Abstract: Flame retardant blend of high impact polystyrene (PS-HI) and bromopolystyrene (BPS) was obtained by melt-compounding. Proper amounts of antimony oxide (AO) and organically modified montmorillonite (OMMT) were introduced to the blend as flame-retarding synergists. The flame retardancy of the blend was investigated by means of horizontal burning, vertical burning, oxygen index, and high temperature pyrolysis experiments. When the mass ratio of PS-HI/BPS/AO was 100/30/10, the oxygen index of the composite reached 28.1 % and the horizontal burning rating and vertical burning rating of the material were FH-1 and FV-0, respectively. When both AO and OMMT were added, the high temperature charring ability of the composite was enhanced obviously, suggesting that the flame retardant mechanism of the composite has changed from gas phase flame retardation to gas phase and condensed phase synergistic flame retardation. However, due to the much longer afterglow time, the composite containing both additives cannot reach FV-0 rating in the vertical burning test.

Key words: bromopolystyrene, high impact polystyrene, flame retardancy