Antimony oxide by itself is not a fire retardant,
and the halogens by themselves, mainly bromine and chlorine, are weak fire
retardants. However, when they are combined they become synergistic and are the
most effective and most widely used flame retardant system for plastics. Usually
three to four parts of halogenated flame retardants are used to one part of
antimony oxide on a weight basis. Using more than the 4: 1 ratio offers little
additional protection. The stoichiometric ratio of chlorine to antimony in
antimony trichloride is 3: 1. Formulations in different applications will depend
on thermal stability, cost, tinting strength, Change in physical properties,
smoke considerations, streaking, blend ability, and the flame retardant
Two mechanisms exist in the synergistic system.
First is the "free radical capture" process that takes place in the vapor phase.
On combustion at a temperature of over 6000
F, the halogen forms
hydrochloric or hydrobromic acid that reacts with the antimony oxide to form
antimony trichloride, antimony oxychloride,
antimony tribromide, or antimony oxybromide.
The flame retarding action takes place in the vapor stage above the burning
material. It is thought that "free radicals propagate" the flame. But, the
antimony trihalides or antimony oxyhalides act as "free radical traps", and take
up free radicals. They inhibit ignition and pyrolysis in the solid, liquid, and
A second process occurs in a solid phase and is the
"char process". The antimony oxide promotes the formation of "char" (essentially
carbon) on the substrate which reduces volatile gas formation. The barrier
between the substrate and the vapor phase reduces the available oxygen to the
underlying substrate. The barrier effect is obtained by almost any inert
additive. In plastics there is a cross linking with antimony to produce a more
stable thermoset polymer. Additionally in the solid phase, the formation of SbCI3
and SbOCI acts as a
dehydrating agent that increases charring. Sometimes phosphorous compounds
(TCP), magnesium oxide, alumina trihydrate, molybdic oxide, zinc borate, or zinc
oxide are used in combination or in place of antimony oxide to reduce costs, to
increase char formation, or to reduce smoke. However, the substitution of the
other retardants greatly reduces the flame retardance normally rendered by
antimony oxide. Testing of the amounts of the halogen and antimony oxide in each
formulation is necessary to optimize the flame retardance and lower costs.
Alumina trihydrate is not synergistic with halogenated flame retardants. It
functions as a flame retardant by the release of its water of hydration and
cannot be used in high temperature processes. Zinc borate, molybdic oxide, zinc
oxide, and magnesium oxide can be used in conjunction with antimony oxide to
augment char formation and decrease smoke. Replacement of the antimony oxide to
meet a smoke requirement compromises the flame retardance.
A relationship exists between particle size and
tinting strength. If the particles are very fine (less than 300 nanometers) they
are below the visual range and there is no tint strength. However, within the
visual range, the smaller the particle size, the higher the tint strength. Or
conversely, the larger the particle size, the lower the tint strength.