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

ABSTRACT 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.