Improved thermally stable poly(ethylene terephthalate) (PET): Solid state polymerization of PET

Solid state polymerization (SSP) reaction kinetics of poly (ethylene terephthalate) (PET) have been investigated in relationship to initial precursor intrinsic viscosity (molecular weight). Evaluations were performed using otherwise equivalent precursors, melt-polymerized to intrinsic viscosities (IV) of 0.50, 0.56, and 0.64 dL/g. Changes in molecular weight and other properties were monitored as functions of reaction times at solid state temperatures from 160°C to 230°C. Precursors with higher initial molecular weights were found to exhibit higher rates of SSP than those with lower initial values, as discussed in relationship to levels of crystallinity as well as carboxyl and hydroxyl end group composition. Activation energies were observed to decrease at temperatures above 200°C, indicating a change of SSP reaction mechanism. At temperatures from 200°C to 230°C, similar activation energies are required for polymerization of all three precursors. Lower temperature polymerizations, from 160°C to 200°C, require higher activation energies for all precursors, with the 0.50 IV material requirement about twice as high as calculated for the higher IV precursors.; A comprehensive modeling studies on the kinetics in the solid state polymerization (SSP) of Poly(ethylene terephthalate) were also performed. The validity of the model could be confirmed by successfully fitting the experimental results for the molecular weight increase with one fitting parameter. The changes of concentrations for the hydroxyl end group, carboxyl end group, vinyl end group, and terephthalic acid (TPA) were simulated using the model. During SSP, the contents of not only hydroxyl and carboxyl end groups but also vinyl ester end group and TPA monomer were simulated to decrease as a function of SSP time and temperature. The effect of the pellet size and the molar ratio of carboxyl end group were also simulated. At the end group molar ratio, [-COON]/[-OH], of around 0.7, a maximum SSP rate was obtained. As the ratio, [-COOH]/[-OH], increased, the contents of vinyl end group and TPA monomer were simulated to increase.; After three different IV poly(ethylene terephthalate) (PET) precursors were solid state polymerized, vinyl ester end group(VE) contents were estimated, by applying reaction kinetics, to be 0.7–5.5 mmol/kg PET for the various samples including precursors and solid stated samples. As the intrinsic viscosity (IV) of the precursor increased, VE content also increased, representing different thermal histories of the samples in the melt phase polymerization for different precursor IV's. VE contents were decreased as SSP time increased from 0 to 12 hours and as the temperature of SSP increased up to 220°C.; A series of acetaldehyde (AA) generation experiments were conducted in the temperature range of 270–300°C with the samples solid stated from different precursors. The rate of AA generation decreased as the final IV of the solid stated PET increased, showing that the SSP process helps to improve thermal stability of PET. We also compared the AA generation rates of the samples of similar final IV, which were solid stated from different IV precursors. When the generation temperature was low, the amount and the rate of AA generation increased as precursor IV increased. This tendency was observed to become obscure as the generation temperature increased, probably due to the interference from the increasing rate of VE formation reaction from the polymer chains at the elevated temperature.