Basic study on treatment of waste polyvinyl chloride plastics by hydrothermal decomposition in subcritical and supercritical regions

Abstract We are developing a process for treating waste plastics in an environmentally friendly way. The increased awareness of possible problems caused by waste PVC plastics is leading to a need to develop a reliable technique for treating them in a safe and environmentally friendly way, that is, in a way that does not lead to the release of chlorinated organic compounds. We focused on using water vapor at sublimation pressure and subcritical and supercritical water as solvents for treatment. We found that the chlorine in PVC dissolved in water as hydrochloric acid, and no harmful chlorinated organic compounds were observed in the liquid and gas fractions after treatment at 300 °C. Between 250 and 350 °C, this technique produced polyene as a residual solid, and low-molecular weight aromatic and aliphatic compounds in the liquid and gas fractions. Further decomposition at over 350 °C in supercritical water produced acetone, phenol, benzene, benzene derivatives, and aliphatic alkane and alkene in the liquid and gas fractions. The combustion enthalpy of the residual solid was 9270 kcal/kg, which is in the same range as the values for coal and coke, so it has good potential as a fuel ingredient. This technique is promising for establishing a non-toxic and almost perfectly closed system for the treatment of waste PVC in a sustainable society.