Structural adhesives are finding increasing use in many applications. However, their utilization at elevated temperature has always been a challenge due to their low thermal and mechanical properties. However, in recent years, the development of high performance polymers have overcome the problem of using adhesive bonding at high temperature to some extent. Polybenizimidazole (PBI) is one such recently emerged high performance polymer with excellent thermal and mechanical properties. It has a tensile strength of 160 MPa and a glass transition temperature (T_g) of 425 °C. Due to its excellent thermal and mechanical properties, it has the potential to be used as an adhesive under various environmental conditions. In the present work, efforts are devoted to explore the potential of using PBI at high temperature and in hot-wet environmental conditions. M21 and DT120 epoxy based carbon fiber composite bonded joints were prepared and tested. Both M21/carbon composite and DT120/carbon composite have exhibited a reduction in joint strength of about 16% and 25% respectively after 1000 h of conditioning in a hot-wet environment. However, a reduction in lap shear strength of 52% and 56% is observed when composite bonded joints were tested at 80 °C.

In Present study, characteristic behavior of PBI as a protective coating for aircraft application is presented. Performance of PBI coating is evaluated after exposure to hot wet environment and liquid immersion. Critical properties including hardness, scratch resistance and adhesion of PBI coating are assessed and results are presented. M21 epoxy based carbon fiber composite is used as substrate material. Atmospheric pressure plasma treatment (APPT) is performed to prepare the surface of substrate prior to application of PBI coating. Lap shear tests demonstrate that APPT has improved the adhesion strength of PBI bonded joints to about 250%. SEM analysis shows that strong composite/coating interface resulted in cohesive failure of the bonded joints. Lap shear test results on conditioned samples reveal that composite bonded joints has depicted about 15% decrease in strength after exposure to hot/wet environment under the conditions of 80 °C and 95% relative humidity (RH). However, mode of failure of bonded joints has not changed even after 1000 h of conditioning. PBI coated samples were also immersed in water and Skydrol to evaluate the effect on hardness and scratch resistance of PBI coating. Scratch tests on conditioned samples reveal that the visco-elastic recovery has increased from 58% (unexposed sample) to 71% with PBI coated panel immersed in skydrol. However, no major effect on visco-elastic recovery is observed with the samples immersed in water and the sample conditioned in hot/wet environment. Nano-indentation test results indicate that PBI coated panel immersed in skydrol has not depicted any decrease in elastic modulus ad hardness; whereas coated panel immersed in water for 1000 h has shown a minor decrease in both modulus and hardness. All these results indicate that PBI coating has great potential to retain its properties under harsh environment and it can be used as protective coating for aerospace application.

Surface wetting properties of polymer based composite materials play a crucial role in the achievement of better adhesive bonding. In the present work, efforts are made to evaluate the effect of surface morphology on adhesion properties of two different epoxy based carbon fiber composite materials before and after performing the atmospheric pressure plasma treatment and hand sanding. In this study, high performance thermoplastic polybenzimidazole (PBI) adhesive having a glass transition of 425 °C is used as an adhesive. PBI adhesive bonded joints were formed using M21 epoxy based and DT120 epoxy based carbon fiber composites. Experimental results demonstrate that surface morphology plays a very critical role in achieving a high lap shear strength. Due to better surface morphology, DT120/carbon composite bonded joints exhibits four times higher lap shear strength even without performing any surface treatment. Furthermore, plasma treated composite bonded joints have shown higher lap shear strength when compared to the hand sanded composite bonded joints. Lap shear strength of bonded joints is correlated with surface morphology of plasma treated and hand sanded samples. SEM analysis of treated surfaces demonstrates that plasma treatment has changed the surface morphology without damaging fibers on the surface; whereas hand sanding treatment has damaged some fibers on the surface which ultimately leads towards a weak substrate/adhesive interface and hence resulted in lower bonded joint strength.

This investigation highlights different surface functionalization processes of multi-walled carbon nanotubes (MWCNTs) and their effects on mechanical properties of polyetherimide nanocomposite. Surfaces of MWCNTs were modified by chemical process and by low-pressure plasma process. There is a significant change in physicochemical characteristics of MWCNTs after chemical and low plasma treatment evident from scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy studies. Due to surface modification of CNTs, there is a significant change in surface morphology and increase in oxygen functionalities such as C=O, C-O, and COOH especially evident in low-pressure plasma treatment; however, differential scanning calorimeter and thermogravimetric analysis studies reveal that thermal properties of the composite do not alter as such. There is a significant increase in mechanical properties of high-performance polymeric nanocomposites when surface-functionalized MWCNTs are dispersed in polymeric matrix; however, surface characteristics of the composite remain almost unchanged evident from contact angle and surface energy studies.

Polybenzimidazole is a recently emerged high-performance polymer exhibiting excellent thermal and mechanical properties. Because of the nature of the chemical structure of polybenzimidazole, it has great potential to be used as high-temperature radiation-resistant material for applications in space environment. In this context, a study is performed to evaluate the thermal, mechanical, and optical performance of polybenzimidazole after exposure to high-energy electron and gamma radiation. Polybenzimidazole films are exposed to gamma radiation and electron radiation foradose of 300 and 1000 kGy, respectively. Thermal gravimetric analysis shows that exposure of polybenzimidazole to high-energy radiation has not deteriorated the thermal stability of the polymer, and it has maintained its thermal stability even up to a temperature of 500°C with a total weight loss of 15%. Dynamic mechanical analysis shows that the exposure of polybenzimidazole to high-energy radiation has even improved the storage modulus in the high-temperature range. Tensile test results indicate that high-energy radiation has a very minor effectonthe tensile strength of polybenzimidazole. However, a decrease in tensile strain is observed. Photospectroscopic results show that polybenzimidazole has not changed its light-transmitting characteristic even after exposure to high-energy radiation. All these results indicate that polybenzimidazole has great potential for application in space and nuclear industry.