Polycarbonate (PC) is a molecular chain containing carbonate group, according to the molecular structure with different ester groups, can be divided into aliphatic, alicyclic, aromatic, of which the most practical value of the aromatic group, and the most important bisphenol A type polycarbonate, the general heavy average molecular weight (Mw) in 20-100,000.

Picture PC structural formula

Polycarbonate has good strength, toughness, transparency, heat and cold resistance, easy processing, flame retardant and other comprehensive performance, the main downstream applications are electronic appliances, sheet and automotive, these three industries account for about 80% of polycarbonate consumption, other in industrial machinery parts, CD-ROM, packaging, office equipment, medical and health care, film, leisure and protective equipment and many other fields have also achieved a wide range of applications, becoming one of the five engineering plastics in the fastest growing category.

In 2020, the global PC production capacity of about 5.88 million tons, China’s PC production capacity of 1.94 million tons / year, production of about 960,000 tons, while the apparent consumption of polycarbonate in China in 2020 reached 2.34 million tons, there is a gap of nearly 1.38 million tons, need to import from foreign countries. The huge market demand has attracted numerous investments to increase production, it is estimated that there are many PC projects under construction and proposed in China at the same time, and the domestic production capacity will exceed 3 million tons/year in the next three years, and the PC industry shows an accelerated trend of transfer to China.

So, what are the production processes of PC? What is the development history of PC at home and abroad? What are the main PC manufacturers in China? Next, we briefly do a comb.

PC three mainstream production process methods

Interfacial polycondensation photogas method, traditional molten ester exchange method and non-photogas molten ester exchange method are the three main production processes in the PC industry.
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1. Interfacial polycondensation phosgene method

It is the reaction of phosgene into inert solvent and aqueous sodium hydroxide solution of bisphenol A to produce small molecular weight polycarbonate, and then condensed into high molecular polycarbonate. At one time, about 90% of industrial polycarbonate products were synthesized by this method.

The advantages of interfacial polycondensation phosgene method PC are high relative molecular weight, which can reach 1.5~2*105, and pure products, good optical properties, better hydrolysis resistance, and easy processing. The disadvantage is that the polymerization process requires the use of highly toxic phosgene and toxic and volatile organic solvents such as methylene chloride, which cause serious environmental pollution.

Melt ester exchange method, also known as ontogenic polymerization, was first developed by Bayer, using molten bisphenol A and diphenyl carbonate ( Diphenyl Carbonate, DPC), at high temperature, high vacuum, catalyst presence state for ester exchange, pre-condensation, condensation reaction.

According to the raw materials used in DPC process, it can be divided into traditional molten ester exchange method (also known as indirect photogas method) and non-photogas molten ester exchange method.

2. Traditional molten ester exchange method

It is divided into 2 steps: (1) phosgene + phenol → DPC; (2) DPC + BPA → PC, which is an indirect phosgene process.

The process is short, solvent-free, and the production cost is slightly lower than the interfacial condensation phosgene method, but the production process of DPC still uses phosgene, and the DPC product contains trace amounts of chloroformate groups, which will affect the final product quality of PC, which to a certain extent limits the promotion of the process.

3. Non-phosgene molten ester exchange method

This method is divided into 2 steps: (1) DMC + phenol → DPC; (2) DPC + BPA → PC, which uses dimethyl carbonate DMC as raw material and phenol to synthesize DPC.

The by-product phenol obtained from ester exchange and condensation can be recycled to the synthesis of DPC process, thus realizing material reuse and good economy; due to the high purity of raw materials, the product also does not need to be dried and washed, and the product quality is good. The process does not use phosgene, is environmentally friendly, and is a green process route.

With the national requirements for petrochemical enterprises’ three waste With the increase of national requirements on safety and environmental protection of petrochemical enterprises and the restriction on the use of phosgene, the non-phosgene molten ester exchange technology will gradually replace the interfacial polycondensation method in the future as the direction of PC production technology development in the world.