Antimony trioxide is a flame retardant synergist that works in conjunction with halogenated compounds to suppress flames. It works by catalyzing the breakdown of halogenated flame retardants, which then react with antimony trioxide to form volatile antimony halogen compounds. These compounds quench radicals that feed the flame.  It can be employed in polypropylene, polyethylene, polystyrene, ABS, etc. when accompanied by halogenated compounds. It can also be used directly in PVC where it reacts with the chloride to suppress the flames. Netchem is an antimony trioxide supplier to the US, Canada, and Mexico.