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Trends in Chemical Engineering   Volumes    Volume 9 
Characteristics and kinetics of degradation and depolymerization of plastics in supercritical fluids
Hidehiro Kumazawa, Hyun Kwak, Seong-Youl Bae
Pages: 1 - 26
Number of pages: 26
Trends in Chemical Engineering
Volume 9 

Copyright © 2005 Research Trends. All rights reserved


The characteristic and kinetic behavior of degradation of such plastics as polystyrene (PS), polyethylene (PE), polypropylene (PP), etc. in supercritical acetone, n-hexane and the similar behavior of depolymerization of polyethylene terephthalate (PET) in supercritical methanol were investigated and discussed along with similar existing works by the other researchers.

The degradation of PS in supercritical acetone and n-hexane as well as the thermal pyrolysis was found to follow the first-order kinetics at the initial stage of the reaction. From the Arrhenius plots, the activation energies were evaluated to be 140 kJ/mol for the degradation in supercritical n-hexane and 224 kJ/mol for the thermal pyrolysis. It has been found that the degradation in supercritical n-hexane can take place at lower temperatures than the thermal pyrolysis. The degradations of high density polyethylene (HDPE) and polypropylene (PP) were carried out in supercritical acetone under the reaction temperatures ranging from 450 to 470°C, pressures ranging from 6.08 to 10.1 MPa and reaction duration till 60 min. The following information was got regarding the degradation characteristics. The degradation of HDPE, PP and their mixtures in supercritical acetone at 450–470°C and 6.08-10.1 MPa was completed in 30 min after reaching the prescribed temperature. The yields of oil form HDPE and PP under high pressures above 7.1 MPa remained almost constant, i.e., 88% for HDPE and 96% for PP. The yields of wax from HDPE and PP decreased with increasing pressure below 7.6 MPa and above the value they remained almost constant, especially zero with PP.

A kinetic model to describe the depolymerization of PET in supercritical methanol was proposed, where the scission of one ester linkage in PET by a methanol molecule results in the formation of one carboxymethyl group and one hydroxyl group. The values of the forward reaction rate constant at different temperatures were determined by comparing the observed time dependence of carboxymethyl group concentration  with that calculated by the proposed model. The activation energy was evaluated to be 49.9 kJ/mol, a value close to a literature value (55.7 kJ/mol) for the depolymerization in subcritical water.

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