Protective char

Flame retardants (qv) are incorporated into the formulations in amounts necessary to satisfy existing requirements. Reactive-type diols, such as A/ A/-bis(2-hydroxyethyl)aminomethylphosphonate (Fyrol 6), are preferred, but nonreactive phosphates (Fyrol CEF, Fyrol PCF) are also used. Often, the necessary results are achieved using mineral fillers, such as alumina trihydrate or melamine. Melamine melts away from the flame and forms both a nonflammable gaseous environment and a molten barrier that helps to isolate the combustible polyurethane foam from the flame. Alumina trihydrate releases water of hydration to cool the flame, forming a noncombustible inorganic protective char at the flame front. Flame-resistant upholstery fabric or liners are also used (27). 

Aluminium hydroxide is essentially non-toxic, but does require high addition levels to be effective. As a result, the physical properties of the compound usually suffer. Its fire retardancy action results from the endothermic reaction which releases water under fire conditions and produces a protective char . The endothermic reaction draws heat from the rubber/filler mass and thus reduces the thermal decomposition rate. The water release dilutes the available fuel supply, cooling the rubber surface and mass. 

In short, an intumescent formulation has to be optimized in terms of physical (char strength, expansion, viscosity,. ..) and chemical (thermal stability, reactivity) properties in order to form an effective protective char that will be able to protect its host polymer (reaction to fire) or a substrate like steel or wood (resistance to fire).16... 

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]. 

Combinations of inorganic and organic flame retardants are discussed hcrc. " Figure 13.6 shows than the addition of regular fillers, such as talc and CaCOs, to ammonium polyphosphate increased the fire resistance of PA-6. The function of filler in these combinations is to increase char yield and increase insulation properties of char. On the other hand, ammonium polyphosphate protects char Ifom oxidation and hinders diffusion of combustible gases to the flame. 

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). 

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)... 

The development of flame retardant additives for polymeric materials that could simultaneously promote both gas-phase and solid-phase types of action could result in products that are both more cost-effective and more environmentally-friendly than those currently in use [69]. These include bromoanilino triazine derivatives and bromoaryl phosphates. Both have the potential to display both solid-phase and gas-phase FR activity. These were evaluated by a variety of thermal methods. Some of these compounds had the potential to display dual functional behaviour as FR, i.e., to maintain the good gas phase activity associated with organohalogen compounds while, at the same time promoting the development of protective char at the solid phase. 

The active species in fire retarding are the halogens, chlorine and bromine, phosphorus, and water. The performance of these primary flame retardants is enhanced by synergists antimony, zinc and other metal salts. Some help to develop a protective char (e.g., phosphorus-based systems), separating the unbumed polymer from the flame and heat source. 

The arrangement of the layers themselves also has an impact. The self-extinguishing properties of these materials, for instance, have been described qualitatively. A protective char layer forms and acts as a diffusion barrier to further combustion. Likewise, before the advents of nanocomposites, models of the barrier properties of glass-ribbon reinforced composites foreshadowed the increased tortuosity arguments often heard with regard to nanocomposite barrier properties. Improvements on these first approximations of bar-... 

Protective Char Layer Charring Polymer Virgin Polymer... 

Keywords intumescent coating, intumescent formulations, fire retard-ance, fire protection, char formation, heat flux, thermal decomposition, carbonization, pyrolisis, antagonistic effect, UL-94, LOI, spumification. 

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. 

Boron compounds act in the condensed phase by redirecting the decomposition process in favor of carbon formation rather than CO or CO2. A second mechanism may involve the formation of a surface layer of protective char, which prevents the oxidation of carbon by limiting the accessible oxygen. In both cases a char is formed [152]. 

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