Study on mechanism of phosphorus–silicon synergistic flame retardancy on epoxy resins

A series of flame-retarded epoxy resins (EPs) loaded with two kinds of silsesquioxanes and DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide), respectively, have been prepared. The thermal stability and quantity of the char of these EPs have been investigated by TGA, and the results have indicated that the combined use of silsesquioxane and DOPO enhanced the quantity of the char more efficiently than when using silsesquioxane or DOPO alone. Details of fire behavior, such as TTI, HRR, p-HRR, TSR, SEA, and THR, have been tested by means of a cone calorimeter. In the EP under combustion, the interactions between DOPO and silsesquioxane in the condensed phase induce a better flame retardancy capability than that with either silsesquioxane or DOPO alone. The char residues of the EP composites have been investigated in detail by FTIR and XPS. The interactions between DOPO and silsesquioxane in the condensed phase involve the formation of –P(O)–O–Si– structures. The formation of a –P(O)–O–Si– structure increases both the quantity and the thermal stability of the char. The flame retardancies of these EPs have been tested according to the LOI and UL-94 standards, and the results have indicated that the interactions of DOPO with the silsesquioxane have important effects on the flame retardancy of EP composites.