Influence of the molecular weight of PPO resins and char-forming behavior of polymeric additives on the flame retardancy of EPDM formulations

The influence of the molecular weight of poly(2,6-dimethylphenylene oxide) (PPO) on the flame retardancy of ethylene–propylene–diene-modified elastomer (EPDM) formulations containing melamine, kaolin, and PPO formulations was studied. The influence of the molecular structures of various char-forming polymers on their flame-retardant effect was also investigated. PPO resins having number-average molecular weight (Mn) from 3200 to 24,800 and weight-average molecular weight (Mw) 9000 to 58,400 affected the oxygen index (OI) values and UL 94 ratings of EPDM formulations, and the preferable molecular weight was found to be about Mn 13,300 and Mw 29,200. Among the char-forming polymeric additives studied, PPO was most effective in providing flame retardancy. The concept of char-forming rate is proposed to explain the variation in the observed flame retardancy. Higher char-forming rate (in contrast to char yield) correlated well with higher OI and better UL 94 ratings in these systems. The melting-before-charring character of char-forming polymers was another important factor that appeared to control char morphology and thus flame retardancy.