Process analysis of depolymerization polybutylene terephthalate in supercritical methanol

The characteristics of depolymerization of PBT in supercritical methanol were investigated in the range of 453-533 K by using a high-pressure reactor. Based on the qualitative and quantitative analyses of the products, a depolymerization-reaction model was proposed to explain the depolymerization and reaction mechanism, i.e. ester exchange reaction occurred randomly along the chain of PBT. It was suggested that the process of depolymerization consisted of subcritical region, transitional region and supercritical region. In the first region, PBT mainly showed a swelling process in the methanol with slow decrease in molecular weight and little conversion. In the second region, PBT dissolved quickly with high depolymerization rate. While in the third region, the molecular weight of PBT decreased quickly with a thorough depolymerization in few minutes. DMT and BG obtained from the depolymerization of PBT in supercritical methanol reached 98.5% and 72.3%, respectively.