Inﬂuence of the Molecular Weight of PPO Resins and Char-Forming Behavior of Polymeric Additives on the Flame Retardancy of EPDM Formulations WEIMING ZHU, EDWARD D. WEIL Polymer Research Institute, Polytechnic University, 6 Metrotech Center, Brooklyn, New York 11201 Received 20 November 1996; accepted 31 May 1997 ABSTRACT: 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 molec- ular weight ( M n ) from 3200 to 24,800 and weight-average molecular weight ( M w ) 9000 to 58,400 affected the oxygen index (OI) values and UL 94 ratings of EPDM formula- tions, and the preferable molecular weight was found to be about M n 13,300 and M w 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.  1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 1405 – 1414, 1998 Key words: ﬂame retardancy; oxygen index; UL 94 test; char-forming polymer ; char- forming rate; EPDM polymer INTRODUCTION substantial amount of char, which may serve as a heat- and mass-transfer barrier to retard ﬁre propagation. Therefore, these polymers have in- The admixture of char-forming polymers with less- char-forming polymers has become an important herently a certain degree of ﬂame retardancy. 3–5 The ﬂame retarding of less charrable or non- approach for creating useful ﬂame-retardant sys- tems. These char-forming polymers are in most charrable polymer by incorporation of a char- forming polymer has been reported in a number cases polymers containing aromatic ring struc- tures, such as poly ( 2,6-dimethylphenylene oxide ) of patents and is represented by some commercial (PPO), polyphenylene sulﬁde (PPS), polyimide examples. One of these examples is a blend of (PI), and poly(ether ketone ketone) (PEKK). high-impact polystyrene ( HIPS ) and poly ( 2,6-di- They typically have high heat resistance and ther- methylphenylene oxide ) ( PPO ) . 6–10 High-impact mal stability due to their aromatic backbones. 1,2 polystyrene is ﬂammable and leaves nearly no Upon burning, these polymers tend to generate a char upon burning, while PPO is a good char- forming polymer. This blend, which normally con- tains more than 40 wt % PPO, has been commer- Correspondence to : E. D. Weil. cialized by General Electric Co. under the trade- Contract grant sponsor: Electric Power Research Institute. mark of NORYL. To attain a stringent ﬂame Journal of Applied Polymer Science, Vol. 67, 1405 – 1414 ( 1998 )  1998 John Wiley & Sons, Inc. CCC 0021-8995/98 / 081405-10 retardancy such as a UL 94 V-0 rating, an organo- 1405 4726 / 8E20$$4726 12-15-97 10:34:31 polaa W: Poly Applied