聚对苯二甲酸丁二醇酯在超临界甲醇中降解机理的研究

在高压间歇无搅拌反应器中研究了聚对苯二甲酸丁二醇酯(PBT)在高温甲醇溶液中的降解行为,通过对降解产物的各种定性和定量的分析,提出了超临界甲醇降解PBT的机理为在甲醇的作用下聚合物分子链的随机断裂和酯交换反应双重作用下发生的降解反应,建立了降解-反应模型.PBT在甲醇溶液中的降解可分为超临界区、非超临界区和中间过渡区三个区域.通过分子量测定考察了PBT在不同的区域中降解规律.在非临界区PBT在溶剂中处于溶胀状态,其数均分子量Mn下降缓慢,解聚程度低;在过渡区PBT的溶解性能提高,聚合物大分子发生断裂,降解速率加快;在超临界区,Mn随反应的进行而迅速下降,聚合物很快完全降解.在超临界区中PBT可实现完全降解,其主要产物为单体对苯二甲酸二甲酯(DMT)和丁二醇(BG),它们的收率可分别可达98.1%和72.3%.

The Depolymerization Mechanism of Polybutylene Terephthalate in Supercritical Methanol

Depolymerization experiments of polybutylene terephthalate (PBT) in supercritical methanol were carried out in a batch-type reactor with the reaction temperature of 453~583K and reaction time of 5~90 min. The degradation products were analyzed by GC/MS and HPLC/MS and their amounts were determined by GC/FID and HPLC. Based on the results of above operations, a depolymerization reaction model was proposed to explain the reaction mechanism, i.e. Random scission and ester exchange reactions occur simultaneously which cause the depolymerization of PBT. The experimental results show that the PBT decomposes mostly to its monomers, dimethyl terephthalate (DMT) and butylene glycol (BG), by methanolysis in supercritical methanol, and the degradation of PBT can be divided into three regions, such as sub-critical region, transitional region and supercritical region. In the sub-critical region, the degradation reaction primarily occurs on the surface of PBT, which mainly shows as a swelling process, the degree of PBT degradation is low and its numerical average molecular weight Mn decreases slowly. In the transitional region, the solubility of PBT is enhanced, the scission of the large molecular polymers occurs and the degradation is speeding up. In the supercritical region, the PBT dissolves in the supercritical methanol quickly and the Mn decreases with the progress of the reaction rapidly, finally the PBT can be thoroughly depolymerized in this region and the yield of monomer DMT and BG can reach 98.1% and 72.3% respectively.