Combustion, flaming, levoglucosan

Condensed-Phase Mechanisms. The mode of action of phosphorus-based flame retardants in cellulnsic sy stems is probably best understood. Cellulose decomposes by a noncalalyzed route lo tarry depolymerization products, notably levoglucosan, which then decomposes to volatile combustible fragments such as alcohols, aldehydes, ketones, and hydrocarbons. However, when catalyzed by acids, the decomposition of cellulose proceeds primarily as an endothermic dehydration of the carbohydrate to water vapor and char. Phosphoric acid is particularly efficaceous in this catalytic role because of its low volatility (see Phosphoric Acids and Phosphales). Also, when strongly heated, phosphoric acid yields polyphosphoric acid which is even more effective in catalyzing the cellulose dehydration reaction. The flame-retardanl action is believed to proceed by way of initial phosphory lation of the cellulose. 

One important thermal degradation mechanism of cellulose fibres (cotton, rayon, linen, etc.) is the formation of the small depolymerisation product levoglucosan (Fig. 8.7). Levoglucosan and its volatile pyrolysis products are extremely flammable materials and are the main contributors to cellulose combustion. Compounds that are able to hinder levoglucosan formation are expected to function as flame retardants for cellulose. The crosslinking and the single type of esterification of... 

It has been shown that the flame-retardant salts alter the course of decomposition of levoglucosan, and promote the charring of this compound. Also, a mixture of tar and a flame retardant (a combination of boric acid and borax), when coated on a glass tape, behaved like a similarly treated cotton fabric, and was not combustible. ... 

Levoglucosan as the degradation product of cellulose and celluloselike materials is at least partially responsible for flaming combustion, which may be important in the spreading of fires22 (for a review, see Ref. 23). Therefore, its formation is indirectly associated with the development of human culture and its environment. 

As already mentioned in the Introduction, one of the most common methods for enhancing the flame retardancy of cellulose, namely cotton, refers to the use of FRs that are able to promote the char formation at the expense of levoglucosan, thereby reducing the combustible-gas formation [4, 58]. The simplest method to describe their action is the dehydration of cellulose, already discussed. This reaction is acid-catalysed by the... 

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