Polymer combustion

Polymeric foamed materials are very specific in terms of ignitability and flame spread. It has been shown that differences in the surface area of foamed polymers and cell size have a larger effect on flammability than do density or differences in chemical structure. The chemical stmcture, of course, may dictate the surface area or porosity in the formation of foam. For example, flexible polyurethane foams can be ignited by a smoldering cigarette. A textile material normally used to enclose the foam, as is common in upholstered furniture and mattresses, actually helps ignition if snitable flame retardant-treated textiles are not used. 

The possibility of extinguishing a polymer flame depends on the mechanism of thermal decomposition of the polymer. Whereas ignition of a polymer correlates primarily with the initial temperatnre of decomposition, steady combustion is related to the tendency of the polymer to yield a char, which is produced at the expense of combnstible volatile fragments. Therefore, the dependence of steady combnstion on the amonnt of char seems to be simple, and in an early study it was established that the oxygen index shows a very good correlation with the char yield. In reality, char also serves as a physical barrier for heat flux from the flame to the polymer surface, as weU as a diffusion barrier for gas transport to the flame. Therefore, the contribution of the char can be more significant than is expected from a simple reduction in combustible gases. 

Four general mechanisms are important for thermal decomposition of polymers (1) random chain scission, in which the polymer backbone is randomly split 

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Combined cycles Combined reforming Combing Comb polymers Combustion... 

Whilst the development of flame retarders has in the past been largely based on a systematic trial-and-error basis, future developments will depend more and more on a fuller understanding of the processes of polymer combustion. This is a complex process but a number of stages are now generally recognised and were discussed in Chapter 5. 

Change the nature of the polymer combustion process to reduce smoke formation. 

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. 

It has been estimated that over 100,000 workers are potentially exposed to acrylonitrile during production and use (NIOSH 1977, 1988). Occupational exposures include plastic and polymer manufacturers, polymer molders, polymer combustion workers, furniture makers, and manufacturers of fibers and synthetic rubber (EPA 1980a). Other populations who could have elevated exposure to acrylonitrile are residents in the vicinity of industrial sources or chemical waste sites. 

Tewarson, A., Jiang, F. H. and Morikawa, T., Ventilation-controlled combustion of polymers, Combustion and Flame, 1993, 95, 151-69. 

The action of organic solvents on natural polymers combustible minerals (coal and brown coal or peat) is intensively studied for a long time due to following reasons. Firstly, this is one of the successful method of studying the structure of combustible materials and the second is their technological application for obtaining of a so-called montan-wax or low-molecular liquid extracts which can be transformed into synthetic liquid fuel due to hydration process. Moreover, an interaction of a coal with the solvents is a basis of the coals liquation processes and coals transformation into liquid fuel. 

The thermodynamic constants of THF polymerization have been investigated by a number of authors. A variety of experimental techniques have been utilized including determinations of conversion to polymer, combustion, heat capacities eind vapor pressure. Comparison of our results with some previously published data shows that our results are within the range of the values reported (Table 3). 

Then, the influences by which the presence of oxygen can affect these processes are discussed. The different structures of the various polymer types influence the end consequence of any decomposition, and this may affect the resistance, if any, to combustion. At this point, the polymer combustion cycle will be described. 

FIGURE 2.2 Schematic representation of the polymer combustion cycle main approaches to flame retardancy are shown in italics. 

FIGURE 4.1 Self-sustained polymer combustion cycle. 

Anhydrous borax (Na20 2B203), commercially known as Dehybor and often called AB, does not rehydrate under ordinary storage conditions, but it can absorb surface moisture. With a melting point of 742°C, it is an excellent flux and glass former.7 Thus, it is an effective additive for cerami-fication during polymer combustion in applications such as wire and cable, sealants, and so on. Its potential as a flame retardant in polymers has not been fully explored. 

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). 

Zinc borate and phosphorus compounds—the synergy between Firebrake ZB and APP is well documented in epoxy. In polypropylene, it has been shown that the synergy, as evidenced by LOI, derives from the formation of zinc phosphate and borophosphate during polymer combustion.110... 

Zinc borate in PC/ABS and polyamide—when zinc borate is used in conjunction with bisphenol-A-bis-diphenyl phosphate in PC/ABS, it was reported that borophosphate and zinc phosphate were generated during polymer combustion.111 The formation of these materials could be beneficial for passing the more stringent Are tests, such as UL-95 5 V. When Firebrake 500 is used in conjunction with aluminum diethylphosphinate and melamine polyphosphate in polyamide, Schartel et al. reported the formation of boron aluminum phosphate in the condensed phase.112... 

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. 

The second approach is to use the formalism of a 6 function, i.e., equation (15) which implies that unstable polymer combustion, once started, will continue until 9 vanishes. Then the integration step can be made arbitrarily s all, which physically corresponds to an infinitely small mixing time in the reactor. Both approaches were shown to give exactly the same results. 

In an overwhelming majority of cases, heated polymers decompose into combustible gaseous products. The latter diffuse into the oxidative environment, mix and react with oxygen. Therefore, the self-enhancing exothermal reaction responsible for the heat generation and initiation of polymer combustion is in most cases a homogeneous gas phase reaction In order to stress this particular feature of the reactions between... 

The linear velocity of polymer combustion propagation (v) may be represented in terms of the ignition temperature concept (Tj J by a simple fundamental formula... 

Detailed information about material flammability derived from macroscopic combustion parameters is given by Petrella who considered the effect of an external heat source on the mass rate of polymer combustion. From the energy conservation law it follows ... 

Consequently the mass transfer rate during diffusion combustion of polymers is determined by the ratio of the heat of combustion to the effective enthalpy of polymer gasffication. The lower the combustion heat and the higher the polymer gasification enthalpy or, in other words the more heat resistant the polymer, the lower is the B value. For polymer combustion in air the B value of e.g., PMMA varies between... 

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. 

The authors of Ref. generalized all the published polymer combustion limits from the viewpoint of the effect of different factors on the cooling of the reaction zone. At the extinction limit of diffusion combustion, the ratio of heat losses from the front edge of the combustion zone to the total heat generation due to the chemical reaction must be proportional to RT, /E here, Tj is the flame temperature at the extinction limit and E the gas-phase reaction activation energy... 

Properties White, granular solid nonvolatile mildly aromatic odor. D 1.20 (25/25C), mp 62-65C, flash p 405F (207C). Soluble in most organic solvents insoluble in water compatible with a large number of polymers. Combustible. 

Properties Colorless liquid. Mp 15-16C, bp 161-162C, d 1.015 (20C), flash p 170F (76.6C). Soluble in water, alcohol, ether, most organic solvents. Polymerizes readily to give water-soluble polymers. Combustible. 

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