Fabrication and Performance of Self-Toughening Benzoxazine Resin and Glass Fiber-Reinforced Composites

A series of self-toughening benzoxazine resins containing amino-terminated polyarylene ether nitrile (APEN) segments were synthesized from bisphenol-A, paraformaldehyde, and a mixed amine source of APEN and melamine. Unlike conventional physical blending toughening, the APEN segments are covalently incorporated into the benzoxazine network via their amino end groups. This chemical integration not only significantly improves toughness but also simultaneously enhances thermal and dielectric properties, overcoming the common trade-off of “toughening without heat resistance”. Meanwhile, melamine serves as one of the amine sources; its excess amino groups can catalyze the ring-opening polymerization of benzoxazine, which helps to reduce the curing temperature. The effects of APEN content and curing temperature on the properties of the resin and glass fiber composites were studied. The incorporation of APEN optimized the crosslinked network, balancing rigid aromatic structures with flexible ether linkages. As the proportion of APEN segments increased, the thermal decomposition thresholds and char residue were notably enhanced, signifying progressively improved thermal resistance. For composite systems cured at 220 °C, flexural strength exhibited a continuous upward trend with rising APEN content, while the flexural modulus remained steadily within a range of 23–25 GPa, and the impact strength was remarkably elevated from 45 kJ/m2 to values spanning 60–73 kJ/m2. A further curing treatment conducted at 300 °C facilitated additional crosslinking of nitrile moieties, yielding a further enhancement in flexural strength, particularly at lower APEN contents. Fracture surface analysis confirmed the toughening effect, evidenced by the transition from smooth brittle fracture to dendritic crack patterns. In addition, the composite achieved its lowest dielectric constant of 4.2 at an APEN loading of 20 wt.% when cured at 300 °C. Overall, this investigation presented a viable and effective strategy for the design and fabrication of high-performance, self-toughened benzoxazine-based composites.