Thermal Decomposition Reactions of Polymers

The composition of the pyrolysis products is primarily determined by the means of disintegration of the macromolecule to the molecules of gas, oil, and solid residue. Thus to anticipate the pyrolysis oil composition of a plastic material, the chemical composition and structure of the polymer and its thermal decomposition reactions should be consistently considered. Typical thermal decomposition pathways of the various polymer kinds are abundantly treated in the relevant scientific literature [18-20]. Thermal decomposition of the polymer component of a plastic material is expected to begin at the weakest chemical bonds of the macromolecule. However, there are decomposition pathways which require lower energy than the direct breakage of the bonds, when rearrangement over four or six neighbouring atoms leads to the elimination of a volatile compound or to the scission of the macromolecular chain. 

Elimination of carbon dioxide from carboxyl, water from alcoholic hydroxyl, carboxylic acid from alkanoate, and hydrogen chloride from chlorine side groups or chain ends are typical thermal decomposition reactions in the temperature range 250-350°C. Hydrogen chloride is an important product of poly(vinyl chloride) because every second carbon atom of the hydrocarbon polymer chain is chlorine substituted. But hydroxyl, alkanoate and free carboxylic acid groups normally occur only at the ends of the macromolecular chains in customary plastics, thus the contribution of their elimination to the volatile pyrolysis products is negligible. 

As a consequence of the variety of thermal decomposition pathways, the composition of the pyrolysis products may be controlled by the temperature when the plastic is made up of different polymers or when the constituent polymer decomposes through varying routes at different temperatures. 

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