Effects of cell structure and density on the properties of high performance polyimide foams

Activity at the NASA Langley Research Center (LaRC) has focused on developing low density polyimide foam and foam structures which are made using monomeric solutions or salt solutions formed from the reaction of a dianhydride and diamine dissolved in a mixture of foaming agents and alkyl alcohol at room temperature. Monomer blends may be used to make a variety of polyimide foams with varying properties. The first foaming process developed consisted of thermal cycling the polymer precursor residuum and allowing the inflation of the particles to interact to create the foam. This process has resulted in foam structures with higher percentages of open cell content. Another innovative foaming process has been developed that begins with partially inflated microspheres, friable balloons, with incomplete polymer molecular weight gain, which when fully cured into a foam results in more closed cell structures. In a research study performed by NASA Kennedy Space Center (KSC) and LaRC, two closely related polyimide foams, TEEK-H series and TEEK-L series, (4,4'-oxydiphthalic anhydride/3,4-oxydianiline and 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride/4,4-oxydianiline) were investigated for density effects and closed versus open cell effects on the thermal, mechanical, and flammability properties. Thermal conductivity data under the full range of vacuum pressures indicate that these materials are effective insulators under cryogenic conditions. Contributing factors such as cell content, density, and surface area were studied to determine the effects on thermal conductivity. Cone calorimetry data indicated decreased peak heat release rates for the closed cell system, TEEK-H friable balloons, compared to the TEEK foams with higher open cell content. Mechanical properties including tensile strength and compressive strength indicated that the materials have good structural integrity. Foams with more open cell content resulted in greater tensile and compressive strengths than the closed cell foams. The maximum closed cell content achieved in the friable balloon system was 3 78% at a foam density of 0.048 gm/cm(3). Published in 2005 by John Wiley & Sons, Ltd.