Design of high temperature resistance interphase and study on thermo-oxidative aging properties of high strength high modulus carbon fiber/bismaleimide composites

High strength high modulus carbon fiber reinforced bismaleimide composites (CF/BMI) have been widely used in aerospace applications for structural components with high temperature resistance. With the continuous improvement in the heat resistance level of high temperature bismaleimide (BMI) resins, the safety and reliability for long-term service at high temperature environments have been highly dependent on the thermo-oxidative aging stability of the interphase of composite. Herein, HBPAA sizing agents with different degrees of branching (DB) were prepared, and used to construct hyperbranched polyimide (HBPI) interphases with high temperature resistance on carbon fibers. The heat resistance of BMI resin and HBPI were investigated, and the thermo-oxidative stability of carbon fiber surface was analysed. The effects of HBPI interphases with different DB on interfacial shear strength and interfacial toughness of CF/BMI composites were explored after thermo-oxidative aging at 25℃, 200℃, 250℃, and 300℃ for 168 h. According to the interfacial properties as well as the interfacial modulus and structure of CF/BMI composites, the thermo-oxidative aging mechanism of the composites were summarized. Results showed that after imidization treatment of HBPAA with three different DB (0.42, 0.61, 0.81), the glass transition temperature of HBPI were increased with the DB increasing reaching 297℃, 310℃ and 312℃. Compared to those of pristine carbon fibers, the chemical activity and thermo-oxidative stability of carbon fibers were improved by HBPI. Compared to p-CF/BMI, the modified composites (HBPI₁-CF/BMI, HBPI₂-CF/BMI and HBPI₃-CF/BMI) exhibited the improved interfacial properties after thermo-oxidative aging. After thermo-oxidative aging at 300℃ for 168 h, the highest interfacial shear strength and interfacial toughness of CF/BMI (HBPI₂-CF/BMI) were obtained. The thickest interphase effectively alleviated the interfacial modulus disparity in HBPI₂-CF/BMI composite, which was attributed to the rigid framework and the hyperbranched topological structure of HBPI₂ with moderate DB (0.61), as well as transitioning the interfacial modulus and dissipating the interfacial thermal stress. In addition, the interfacial debonding under thermo-oxidative conditions was suppressed due to chemical bonding between HBPI₂ and BMI resin, ultimately endowing the better interfacial performance and resistance to thermo-oxidative aging of the composites.