In 1990 approximately 4500 metric tons of boron flame retardants were used in the United States to impart flame retardancy to plastics. The most widely used is zinc borate [1332-07-6], prepared as an insoluble double salt from water-soluble zinc and boron compounds. Compounds having varying amounts of zinc, boron, and water of hydration are available. The ratio of these components affects the temperature at which the flame-inhibiting powers are activated, as well as the temperature at which they can be processed. Zinc borates can either be used alone or in combination with other halogen synergists, such as antimony oxide. In some instances zinc borate is also used with alumina trihydrate to form a glass-like substance that inhibits polymer degradation. Manufacturers of boron flame retardants are listed in Table 3. The cost of the borates varied from $2.00–2.50/kg in the early 1990s. Manufacturers and Trade Names of Boron Flame Retardants
Barium metaborate is used both as a flame retarder and as an antifungicide for many flexible poly(vinyl chloride) applications. Boric acid [10043-35-3] and sodium borate [1303-96-4], also known as borax, have been used as flame retardants for cellulose since the 1800s. They are quick-fix, nondurable flame retardants. Each is applied by passing the fabric through a solution of the flame retardant. For batts, eg, cotton, a spray applicator is used. Excess solution is removed by passing the fabric through squeeze rollers. Usually an “add-on” of 20% is needed to obtain flame retardancy. However, because the flame retardants are water-soluble, they are removed after several washings or when used in high humidity atmospheres. Boric acid and borax are available from Ashland Chemical, U.S. Borax, and J.H. Henry Chemical Co. Ammonium fluoroborate [13826-83-0], NH4BF4, is unique in that when it is exposed to a flame, it generates both a halogen and a boron flame retardant . Antimony oxide is usually recommended as a co-synergist. Boron Mechanism Boron functions as a flame retardant in both the condensed and vapor phases. Under flaming conditions boron and halogens form the corresponding trihalide. Because boron trihalides are effective Lewis acids, they promote cross-linking, minimizing decomposition of the polymer into volatile flammable gases. These trihalides are also volatile; thus they vaporize into the flame and release halogen which then functions as a flame inhibitor. Boron also reacts with hydroxyl-containing polymers such as cellulose. When exposed to a flame the boron and hydroxyl groups form a glassy ester that coats the substrate and reduces polymer degradation. A similar type of action has been observed in the boron–alumina trihydrate system. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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