A novel phosphorus-, nitrogen- and sulfur-containing macromolecule flame retardant for constructing high-performance epoxy resin composites

Epoxy resin (EP) has two shortcomings: flammability and brittleness. Previous studies have improved brittleness or flame retardancy but they will compromise the glass transition temperature (Tg) and thermal stability. Herein, a novel DOPO based phosphorus-, nitrogen- and sulfur-co-contained macromolecular flame retardant (DOPONH2-S) was synthesized and incorporated into EP to improve its flame retardancy and toughness with not damaging Tg and thermal stability. After curing, both Tg and the tensile strength of EP-2 improved at the addition of DOPONH2-S is 6.75 wt%. Simultaneously, the thermogravimetric analysis (TGA) results showed DOPONH2-S is helpful increasing the thermal stability of EP composites and protecting the polymer from further decomposition. In the fire safety test, EP-2 achieved V-0 rating in the vertical burning test and obtained a limiting oxygen index value of 30.5%. In comparison with pure EP, the peak heat release rate, total heat release and effective heat of combustion of EP-4 incorporated with 10.0 wt% DOPONH2-S were reduced by 59.1%, 58.5% and 46.3%, respectively. A two-phase mechanism was discussed to explain the combination effect in DOPONH2-S. In condensed-phase, rich char residue prevents the exchange of mass and heat during combustion. It also obstructs the release of combustible gases into the combustion zone. In gas-phase, the produced phosphorus- and sulfur-containing free radicals can capture active free radicals (H· and OH·), thus ending the chain reaction of combustion. Moreover, the addition of DOPONH2-S apparently improved the impact strength (62.5%). This work provided ideas for the development of multifunctional EP.