Processable Main-chain Benzoxazine Resin Derived from Daidzein with Intrinsic Flame Retardancy

The development of intrinsically flame-retardant, low-smoke, low-toxicity, and halogen-free polymers represents a critical challenge. In this study, bio-based daidzein was employed as a partial phenolic source, and a series of main-chain benzoxazine resins (Dz-Ph-ddm) was synthesized via Mannich condensation with phenol, 4,4′-diaminodiphenylmethane, and paraformaldehyde. The effects of daidzein content on the thermal stability, flame retardancy, mechanical properties, and char-forming behavior of cured resins were investigated. The results indicate that the rigid aromatic structure and benzopyranone unit of daidzein significantly promoted the formation of a dense and continuous char layer, achieving gas- and condensed-phase synergistic flame retardancy. With 20 mol% of daidzein addition, the resin exhibited a glass transition temperature ( T g ) of 259.6 °C, the char yield at 800 °C increased by 42.5% compared to the control group, a UL-94 V-0 rating, and a 30.9% reduction in total heat release (THR) in cone calorimetry tests. The glass fiber-reinforced composite prepared using this resin as the matrix showed a 100.6% improvement in impact strength, with flexural and compressive strengths reaching 748.4 and 340.8 MPa, respectively. Moreover, it demonstrated good fire safety performance even in an 80 kPa low-pressure combustion environment. This work provides a facile strategy for the preparation of high-performance eco-friendly flame-retardant composites. The alternative text for this image may have been generated using AI.