Fabrication of flame-retardant, strong, and tough epoxy resins by solvent-free polymerization with bioderived, reactive flame retardant

Replacing petroleum-derived chemicals with biobased ones to develop high-performance polymers is an important direction of sustainable development strategy. However, the current bio-based polymers suffered from complex preparation and unsatisfactory comprehensive properties. Herein, the bio-based 9,10-dihydro-10-(2,3-dicarboxypropyl)-9-oxa-10-phosphaphenanthrene 10-oxide (DDP) was covalently introduced into epoxy resin (EP)/anhydride system via solvent-free, green polymerization. The resulting EP1.2 sample (phosphorus content = 1.2 wt%) achieved a UL-94 V-0 rating with a limiting oxygen index (LOI) of 32.6%, indicative of outstanding flame retardancy. The peak heat release rate and total heat release of EP1.2 sample were 50.5% and 22.6% lower than virgin EP. DDP featured the in-situ reinforcing and toughening actions on EP due to the π-π stacking of its biphenyl group. The impact strength, tensile strength and elongation at break of EP1.2 were 218.2%, 46.5%, and 98.1% higher than those of EP, indicating exceptional mechanical strength and toughness. The high transparency and thermal stability of EP1.2 were well-preserved after adding DDP. Obviously, EP1.2 outperformed many previous flame-retardant EP systems due to its ease of fabrication and great overall performance. Therefore, this work offers a feasible and green strategy for the fabrication of advanced EPs with exceptional flame retardancy, mechanical properties, optical performances, and thermal stability by bioderived, reactive flame retardants, which is conducive to the sustainable development.