Flame retardancy decomposition

Antimony Oxide as a Primary Flame Retardant. Antimony oxide behaves as a condensed-phase flame retardant in cellulosic materials (2). It can be appHed 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 (see Flaa retardants for textiles). 

Molybdenum trioxide is a condensed-phase flame retardant (26). Its decomposition products ate nonvolatile and tend to increase chat yields. Two parts of molybdic oxide added to flexible poly(vinyl chloride) that contains 30 parts of plasticizer have been shown to increase the chat yield from 9.9 to 23.5%. Ninety percent of the molybdenum was recovered from the chat after the sample was burned. A reaction between the flame retardant and the chlorine to form M0O2 012 H20, a nonvolatile compound, was assumed. This compound was assumed to promote chat formation (26,27). 

In order for a soHd to bum it must be volatilized, because combustion is almost exclusively a gas-phase phenomenon. In the case of a polymer, this means that decomposition must occur. The decomposition begins in the soHd phase and may continue in the Hquid (melt) and gas phases. Decomposition produces low molecular weight chemical compounds that eventually enter the gas phase. Heat from combustion causes further decomposition and volatilization and, therefore, further combustion. Thus the burning of a soHd is like a chain reaction. For a compound to function as a flame retardant it must intermpt this cycle in some way. There are several mechanistic descriptions by which flame retardants modify flammabiUty. Each flame retardant actually functions by a combination of mechanisms. For example, metal hydroxides such as Al(OH)2 decompose endothermically (thermal quenching) to give water (inert gas dilution). In addition, in cases where up to 60 wt % of Al(OH)2 may be used, such as in polyolefins, the physical dilution effect cannot be ignored. 

C2HgNg H4O2P2 (60). The pyrophosphate is reported to be only soluble to the extent of 0.09 g/100 mL water, whereas melamine orthophosphate is soluble to 0.35 g/mL. The pyrophosphate is the most thermally stable. Melamine orthophosphate is converted to the pyrophosphate with loss of water on heating. AH three are available as finely divided soflds. AH are used commercially in flame-retardant coatings (qv) and from patents also appear to have utihty in a wide variety of thermoplastics and thermosets. A detaHed study of the thermal decomposition of the these compounds has been pubHshed (61). 

Dehydration or Chemical Theory. In the dehydration or chemical theory, catalytic dehydration of ceUulose occurs. The decomposition path of ceUulose is altered so that flammable tars and gases are reduced and the amount of char is increased ie, upon combustion, ceUulose produces mainly carbon and water, rather than carbon dioxide and water. Because of catalytic dehydration, most fire-resistant cottons decompose at lower temperatures than do untreated cottons, eg, flame-resistant cottons decompose at 275—325°C compared with about 375°C for untreated cotton. Phosphoric acid and sulfuric acid [8014-95-7] are good examples of dehydrating agents that can act as efficient flame retardants (15—17). 

Bromine compounds are often used as flame retardant additives but 15-20ptsphr may be required. This is not only expensive but such large levels lead to a serious loss of toughness. Of the bromine compounds, octabromo-diphenyl ether has been particularly widely used. However, recent concern about the possibility of toxic decomposition products and the difficulty of finding alternative flame retarders for ABS has led to the loss of ABS in some markets where fire retardance is important. Some of this market has been taken up by ABS/PVC and ASA/PVC blends and some by systems based on ABS or ASA (see Section 16.9) with polycarbonates. Better levels of toughness may be achieved by the use of ABS/PVC blends but the presence of the PVC lowers the processing stability. 

Substances applied to or incorporated in a combustible material (e.g. organic polymers, nylon, vinyl and rubber, etc.) to reduce flammability. Act by retarding ignition, control/douse burning, reduce smoke evolution. Slow down or interrupt the self-sustained combustion cycle when the heat-flux is limited. Flame retardants (FRs) improve the combustion behaviour and alter the combustion process (cool, shield, dilute, react) so that decomposition products will differ from nonflame retarded articles. FRs are usually divided into three classes ... 

Major disadvantages of inorganic FRs are their relatively low decomposition temperature and requirement of a large fraction of material to give sufficient flame retardant performance in most polymers. 

The flame retardant mechanism for phosphorus compounds varies with the phosphorus compound, the polymer and the combustion conditions (5). For example, some phosphorus compounds decompose to phosphoric acids and polyphosphates. A viscous surface glass forms and shields the polymer from the flame. If the phosphoric acid reacts with the polymer, e.g., to form a phosphate ester with subsequent decomposition, a dense surface char may form. These coatings serve as a physical barrier to heat transfer from the flame to the polymer and to diffusion of gases in other words, fuel (the polymer) is isolated from heat and oxygen. 

Chlorinated additive flame retardants, 11 468-470, 471-473t Chlorinated aromatics, 6 242 decomposition using microwaves, 16 555 Chlorinated butyl rubber, 4 436 development of, 4 434 manufacture, 4 400, 442-444 Chlorinated ethanes... 

The use of phosphorus compounds as flame retardants has been reviewed by Lyons and others (1, 2, 3, 4 5). The mechanism of the action of this element is generally accepted to involve decomposition to produce acids which function as char promoters. Phosphorus compounds are particularly effective flame retardants for polyesters where they function to increase the char yields. 

The mechanism of the action of the phosphonate as a flame retardant is generally believed to be decomposition into acid fragments which contribute to char formation. These acidic species catalyze decomposition of the polyester, and give rise to species which on reaction with the phosphorus moiety cause char formation. TGA curves of the copolymers confirm that the incorporation of phosphorus into the polymer increases the char residue (Figure 4). These curves, however, show little evidence that the presence of phosphorus has any effect upon the temperature or rate of decomposition of the polyester. The curves are all fairly similar up to about 450°C. After that point, the amount of residue is proportional to the amount of phosphorus in the terpolymer. 

The PBDEs (decaBDE, octaBDE, and pentaBDE) and are used as flame retardants in plastics, electronic equipment, printed circuit boards, vehicles, furniture, textiles, carpets, and building materials. Global demand has increased rapidly since the 1970s with 70,000 tonnes produced in 2001. Their flame retardant activity relies on decomposition at high temperatures, leading to the release of bromine atoms. This slows the chemical reactions that drive 02-dependent fires. HBCDs are a flame retardant added to extruded and expanded polystyrene that is used as thermal insulation in buildings. 

It should be noted that both KNO3 and K2SO4 are useful additives for eliminating the luminous flame generated at a rocket nozzle and also for suppressing the formation of muzzle flash generated at the exit of a gun barrel. The potassium atoms generated in the gun barrel by the decomposition of these potassium salts are believed to act as a flame retardant. 

Goethite is used in flame retardants and smoke suppressants. Both laboratory and large scale pilot tests showed that goethite is the most active smoke suppressant when polymers and plastics are burned (Carty and White, 1999 Carty et al., 1999). It reduces the amount of smoke produced during pyrolysis in air of chlorinated PVC plasticized with dioctylphthalate, by changing the decomposition pathway followed by phthalate, so that benzene, which is produced in the absence of the smoke suppressant, is not formed (Carty et al., 1999). 

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