Abstract: Polyborosiloxane flame retardants (PB) were prepared through a two-step synthesis. The flame retardancy of the polyborosiloxane in polycarbonate/acrylonitrile- butadiene-styrene copolymer (PC/ABS) alloy was studied using thermogravimetric analysis, cone calorimeter analysis, and limited oxygen index. It was found that the PB prepared from a 1:1 ratio of silane/boric acid provided the most effective flame retardancy in PC/ABS system. Introducing 5 wt % of PB into PC/ABS alloys increased the limited oxygen index from 24.1 % to 34.1 %. The presence of PB reduced the thermal degradation rate and promote the formation of carbon residue during combustion. Moreover，adding PB increased the fire performance index of the PC/ABS alloy, significantly reduced the generation of heat and the emissions of smoke, CO, CO2 during the combustion.

Key words: polycarbonate, acrylonitrile-butadiene-styrene copolymer, polyborosiloxane, limited oxygen index, flame retardancy