Synthesis of (1,4-Methylenephenylphosphinic acid) Piperazine and Its Application as a Flame Retardant in Epoxy Thermosets

An efficient flame retardant (1,4-methylenephenylphosphinic acid) piperazine (MPPAP) is successfully synthesized. The application of MPPAP in epoxy (EP) thermosets increases limited oxygen index values of composites to 30.2% and 6 wt% or more loading makes the materials get through UL94 V-0 rating. The investigation of the combustion process indicates that MPPAP clearly changes the combustion behavior of EP composites, forms a char layer on the surface of the composites, and presents an effective flame inhibition effect, consequently resulting in clear reduction of the flammability and smoke production. The thermal degradation process of MPPAP/EP indicates that MPPAP promotes the matrix to decompose earlier and increases the char-forming ability of composites. The morphology and structure of the final char shows MPPAP can promote the formation of cross-linking structures and increase the graphitization degree of char residues acting as an excellent barrier. Furthermore, the flame-retardant mechanism illuminates that MPPAP can reduce the generation of volatile fuels, and the pyrolysis products of MPPAP contain PO and PO2, which exert free radical quenching effect in the gas phase. Therefore, MPPAP is a terrific flame retardant for epoxy thermosets which manifests a flame retarding action in the gas phase and condensed phase.