Depolymerization, cellulose retardation

Magnesium compounds retard cellulose depolymerization by deactivating the transition metal catalysts. In alkaline media where hydroxy-acids or products of polysaccharide degradation are present, magnesium forms stable complexes with transition metals. The formation of iron-magnesium complexes in particular is supported by interactions approximating the coordination number of iron (6) and one half the coordination number (3). 

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. 

Lignin is the second most abundant natural polymer in the biosphere and the most abundant renewable aromatic material. It comprises 15-30% of woody plant cell walls, forming a matrix surrounding the cellulose. This encrusting matrix (1,2) significantly retards the microbial depolymerization of cellulose and thus lignin plays a key role in the earth s carbon cycle (1-3). 

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. 

Researchers (39,41) have investigated the addition of various amines to the carbanilation reaction mixtures to decrease the reaction time needed for derivatization of cellulose, especially the reaction time required for a sample with high molecular weight. In DMSO and DMF, the amines catalyzed the conversion of the phenylisocyanate to its trimer phenylisocyanurate. In addition, several amines actually retarded the carbanilation reaction. Most significant was that the presence of several amines in the DMSO-phenylisocyanate reaction mixture caused depolymerization of the cellulose, especially high-molecular-weight cellulose. In some cases, the depolymerization was severe. All three components (amine, phenylisocyanate, and DMSO) were required for depolymerization to take place. 

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