Kinetic and Catalytic Studies in Polyethylene Terephthalate Synthesis

Abstract

of Dissertation Polyethylene terephthalate synthesis by polycondensation of bis (hydroxyethylene) terephthalate and its low molecular weight oligomers catalyzed by different titanium (IV) based catalysts was investigated. An industrial catalyst, antimony triacetate, was used as a reference catalyst. Polycondensation was carried out in a stirred tank reactor made of aluminium in the temperature range of 250°C to 280°C under 1 mbar vacuum. The products were characterized with respect to conversion of reaction, molecular weight and concentration of side products. For further investigation, differential scanning calorimetry and thermogravimetric analysis techniques were used in nonisothermal mode under nitrogen purging. Differential scanning calorimetry is an appropriate technique for catalyst fast screening of polycondensation reaction. However, some critical points like catalytic activity of sample holder and mass transfer of by-products should be carefully optimized. Seven different commercially available titanium (IV) compounds were applied which can be mainly classified as chelated and non-chelated titanium derivatives. It was found that nonchelated titanium catalysts were highly active in the synthesis of polyethylene terephthalate nevertheless accelerates the formation of undesired side products. Chelated titanium catalysts showed less activity and more selectivity in the polycondesation reaction. It was also found that the original used titanium compounds were precursors. The catalysts active sites is formed in the beginning of reaction. The exact structure of active species is not known. Probably, the active species is formed by exchange reaction between hydroxyl end groups of monomer and ligands of titanium. The kinetics of polycondensation reaction catalyzed by titanium tetrabutoxide in melt phase, obeys a second order rate law with respect to the concentration of functional end groups. The overall activation energy of polycodenation reaction is 63 kJ mol and that is about 28% less than of antimony triacetate catalyzed polycondensation reaction. A mathematical model was developed to describe kinetic of polycondensation reaction, progress of molecular weight and concentration of side products. The model employed different chemical reactions like reversible polycondensation, degradation reaction and also physical processes like mass transport of ethylene glycol and water. The experimental data were fitted very well by model with respect to conversion, molecular weight and concentration of side products by using of software package PREDICI. Modeling of molecular weight distribution required modification of the reaction scheme to consider formation of short and long chains of polymers. Good fitting was achieved at lower reaction temperature.