Molecularly engineered bio-benzoxazine: phosphorus–nitrogen integrated cardanol derivative for enhanced fire retardancy in epoxies

Abstract

Recent advances in sustainable materials have intensified the development of high-performance thermosetting resins incorporating bio-derived components, particularly those exhibiting superior flame resistance via synergistic phosphorus–nitrogen (P–N) functionality. In this investigation, a novel P–N-rich benzoxazine monomer (Benz) was synthesized from cardanol, phosphoric acid, and paraformaldehyde through a solvent-free Mannich condensation. The molecular architecture of Benz was elucidated through FTIR and NMR spectroscopy, corroborated by amine value determination. Thermal and flame-retardant behaviors of the resulting films were interrogated using TGA, DSC, LOI, and UL-94 protocols. The inclusion of Benz markedly enhanced thermal robustness and char formation, indicating efficient flame-retardant synergy. Benz was subsequently introduced as a co-curing agent substituting 10–50 wt% of a conventional polyetheramine hardener into a commercial epoxy matrix. The resultant poly(Benz-epoxy) hybrid thermosets were subjected to a comprehensive suite of characterization techniques to assess thermal endurance, mechanical integrity, surface topology, and fire resistance. The outcomes reveal that Benz imparts significant enhancements in thermal stability and flame retardancy, underscoring its potential as a multifunctional, bio-derived curing agent for next-generation epoxy systems.