Hydrothermal Cracking of Polyethylene, Polypropylene, and/or Polystyrene Mixtures under Supercritical Water Condition

In order to clarify the mechanism and the effect of mixed-polymer treatment on the reaction rate of hydrothermal cracking in supercritical water, experimental investigation were carried out with mixtures of PE, PP and/or PS at 430°C and a pressure of ca. 42 MPa. Following results were obtained. (1) When a mixture of different kinds of plastics are treated hydrothermally, the plastics, which are more hardly decomposed, can be easily degraded into low molecular weight oily materials by attack of radicals which are generated from the plastics more easily decomposed. (2) The mechanism of attack of radicals on the plastics, which are more hardly decomposed, in the mixed treatment is effective no matter whether the cracking mechanism of these plastics is random scission or depolymerization at the end of main chain. (3) In mixed treatment, the main mechanism of degradation for each plastic is the same as that in separate treatment, so that the main composition of oily product in mixing treatment is similar to that of the mixture of oily products in each separate treatment. But some new materials were also found in the oily product. They are formed by the reaction of radicals from plastics easily decomposed and the products from plastics hardly decomposed, so that they retain the similar structures of main chain structure of the plastics. (4) In either mixed or separate treatment, the composition of oily product by thermal cracking does not differ much from that by hydrothermal cracking in supercritical water. However, much less polymerization among reactive products occurs in the hydrothermal treatment of mixture.