Approximately 20,000 metric tons of antimony trioxide [1309-64-4] (commonly referred to as antimony oxide), Sb₂O₃, was used in the United States in 1990 to impart flame retardancy to plastics . Although antimony trioxide is found in nature, it is too impure to be used. Flame-retardant grades of antimony oxides are manufactured from either antimony metal or the sulfide ore by oxidation in air at 600–800°C. The particle size and chemical reactivity is determined by the processing conditions, enabling the production of several different grades. The physical properties of various grades are listed in Table. Antimony trioxide is from 99.0–99.9 wt % Sb₂O₃. The remainder consists of 0.4–0.01 wt % arsenic; 0.4–0.01, lead; 0.1–0.0001, iron; 0.005–0.0001, nickel; and 0.01–0.0001,sulfates. It is insoluble in water and the loss on drying at 110°C is 0.1 wt % max.

Antimony trioxide has been used as a white pigment since ancient times. The pigmentation from antimony oxide in plastics can be controlled and adjusted by the judicious selection of a Sb₂O₃ grade having a specific particle size. The product with the smallest particle size and the narrowest particle-size range imparts the whitest color and highest opacity. Translucent plastics can be made by using low tint grades with relatively large particles.

Particle size during manufacture is controlled by adjusting the temperature and rate at which the antimony vapors are precipitated as these vapors exit the furnace. The lower the temperature and the slower the precipitation rate, the larger the particles. Figure 1 shows the particle size distribution of the various commercially available grades of antimony trioxides. Although particle size affects pigmentation, it does not appear to affect flame retardancy efficiency. Suppliers and the grades offered are 

 Antimony Oxide as a Primary Flame Retardant

Antimony oxide behaves as a condensed-phase flame retardant in cellulosic materials. It can be applied by impregnating a fabric with a soluble antimony salt followed by a second treatment that precipitates antimony oxide in the fibers. When the treated fabric is exposed to a flame, the oxide reacts with the hydroxyl groups of the cellulose (qv) causing them to decompose endothermically. The decomposition products, water and char, cool the flame reactions while slowing the production and volatilization of flammable decomposition products.

 Antimony Pentoxide

The second most widely used antimony synergist is antimony pentoxide [1313-60-9], Sb₂O₅, produced by the oxidation of the trioxide using either a peroxide or nitric acid. Antimony pentoxide is available as a nonpigmenting colloidal suspension in either water or organic media or as an agglomerated powder. It is insoluble in water, but soluble in hot concentrated acids. Properties of this unique flame retardant synergist are listed in Table.

Submicrometer antimony pentoxide is primarily used to impart flame retardancy to fibers and fabrics. It can be added to the molten or dissolved polymer prior to forming the fiber. The antimony in this form can easily pass through the spinnerets without clogging the openings, whereas standard grades of antimony trioxide would rapidly clog the openings and necessitate frequent shutdowns for cleaning. The submicrometer antimony pentoxide is also more evenly dispersed in the fiber, resulting in better physical properties.

Powdered antimony pentoxide is used primarily in plastics. Stabilizers used to prevent the particles from growing are caustic, and can react with the halogen in the formulation. This can result in color formation and a lower flame-retarding efficiency of the system.

Antimony pentoxide is priced about two to three times higher than the trioxide. However, because it is more efficient than the trioxide, the pentoxide is at least cost-equivalent. Antimony pentoxide is manufactured by both Philadelphia Quartz and Laurel Industries under the Nyacol and Fireshield trade names.

Sodium antimonate [15593-75-6], Na₃SbO₄, another antimony synergist of commercial importance, has an antimony content of 61–63 wt % and a bulk density of 39.4–46.4 kg/m³. Properties are given in Table 2. It is made by oxidizing antimony trioxide using sodium nitrate and caustic. It is a white powder and has a pH of around 9–11 when dissolved in water.

Sodium antimonate contains less antimony than either antimony trioxide or pentoxide and is thus less effective. However, its unique pH and low refractive index makes the antimonate the most desirable synergist for polymers that hydrolyze when processed with acidic additives or in polymers for which deep color tones are specified. Sodium antimonate costs approximately $3.30–4.40/kg and can be obtained from either Elf Atochem NA under the Thermoguard name or from Anzon Inc. as a Timinox product.

Antimony has been found not to be a carcinogen or to present any undue risk to the environment . However, because antimony compounds also contain minor amounts of arsenic which is a poison and a carcinogen, warning labels are placed on all packages of antimony trioxide.

Worldwide scarcities of antimony have prompted manufacturers to develop synergists that contain less antimony. Other metals have been found to work in concert with antimony to form a synergist that is as effective as antimony alone. Thermoguard CPA from Elf Atochem NA, which contains zinc in addition to antimony, can be used instead of antimony oxide in flexible poly(vinyl chloride) (PVC) as well as somepolyolefin applications. The Oncor and AZ products which contain silicon, zinc, and phosphorus from Anzon Inc. can be used in a similar manner. The mixed metal synergists are 10 to 20% less expensive than antimony trioxide.

Antimony synergists are used almost exclusively with either brominated or chlorinated organic flame retardants. These work in concert with one another and provide a highly effective flame-retardant system. Antimony and the halogens react at flame temperatures to form the corresponding trihalide or oxyhalide. The product formed depends on the mole ratios of the reactants and the structure of the organic halogen compound. The active flame-retarding species, ie, the tribromide and the trichloride  are formed directly when the mole ratio of halogen to antimony is at least 3-to-1 and the halogen compound is capable of dehydrohalogenating.