Piperazine/Alkene-Containing Phosphoramide Oligomer for the Intumescent Flame Retardation of EPDM Rubber

Ethylene propylene diene monomer (EPDM) rubber is highly flammable, facing potential fire hazard when it is used as electric cable sheaths, automobile parts, etc. In this work, aiming to reduce fire hazard of EPDM rubber, a piperazine/alkene-containing phosphoramide oligomer poly (2-butene-1,4-dione piperazine fumaroyl phenylphosphonate) (PPFPP) was synthesized and used as charring agent to fabricate intumescent flame retardant with ammonium polyphosphate (APP). When 20 wt% APP/PPFPP (weight ratio: 3:1) was incorporated into EPDM, the flame-retarded EPDM achieved the UL-94 V-0 rating and the limiting oxygen index (LOI) of 27.0%, significantly superior to flame-retarded EPDM consisting of controlled flame retardant PPSPP without alkene and PPFA without phenyl phosphine. In cone calorimeter test, flame retardant consisting of 6.25 wt% PPFPP and 18.75 wt% APP strongly reduced the peak of heat release rate (PHRR) and the peak of smoke production rate (PSPR) of EDPM, diminished approximately by 72% and 60%, respectively. In the case of meeting flame retardancy, mechanical property of flame-retarded EPDM maintained at a high level. Different measurements confirmed that the flame-retardant mechanism of PPFPP is attributed to two aspects. The first one is ascribed to the char-forming action of PPFPP in the presence of alkene and piperazine, playing a barrier role in condensed phase; the second one is due to the gaseous action of phenyl phosphine groups originating from the pyrolysis of PPFPP. In summary, the combination of piperazine and alkene with phenyl phosphine may fabricate an efficient charring agent for the intumescent flame retardation for EPDM.