Efficient flame retardancy, good thermal stability, mechanical enhancement, and transparency of DOPO-conjugated structure compound on epoxy resin

A novel multifunctional flame retardant EAD with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and conjugated structure was successfully synthesized via addition reaction of P–H bond and alkynyl group. The structure was characterized by Fourier transform infrared (FTIR) spectra and nuclear magnetic resonance (NMR) spectra. Thermogravimetric analysis (TGA) showed that EAD possessed high thermal stability with 290 ℃ T_onset and 20.4% char yield at 700 ℃ due to the presence of rigid structure and crosslinking reaction. When incorporating 2 wt% EAD into epoxy resin, the results from TGA indicated that the modified sample had good thermal stability with a T_onset value of 354 ℃ and char-forming performance with 22.9% char yield at 700 ℃, as well as glass transition temperature without any deterioration was obtained. Compared with pure epoxy, the same EAD content with only 0.186 wt% phosphorus loading improved effectively the flame retardancy, reflected by the increased limiting oxygen index (LOI) value of 31.0% and V-0 rating in UL-94 vertical burning test. Results from cone calorimeter test (CCT) further confirmed the positive effect on the fire retardancy, which was expressed as the heat release reduction and smoke suppression. The high-efficient flame retardancy was investigated by analyzing the pyrolysis behavior and char residue, which demonstrated the multiple actions of EAD in gas and condensed phases. The low addition of EAD also brought out the reinforcing and toughening effect for epoxy resin. Moreover, the modified epoxy samples with no more than 2 wt% EAD presented good transparency and UV-shielding performance.