Boron compounds retardants

Inorganic boron compounds are generaHy good fire retardants (59). Bode acid, alone or in mixtures with sodium borates, is particularly effective in reducing the flammabHity of ceUulosic matetials. AppHcations include treatment of wood products, ceUulose insulation, and cotton batting used in mattresses (see Flame retardants). 

Economic Aspects and Uses. The principal producers in the United States are U.S. Borax and Chemical Corp., North American Chemicals Co., and American Borate Corp. Their combined aimual capacity in 1989 was reported to be 735,000 metric tons of equivalent boron oxide [1303-86-2], B2O2 (20). Of this toimage, 50% is exported. About 30% of boron compounds are used in glass fiber insulation. Another 30% is used in other type fibers and borosihcate glasses. Boron is also used in soaps and detergents, fire retardants, and agriculture (see Boron compounds). 

Fuel, oxygen, and high temperature are essential for the combustion process. Thus, polyfluorocarbons, phosphazenes, and some composites are flame-resistant because they are not good fuels. Fillers such as alumina trihydrate (ATH) release water when heated and hence reduce the temperature of the combustion process. Compounds such as sodium carbonate, which releases carbon dioxide when heated, shield the reactants from oxygen. Char, formed in some combustion processes, also shields the reactants from a ready source of oxygen and retards the outward diffusion of volatile combustible products. Aromatic polymers, such as PS, tend to char and some phosphorus and boron compounds catalyze char formation aiding in controlling the combustion process. 

Char also shields the reactants from oxygen and in addition retards the outward diffusion of volatile combustible products. Aromatic polymers tend to char, and some phosphorus and boron compounds tend to catalyze char formation. 

Boron compounds such as borax and boric acid are well-known fire retardants in cellulosic products and coatings.12 However, the use of boron compounds such as zinc borate, ammonium pent-aborate (APB), melamine borate, boric oxide, boron phosphate, and other metal borates in polymers has become prominent only since early 1980s.3 6 This chapter will review the chemical and physical properties, the end-use applications, as well as the mode of actions of major boron compounds as fire retardants in different applications. Since boron-based flame retardants are extensively used and quoted in literature, only those formulations of commercial importance and representative literature examples will be discussed and/or cited in this chapter. 

Wood Composites—these are resin-bonded composite boards where the particles are wood shavings, flakes, chips, or fibers bonded with thermosetting adhesives that can be urea formaldehyde, melamine formaldehyde, phenol formaldehyde, or diisocyanate. In recent years, the markets for OSB and MDF board have been rapidly increasing. Most particle board production uses urea-formaldehyde as a binder that is acid setting. Hence, sodium borates (alkaline) can interfere with the setting. As a result, boric acid has been the major boron compound used as the flame retardant in particle board.28 29 Typically, a loading of 12%-15% of boric acid in MDF is required to meet the ASTM E-84 Class A rating. If sodium borate is used as a flame retardant, phenol-formaldehyde binder, that is compatible with alkaline chemicals, is commonly used. 

FIRE RETARDANCY MECHANISM OF BORON COMPOUNDS 9.3.1 Borates in Wood/Cellulose... 

Shen, K.K. 1996. Boron compounds as fire retardants. Fire Retardant Chemical Association Spring Conference, Baltimore, MD. 

Boron. Boron compounds have been used to treat wood for fire retardancy. Borax and boric acid, the primary fire-retardant compounds, have low melting points and form glassy films on exposure to high temperature. Borax, also known as sodium tetraborate deca-hydrate, is available in other hydrated states. Sodium tetraborate pentahydrate can be used in place of the decahydrate at a weight ratio of 74 (pentahydrate) to 100 (decahydrate) (81). 

Boron compounds have been used in several ways to achieve reduced flammability of wood products. Borax and boric acid can be incorporated into particle board chips before addition of a dicyan-diamide, phosphoric acid, amino-resin system 85). They can also be used to produce a fire-retardant hardboard. Riem and Dwars 86) added water-soluble ammonium borate to wood fibers before the board was formed. A 6-7% boron content produced a hardboard that had a flame spread of 25 or less. 

The most important use of boron is still in glass manufacture. Agricultural products, fire retardants, and soaps and detergents are all made of boron compounds. 

Small amounts of boron compounds are also used to control the growth of weeds in agriculture, and as insecticides, fertilizers, and flame retardants. A flame retardant is a material that prevents another material from catching fire and burning with an open flame. 

Urea-formaldehyde Foams. While urea-formaldehyde (UF) foams can be rated as difficult to bum, blending of UF with another polymer can decrease the resistance of the foam to burning. Fire retardants, including phosphorus and boron compounds, have been added to decrease the flammability of UF foams (42). According to Frisch (42) phosphonates, furfuryl alcohol and ethylene glycol have been used as fire retardants. 

Common flame retardants are aluminum trihydrate, brominated compounds, phosphorous compounds, antimony oxide, chlorinated compounds, and boron compounds. Brominated flame retardants are preferred for thermoplastic resins such as polystyrene, polyesters, polyolefins and polyamides but are also used in epoxies, ABS and polycarbonates. Decabromodiphenyl oxide is the most common brominated flame retardant used. 

Boron, an element, occurs in many compounds, including borax, borates, boric acid, and carboxyboranes used in glass, ceramics, detergents, bleaches, fire retardants, disinfectants, alloys, specialty metals, preservatives, pesticides, and fertilizers (Mastromatteo and Sullivan 1994). Boron compounds also constitute an important group of dopants in the semiconductor industry. Dopants alter crystalline substrates electrical conductivities in the manufacturing of diodes, transistors, and capacitors (Lewis 1986). 

By acting as char formers, as phosphorous flame retardants do. They are also subdivided into nonhalogenated organophosphate esters, ammonium polyphosphate, and others. When heated, they produce a solid form of phosphoric acid that in turn chars the material and shields it from releasing of flammable gases feeding flames. Phosphorous flame retardants account for about 20% of flame retardants in the industry (mainly not with polyolefins). Boron compounds also work as char formers [2]. 

Disodium tetraborate is the source of many industrially important boron compounds. such as barium borate (fungicidal paints), zinc borate (fire-retardant additive in plastics), and boron phosphate (heterogeneous acid catalyst in the petrochemicals industry). 

Aqueous solutions of boric acid (H3BO3) and borax (Na2B207 x IOH2O) at a mass ratio of 5 to 6 were successfully used for fire-retardance of cellulosic textiles in the twenties. Sodium, potassium, and lithium borates are readily water-soluble. They therefore, continue to be used in the textile, wood, and paper industry. The increasing demand for and the rising price of antimony trioxide is another factor which has promoted research into the use of boron compounds for the flame-retardancy of plastics. 

Some organo-boron compounds have also been recently introduced as flame-retardants. 

Some flame-retardant boron compounds are shown in Table 5.13. 

Zinc borate. Zinc borate is the major boron compoimd used as a flame retardant in plastics. It competes with antimony oxide when antimony prices are high. The largest application for boron compounds as a flame retardant is in cellulose insulation. The flame-retardant categories and the major plastics where they are used are summarized in Table 4.11. 

The chemistry of flame-retardant additives is highly varied and is optimised not only for specific polymer chemistries, but also to address flammability effects such as flame spread, dripping, smoke release and so on. Flame-retardant chemistry includes classes of compounds such as halogenated organics, char formers, crosslinking compounds, mineral fillers, intumescent packages, phosphorus compounds, nitrogen-based compounds and even certain metal and boron compounds. 

---