Retardation, chemical mechanical

The antimony oxide/organohalogen synergism in flame retardant additives has been the subject of considerable research and discussion over the past twenty-five years (1-17). In addition to antimony oxide, a variety of bismuth compounds and molybdenum oxide have been the subject of similar studies (18-20). Despite this intensive investigation, relatively little has been conclusively established about the solid state chemical mechanisms of the metal component volatilization, except in those cases where the organohalogen component is capable of undergoing extensive intramolecular dehydrohalogenation. 

Polyamide-imides are appreciated for good mechanical and electrical properties high service temperatures (up to 220°C with possible long service times at 260°C) rigidity good creep behaviour fatigue endurance low shrinkage and moisture uptake inherent flame retardancy chemical resistance usability down to -196°C. 

The most widely accepted theory of the mechanism of fire-retardant chemicals in reducing flaming combustion of wood is that the chemicals alter the pyrolysis reactions with formation of less flammable gases and tars and more char and water (4,5,8,21,24-29). Some fire retardants start and end the chemical decomposition at lower temperatures. Heat of combustion of the volatiles is reduced. Shafizadeh (21) suggests that a primary function of fire retardants is to promote dehydration and charring of cellulose. 

The different flame-retardant (FR) mechanisms of action of current nanoparticles, such as layered silicates, carbon nanotubes (CNTs), and nano-oxides or -hydroxides, according to their nature and interfacial modifications, are relatively well known and detailed in numerous works.5 13 These mechanisms are rather different from those exhibited by usual FRs and correspond mainly to the following physical, physicochemical, or chemical actions ... 

Lewin M. Physical and chemical mechanisms of flame retarding of polymers. In Fire Retardancy of Polymers. The Use of Intumescence. Le Bras M, Camino G, Bourbigot S, Delobel R, Eds. Royal Society of Chemistry London, 1998 pp. 3-32. 

Free radical trap theories. Fire-retardant chemicals release free radical inhibitors at pyrolytic temperatures that interrupt the chain propagation mechanism of flammability. 

Basic Mechanisms. Finally, further work is necessary on fundamental mechanisms of individual fire retardants. These mechanisms are a function of the particular chemicals involved and the environmental conditions of the fire exposure. There is a need to establish common methods and conditions for determining these mechanisms in order to compare different treatments. This would give us a better understanding of how these compounds work in action and would provide a more efficient approach for formulating fire-retardant systems than a trial and error approach. Correlations also need to be established between rapid precise thermal analysis methods and standard combustion tests. Retardant formulations could be evaluated initially on smaller (research and development size) samples. The more promising treatments could be tested for flame-spread index, heat release rate, and toxic smoke production. 

In addition to the chemical mechanisms of fire retardants, thermal or barrier-type mechanisms may be operative. Coatings may prevent oxygen from reaching the wood surface. Dilution of combustible gases by noncombustible gases and inhibition of flaming by free radicals can also be in effect. Therefore, fire retardancy of wood involves many complex reactions. The effectiveness of a particular fire retardant depends on the overall summation of these competitive and sequential reactions and the thermal and physical environment of the material. 

As in polyester resins, reactive halogens containing fire-retardant chemicals are most often used in epoxy materials. Tetrabromobisphenol A is perhaps the most widely used component for flame-retarding epoxy resins. Nara and Matsuyama (24) and Nara et al. (25) described the thermal degradation and flame retardance of tetrabrominated bisphenol A diglycidyl ether compared to the nonbrorainated structure. Their results indicate that bromine acts by vapor-phase as well as condensed-phase mechanisms of flame inhibition. 

Biomass-based fiiran resins use furfuryl alcohol (FA) [17]. It has affected the success of the rather old biomass-based chemical industry. This reactive alcohol plays a vital role in the production of foundry sand binders. For over 30 years these fiiran resins have been extensively used in the foundry industry to produce cores that yield high-quality castings. Despite the good chemical, mechanical and thermal properties of FA resins/polymers such as corrosion resistance, fiame retardancy, low smoke emission and excellent char formation, the consumption of these bio-based resins in other markets is of minor significance ... 

PI with polyimide-type LCP, Tm > 300 °C Processability, chemical resistance, flame retardancy and mechanical strength Asanuma et al. 1993... 

With the need to improve both performance and toxicity of fire retardants, there is an emerging focus onnanomaterials. The chemical mechanisms offire retardance using nanomaterials are a subject with a great potential for exploration in the coming years. 

There are two basic types of flame-retardant chemicals reactive FRs and additive FRs. Reactive FRs are usually introduced during the polymerization stage and copolymerized, together with other monomers. They therefore have only minimal effect on mechanical properties. Typical examples are tetrabromobisphenol A, dibromoneopentyl glycol, vinyl chloride, and bromo- or dibromostyrene. 

More attractive, however, is the question of what the chemical mechanism of unaccelerated sulfur vulcanization is. A free radical mechanism as first assumed [119-121] had to be abandoned because no evidence was found that free radicals are envolved [116,117] these sulfur-olefin reactions are insensitive to free-radical initiators and do not respond to free-radical retarders or inhibitors. 

Chlorine is believed to perform its flame retardant function in both the gaseous and condensed phases. In the gaseous phase, it employs the chemical mechanism of redirection or termination of the chemical reactions involved in combustion, and the physical mechanism of evolution of... 

The major applications of LCPs are in metal and ceramic replacements that require resistance to high temperatures, chemicals, mechanical stress, creep resistance, and so forth. LCP parts include electronic and electrical connectors, sockets and pin-grid arrays exposed to high-temperature manufacturing or service conditions, automotive and aerospace parts that require the ability to withstand high temperatures and flame retardance, and chemical-processing components that exist in aggressive environments. 

Lubricating greases are essential for the performance of friction units in various machines and mechanisms. In these cases, surfactants play a dual role at early stages of their use, they enhance wear and thus facilitate a breaking-in of the friction surfaces, while at later stages, they form protective adsorption layers that help to retard wear in the parts. The role of surface-active media in the processes of friction and wear is the subject of tribology, which constitutes a separate area of physical-chemical mechanics [95-106]. 

Examples are aluminum hydroxide and magnesium hydroxide. On the other hand, there is no single flame retardant that will operate exclusively through a chemical mode of action. Chemical mechanisms are always accompanied by one or several physical mechanisms, most commonly endothermic dissociation or dilution of fuel. Combinations of several mechanisms can often be synergistic. 

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