Antimony trioxide

 Antimony trioxide is not a flame retardant  per se, but it is
    used as a synergist.  It is utilized in plastics, rubbers, textiles,
    paper and paints, typically 2-10% by weight, with organochlorine and 
    organobromine compounds to diminish the flammability of a wide range
    of plastics and textiles (IARC, 1989).

         Antimony oxides and antimonates must be converted to volatile
    species.  This is usually accomplished by release of halogen acids  at 
    fire temperatures.  The halogen acids react with the antimony-
    containing materials to form antimony trihalide and/or antimony halide
    oxide.  These materials act both in the substrate (condensed phase) 
    and in the flame to suppress flame propagation.  In the condensed
    phase, they promote char formation, which acts as a physical barrier
    to flame and inhibits the volatilization of flammable materials.  In 
    the flame, the antimony halides and halide oxides, generated in
    sufficient volume, provide an inert gas blanket over the substrate,
    thus excluding oxygen and preventing flame spread.  These compounds 
    alter the chemical reactions occurring at fire temperatures in the
    flame, thus reducing the ease with which oxygen can combine with the
    volatile products.  It is also suggested that antimony oxychloride or 
    trichloride reduces the rate at which the halogen leaves the flame
    zone, thus increasing the probability of reaction with the reactive
    species.  Antimony trichloride probably evolves heavy vapours which 
    form a layer over the condensed phase, stop oxygen attack and thus
    choke the flame.  It is also assumed that the liquid and solid
    antimony trichloride particles contained in the gas phase reduce the
     energy content of the flames by wall or surface effects (Troitzsch,
    1990).

         Other antimony compounds include antimony pentoxide, available
    primarily as a stable colloid or as a redispersible powder.  It is 
    designed primarily for highly specialized applications, although
    manufacturers suggest it has potential use in fibre and fabric
    treatment.

         Sodium antimonate (Na2OSb2 O5Ê´H2O) is recommended for
    formulations in which deep tone colours are required or where antimony
    trioxide may promote unwanted chemical reactions.