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Tin has been used as a flame retardant for cellulose since the latter 1800s. Only since the 1970s has it been used as a synergist for halogen flame retarders in the same manner as antimony oxide. Anhydrous and hydrated zinc stannate and stannic oxide are the three most important tin flame retardants. Properties are given in Table.

Properties of Tin Flame Retardants

ParameterZinc hydroxy stannateZinc stannateStannic oxide

molecular formulaZnSn(OH)6ZnSnO3SnO2·xH2O
CAS Registry Number[12027-96-2][12036-37-2]
tin, wt %475167
zinc, wt %2128
specific gravity3.44.25
decomposition temperature, °C180180

a All materials are white powders. Particle sizes range from 1–10 µm.

The mechanism by which tin flame retardants function has not been well defined, but evidence indicates tin functions in both the condensed and vapor phases. In formulations in which there is at least a 4-to-1 mole ratio of halogen to tin, reactions similar to those of antimony and halogen are assumed to occur. Volatile stannic tetrahalide may form and enter the flame to function much in the same manner as does antimony trihalide.

If the tin source is anhydrous, very little volatile tin tetrahalide is formed. If the mole ratio of halogen to tin is less than 4-to-1 or if there is no halogen present, yet flame retardancy is observed, condensed-phase activity is assumed. The only supplier of tin flame retardants is Alcan Inc. The price in 1992 was $7.70/kg.