Supercritical Fluid Technology Depolymerization Of Waste Polystyrene

A new approach for polymer recycling is to depolymerize polymers into monomers, dimers, and useful hydrocarbon materials or liquid fuels in supercritical fluids. The paper introduces the basal characteristics of supercritical fluids and its application on chemical recycling of waste plastics. Recent studies progress on recycling of polystyrene(PS), especially on chemical recycling, are summarized.Depolymerization of waste polystyrene (PS) in supercritical xylene was carried out in a stirred batch autoclave at temperature ranging from 340-390℃, at different pressures of 4.0-27.0MPa, respectively. The depolymerization products were examined by gas chromatography-mass spectrometry, gas chromatography respectively. Total 107 components including isomers were identified by GC-MS in the liquid product. The main reaction products were dipolymer of styrene, isopropyl benzene and dipolymer of p-methyl styrene. Conversion and content of dipolymer of styrene in the liquid product increases with the increase of temperature. The results of kinetic analysis showed that the depolymerization reaction rate was first order to the polystyrene concentration. The dependence of the rate constant on the reaction temperature was correlated by the Arrhenius relation, which shows activation energy of 47.6-4 1.9kJ·mol-1and frequency factor of 1.181-0.377 s-1.Depolymerization of PS in supercritical benzene was also carried out at temperature ranging from 300-360℃, at different pressures of 4.5-10.7MPa, at reaction time of 10-50min, respectively. The main reaction products were .alpha.-methylstyrene and dimmer by gas chromatography-mass spectrometry, gas chromatography and FT-IR spectrometry, respectively. Reaction temperature, pressure, time play a role on component contents of the liquid products. It is found out that the optimized conditions of the reaction are at temperature of 340 ℃, reaction time of 40min and PS/benzene ratio of 15g/187.5mL-20g/250mL.By discussing mechanisms of depolymerization, Random scission is main depolymerization path in supercritical benzene, whereas chain-end scission is negligible compared to polystyrene melt. Studies show depolymerization path in supercritical benzene is a typical free radical reaction, where initiation(random scission, carbon-hydrogen bond fission), propagation(intramolecular H abstraction, intromolecular H abstraction) and termination(recombination, disproportionation) reactions are the relevant reaction classes.Excellent solubility and mass transfer of supercritical fluids improve greatly molecular fluidity, and enhance hydrogen transfer and engaging in free radical reaction. Meanwhile, in thermal pyrolysis the crystalline restricts hydrogen transfer and therefore favors chain-end scission.