Polycarbonate Production from BPA and Phosgene

The process under analysis comprises three major sections: (1) Polymerization; (2) Purification; and (3) Finishing.

Polymerization. Initially, A caustic soda solution, 50 wt%, is mixed with the Bisphenol A and demineralized water, to create a solution of bisphenol-sodium salt, so Bisphenol A solubility in the aqueous phase increases. Phosgene is prone to suffer hydrolysis in an aqueous medium, so it is rather dissolved in an inert organic medium, in this process dichloromethane is used. The reaction is then conducted by mixing these two phases under severe turbulent flow. It is promoted by the addition of a phase transfer catalyst and alkaline medium. A chain stopper agent is added to control the polymer molecular weight. This reaction is conducted in two steps, first the reaction between the bisphenol-sodium salt and phosgene is favored, producing a variety of oligomers, primary chlorocarbonic esters. In the second step these oligomers are condensed into high molecular weight polycarbonate.

Purification. The final output from the polymerization reaction is then fed to a decanter, from which a dichloromethane phase rich in polycarbonate is recovered. This organic stream is then subjected to a series of washes to remove unconverted raw materials and other contaminants. Then, a series of drying operations are conducted to recover a dry polycarbonate solid. Aqueous stream from caustic wash are mixed with an an organic stream from drying operations and are fed to a distillation column, where a top product containing dichloromethane is obtained and recycled to oligomerization.

Finishing. The flakes are mixed with additives and fed to twin-screw extruder, where they are homogenized and pelletized. The pellets are homogenized, packed and stored.