UNITED STATES ANTIMONY CORPORATION (USAC) has produced various antimony products for more than
  thirty-six years.  Production was originally from a large underground mine reserve of antimony ore in Thompson
  Falls, Montana, and now from other sources.  Additional raw material supplies are being developed by United
  States Antimony, Mexico, S.A. de S.V. (ADM), a USAC joint venture with a Mexican Corporation, from several
  mines in Mexico.
 

                                                                     USAC FACILITIES

  The manufacturing facility is located near the mine at Thompson Falls, Montana, and the product is warehoused
  throughout the United States. The plant is unique, because it has the capability of treating a diverse supply of raw
  materials. Unlike other producers, USAC dedicates a production line to each different product to insure consistent
  quality. Besides Antimony Oxide VF, MP, HT,  LT, Glass Grade, FR Grade and Catalyst Grade, the Company
  produces antimony metal, sodium antimonate, and a wide variety of antimony specialty compounds. USAC has
  an ongoing research program for new antimony products development. The Company has focused on the supply
  of antimony products, and has avoided competing with the consumer in compounding operations.

FLAME RETARDANT (FR) 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. 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 specification.

  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 600^o 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 vapor phases.

  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  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 retraces 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.