Char layers, formation

Brown, R. J. Practical reduced combustibility of rigid PVC compounds with improved protective char layer formation. J. Fire Retardant Chem., 9, No. 5, 102 (1982)... 

At present there is no small-scale test for predicting whether or how fast a fire will spread on a wall made of flammable or semiflammable (fire-retardant) material. The principal elements of the problem include pyrolysis of solids char-layer buildup buoyant, convective, tmbulent-boundary-layer heat transfer soot formation in the flame radiative emission from the sooty flame and the transient natme of the process (char buildup, fuel burnout, preheating of areas not yet ignited). Efforts are needed to develop computer models for these effects and to develop appropriate small-scale tests. 

Figure 1. Fire penetration into wood and formation of char layer under the fire exposure conditions of...

Fire-retarded materials functioning in the condensed phase, such as intumescent systems, form, on heating, foamed cellular charred layers on the surface, which protects the underlying material from the action of the heat flux or the flame. It is recognized that the formation of the effective char occurs via a... 

Since the majority of research carried out on flame retardancy of nanocomposite has dealt with OMLS, the most investigated combinations have concerned the corresponding class of nanocomposites of polymers, particularly EVA copolymer, PP, and polystyrene. The great interest taken in the development of IFR systems has also entailed the development of various and complex compositions in which OMLS have been associated with different intumescent systems containing APP and co-synergists able to promote the formation of a stable and expanded char layer reinforced by aluminophosphate species formed by reaction between APP and OMLS. 

Formation of a protective char layer is another important condensed-phase mechanism. Unfortunately, polystyrene does not form any appreciable levels of char during burning, even in the presence of charring catalysts. Some progress has been made in enhancing char formation of polystyrene by the use of Friedel-Crafts chemistry [18]. 

Wood burns because the cell wall polymers undergo hydrolysis, oxidation, dehydration, and pyrolysis reactions with increasing temperature to give off volatile, flammable gases. The lignin component contributes more to char formation than do the cellulose components, and the charred layer helps insulate the wood from further thermal degradation see Chapter 13). 

When a char layer is formed on the outside surface of the foam the inside virgin foam is protected by the char layer having a high degree of heat insulation. This char-formation mechanism is shown in Figure 64 (14). 

The fire-retardant mechanism associated with nanoclays has recently been studied and is likely to involve the formation of a ceramic skin which catalyzes char formation by thermal dehydrogenation of the host polymer to produce a conjugated polyene structure. " The nanocomposite structure present in the resulting char appears to enhance the performance of the char through reinforcement of the char layer. These effects would explain the apparent fire-retardant synergy observed when nanoclays are incorporated into polymer formulations containing condensed phase fire-retardant systems, including coated fillers. 

Even minor loadings of layered clay in PNCs result in substantial reduction in flammabifity which has been attributed to the build-up of a protective char layer involving the clay [260]. Substantial improvement in the ablative properties of materials due to a. similar char formation has also been reported [261 ]. These properties could lead to the development of novel fire resistant composites without the use of halogenated compounds that have negative environmental impact and hydrated inorganic oxides that require substantial loading. 

Fire-retardant treatment Hammability of acrylic textiles can be reduced by using (meth)acrylates containing phosphorus monomers known to be effective as flame-retardant compounds (Price et al., 2002). Tsafack et al. (2004) smdied the plasma-grafted thin layer of phosphorus polymer by plasma-induced graft-polymerization. The formation of a characteristic protective char layer during the burning test was observed for the treated compounds whereas the untreated ones burned without residuals (Tsafack et al., 2004). 

Both chemical and physical mechanisms may be involved during their functioning. The former include the interruption of the gas phase reaction and the prevention of the formation of CO or CO2 through the redirection of carbon to a char layer. The latter are the cooHng of the sohd polymer through latent heat absorption, dilution of combustible volatile gases, and the insulation of heat through the formation of a char layer [5]. 

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