Mechanism Investigation And Molecular Weight Change In Hydrolytic Depolymerization Of Pet Under Microwave Irradiation

In recent years, the recycling of Poly(ethylene terephthalate)(PET) hasattracted more attention of researchers and many different recycling methodshave also appeared with the wide use of PET. But to achieve theindustrialization of PET depolymerization, the investigation ofdepolymerization mechanism becomes important. The study on mechanismof hydrolysis and catalysis hydrolytic depolymerization of PET undermicrowave irradiation will not only have an important significance inimproving the PET depolymerization rate and the full use of microwaveenergy, but also provide effective theoretical support to the industrializationof PET depolymerization under microwave irradiation.In this thesis, gel permeation chromatography (GPC) was adopted todetermine the molecular weight (MW) of the residue in the process ofhydrolysis and catalytic hydrolysis of PET under microwave irradiation atdifferent temperature of180℃,190℃and200℃. The depolymerizationprocess was studied in detail by investigating the change of the MW and themolecular weight distribution (MWD) of the depolymerization residueduring the depolymerization process. Meantime, the mechanism of PEThydrolysis and catalytic hydrolysis under microwave irradiation were furtherresearched by combining the change of PET depolymerization rate and theanalysis results of infrared spectroscopy (IR), differential scanningcalorimetry (DSC) and scanning electron microscopy (SEM) of thedepolymerization residue.During the research, it is found that the MW of the residue decreasedwith the reaction time increasing, then finally became stability, and themolecular weight distribution index (PD) was reduced to about1; but the MWD of the residue derived gradually towards the low molecular weights,in which two or many peaks derived from oligomers were observed, finallythe location and shape of the peaks remained almost unchanged. In addition,it is also found that the higher the reaction temperature raised, the faster theMW decreased, and the higher the PET depolymerization rate was. At theend of the depolymerization reaction, the MW of the residue at the lowtemperature in PET hydrolysis was nearly the same as that of the hightemperature, while in the catalytic hydrolysis of PET the MW of the residueat the low temperature was even lower than that of the high temperature.All the research results showed that the mechanism of PET hydrolysiswas similar with that of the catalytic hydrolysis of PET under microwaveirradiation, and the depolymerization process was divided into three differentstages: in the fist stage, PET was degraded into the polymer with a smallerMW by random scission; in the second stage, the polymer with a smallerMW was continuously depolymerized into oligomers and monomers byrandom scission and specific scission; in the third stage, the producedoligomers reacted further with water by specific scission. It is also found thatzinc acetate didn’t change the depolymerization process, but accelerated thereaction speed, and the catalysis of zinc acetate was more obvious at the lowreaction temperature. Meantime, the experimental results also suggested thatthe thermal effect and non-thermal effect of microwave irradiation bothexisted in hydrolysis and catalytic hydrolysis of PET under microwaveirradiation, and the non-thermal effect of microwave irradiation wasobserved easily at the lower temperature. What’s more, the synergistic effectof ZnAc2and the non-thermal effect of microwave irradiation made the MWof the residue at the lower temperature much smaller at the end of thedepolymerization reaction.