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A phosphorus-rich compound 2-methyl-1,2-oxaphospholan-5-one 2-oxide (OP) (Clariant, Germany), and poly(2,6-dimethylphenylene oxide) (PPO) were mixed with high impact polystyrene (HIPS). The combustion behavior of the formulations was studied by limiting oxygen index (LOI) and Underwriters Laboratories UL94 test. The mechanism of the fire-retardant action of the additives was studied by thermogravimetry, IR characterization of the solid residues and high boiling degradation products (HBPs) and GC/MS characterization of the HBPs. OP was found to improve fire retardant performance of the PPO/HIPS blend providing the UL94 V-0 rating. Upon Downloaded from pyrolysis, it evaporates and forms phosphorus-containing volatiles. Thermogravimetry and FT-IR provide evidence of an interaction between PPO and OP. The synergistic fire retardant effect is mostly attributed to the condensedand vapor-phase activity of OP and condensed-phase effect of PPO.KEY WORDS: 2-methyl-1,2-oxaphospholan-5-one 2-oxide, poly(2,6-dimethylphenylene oxide), UL94 V-0, high impact polystyrene, synergistic effect, combustion performance, thermogravimetry, FT-IR, GC/MS.

SUMMARYThe flammability of eight halogen-free styrene resins and one halogen-containing styrene resin was characterized by UL 94 VB, LOI and cone calorimeter tests. Their burning behaviour was also measured when exposed to three external ignition sources (methenamine tablet, candle, paper ball). Five resins were used for 19 00 monitor housings and the others for 25 00 TV. The LOI values of UL 94 V-2, V-1 and V-0 rated resins were higher than that of HB. The heat release rate decreased as the UL 94 ratings increased from HB to V-0. When these resins were exposed to three external ignition sources, UL 94 V-1 and V-0 rated resins showed a self-extinguishing property after removal of the fire and did not cause fire growth in either 19 00 monitor or the 25 00 TV housings in all cases. However, UL 94 V-2 and HB rated resins were easily ignited and spread fire by dripping burning trickles. The burning rate of V-2 resin was slower than that of HB.

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