Antimony is mainly used as its trioxide in making flame @-@ proofing compounds . It is nearly always used in combination with halogenated flame retardants , with the only exception being in halogen @-@ containing polymers . The formation of halogenated antimony compounds is the cause for the flame retarding effect of antimony trioxide , due to reaction of these compounds with hydrogen atoms and probably also with oxygen atoms and OH radicals , thus inhibiting fire . Markets for these flame @-@ retardant applications include children 's clothing , toys , aircraft and automobile seat covers . It is also used in the fiberglass composites industry as an additive to polyester resins for such items as light aircraft engine covers . The resin will burn while a flame is held to it but will extinguish itself as soon as the flame is removed .