Charring fire retardance

Solutions of these fire retardant formulations are impregnated into wood under fliU cell pressure treatment to obtain dry chemical retentions of 65 to 95 kg/m this type of treatment greatly reduces flame-spread and afterglow. These effects are the result of changed thermal decomposition reactions that favor production of carbon dioxide and water (vapor) as opposed to more flammable components (55). Char oxidation (glowing or smoldering) is also inhibited. 

Proper selection of plastic matrix fire retardants and property enhancers offers acceptable combinations of impact properties and heat-distortion temperature (HDT) values for fire retardant plastics. This can be demonstrated by fire retardant styrenics. Fire retardant enhancers have special interest as property enhancers for example the addition of a highly flammable material such as ethylene propylene diene terpolyer (EPDM), dramatically improves the bromine efficiency of octabromodiphenyl oxide in ABS by increasing char-forming without changing the Sb-Br reaction. 

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. 

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. 

The symposium was planned as a state-of-the-art meeting, focusing on the basic science. Program areas included high heat polymers, fire performance of polymers, hazard modeling, mechanism of flammability and fire retardation, char formation, effects of surfaces on flammability, smoke assessment and formation mechanisms, and combustion product toxicity. 

The third composition in Table IV seems to be related to the aromatic sulfonate/polycarbonate technology just discussed with some modifications being necessary in order to compensate for the aliphatic nature of the polypropylene (17. 181 substrate. In this case the aromatic sulfonate is replaced with a metal salt (preferably magnesium stearate). A silicone oil and or gum has been added to enhance the intumescent character and a small amount of inert filler and decabromodiphenyl oxide is included probably to improve the molding characteristics of the total composition. Fire retardant compositions with a good surface char can be obtained at total loadings only about half that required for the halogen/antimony oxide composition. 

Additives, such as fire retardants, can have a major effect on pyrolysis, and even trace amounts of ash have been shown to influence pyrolysis (6 ). Generally, fire retardants work by increasing the dehydration reaction rate to form more char and as a direct result give fewer flammable volatile compounds (1,3,7). Several papers have noted that phosphoric acid and its salts decrease the Efl (13,18,22,29), aluminum chloride has little effect (22) on Efl and boric acid increases the Efl (12,18). The reaction order for treated samples has been generally reported as lst-order (12,13,18,29) which is also the most commonly used rate expression for analysis of TGA data of untreated cellulose. 

The mass loss rates for the boric acid samples were comparable to the untreated samples, despite a higher char yield (Table IX) for the treated samples. This was unexpected since the role of a wood fire retardant is to increase the char by increasing the dehydration reaction (1,3,7). Thus, a fire retardant treated sample will actually pyrolyze at a lower temperature. Data from Table III suggests that boric acid may form more char by suppressing formation of flammable volatiles instead of by increasing the dehydration rate. 

Borates, through their ability to act as glass network formers, can act as excellent char formers and drip suppressants in fire retardant applications. In many cases this involves processing into polymeric materials, leading to specific requirements for thermal stability and particle size. Most common borate materials, however, exhibit relatively low dehydration temperatures and may be unsuitable for use in many polymer systems. Zinc borates are often used because they have unusually high dehydration onset temperatures and can be produced as small particle size powders. 

Of the pigments and extenders tested, zinc borate is the most effective as a fire retardant. Calcium carbonate has good fire-retardant properties, especially in the limitation of the char area. Antimony oxide, whether in presence or absence of chlorine, is not particularly effective as a fire-retardant pigment in paint. 

The most widely accepted theory of the mechanism of fire-retardant chemicals in reducing flaming combustion of wood is that the chemicals alter the pyrolysis reactions with formation of less flammable gases and tars and more char and water (4,5,8,21,24-29). Some fire retardants start and end the chemical decomposition at lower temperatures. Heat of combustion of the volatiles is reduced. Shafizadeh (21) suggests that a primary function of fire retardants is to promote dehydration and charring of cellulose. 

Commercial fire-retardant treatments generally do not add significantly to the fire endurance of assemblies. It is often more advantageous from the cost standpoint, either to use thicker wood members or to select species with lower charring rates, than to add the cost of the fire-retardant treatment. In some assemblies, however, it has been found worthwhile to use some fire-retardant-treated components in order to gain the extra time which will bring the fire endurance time up to the goal desired. For example, treated wood studs in walls and treated rails, stiles, and cross bands in solid wood doors have been used. 

The use of polyols such as pentaerythritol, mannitol, or sorbitol as classical char formers in intumescent formulations for thermoplastics is associated with migration and water solubility problems. Moreover, these additives are often not compatible with the polymeric matrix and the mechanical properties of the formulations are then very poor. Those problems can be solved (at least partially) by the synthesis of additives that concentrate the three intumescent FR elements in one material, as suggested by the pioneering work of Halpern.29 b-MAP (4) (melamine salt of 3,9-dihydroxy-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]-undecane-3,9-dioxide) and Melabis (5) (melamine salt of bis(l-oxo-2,6,7-trioxa-l-phosphabicyclo[2.2.2]octan-4-ylmethanol)phosphate) were synthesized from pentaerythritol (2), melamine (3), and phosphoryl trichloride (1) (Figure 6.4). They were found to be more effective to fire retard PP than standard halogen-antimony FR. 

Furthermore, the effect of hydrated fillers on polymer fire retardancy will depend not only on the nature of the filler, including its particle characteristics (size, shape, and purity) and decomposition behavior, but also on the degradation mechanism of the polymer, together with any filler/ polymer interactions that might occur, influencing thermal stability of the polymer and possible char formation. 

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