Melt polycondensation of bisphenol A polycarbonate by forced gas sweeping process II. Continuous rotating-disk reactor

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 h.-v. 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.