Why is antimony oxide used as a flame retardant?
According to FLAME RETARDANT THEORY:

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.

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Two mechanisms exist in the synergistic system.
First is the &quot;free radical capture&quot; process that takes place in the vapor phase. On combustion at a temperature of over 600 [degrees] 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 &quot;free radicals propagate&quot; the flame. But, the antimony trihalides or antimony oxyhalides act as &quot;free radical traps&quot;, and take up free radicals. They inhibit ignition and pyrolysis in the solid, liquid, and vapor phases.
A second process occurs in a solid phase and is the &quot;char process&quot;. The antimony oxide promotes the formation of &quot;char&quot; (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 SbCl3 and SbOCl 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.

See also Flame retardant mechanism of antimony trioxide