Char formation temperature effects

The mechanism of the action of the phosphonate as a flame retardant is generally believed to be decomposition into acid fragments which contribute to char formation. These acidic species catalyze decomposition of the polyester, and give rise to species which on reaction with the phosphorus moiety cause char formation. TGA curves of the copolymers confirm that the incorporation of phosphorus into the polymer increases the char residue (Figure 4). These curves, however, show little evidence that the presence of phosphorus has any effect upon the temperature or rate of decomposition of the polyester. The curves are all fairly similar up to about 450°C. After that point, the amount of residue is proportional to the amount of phosphorus in the terpolymer. 

Because the mechanisms of oxidation are complex and apparently differ above and below temperatures of 340 to 350 K (2.9-12V this study investigates the comparative differences between and the effects of oxidation at 373 K and weathering, due to exposure to atmospheric conditions, upon char formation and char combustion. 

The Lewis and Bronsted acids lower the starting temperature of wood decomposition and charcoal formation and along with an increased char formation, diminish the yield of some volatile products of pyrolysis. The latter fact is an undesirable i enomenon if an energetic self-sufficiency in the carbonisation of wood is striven for (10), Therefore, the objective of the present investigation was to elucidate not only the effect of different available catalysts upon the yield of charcoal, but also that on the yield of volatile products. 

In an environment without adequate humidity, the initial effect of heating wood is dehydration. As temperatures approach 55-65 °C for extended periods (2-3 months), hemicellulose and cellulose depolymerization begins (28), Pyrolysis and volatilization of cell wall polymers occur at about 250 °C followed by char formation in the absence of air and combustion in the presence of air. 

These observations also clearly indicate that in this flame-retardant system, brominated phenols are first evolved at slightly lower temperatures than those of the flammable product such as 1,3-butadiene and butylene terephtha-late evolved from the substrate polymer, PBT, to cause the initial flame-retarding effect. In addition, HBr would then be evolved over the whole degradation temperature range for PBT to cause free radical trap in volahle phase and char formation to decrease flammable products. 

Finally, the already mentioned investigations on the Mediterranean pine Pinus halepensis shall be cited again in which commercial cellulose as a model compound for studies of forest fuel pyrolysis, pine needles, pine needle lignin and extractives were analyzed with and without two ammonium salts as fire retardants [46]. Both salts provoked a lowered pyrolysis temperature of cellulose and a significant increase in char formation for cellulose (up to 2.4 times) and intact needles (up to 1.7 times), but they had negligible effects on lignin and extractives. 

In die presence of oxygen, more complex thermo-oxidative processes occur in polyesters containing aliphatic moieties. They result in crosslinked products and in the formation of compounds such as aldehydes, carboxylic acids and vinyl esters, as reported in the case of PET.93,94 On the other hand, the presence of oxygen has little effect on the thermal resistance of wholly aromatic polyesters below 550°C. Above this temperature a char combustion process takes place.85... 

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