Dehydrochlorination of poly(Vinyl Chloride) with substitute reaction and possibility of applications

Polyvinylchloride (PVC) is, besides polyethylene, polypropylene, polystyrene, and polyethylene terephthalate, one of the most common plastics in the municipal waste stream. In opposite to the other common plastics, it owns a high chlorine content of about 56 wt%. This makes it necessary to treat PVC separately from other plastics or, if not possible, to dechlorinate the entire plastic waste. In this review, we will describe the possibilities for the wet treatment of PVC. There are already processes in use (Stignaes, Denmark), applying aqueous sodium hydroxide solutions (NaOH(aq)) at temperatures between 200 and 250 °C at high pressure. The chlorine is removed mainly by the elimination of HCl, which is directly neutralized by NaOH, and NaCl is obtained. The dehydrochlorinated residue is a black solid material, which can be converted by thermolysis into char and oil. By replacing water by ethylene glycol (EG), high pressure can be avoided, making the whole process less expensive. The reaction can be carried out at a temperature just below the boiling point of EG at 196 °C. The lower polarity of the organic solvent offers also the advantage of a better contact between the solution and the solid PVC, leading to a more effective and faster dehydrochlorination of the PVC. The efficiency can be further improved by applying a ball mill or phase-transfer catalysts. Besides the elimination of HCl, the substitution of chlorine by hydroxyl groups takes place in a minor extent. More than 20% of the chlorine groups can be substituted at 190 °C, while more than 70% of the chlorine is eliminated in a NaOH solution. Other nucleophiles, such as azide and thiocyanate, have a similar effect on the elimination and substitution of chlorine. The reduction of the temperature increases the ratio between substitution and elimination, making it possible to obtain modified PVC from waste PVC with new properties.