New Mechanistic Aspects ofthe Dehydrochlorination of PVC - Application of Dehydrochlorination to Plastic Mixtures and Electronic Scrap

Abstract

The dehydrochlorination of poly(vinyl chloride) is of interest from several points of view, e.g., the evolution of harmful substances during fires or waste incineration, reclaiming chemical raw materials from plastic waste, electronic scrap management and recycling of plastic waste mixtures. In this work some new mechanistic aspects of the decomposition of poly(vinyl chloride) are investigated by the combined application of TG-MS, isothermal experiments in a gradient-free reactor and DSC. Poly(vinyl chloride) degrades in two steps. At lower temperature (220-350°C) HCl and small amounts of benzene are formed. At higher temperature ( > 400°C) the residue decomposes into mostly aromatic compounds. Dehydrochlorination at moderate temperatures consists of an endothermic and exothermic path. The benzene formation is identified to be a second order reaction. The kinetic data for decomposition confirm that in mixtures of different plastics the dehydrochlorination of poly(vinyl chloride) can be conducted at moderate temperature and prior to the thermal degradation of most other plastics. This has been demonstrated in this work in a cascade of well stirred reactors at laboratory scale. The degree of conversion of chlorine from PVC into hydrogen chloride in the first reactor is about 99.6% at 330°C after a residence time of 26 min. Similar results are obtained during the thermal degradation of electronic scrap. The hydrogen chloride evolution from poly(vinyl chloride) occurs in the same way as in mixtures of commodity plastics. Brominated flame retardants are decomposed or evolve at higher temperatures ( > 300°C). The conversion of bromine into copper salts by means of copper from copper wires is observed as well.