Study On Depolymerization Of Polyethylene Terephthalate(PET) By FTIR In Situ

Polyethylene terephthalate (PET) is a semi-crystalline thermoplastic polyester with a wide range of applications. In recent years, with the rapid development of the beverage packaging industry, a large amount of PET was used for the production of fast-growing disposable consumer goods, such as beverage bottles. These products were discarded immediately once they were used, resulting in a very serious waste and environmental pollution, In this study, recycled PET flakes as the main research object, FTIR in-situ analysis as the main research tool, the depolymerization of PET by different alcohols were investigated thoroughly and systematically.Through FTIR in situ tracking the PET depolymerization, the C-O and C=O groups of the product molecules at1271cm-1and1720cm-1can been observed obviously on the IR spectra. The trends of the absorption intensities of two groups are the same. These results indicated that the two groups were from the same ester products. Based on the law of Beer-Lambert, the group concentration could be characterized by the groups peak intensity. For the system of PET depolymerization reaction, the PET depolymerization degree vs. time could be characterized by the C-O or C=O groups concentration vs. time.Firstly, The catalytic activity of several acetic acid salts (acetyl acetone salts) for the process of PET methanolysis was investigated. The catalytic activities of acetic acid salts are in a decreasing order of Zn2+> Mn2+> Co2+. The reactivities of four monohydric alcohols with PET were compared. The reactivity order is methanol> ethanol>propanol>isopropanol. Thus the PET depolymerization reaction rate was related to the nucleophilic activity of the alcoholic hydroxyl groups. The stronger the nucleophilic activity of alcoholic hydroxyl groups is, the faster the depolymerization rate is. The nucleophilic activities of monohydric alcohols were mainly affected by the intramolecular steric effects.For the process of PET depolymerization, a "shrinking flake model", based on the "shrinking grain model" of solid reaction, was established to describe the dynamic process of PET flakes depolymerization. The process was divided into two stages, at the earlier stage(X<0.8), the flake became thinner, the change of the reaction area was not significant; at the latter stage(X>0.8), the flakes dissolved in the liquid gradually, the reaction area became smaller. Using this model, the apparent activation energy of PET depolymerization reaction with methanol catalyzed by ZnAc2was calculated as101.1kJ·mol-1.To study the process of PET glycolysis with binary alcohols, the catalytic activities of several acetic acid salts (acetyl acetone salts), ZnAc2, MnAc2etc, was investigated. The catalytic activity order of MnAc2and ZnAc2were different at different temperature. Using ZnAc2as a catalyst, the "induction period" was existed at the earlier stage. The mixing of different acetate salts had a synergistic catalytic effect, the induction period could be eliminated and PET depolymerization rate could be increased.Using ZnAc2as a catalyst, The reactivities of different binary alcohols with PET were compared, the reactivity order is1,2-propanediol>ethylene glycol>1,4-butanediol, the optimum amount of acetate salts catalyst is between1%-3%of PET usage. The attempt of using ionic liquids of [Bmim]Br and [Bmim]Br-ZnCl2to catalyze the reaction of PET depolymerization indicated that both of them had the catalytic activities.Under the conditions of using the selected catalyst, the influence of reaction temperature and surface area of PET raw material were studied. The depolymerization reaction rate could be accelerated greatly through increasing the temperature or the surface area. Based on the "shrinking flake model", data from the dynamic process of PET depolymerization by ethylene glycol and propylene glycol were fitted to get the apparent activation energies. They were148.8kJ·mol-1and152.4KJ·mol-1. The PET glycolysis dynamic process was studied by the nonlinear fitting method; the kinetic parameters from the two methods were compared. The results were consistent with the methods, which indicated that the model established could describe the kinetic process well. Finally, the relationship between the molecular structure and reactivities of the monohydric alcohol and binary alcohol were discussed, which shows that there isn’t a direct causal relationship between the reactivities of alcohols and the number of hydroxyl group on the carbon chain. The reactivities of alcohol mainly depend on steric effects and electronic effects.