Synthesis and flame-retardant analysis of a novel P/N containing flame retardant on epoxy resin composites

A new reactive flame-retardant N′-(1-(4-hydroxyphenyl)-1-(6-oxido-6H-dibenzo[c,e][1,2]oxaphosphinin-6-yl)ethyl) benzohydrazide (DOPO-BD) was synthesized via a two-step reaction involving the Schiff base reaction between benzohydrazide and 4-hydroxy acetophenone, followed by a reaction between the imine-based condensation product and 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The chemical structure of DOPO-BD was well characterized by Fourier transform infrared spectra (FTIR), ¹H, ¹³C, ³¹P NMR, and high-resolution mass spectrometry (HRMS). The DOPO-BD was utilized as a reactive flame retardant and co-hardener in DGEBA/DDS. The epoxy composites of varying phosphorus weight percentages were synthesized. UL-94 and LOI tests were used to investigate the flammability of composites. The epoxy composites retained good transparency, which was measured by UV–vis spectroscopy. Thermogravimetric analysis (TGA) observed an increase in the thermal stability of epoxy composites in a higher temperature range with increased char yield. In the UL-94 and LOI tests, EP/DOPO-BD(1.5%P) sample gave a V-0 rating and 30% LOI value. The cone calorimeter test indicated a decrease in total heat released (THR), peak of heat release rate (pHRR), and total smoke production (TSP) for EP/DOPO-BD(1.5%P) in comparison with epoxy (EP). The mechanical properties and glass transition temperature (T_g) of the epoxy composites were determined by Dynamic mechanical analysis (DMA), which showed an increase in storage modulus and a decrease in T_g.