Modification of high-performance polymer composite through high-energy radiation and low-pressure plasma for aerospace and space applications

In this investigation, attempts are made to modify a high-performance polymer such as polybenzimidazole (PBI) (service temperature ranges from -260 degrees C to +400 degrees C) through high-energy radiation and low-pressure plasma to prepare composite with the same polymer. The PBI composites are prepared using an ultrahigh temperature resistant epoxy adhesive to join the two polymer sheets. The service temperature of this adhesive ranges from -260 degrees C to +370 degrees C, and in addition, this adhesive has excellent resistance to most acids, alkalis, solvents, corrosive agents, radiation, and fire, making it extremely useful for aerospace and space applications. Prior to preparing the composite, the surface of the PBI is ultrasonically cleaned by acetone followed by its modification through high-energy radiation for 6 h in the pool of a SLOWPOKE-2 (safe low power critical experiment) nuclear reactor, which produces a mixed field of thermal and epithermal neutrons, energetic electrons, and protons, and gamma-rays, with a dose rate of 37 kGy/h and low-pressure plasma through 13.56 MHz RF glow discharge for 120 s at 100 W of power using nitrogen as process gas, to essentially increase the surface energy of the polymer, leading to substantial improvement of its adhesion characteristics. Prior to joining, the polymer surfaces are characterized by estimating surface energy and electron spectroscopy for chemical analysis (ESCA). To determine the joint strength, tensile lap shear tests are performed according to ASTM D 5868-95 standard. Another set of experiments is carried out by exposing the low-pressure plasma-modified polymer joint under the SLOWPOKE-2 nuclear for 6 h. Considerable increase in the joint strength is observed, when the polymer surface is modified by either high-energy radiation or low-pressure plasma. There is further significant increase in joint strength, when the polymer surface is first modified by low-pressure plasma followed by exposing the joint under high-energy radiation. To simulate with spatial conditions, the joints are exposed to cryogenic (-196 degrees C) and high temperatures (+300 degrees C) for 100 h. Then, tensile lap shear tests are carried out 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, to understand the behavior of ultrahigh temperature resistant epoxy adhesive bonding of PBI, the fractured surfaces of the joints are examined by scanning electron microscope. It is observed that there is considerable interfacial failure in the case of unmodified polymer-to-polymer joint whereas surface-modified polymer essentially fails cohesively within the adhesive. Therefore, this high-performance polymer composite could be highly useful for structural applications in space and aerospace. (c) 2006 Wiley Periodicals, Inc.