Decomposed Behavior And Kinetic Analysis Of PBT In Supercritical Alcohol

Poly(butylene terephthalate) can be decomposed in supercritical methanol efficiently, and the decomposed products are dimethyl terephalate (DMT) and butylene glycol (BG) which could be recycled as the monomers of PBT. As a result, the studies of decomposition of PBT in supercritical alcohol mediums have received wide attention for the preservation of resources.Supercritical ethanol, 1-propanol and 1-butanol are used for the decomposition of PBT in this paper. Frontier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), High Performance Liquid Chromatography (HPLC) and Gas Chromatography-Mass Spectrometry (GC-MS) analytic techniques are taken to identify the ingredients of products. The effectiveness of the decomposition is represented by the amount of solid products that could not be dissolved in KOH solution. The experimental results indicate that the products are composed of alkyl hydroxybutyl terephthalate, bis(hydroxybutyl)terephthalate(BHBT), monomer-dialkyl terephthalate, butylene glycol(BG), tetrahydrofuran(THF) and a series of different molecular-weight polymers with the same structure to PBT. The discussion of decomposition ratio shows that pressure above critical point has little effect on the decomposition while reaction time and temperature have active influence on the PBT decomposition. The result of comparison on decomposition ratio in alcohols is methanol > ethanol >1-propanol >1-butanol.Furthermore, we study the kinetic behavior of PBT in these three supercritical alcohol mediums according to the end-group model. At conditions of T=1.25Tc in a ratio of 5.8, the rate constants and reaction equilibriums are obtained. The results are as followed: kethanol (0.2907 g-PBT·(mol·min-1)) > k1-propanol(0.1979 g-PBT·(mol·min-1)) >k1-butanol(0.1772 g PBT·( mol·min-1)). The apparent activation energy is calculated in 300~400℃with Arrhenius relationship, which gives the values lower than that of PBT in supercritical methanol.Ethylene glycol is chosen as the medium for decomposing PBT at high temperature in order to discuss the effects on the decomposition effectiveness at different experimental parameters which can show us the differences in using different alcohol mediums. FTIR and GC-MS analytic techniques are taken to identify the ingredients of products. The experimental results indicate that the products are composed of bis(hydroxybutyl)terephthalate(BHBT) , butylene glycol(BG), tetrahydrofuran(THF) and a series of different molecular-weight polymers with the same structure to PBT. The reaction time and temperature have active influence on the PBT decomposition. The decomposition ratio is 82.60% in the condition of 400℃, ratio of 5.8 and 25min.