Abstract: In order to develop highly efficient halogen-free flame retardants of epoxy resins (EP), a novel flame retardant DOPPO, which contains two phosphorus oxidation states and hydroxyl groups, was synthesized from p-Hydroxybenzaldehyde, phosphorus oxychloride, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The influence of DOPPO on the thermal, flame-retardant, and fire-resistant performance of EP composites was studied. Tests showed that the flame retardance and fire resistance of EP composites are influenced by the molecular structure of organic phosphorus-containing flame retardant. Compared with DOPh and TPP, DOPPO shows obvious synergistic flame retardant effect, and the most efficient flame retardance and fire resistance in EP. In detail, when the phosphorus content in EP is low at only 0.42%, the EP/DOPPO acquired the UL 94 V-0 rating and obtained a higher LOI value of 34.6%, the formation of more compact expanded char residues can inhibit the release of heat and toxic smoke. TG-IR analysis shows the generation of phosphorus containing volatiles, which may provide DOPPO with gas phase flame retardant function for EP. The better flame-retardant performance of DOPPO is ascribed to the quenching effect of the phosphaphenanthrene group in the gas phase and the charring effect of phosphate ester group and phosphaphenanthrene group with hydroxyl group in the condensed phase. And, the results from the large-plate combustion test reveals that DOPPO was superior to DOPh and TPP in terms of expansion ratio as well as fire-resistant performance of the fireproof coatings.

Key words: epoxy resin, flame retardant, multi-phosphorous oxidation states, flame-retardant property, fire-resistant property