Condensed phase fire-retarding mechanisms

A condensed phase fire retardant mechanism is proposed for APP in nylon-6 [141]. In fact, an intumescent layer is formed on the surface of burning nylon-6/APP formulations which tends to increasing content of APP. 

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. 

Their fire-retardant mechanism is predominantly due to condensed phase action involving a combination of endothermic decomposition, water release, and oxide residue formation. 

Solid (Condensed) Phase Sohd phase fire retardant(s) alter the physical burning characteristics by either forming an insulative fire barrier or by changing the surface morphology to interfere with the release or generation of combustible gases. This mechanism is commonly seen with phosphorus-based flame retardants. 

Mechanistic studies described above show that halogenated fire-retardant systems can act by a condensed phase mechanism that in some cases could be induced by a halogen-free compound. 

Costa, L. Camino, G. Luda, M. P. Mechanism of condensed phase action in fire retardant bismuth compound-chloroparaffin-polypropylene mixtures Part I—The role of bismuth trichloride and oxychloride, Polymer Degradation and Stability, 1986, 14(2), 159-164. 

Phosphorus-containing fire retardants can be active in the condensed phase or in the vapor phase, or in both phases.88 The relative predominance of the different mechanisms actually operating in a... 

Mechanism. No single mechanism explains the action of all fire retardants, so they probably work through a combination of several mechanisms. The mechanisms of fire retardants in wood involve a complex series of simultaneous reactions whose products may affect subsequent reactions. Pyrolysis of cellulose involves dehydration, depolymerization, decarbonylation, decomposition of smaller compounds, condensation, and other reactions. These pyrolysis reactions occur both in the solid phase and vapor phase. Addition of fire retardants will alter the reactions however, this alteration will depend on the additives, the material, and the thermal-physical environment. The presence of oxygen adds subsequent and competitive oxidation reactions to the above series. These oxidative reactions can take place in both the solid and vapor phases. Evidence indicates that most fire retardants reduce combustible volatiles production and limit combustion to the solid phase. The best retardants also inhibit solid-phase oxidation to effectively remove the fuel from the fire. 

Although much work has been carried out on the mode of action of flame retardants generally, the mechanisms associated with tin additives are only partially understood. It is clear that tin-based fire retardants can exert their action in both the condensed and vapor phases, and that the precise action in any particular system depends on a number of factors, including incorporation level, the amount and chemical nature of other additives present, and the nature of the polymer itself. 

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. 

Wool has been regarded as a relatively safe fiber from the flammability point of view. However, it could be flame retarded to a higher degree if required. Hendrix et al. (26) suggested a large improvement in fire resistance of wool by treatment with 15% H PO. Beck et al. (27) showed that weak acidic materials, such as boric acid and dihydrogen phosphate, are effective additives for flame retarding wool by the condensed-phase mechanism (increased char residue). 

The mechanism of burning for polymers is believed to take place through thermal pyrolysis of the solid plastic to produce gases that act as fuel for the fire (45). Fire retardants work in both the condensed and the vapor phase to interrupt melting of the polymer and burning of the gases. Triaryl phosphates function well in the vapor phase. Alkyl aryl phosphates are believed to decompose in the flame front to form polyphosphoric acid, which stays in the condensed phase to form char, which reduces flammability and smoke evolution (46. 47). 

Another solution to improve the fire-retardant (FR) properties of polymers is the use of intumescent additives [2, 3]. Intumescent technology [4, 5] has found a place in polymer science as a method of imparting flame retardancy to polymeric materials. On heating, FR intumescent materials form a foamed cellular charred layer on their surfaces [6, 7], which protects the underlying materials from the action of heat flux and flame. The proposed mechanism [8] is based on the charred layer acting as a physical barrier, which retards heat and mass transfer between the gas and the condensed phase. 

Borates with zinc, calcium, magnesium, melamine or barium as the cation, combine char formation with endothermic decomposition as a dual mechanism that provides an efficient flame retardant system. Although the exotherm reduction is not as great as that achieved with ATH, there is enough endotherm to delay the initiation of the polymer exotherm by as much as 100 °C in some systems. It is this endothermic reaction in the polymer condensed phase and the formation of a glassy char that makes borate products good s mergists with the fire retardant antimony trioxide. 80% of the boron from zinc borate remains in the char after PVC combustion. 

Phosphorus is believed to perform most of its flame retardant function in the condensed phase (including both the solid and liquid phases, because various degrees of melting are involved at fire temperatures). Phosphorus-containing compounds increase the amount of carbonaceous residue or char formed by one or both of two mechanisms redirection of the chemical reactions involved in decomposition in favor of reactions yielding carbon rather than carbon monoxide or carbon dioxide and formation of a surface layer of protective char. 

Thus, the condensed phase mechanism of fire retardant intumescent systems aims at reducing the rate of pyrolysis of the polymer below the threshold for self-sustained combustion. This limits the production of volatile moieties and hence reduces undesirable secondary effects of volatiles combustion such as visual obscuration, corrosion and toxicity, which are typical of the widely used halogen containing fire retardants. 

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