【作者】 苏磊；

【导师】 经福谦；

【作者基本信息】 西南交通大学 ， 固体力学， 2007， 博士

【摘要】 塑料高分子的科技进步给人类带来了巨大文明，但大量废弃物的出现也给人类提出了严峻的挑战。超临界流体(Supercritical Fluid，SCF)技术是一种有效处理塑料废弃物的绿色环保方法，然而，超临界流体相对苛刻的反应条件限制了该技术的工业应用。本文在自行设计的高压反应装置中，以聚烯烃塑料—聚乙烯、聚丙烯为原料，以降低生产成本为出发点，同时积极探索聚烯烃降解的机理，为超临界流体在塑料废弃物处理领域的应用提供有价值的依据。本文主要在以下几方面开展了研究和探讨：(1)以聚丙烯为研究对象，研究了压力、温度和反应时间等对聚丙烯在超临界水中降解行为的影响。用乌氏黏度计测量产物的黏均分子量，色谱-质谱检测分析液体油的组成结构和成分。实验结果显示：在本实验的条件下，聚烯烃类塑料—聚丙烯在超临界水中可以完全转化成单体和低分子；在反应的前30分钟内，降解速度最快：温度是影响降解反应的重要因素。(2)选取一种常用的工业自由基引发剂——过氧化苯甲酰(BPO)作为添加剂，研究了其对聚丙烯在超临界水中降解行为的影响。对使用添加剂和未使用添加剂的实验产物采取多种方式检测对比，结果表明：在反应温度较低和反应时间较短的情况下，添加BPO可以有效促进聚丙烯的降解，达到与未添加BPO实验中高温和长时间反应相比拟的效果。同时讨论了聚丙烯在超临界水中降解反应机理，对BPO在反应中的促进作用也进行了分析。因为BPO热分解温度很低，过氧键(O—O)容易断裂产生自由基C6H5COO·。BPO的降解反应为放热反应，该初始反应放出的能量部分提供给了链断裂所需要的能量，导致自由基C6H5COO·容易裂解形成一个新的自由基C6H5·。两种自由基均比较稳定，可进攻聚烯烃大分子链，进一步形成大分子自由基，且在大分子链上产生活性点。随着反应时间的增加，引发剂BPO不断分解，自由基浓度增加，造成聚烯烃的大规模降解。因此在相同反应温度和反应时间条件下，加有BPO的实验产物平均分子量要明显小于对照实验产物测量结果。(3)讨论了升温升压过程对聚丙烯在超临界水中降解的影响，认为升温升压过程也是影响聚丙烯在超临界水中降解的一个重要因素。在此研究中详细记录了聚丙烯降解的升温升压过程，升温升压过程经过气相区进入超临界区的称为过程一，升温升压过程经过液相区进入超临界区的称为过程二。在本实验的条件下，不同的升温升压过程将导致实验结果产生很大的差异；适当控制反应中的升温升压过程使其经过液相区进入超临界区，将有效促进聚烯烃塑料的降解，从而降低生产成本。(4)以聚乙烯为研究对象，详细讨论了结晶度对聚乙烯在超临界流体中降解的影响。结果表明：超临界条件下塑料的降解与一般水热条件下的降解有很大的不同。高分子材料一般由晶体和非晶体两部分组成。它们的分子都是长链状，链内碳原子间靠共价键连接，具有很强的结合力。而链与链之间则靠范德瓦耳斯力相互结合，在晶体情况下由于分子整齐排列，分子间的范德瓦耳斯力比在非晶中要强得多。当这样的材料处于一般水热反应条件时，链间结合较弱的非晶体便较容易作为单链溶解，进而非晶部分发生降解，但晶体部分仍然存在，从而结晶度提高。当这种材料处于超临界水中时，由于压力和温度的提高使溶剂和溶质的分子间相互作用大大增强以及超临界流体中大量自由基的存在，塑料高分子中无论晶体和非晶体的分子长链都会受到攻击，从而降解成为短链小分子量的低聚物，这种过程对晶体和非晶体都是同等的，链间结合力的差别在这样的条件下已经显示不出来。更多还原

【Abstract】 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.更多还原