Synthesis of DOPO Derivatives for the Fabrication of Flame Retardant Epoxy Composites

Four flame retardant DOPO derivatives were succesfully synthesized by Pudovik reaction, including 6,6'-(([1,1'-biphenyl]-4,4'-diylbis(azanediyl))bis (thiophen-2-ylmethylene))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) 5a, 6,6'-(((sulfonylbis(4,1-phenylene))bis(azanediyl)) bis(thiophen-2-ylmethylene))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) 5b, 6,6'-((1,4-phenylenebis(azanediyl))bis(furan-2ylmethylene))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) 5c and 6,6'-((oxybis(4,1-phenylene))bis(1-(thiophen-2-yl)ethane-2,1-diyl))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) 5d. Among them, flame retardants 5a and 5d were synthesized for the first time. DOPO derivatives were combined with functionalized ammonium polyphosphate using polyethyleneimine (PEI-APP) as flame retardant systems for epoxy composites. The combination of compound 5a and PEI-APP gave the materials with the best flame retardancy. 6wt% of PEI-APP/6% of 5a promoted the composite material to V-0 rating in UL-94 vertical burning test and the limiting oxygen index reached 28.9%. The impact of flame retardants towards the mechanical and physical properties of epoxy resin was also evaluated. The combination of 5a with PEI-APP showed the least decrease in mechanical strength of epoxy composite.