A Novel P–N Bonded Pyrimidine-Functionalized DOPO Derivative for Epoxy Resin With Improved Flame Retardancy and Mechanical Strength: Synthesis, Performance, and Mechanism Study

To leverage strong P/N synergistic effects to impart high flame retardancy to epoxy resin without compromising overall performance, a novel flame retardant, 6-(pyrimidin-2-ylamino)dibenzo[c,e][1,2]oxaphosphinine 6-oxide (DAP), was synthesized by forming a P–N bond between 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) and 2-aminopyrimidine. Its structure was confirmed by NMR and mass spectrometry. Non-isothermal DSC tests demonstrated that DAP acted as a co-curing agent with 4,4′-diaminodiphenylmethane for diglycidyl ether of bisphenol-A and enhanced the curing rate. DAP significantly enhanced the flame retardancy of the epoxy resin at a low loading of 4 wt% (0.40 wt% phosphorus), achieving a limiting oxygen index (LOI) of 32.1% and a UL-94 V-0 rating. It also reduced heat and smoke release. The results from dynamic mechanical analysis (DMA) and mechanical property tests showed that the epoxy resin containing 4 wt% DAP retained a glass transition temperature comparable to the neat resin, along with slight improvements in mechanical properties, including flexural strength and modulus, non-notched impact strength, and tensile strength. The analyses of UL-94, cone calorimetric test (CCT), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis/infrared spectroscopy (TGA/IR), and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) revealed that the flame-retardant mechanism involved both a condensed-phase char barrier and gas-phase quenching and dilution effects.