In this investigation, attempts are made to prepare nano adhesive bonding of high performance polymer such as Polybenzimidazole (PBI) (service temperature is –260<sup>0</sup> C to +400<sup>0</sup> C) for its essential applications to aerospace. In order to prepare high performance adhesive, nano adhesive is prepared by dispersing silicate nano particles into the ultra high temperature resistant epoxy adhesive (DURALCO 4703, the service temperature of the adhesive is –260<sup>0 </sup>C to +325<sup>0</sup> C) at 10% weight ratio with the matrix adhesive followed by modification of the nano adhesive after curing under high-energy radiation for 6 hours in the pool of SLOWPOKE-2 nuclear reactor with a dose rate of 37 kGy/hr in order to essentially increase the crosslink density within the nano adhesive resulting in much improved cohesive properties of the adhesive. Prior to bonding, the surface of the Polybenzimidazole is ultrasonically cleaned by acetone followed by its modification under low-pressure plasma using nitrogen as process gas under RF glow discharge, in order to essentially increase the surface energy of the polymer leading to substantial improvement of its adhesion characteristics. First, the polymer surfaces are characterized by estimating surface energy and then the polymer surface is characterized by Electron Spectroscopy for Chemical Analysis (ESCA). The thermal characteristics of the basic ultra high temperature resistant epoxy adhesive and the high performance ultra high temperature resistant radiation cross linked silicate nano adhesive are carried out by TGA and DSC and the physicochemical characteristics of these adhesives are carried out by the studies under solid state NMR. The TGA studies clearly shows that for the basic adhesive, there is a weight loss of the adhesive of about 10% when the adhesive is heated up to 325<sup>0</sup> C resulting in deterioration of cohesive properties of the adhesive over the range of temperatures. However, in the case of the radiation cross linked epoxy-silicate nano adhesive, there is a perfect 100% retention of weight of the adhesive when the adhesive is heated up to 325<sup>0</sup> C resulting in significant improvement of cohesive properties of the adhesive over the range of temperatures. In order to determine the joint strength, tensile lap shear tests are performed according to ASTM D 5868-95 standard. Considerable increase in the joint strength is observed, more than 15 times when the polymer surface is modified prior to joining. Joints prepared with the unmodified polymer show only a joint strength of 1 MPa and increases up to 15.50 MPa with the surface modified polymer. There is a further massive increase in joint strength up to 25 MPa, when the joint is prepared by nano silicate epoxy adhesive and further modification of the adhesive joint under high-energy radiation results a further significant increase in joint strength up to 30 MPa. Therefore, with all the combinations there is about 30 times increase in joint strength. In order to simulate with aerospace climatic conditions, the joints are exposed to cryogenic (-80<sup>0</sup> C) and elevated temperature (+300<sup>0</sup> C) for 100 hours and further, thermal fatigue tests of the joints are carried out under 10 cycles by exposing the joint for 2 hours under the above temperatures. When the joint completely kept at ambient condition and the joint strength compared with those joints exposed to aerospace climatic conditions, it is observed that there is no difference in joint strength. Finally, to understand the behavior of high performance silicate epoxy nano adhesive bonding, the fractured surfaces of the joints are examined by scanning electron microscope. It is observed that the joint essentially fails cohesively within the adhesive even when the joints are exposed to cryogenic, elevated temperature and thermal fatigue conditions. Therefore, this nano adhesive bonding of high performance polymer could be highly useful for structural application in future generation aerospace.
Shantanu Bhowmik, Bonin, H. W., Bui, V. T., and Benedictus, R., “Nano Adhesive Bonding of High Performance Polymer for Aerospace Applications”, in 8th International Conference on Flow Processes in Composite Materials (FPCM8), Ecole des Mines de Douai, Douai, France, 2006.