Synthesis and characterization of intrinsic flame-retardant epoxy resins with excellent mechanical and dielectric properties

Epoxy resins (EP) are utilized across numerous fields owing to their exceptional material characteristics. However, their flammable nature and insufficient dielectric properties restrict their application in high-performance sectors. To address this, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was chemically grafted onto the phenolic resin (BPF) skeleton via covalent bonding, successfully synthesizing a novel phosphorus-modified curing agent (BPF-DOPO). This prepared BPF-DOPO was then employed to modify the curing process of EP, producing a series of composite materials. The successful synthesis of BPF-DOPO was verified through Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (¹H/³¹P NMR), and X-ray photoelectron spectroscopy (XPS) analyses. Flame retardancy tests demonstrated that EP/BD composites containing 2.6 % phosphorus achieved a UL-94 V-0 rating and exhibited a limiting oxygen index (LOI) of 29.4 %. Cone calorimeter analysis revealed dramatically reduced fire hazards in EP/BD composites, with peak heat release rate (pHRR), total heat release (THR), and peak CO₂production rate (pCO₂PR) decreasing by 73.4 %, 28.6 %, and 75.8 %, respectively. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) analysis demonstrated that BPF-DOPO exerts its flame-retardant effect through a synergistic mechanism integrating the quenching of gas-phase radicals with the formation of condensed-phase char. Furthermore, BPF-DOPO's bulky rigid structure simultaneously enhances the mechanical properties of EP and improves its dielectric characteristics, achieving a dielectric constant (D_k) of 2.36 and a dielectric loss (D_f) of 0.008 at 10⁶ Hz. The BPF-DOPO curing agent developed in this work thus flame retardancy, mechanical reinforcement, and low dielectric loss, providing novel approaches for the design of high-performance EP.