Metal hydroxide flame

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. 

Inert Gas Dilution. Inert gas dilution involves the use of additives that produce large volumes of noncombustible gases when the polymer is decomposed. These gases dilute the oxygen supply to the flame or dilute the fuel concentration below the flammability limit. Metal hydroxides, metal carbonates, and some nitrogen-producing compounds function in this way as flame retardants (see Flame retardants, antimony and other inorganic compounds). 

Thermal Quenching. Endothermic degradation of the flame retardant results in thermal quenching. The polymer surface temperature is lowered and the rate of pyrolysis is decreased. Metal hydroxides and carbonates act in this way. 

In 2000, NEC developed an epoxy resin with what it describes as a fire-retardant structure that avoids the need for either TBBA or phosphorus-based flame retardants in circuit boards. The new resin contains a metal hydroxide retardant. The company claims the new board is almost totally free of pollutants, and is easy to process and thermally recycle. By also integrating flame retardant properties within the board, use of the metal hydroxide is minimised, while offering good electrical properties, higher heat resistance and improved processing characteristics. ... 

Complex organo-silane products are now emerging which seem able give significant improvements in the impact strength of highly filled polyolefins, especially when used in conjunction with metal hydroxide flame retardants [62]. This may significantly increase their use. 

Metal hydroxides in combination with various silicon-containing compounds have been used to reduce the amount of additive required to achieve a required level of flame retardancy in a variety of polymeric materials, including polyolefins.62-63 Systems that have been used contain a combination of reactive silicone polymers, a linear silicone fluid or gum, and a silicone resin, which is soluble in the fluid, in addition to a metal soap, in particular magnesium stearate. However, there is little insight given into how these formulations work. 

It has been shown that the required loading levels of metal hydroxides to flame retard polyolefins can be reduced by the addition of transition metal oxides as synergistic agents. For example, a combination of 47.6% MH modified with nickel oxide in PP gave a UL94 V-0 flammability rating, which would require -55% of unmodified MH.4 These systems, however, can only be used where the color of the product is not important. 

The addition of metal nitrates to improve the flame retardancy of metal hydroxides and EVA has been reported.64 Synergistic behavior was observed by an addition of 2% of copper nitrate to EVA containing only 33% ATH, in which the oxygen index was raised from 19.9% to 30.0%. 

The addition of silane cross-linkable PE copolymer to PE/metallic hydroxide systems can significantly improve the flame-retardant properties of these materials allowing lower filler levels to be used.69... 

The combination of melamine with hydrated mineral fillers can improve the fire retardancy behavior of PP, eliminating at the same time the afterglow phenomenon associated with these fillers used in isolation.70 Similarly in EVA copolymer, antimony trioxide used in combination with metal hydroxides has been reported to reduce incandescence.56 Chlorinated and brominated flame retardants are sometimes used in combination with metal hydroxides to provide a balance of enhanced fire-retardant efficiency, lower smoke evolution, and lower overall filler levels. For example, in polyolefin wire and cable formulations, magnesium hydroxide in combination with chlorinated additives was reported to show synergism and reduced smoke emission.71... 

Shen, K.K, Olson, E., Amigouet, P., and Tong, C. 2006. Recent advances on the use of metal hydroxides and borates as fire retardants in halogen-free polyolefins. The 17th Annual BCC Conference on Flame Retardancy, Stamford, CT, May. 

New developments in the use of silicates to improve flame retardancy have arisen from the use of synthetic anionic clays that correspond to the family of lamellar mixed metal hydroxides, commonly named layered double hydroxides (LDH) or hydrotalcite-like compounds.17... 

Metallic interlayer (MIL) influences the chemical processes of FR. The role of metal ions in the degradation process has been summarized by Lewin and Endo [30], The advantageous effect of a MIL around metal hydroxide flame retardants was utilized at first by Hornsby et al. [31]. They proposed a zinc-hydroxy-stannate (ZnHSt) layer, the detailed chemical-physical structure and 4.7 nm thickness... 

Marosi, G., Keszei, S., Matko, S., and Bertalan, G. 2006. Effect of interfaces in metal hydroxide-type and intumescent flame retarded nanocomposites. In Fire and Polymers TV Materials and Concepts for Hazard Prevention, Vol. 922, eds. Wilkie, C. and Nelson, G. Washington, DC ACS, pp. 117-30. 

Halogenated compounds such as bis(alkyl ether)tetrabromobisphenol A or decabromodiphenyl oxide (DECA) may be used as flame-retardants for polyolefin foams, eventually using antimony oxide, metal oxides, boric acid salts, and metal hydroxides as synergist.92 For example Weil and Levchik93 reported that using suitable amounts of DECA and Sb203, polyethylene foams rated UL94 HF-1 are obtained. 

Lin, T.S., Bunker, S.P., Whaley, P.D., Cogen, J.M., and Bolz, K.A. Evaluation of metal hydroxides and coupling agents for flame resistant industrial cable applications, 54th TWCSlFocus International Wire and Cable Symposium, Providence, RI, 2005, pp. 229-236. 

There are a number of flame-retarding mechanisms that operate in the solid phase of polymers. One is to use additives that absorb some of the heat of combustion by endothermic reactions this was mentioned in the previous section in connection with metal hydroxides. 

A number of flame-photometric methods have been developed by Sugden and co-workers [131—134] to measure hydrogen atom concentrations in the burnt gas from hydrogen—oxygen flames. When small quantities of a sodium (or similar) salt are added to a flame, and if the flame temperature is high enough, thermal sodium D-line emission occurs. At low concentrations this emission is proportional to the concentration of the metal atoms. However, if lithium salts are added to the flame, some hydroxide is formed [135] by the process... 

The bond dissociation energies of the alkali metal hydroxides have been determined by several workers from flame studies (9 -11) and by mass spectrometry (1 ). The data for KOH are summarized below. 

The equilibrium data of Sugden and co-workers (1, 2) are likely to contain significant errors, since the dissociation constants were calculated with OH concentrations determined from measured flame temperatures and known gas compositions. It is now well established (8, 9) that flame radical concentrations vary greatly with the distance from the reaction zone of the flame. The dissociation pressure data of Johnston and Ditmars and Johnston (see LiOH(t) table) show excessive drifts which usually are indications of nonequilibrium measurements. However, even the remaining flame work (3- ) and vapor pressure data still show a scatter of over 3 kcal mol in AjH (298.15 K) of LiOH(g). Cotton and Jenkins (4) investigated the other alkali metal hydroxides, and their data lead to A H (298.15 K) values for these compounds which are quite consistent with JANAF data (10). On the other hand, the flame studies of McEwan and Phillips (5) as a function of temperature do not show significant drift yet, the... 

The bond dissociation energies of the alkali metal hydroxides have been the subject of a number of investigations (1[ -4) in recent years. The majority of this work has involved equilibrium studies on the reaction A(g) + HpO(g) = AOH(g) + H(g), where A is an alkali metal, in hydrogen-oxygen-nitrogen flames containing water. In the case of NaOH, data are summarized below ... 

Sugden and his co-workers " have developed a number of special techniques for the estimation of intermediates in flames, with particular reference to atomic hydrogen and hydroxyl radicals. In each case the technique involves the addition to the reaction mixture of traces of metal salts, which lead to the emission of radiation in the flame. The basis of the first method is a comparison of the relative intensities of the lithium and sodium resonance lines emitted when salts of these metals are added in equal concentrations to the flame. Lithium hydroxide is stable at the flame temperatures, and since water is one of the combustion products the lithium concentration is modified by the equilibrium reaction... 

The most common reagents for open-vessel decomposition of inorganic analytical samples are the mineral acids. Much less frequently, ammonia and aqueous solutions of the alkali metal hydroxides are used. Ordinarily, a suspension of the sample in the acid is heated by flame or a hot plate until the dissolution is judged to be complete by the total disappearance of a solid phase. The temperature of the decomposition is the boiling (or decomposition) point of the acid reagent. 

Since all the free radicals derived from the flame gas matrix are buffered to their equilibrium proportions by the reservoir of the major constituents, they can themselves act as buffers to the minor constituents which will distribute themselves over the possible states accordingly. The interactions may however be with the radical excess or against it. For example, a metallic hydroxide may be made by two alternative processes ... 

Isarov A, et al. Non-halogen flame retardant polyolefin compounds via synergistic blends of metal hydroxides and mineral fillers. International Polyolefins, conference proceedings. Society of Plastics Engineers 2007. 

Hydroxides also are known to be formed in flames. Species such as LiOH, CsOH, CuOH, MnOH, InOH, and CaOH all have been observed. In some cases as much as 80% of the element can be present as the metal hydroxide, depending on flame temperatures and composition. 

Spectral band interference also can be troublesome. At low dispersion a spectral band appears as a single, broad band. With high dispersion the fine structure of the band appears. Frequently encountered bands in flame emission spectroscopy include those produced by OH, C2, CH, metal oxides, and metal hydroxides. 

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