Publication

Abstract : In this investigation, attempts are made to modify space durable polymer such as polybenzimidazole (PBI) (service temperature ranges from -260 °C to +400 °C, and also exhibits extreme fire and high energy radiation resistance) through low-pressure plasma inorder to prepare composite with the same polymer. The Polybenzimidazole composites are prepared using an ultra high temperature resistant epoxy adhesive in order to join the two polymer sheets. The service temperature of this adhesive ranges from -260 °C to +370 °C and, in addition, this adhesive has excellent resistance to most acids, alkalis, solvents, corrosive agents, radiation and fire, making it extremely useful for space applications. Prior to preparing the composite, the surface of the polybenzimidazole is ultrasonically cleaned by acetone followed by its modification through low-pressure plasma through 13.56 MHz RF Glow Discharge for 30, 60, 120, 240 and 360 seconds at 100 W of power using nitrogen as process gas. Prior to adhesive bonding, the polymer surfaces are characterized by estimating surface energy and Electron Spectroscopy for Chemical Analysis (ESCA). It is observed that polar component of surface energy leading to total surface energy of the polymer increases significantly up t o 120 seconds of exposure and then it saturates. In order to determine the joint strength, tensile lap shear tests are performed according to ASTM D 5868-95 standard. It is observed that joint strength increases significantly with increasing exposure of low pressure plasma up to 120 seconds followed by its saturation. Another set of experiments is carried out by exposing the low-pressure plasma modified polymer joint, under the SLOWPOKE-2 safe low power critical experiment) nuclear reactor for 6 hours, which produces a mixed field of thermal and epithermal neutrons, energetic electrons and protons, and gamma rays, with a dose rate of 37 kGy/hr. It is observed that there is further significant increase in joint strength, when the polymer surface is first modified by low-pressure plasma of 120 sec of exposure followed by exposing the joint under high-energy radiation. In order to simulate with spatial conditions, the joints are exposed to cryogenic (-196 °C) and elevated temperature (+300 °C) for 100 hours. Then tensile lap shear tests are carried out in order to determine the effects of these environments on the joint strength. It is observed that when these polymeric joints are exposed to these climatic conditions, the joints could retain their strength of about 95% of that of joints tested under ambient conditions. Finally, in order to understand the failure modes of the joint, the fractured surfaces of the joints are examined by scanning electron microscope. It is observed that unmodified polymer-to-polymer joint fails completely through the interface; whereas the surface modified polymer essentially fails cohesively within the adhesive resulting in significant increase in joint strength. Therefore, this space durable polymeric composite could be highly useful for structural applications in spacecraft.