Production process flow of polycarbonate

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1 Interface phosgene method The interface phosgene process begins with the reaction of bisphenol A and a 50% sodium hydroxide solution to produce bisphenol A sodium salt, which is then fed into a phosgenation reactor. Using dichloromethane as a solvent, phosgene is introduced, allowing it to react with the bisphenol A sodium salt at the interface to form low-molecular-weight polycarbonates, which are subsequently polycondensed into high-molecular-weight polycarbonates. The reaction is carried out at atmospheric pressure, with triethylamine generally used as a catalyst. The materials separated after the polycondensation reaction, the centrifuged mother liquor, dichloromethane, hydrochloric acid, and the like all need to be recycled. This method features mature technology and high product quality. 2 Solution phosgene method: The solution phosgene process involves introducing phosgene into a dichloromethane solvent containing bisphenol A and an acid acceptor (calcium hydroxide, triethylamine, and p-tert-butylphenol), allowing a reaction to take place, and then separating the polymer from the solution. GE used this process in its first plant in the United States. This process is less economical and lacks competitiveness compared to the phosgene-based process. 3 Ordinary melt transesterification method: The melt transesterification process uses phenol as a raw material to produce diphenyl carbonate (DPC) through phosgenation. Diphenyl carbonate then undergoes a transesterification reaction with diacid A in the presence of catalysts such as lithium halides, lithium hydroxide, lithium aluminum halides, and boron hydroxide, as well as various additives, to yield oligomers; these oligomers are further polycondensed to produce PC products. The production cost of the transesterification method is lower than that of the phosgene coupling method, but certain drawbacks of this process hinder its industrial application. Such as poor optical properties of the product, a limited molecular weight range, and catalyst contamination. Currently, Bayer is still conducting research on this process, attempting to recover chlorine from the by-product sodium chloride using electrolysis, and to recycle this chlorine for the production of phosgene. 4 Non-phosgene melting processes: Due to the high toxicity and severe pollution associated with the phosgene method, new processes for producing polycarbonate without using phosgene have been developed in recent years and have been put into industrial use, representing a major breakthrough in the production of polycarbonate. Unlike the conventional melt transesterification method, the phosgene-free melt process does not use the highly toxic phosgene to produce diphenyl carbonate; instead, it employs dimethyl carbonate (DMC) and phenol in a transesterification reaction to yield diphenyl carbonate. This diphenyl carbonate is then polycondensed with diacid A to produce polycarbonate. The production method of dimethyl carbonate, the raw material in this process, generally utilizes a patent from the Italian company Eni; it is obtained through oxidative carbonylation using methanol, carbon monoxide, and oxygen as raw materials. GE has built polycarbonate production plants using the non-phosgene process with capacities of 40 kt/a and 130 kt/a in Japan and Spain, respectively. The phosgene-free melting process does not use the highly toxic phosgene, which is beneficial to environmental protection, and the resulting products are more suitable for producing high-value optical discs. During the production process, methanol and phenol are recycled, reducing raw material costs. Compared with the phosgene-based interface method, the phosgene-free melting method offers advantages in terms of investment and production costs.