unique since it does not require any semi-empirical constants to be determined, as is required in other foam tensile modulus models. The Mori-Tanaka model was shown to predict the tensile modulus and yield stress of high relative density foams well. (Abstract shortened by UMI.)...