Fire polymers combustion

Unfortunately, most fatalities in fires occur by inhalation of toxic vapours. These can be carbon monoxide (which arises from incomplete combustion), cyanides (from nitrogen-containing polymers) and chlorides (from chloropoly-mers). These are the adverse consequences of flammable polymer combustion. They can be overcome by using breathing apparatus, face masks, etc. 

Zinc borates are predominately a condensed phase fire retardant. In a halogenated system such as flexible PVC, it is known to markedly increase the amount of char formed during polymer combustion whereas the addition of antimony trioxide, a vapor-phase flame retardant, has little effect on char formation. Analyses of the char show that about 80%-95% of the antimony is volatilized, whereas the majority of the boron and zinc from Firebrake ZB remains in the char (80% and 60%, respectively).48-56 The fact that the majority of the boron remains in the condensed phase is in agreement with the fact that boric oxide is a good afterglow suppressant. The mode of action can be summarized in the following equation (not balanced). 

Price D, Anthony G, Carty P. Introduction Polymer combustion, condensed phase pyrolysis and smoke formation. In Fire Retardant Materials. Horrocks AR, Price D, Eds. Woodhead Publishing Cambridge, U.K., 2001 chap. 1, pp. 1-30. 

Kishore, K. Mohandas K. Mechanistic studies on the action of ammonium phosphate on polymer fire retardancy Combust. Flame 1981, 43, 145-153. 

Flammability is the ease with which a substance will ignite, causing fire or combustion. Combustion or burning of solid polymers is a complicated process involving physical and chemical phenomena that are only partially understood (Nelson, 2002). It is a sequence of... 

The investigation of critical conditions for polymer combustion is of great interest for the further development of the combustion theory, as well as for practical reduction of flammability of materials, fire prevention, and extinction. 

Being organic substances, polymers provide an excellent fuel source for the propagation of fire. The combustion of polymers occurs via a free-radical mechanism the heat from the fire vaporizes and ionizes the constituents of the gas forming a cloud of free radicals. The spread of combustion occurs via the well-known free radical mechanisms of propagation chain branching and termination. There are a number of ways to interrupt the chain of events involved in fire propagation ... 

EXPLOSION and FIRE CONCERNS combustible liquid NFPA rating (not rated) vapors may form explosive mixtures with air flashback along vapor trail may occur vapor explosion hazard indoors, outdoors or in sewers containers may explode in fire runoff to sewer may create fire or explosion hazard liquid is flammable when exposed to heat, sparks, or flame violent reaction with oxidizers, such as perchlorates, peroxides, chlorates, nitrates, and permanganates poisonous gases are produeed in fire, including carbon monoxide use dry ehemical, carbon dioxide, or alcohol or polymer foam for firefighting purposes. 

The oxygen index test is used for molded polymers, fabrics, expanded polymers, thin films, polymers which form char, drip, or soften, and for liquids. The data are reproducible. The test is used to study polymer combustion chemistry, fire retardant treatment of the polymers and for screening the polymers. No relationships have been established between LOI and the flame heat flux, CHF, TRP, and fire propagation rate. The application of the oxygen index test data to predict the fire propagation behavior of polymers expected in actual fires is thus uncertain. 

T. Kashiwagi, Polymer combustion and flammability - Role of the condensed phase. Presented at Twenty-Fifth Symposium (International) on Combustion, Httsburgh, 1994, pp. 1423-37. Available at http //www.fire.nist.gov/bfrlpubs/fire95/artl04.html. 

The burning characteristics of polymers cannot be thoroughly evaluated by determining a few simple fire parameters in the laboratory. Fire and combustion is very complex and at least four major features interact during the overall burning process ... 

The use of flame retardants came about because of concern over the flammabiUty of synthetic polymers (plastics). A simple method of assessing the potential contribution of polymers to a fire is to examine the heats of combustion, which for common polymers vary by only about a factor of two (1). Heats of combustion correlate with the chemical nature of a polymer whether the polymer is synthetic or natural. Concern over flammabiUty should arise via a proper risk assessment which takes into account not only the flammabiUty of the material, but also the environment in which it is used. 

Flammability. The results of small-scale laboratory tests of plastic foams have been recognized as not predictive of their tme behavior in other fire situations (205). Work aimed at developing tests to evaluate the performance of plastic foams in actual fire situations continues. All plastic foams are combustible, some burning more readily than others when exposed to fire. Some additives (131,135), when added in small quantities to the polymer, markedly improve the behavior of the foam in the presence of small fire sources. Plastic foams must be used properly following the manufacturers recommendations and any appHcable regulations. 

Acryhc elastomers are normally stable and not reactive with water. The material must be preheated before ignition can occur, and fire conditions offer no hazard beyond that of ordinary combustible material (56). Above 300°C these elastomers may pyrolize to release ethyl acrylate and other alkyl acrylates. Otherwise, thermal decomposition or combustion may produce carbon monoxide, carbon dioxide, and hydrogen chloride, and/or other chloiinated compounds if chlorine containing monomers are present ia the polymer. 

The use of fire retardants in polymers has become more complicated with the realisation that more deaths are probably caused by smoke and toxic combustion products than by fire itself. The suppression of a fire by the use of fire retardants may well result in smouldering and the production of smoke, rather than complete combustion with little smoke evolution. Furthermore, whilst complete combustion of organic materials leads to the formation of simple molecules such as CO2, H2O, N2, SO2 and hydrogen halides, incomplete combustion leads to the production of more complex and noxious materials as well as the simple structured but highly poisonous hydrogen cyanide and carbon monoxide. 

One possible solution to the problem is to make greater use of intumescent materials which when heated swell up and screen the combustible material from fire and oxygen. Another approach is to try to develop polymers like the phenolic resins that on burning yield a hard ablative char which also functions by shielding the underlying combustible material. 

A number of other monomers may be employed as variations on the materials mentioned so far, to introduce specific properties into the finished resin. For example, halogenated molecules containing either chlorine or bromine atoms may be used to confer fire resistance. As described in Chapter 8, the effect of halogens in the polymer structure is to make the resins difficult to ignite and unable to sustain combustion. 

Thermosets and thermoplastics behave differently from each other in fires. Thermosets do not melt when heated but may well undergo further crosslinking. The presence of such additional crosslinks hinders movement of any volatile degradation products through the polymer matrix. Hence the combustion zone tends to be starved of fuel and for this reason thermosets tend to be relatively non-flammable. 

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