Initial Pyrolysis Reactions in Unmodified and Flame-Retardant Cotton

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Abstract
Thermogravimetric data were used to make Arrhenius plots for the vacuum pyrolyses of unmodified cotton and of cotton finished with various add-ons of THPOH-NH3 and THPS-urea-Na2HPO4 flame retardants. These plots show that all pyrolyses occurred in consecutive stages: The first and second initial stages, associated with the less ordered regions of the cotton fibers, and the main cellulose pyrolysis reaction, associated with cellulose crystallites. Cotton decrystallized by ball milling showed only the two initial pyrolysis stages. The second stage followed first-order kinetics. The first pyrolysis stage in unmodified cotton was characterized by a moderately low activation energy and by a large negative entropy of activation; the second stage showed a larger activation energy and a less negative entropy of activation. Mechanisms involving cellulose chain scission and chain unzipping are proposed for the first and second stages, respectively. Add-on of the two flame retardants had contrasting effects on the two initial pyrolysis stages. These effects are explained in terms of the way in which the flame retardants are deposited in the less ordered regions of cotton fibers.

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