| Plastic brings us civilization and convenience. However, lots of waste plastics have been the serious environmental problem. In recent years, SCF technology has been focused as a "super green" solvent for the conversion of waste materials into resources. But the reaction conditions, such as temperature and pressure, are so hard that they prevent us using in the production of industry. In order to find new ways for lowering the reaction conditions, in this research, a supercritical fluid reaction system was constructed and used in the study of the major characters of polypropylene and polyethylene degradation in supercritical water, expecting to lay the foundation of using supercritical water in the Plastic wastes treatment. This work includes the following parts:(1) Effect of temperature, pressure and reaction time on polypropylene (PP) degradation in supercritical water was investigated with the aim of developing a process for recycling of waste plastics. A series of experiments were carried out in a reaction system at temperatures of 653K and 673K under pressure about 26 MPa for 30, 75 and 120 min respectively. Products were analyzed by Ostward-type viscometer, gas chromatography and mass spectrometers (GC/MS) etc. The results indicated that mean molecular weight (Mw) of the samples decreased greatly along with the time prolonging or the temperature increasing, and PP was decomposed to hydrocarbon of methane series, hydrocarbon of ethylene series, cycloparaffinic hydrocarbons and few benzenoid hydrocarbons by solvolysis of supercritical water. In the process, efficiency of the reaction is utmost in the first 30 minutes and reaction temperature is an effective factor on PP degradation.(2) Effect of benzoylperoxide(BPO) on the depolymerization behavior of pp in supercritical water was reported in the article. Two series of experiments with and without BPO were run at temperature of 380℃, 400℃and pressure about 26MPa for 30min, 75min or 120min respectively. The color and phase character of products were observed and compared, the mean molecular weight of samples was estimated by Ostwald viscometer, the components and configurations of products were analyzed by IR and GC-MS. The results show that PP can be entirely decomposed to oligomers and monomers under the experimental conditions, the decomposition of PP can be promoted with temperature increase in supercritical water condition. At lower temperature and shorter reaction time, effect of additive BPO on PP depolymerization is equivalent to that at higher reaction temperature and longer time in the experiments without BPO. The reaction mechanism was also discussed in the paper. BPO is an important source of free radicals in modern industry. In supercritical water the 0-0 bond broke easily, and then BPO was decomposed to free radical C6H5COO·. Because the depolymerization of BPO is an exothermic reaction, the free radical C6H5COO·further decomposed to a new free radical C6H5·. Those free radicals (including HO) could attack long chains of PP, and PP was cracked to monomers with low molecular weight. As the reaction time was longer, density of the free radicals increased, and then the degradation of PP in supercritical water was accelerated. So Mw of the product in the presence of BPO is smaller obviously than that in the absence of BPO under the same reaction conditions.(3) Effect of increasing course of temperature and pressure on polypropylene (PP) degradation in supercritical water was investigated for developing a process of recycling waste plastic. A group of experiments was carried out in a reaction system at a pressure of 26 MPa, temperature of 380 or 400℃for 30, 70, and 120 min by Course One (the increasing course of temperature and pressure is via gaseous regions to supercritical regions), and the other group was carried out at corresponding holding conditions by Course Two (the increasing course of temperature and pressure is via liquid regions to supercritical regions). The time of the increasing courses was about 30 min. Products were analyzed by Ostward-type viscometer, gaseous chromatography, and mass spectrometers (GC/MS). Characterization results suggested that different increasing courses of temperature and pressure would give rise to different results, although they were treated under the similar holding conditions. It was also found that Course Two was more effective on PP degradation in supercritical water. Therefore, controlling the increasing course of temperature and pressure is a promising way to advance efficiency and decrease cost in the industrial process for recycling of waste plastics.(4) polyethylene (PE) was chosen as the initial material, and the effect of crystallinity on depolymerization of PE in supercritical water was investigated in this work. Products were analyzed by FI-IR, GPC and DSC etc. The results indicated that the reaction mechanism of PE degradation in supercritical water was different from its depolymerization under hydrothermal condition. In general, there is dissolution course before the degradation of the plastic. That is, the macromolecules dissolve in the solvent firstly, and then decompose to low molecular weight. The solvency of supercritical fluid is very high, so most of the molecules of the plastic dissolve in it. The free radicals of SCF attack long chains of plastic and then the macromolecules crack into oligomers and monomers. However, the solvency of liquid under hydrothermal condition is low relatively. It makes the amorphous part of the plastic dissolve it and then decompose, whereas the other part is difficult to dissolve in. As a result, the crystallinity of plastic was increased during the whole reaction process. It indicates that depolymerization was firstly proceed in the amorphous phase underhydrothermal condition.
|
PDF Full Text Request