Experimental and modeling studies are presented on the synthesis of bisphenol A polycarbonate at ambient pressure by a novel forced gas sweeping process. Unlike in the conventional highvacuum melt transesterification process, the condensation byproduct (phenol) is removed from a highly viscous polymer melt in the forced gas sweeping process by forcing inert gas bubbles to flow through the polymer melt phase. As the inert gas bubbles rise in the melt phase, dissolved phenol molecules diffuse to the bubbles and are removed from the polymer melt, and the polymer molecular weight increases. To examine the feasibility of the proposed method, the effects of reaction temperature and gas flow rate on the polymer molecular weight were investigated at 260-280 °C and ambient pressure using a small semibatch laboratory reactor. It has been found that medium-range molecular weight polycarbonate can be readily prepared in a relatively short reaction time. The semibatch polymerization process is also modeled by a mass-transfer reaction model in which bubble size and bubble rising velocity are used to estimate the interfacial masstransfer area and gas-liquid contact time. The experimental data suggest that the forced gas sweeping process can be a potential alternative to a high-vacuum melt polycondensation process for the synthesis of bisphenol A polycarbonate at ambient pressure.
Experimental and theoretical modeling studies are presented on the forced gas sweeping process for the continuous melt polycondensation of bisphenol A polycarbonate. In this process, unlike in the conventional high-vacuum melt polycondensation (transesterification) process, the condensation byproduct (phenol) is removed from a highly viscous polymer melt by forcing inert gas bubbles to flow directly through the polymer melt phase. As the gas bubbles rise in the polymer melt phase, dissolved phenol molecules diffuse to the bubbles and are removed from the polymer melt, and the polymer molecular weight increases. In this study, the feasibility of continuous reactor operation is investigated using a continuous rotating-disk reactor at 260-300 °C and ambient pressure. With a low-molecular-weight polycarbonate prepolymer (M n ) 5050) as the feed, polycarbonate of molecular weight up to 20 000 has been obtained at steady state. To investigate the effect of reactor operating conditions on the polymer molecular weight, a multicompartment dynamic mass-transfer reaction model has also been developed. In the model, bubble size and bubble rising velocity are used to estimate the interfacial mass-transfer area and average gas-liquid contact time. Both model simulations and experimental results indicate that the forced gas sweeping process can be a potential alternative to a high-vacuum continuous melt polycondensation process for the synthesis of bisphenol A polycarbonate.
ABSTRACT:The experimental and modeling studies are presented on the melt polycondensation of poly(ethylene terephthalate) by a gas sweeping process. In this process, low molecular weight prepolymer is polymerized to a higher molecular weight polymer in a molten state at ambient pressure as ethylene glycol is removed by nitrogen gas bubbles injected directly to the polymer melt through a metal tube. In the temperature range of 260 -280°C, the rate of polymerization by the gas sweeping process is quite comparable to that of conventional high vacuum process. The effects of nitrogen gas flow rate and reaction temperature on polymerization rate and polymer molecular weight were investigated. Polymer molecular weight increases with an increase in gas flow rate up to certain limits. A dynamic mass transfer-reaction model has been developed, and the agreement between experimental data and model simulations was quite satisfactory. The effect of ethylene glycol bubble nucleation on the polymerization has also been investigated. It was observed that the presence of nucleated ethylene glycol bubbles induced by the bulk motion of polymer melt has negligible impact on the polymerization rate and polymer molecular weight.
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