1. Field of the Invention
The present invention relates to a process for producing a colorless (co)polycarbonate having a high molecular weight by polycondensing a dihydroxy compound with a carbonic diester in the presence of a transesterification catalyst.
2. Description of the Related Art
A polycarbonate having a high molecular weight is a general-purpose engineering thermoplastic which is useful in various fields, particularly as injection molding material or sheet material substituting for window pane. It is said that the polycarbonate usually has excellent thermal resistance, transparency and impact resistance.
Generally known processes for producing a polycarbonate include, for example, a phosgene process wherein a dihydroxy compound is reacted with phosgene by interfacial polycondensation and a transesterification process wherein a dihydroxy compound is reacted with a carbonic diester in a molten state.
A representative transesterification process comprises reacting a dihydric phenol with a carbonic diester in the presence of a transesterification catalyst under heating and reduced pressure while distilling off a formed phenol to prepare a prepolymer and then reacting the prepolymer under heating to 290.degree. C. or above in a high vacuum and distilling off a formed phenol to obtain a polycarbonate having a high molecular weight (see U.S. Pat. No. 4,345,062).
It is known that in the transesterification process, a prepolymer is prepared in an ordinary tank reactor having stirring blades during the initial stage of the reaction and then the polycondensation reaction is conducted in, for example, a vented horizontal extruder in order to efficiently conduct the reaction to thereby obtain a polycarbonate having a high molecular weight.
However, the transesterification process has the problem that a polycarbonate having a high molecular weight has such an extremely high melt viscosity, unlike other engineering plastics, that a temperature as high as 280.degree. C. or above is necessitated for the reaction and so is a high vacuum (1 to 10.sup.-2 Torr) for distilling off the formed monohydroxy compound having a high boiling point, which makes the industrialization of the process difficult from the viewpoint of the equipment. Another problem of the transesterification process is that the high reaction temperature and so forth exert unfavorable influences on the hue, thermal resistance, residence stability in the molding machine, water resistance and weather resistance of the formed polycarbonate.
It is described in "Polycarbonate Resin", third edition, p. 64 (published by the Nikkan Kogyo Shimbun Ltd.) that when an alkaline catalyst is used, bisphenol A as the monomer is decomposed and thereby a colored substance and a resinous substance are formed. It is also described therein that when a polycarbonate is prepared in the presence of an alkali at a high temperature, a side reaction similar to the Kolbe-Schmitt reaction occurs to form branched or crosslinked products.
Under these circumstances, it has been eagerly demanded to develop a process for producing a polycarbonate having a high molecular weight and excellent thermal resistance, hue and impact resistance by the polycondensation reaction of a dihydroxy compound with a carbonic diester in the presence of a transestcrification catalyst.