PC hollow sheet extrusion line

Research and market analysis of polymerisation methods for polycarbonate PC

PC hollow sheet extrusion line：Polycarbonate (PC) is a polymer containing carbonate groups in the molecular chain, the molecular structure formula is shown in formula.

PC is an almost colourless, glassy, amorphous polymer with good optical properties, heat resistance, impact resistance, resistance to weak acids and alkalis and neutral oils. High relative molecular mass PC has high toughness, but it is poorly resistant to hydrolysis, sensitive to chipping, poorly resistant to scratching, susceptible to certain organic solvents, yellowing on long-term exposure to ultraviolet light and cannot be used for products repeatedly subjected to high pressure steam.

PC can be divided into aliphatic, aromatic, aliphatic-aromatic and other types according to the structure of the ester group, among which aliphatic and aliphatic-aromatic PC have lower mechanical properties, limiting their application in engineering plastics; only aromatic PC has been industrially produced so far. Due to its structural peculiarities, PC has become one of the fastest growing general engineering plastics with excellent overall performance and is widely used in the fields of electronics and electrical appliances, sheet containers, automotive industry, medical devices and protective equipment, and is rapidly expanding into new fields such as aerospace and aviation, optical components and optoelectronic information.

PC production process

At present, there are three production processes for PC, namely photogas interface polycondensation (also known as photogas method), molten ester exchange polycondensation and non-photogas molten ester exchange polycondensation.

1.Photogas interfacial polycondensation

First, CO is obtained by partial oxidation and purification of coking coal or low-carbon alkanes, while aqueous NaCl is electrolysed to obtain Cl2 and NaOH. the resulting CO is reacted with Cl2 to produce the raw material phosgene, which undergoes interfacial condensation with BPA in CH2Cl2 solution to produce PC.

In addition, the NaOH produced by the electrolysis serves as a catalyst in the reaction process. The PC solution obtained is first purified by washing with large amounts of water, then adding hexane or toluene and distilling to remove solvents such as CH2Cl2 to obtain PC pellets. The dried PC is granulated using a melt granulator. The reaction is carried out at atmospheric pressure and the separated materials after the polycondensation reaction, the mother liquor from centrifugation, dichloromethane and hydrochloric acid are recycled.

The process is mature, the quality of the product is high, there are no special requirements for the purity of the raw material, the reaction equipment is relatively simple and can be carried out under low temperature and low pressure conditions, but the residual chloride ions in the preparation process will have an impact on the quality of the product. In addition, the raw material is the highly toxic phosgene, which is relatively dangerous, while the amount of waste gas and waste water generated is large.

GE (General Electric) Plastics, Bayer and Mitsubishi Chemical combine evaporation and precipitation in the post-treatment process to remove soluble impurities with the help of solvents and precipitants and improve the purification effect.

2. Melt Ester Exchange Polycondensation
Melt Ester Exchange Polycondensation is based on phenol as raw material, the DPC is first prepared by interfacial photogasification reaction, then in the presence of catalysts (such as lithium halide, lithium hydroxide, lithium aluminium halide and boron hydroxide, etc.) and additives, then the oligomer is obtained by ester exchange reaction with bisphenol A, and further polycondensation to obtain PC products, the reaction process is shown in equation (3).

The process is less expensive than photogas interfacial polycondensation and is relatively simple and produces less waste water and gas. However, the process still requires the use of the highly toxic phosgene in the preparation of the raw material DPC, and the resulting Cl-containing impurities can deactivate the alkaline catalyst through neutralisation reactions, reducing the performance of the PC while also causing the material to yellow.

3. Non-phosgene melt ester exchange polycondensation method
In 1978 GE Plastics first disclosed a patent for the oxidative carbonylation of phenol with CO and O2 using Pd as a catalyst to produce DPC, which was then exchanged with BPA in the molten state to form PC, as shown in equation (4).

The process of oxidative carbonylation of methanol with CO and O2 to form dimethyl carbonate (DMC) was first disclosed by Romano et al. in 1980. DMC then underwent an ester exchange reaction with phenol to form toluene carbonate (MPC), MPC disproportionated to form DPC, and DPC underwent an ester exchange polycondensation reaction with bisphenol A in the molten state to produce PC products, as shown in formula (5).

In 1993, GE Plastics took the lead in industrial production of PC by a non-phosgene process using a biaxial reactor with a capacity of 25,000t/a using CO as a raw material.

CO2 is a non-toxic, non-flammable, bio-renewable raw material, and its activity is lower compared to CO, but it can also be used as a copolymer monomer to produce PC. ningbo zhejiang iron dafeng chemical co., ltd. in China has independently developed a non-phosgene method PC production process using CO2 and propylene oxide as raw materials, and the current annual production capacity is 100,000 t. Asahi Kasei in Japan has also developed a process for the production of PC by non-phosgene method using CO2, as shown in Figure 2. This is shown in Figure 2.
The process is carried out in an anhydrous environment (except cooling water) without the use of any corrosive raw materials. The by-products methanol and phenol can be recycled and the reaction process yields 2 products of high quality PC and high purity MEG without any waste or effluent generation.
The process does not use toxic substances, has a high atomic utilisation rate and is basically pollution-free. The excess raw materials and by-products can be recycled during the production process, which effectively controls production costs.

PC market and development prospects

In recent years, with the rapid development of the economy and the expansion of new applications, PC has become one of the fastest growing engineering plastics, with the main producers concentrated in the United States, Western Europe and Japan, where Bayer, GE Plastics, Dow Chemical and Teijin control the world’s PC production and market.

In 2020, the Asia-Pacific region will account for around 72.6% of global PC demand, and China has become the world’s largest PC producer and consumer. With the strong support of national policies, and the introduction of PC technology in the form of joint ventures and the continuous maturation of domestic self-developed production technology, major PC production projects have been invested in, greatly increasing the domestic PC production capacity, so that this industry has ushered in new development opportunities.

The technical barriers to the PC production process are relatively high, and China started late. before 2015, all domestic PC production plants were built by joint ventures or technology introduction, and the production capacity was highly concentrated. Since then, enterprises such as Wanhua Chemical and Sichuan Zhonglan Guohua New Material have developed various PC synthesis technologies with independent intellectual property rights, which have enabled China’s PC industry to develop and grow.

At present, the world’s new production capacity is mainly concentrated in China. 2019 China’s PC production capacity is 1.66 million t, an increase of 31.74% year-on-year, with a capacity utilization rate of 58%. 2020 China’s PC production capacity increases to 2.44 million t/a and production increases to 1.185 million t.

PC has been included in the national high-tech industry, key strategic emerging industries and the ten key areas described in “Made in China 2025”. Encouraged by national policies and the massive investment in domestic production capacity, the future domestic PC market is bound to show a situation where supply exceeds demand, with a conservative estimate of 2.41 million t/a of PC production capacity to be released in China by 2023, by which time China’s total PC production capacity will be close to 3.6 million t/a.

Compared with the fast-growing PC production capacity, the downstream demand has not been released simultaneously. Overall, PC consumption growth is sluggish and the industry has entered a period of low development. Ordinary PC has poor impact resistance and chemical resistance and cannot be applied in many fields, but the technical barriers to the production of high-end products are high. While expanding production at high speed, domestic enterprises should strengthen the quality of their products and change the dilemma of excess low-end products and lack of high-end products in order to grow and develop in the fierce market competition.

Among the three major production processes of PC, the non-phosgene method is non-polluting to the environment, no by-products, low investment, good efficiency and high quality of the resulting products, which will definitely be the future development direction of PC. At present, the production cost of high-purity DPC can be reduced by developing efficient catalysts, and in addition, high-efficiency reactors can be designed to make them suitable for the production of PC prepolymers with high melt viscosity.

Although the production capacity and consumption of PC in China are increasing, the products are generally basic materials, mainly used in packaging, films and sheets, etc., with narrow applications and low profits. The high-end products used in automotive, home appliances, electronic products and other fields are still dependent on joint ventures or foreign enterprises. With the further release of production capacity, supply and demand are gradually approaching equilibrium, competition in the PC industry is intensifying and companies are facing great challenges. Eliminate environmentally unfriendly technologies, reduce the production cost of PC raw materials, regulate and improve product performance, develop high-performance products, reduce energy consumption of devices, improve the competitiveness of devices, and increase the independent research and development force of domestic technology is the future direction of development of the PC industry.