Synergistic effect of phenyl phosphoric acid derivatives and DOPO on multifunctional epoxy resin: Fire safety, mechanical properties, transparency and hydrophobicity

A novel reactive flame retardant named PPDA was synthesized by neutralizing phenyl phosphate (PPOA) with 4,4′-diamino diphenyl ether (ODA). When 2.5 wt% of PPDA was incorporated into epoxy resin (EP) alongside 2.5 wt% of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), EP/PPDA/DOPO composite possessed UL-94 V-0 rating and a limited oxygen index (LOI) of 34.2% while exhibiting a remarkable synergistic effect between PPDA and DOPO. In contrast to pure EP, EP/PPDA2.5/DOPO2.5 exhibited a significant reduction in peak heat release rate, total heat release, and total smoke production by 56.3%, 23.0%, and 26%, respectively, when increasing char yield to 30.2%. The synergistic system formed char with organic phosphorus structures containing P-O-C, P-N-C, and P=O bonds, along with benzene rings in the condensed phase, meanwhile forming P-containing radicals and non-flammable nitrogen-containing compounds in the gas stage. Additionally, it was discovered that the presence of the NH bond in PPDA and the P-H bond in DOPO facilitated the ring-opening reaction of the epoxy groups, which contributed to the preservation of the mechanical properties and transparency of the EP. PPDA/DOPO was a halogen-free, low-cost alternative for flame retardants, demonstrating excellent performance and potential for future applications.