Greening Biobased Polybenzoxazine Network: Three Benefits in One Go

Eco-friendly initiators and environmentally safe flame-retardant (FR) adhesives are highly desired to empower sustainability in polymers. However, incorporating all of the properties simultaneously in a single step is challenging to achieve high-performance biomass sourced polymers. Herein, biobased benzoxazine monomers sourced from agro-waste phenol (cardanol, C) were synthesized using a facile solventless catalyst-free approach. Owing to their requirement of high ROP temperature (∼ 250 °C), prolonged polymerization cycle, issues of high flammability, and poor adhesive strength of the resultant polybenzoxazines limit their applications. To alleviate these issues, blends of naturally occurring phytic acid (PA) with the monomers resulted in fast polymerization kinetics (∼90% monomer conversion in 1 h) with the formation of hybrid polybenzoxazine networks at a mild temperature (∼100 °C). PA enacted as a green initiator and reactive halogen-free biobased FR and revealed favorable interactions across the polymer framework. Benefitting from the design, physical, ionic, and covalent cross-linking network of polymers with PA accounted for the extra-strong adhesive strength (71.1 kg/cm 2, nearly 3.7 times higher than that of the pristine polybenzoxazine). Upon burning, the hybrid polybenzoxazines exhibited extremely low flammability, superior smoke suppression and formed intumescent char as supported by the limiting oxygen index, vertical burning, and cone calorimetry measurements. The present study indicated that the benign by design approach is a way forward to green thermosets offering excellent combination properties and potential applications in the future.