Benzoxazine-Modified Phenolic Triazine Resin with Improved Curing, Toughness, and Balanced Thermomechanical Properties, Enabling Hot-Melt Processing

Abstract Phenolic triazine (PT) resins suffer from high curing temperatures, inherent brittleness, and poor adaptability with hot-melt prepreg processing. To address these restrictions, benzoxazine (BZ) was introduced as a modifier for low-temperature curing, toughening, good hot-melt processability, and balanced thermomechanical characteristics. A PT/BZ hybrid resin system was synthesized and its curing behavior, thermomechanical characteristics and network structure were comprehensively characterized. Then, hot-melt prepreg technique was used to manufacture carbon fiber composites and their mechanical and interfacial properties were examined. The results indicate that the phenolic hydroxyl groups generated in situ during the BZ ring-opening can effectively catalyze the cyclotrimerization of cyanate esters, lowering the curing temperature from 265 to 210 °C, shortening the curing time by 52.1%, and achieving a comparable curing degree to that of PT resin. The PT/BZ resin also exhibits good thermal stability, with a 5% weight loss temperature of up to 430.5 °C. Its onset glass transition temperature (Tg) reaches 207.7 °C, and the ratio of the Tg to the maximum curing temperature increases by 15.6% compared with PT resin, realizing the ideal balance of “low-temperature curing with high-temperature serviceability.” Moreover, the Mannich bridges derived from BZ act as flexible segments that reduce the cross-linking density by 41.0%, significantly improving the toughness of the PT matrix. As a result, the tensile strength, elongation at break, and impact toughness of PT/BZ castings increase by 28.2, 28.8, and 19.1%, respectively. For carbon fabric-reinforced PT/BZ composites, the interlaminar normal tensile strength reaches 20.3 MPa, which is 3.8 times that of a conventional carbon fabric/phenolic system. Through catalytic low-temperature curing and synergistic network design, the PT/BZ hybrid system greatly improves processability and toughness while maintaining a high Tg and excellent thermal stability, demonstrating strong potential as an ideal matrix for high-performance hot-melt prepregs and integrated ablative/load-bearing composites.