Effect of thermo-oxidative aging on surface characteristics of benzoxazine and epoxy copolymer

The copolymer of bisphenol A-based benzoxazine and cycloaliphatic epoxy resin possesses greater processability and glass-transition temperature compared to that of benzoxazine homopolymer, making it suitable for high-temperature aerospace applications. The aim of this work is to investigate the behavior of the copolymers under thermos-oxidative aging conditions using the surface characterization techniques. The benzoxazine homopolymer and copolymer samples were thermally aged in an air-circulating oven under controlled temperatures of 180 and 200 degrees C for various periods of time up to 24 weeks. The thermo-oxidative-induced weight change and surface cracking were monitored for samples during the aging period. As an alternative method to characterize thermal aging, static contact angle, and contact angle hysteresis of samples were measured over time. Moreover, droplet impact tests using deionized water were performed on stationary and moving samples to investigate the effects of aging time and temperature on impact dynamics of droplets. In addition, FTIR analysis was performed to evaluate the chemical changes on the surface of samples. The results indicated the extent of degradation of aged samples increased with increasing the aging time and temperature. In addition, the copolymer of cycloaliphatic epoxy and benzoxazine showed a greater amount of thermos-oxidative degradation than the one for benzoxazine homopolymer. More importantly, the results obtained from the surface analysis measurements (e.g., contact angle) followed a similar trend to the one obtained from the traditional aging experiments (e.g., weight change). This showed the former nondestructive and simple method can be used to monitor thermal aging of materials in service.