Issue 12/2012 - Non-phosgene green process will become a trend in PC technology

State of the technology

At present, the production methods of polycarbonate (PC) mainly include interfacial
polycondensation process and melt polycondensation process.

Interfacial polymerization process (phosgene method) The whole reaction is divided into two stages: phosgene and polycondensation: the sodium salt solution of bisphenol A is sent to the phosgene reactor, with dichloromethane as the solvent, passed into phosgene, and polymerized into low molecular weight PC at the interface; Then add triethylamine and caustic soda solution to condensation into polymer PC
.
The advantages of the interface polycondensation process are mature process, high product quality, easy to scale and continuous production, good economy, so it has occupied the dominant position of PC production for a long time (about 90% of the world's PC production currently uses this method), and some new devices still use this process
.
The disadvantage is the widespread use of highly toxic phosgene, and the use of dichloromethane also poses an environmental problem
.

The melt polymerization process (also known as non-phosgene method) is melted polycondensation
by diphenyl carbonate and bisphenol A.
This method does not require washing and drying process, the operation is simpler than the interface process, no by-products, basically no pollution, high product purity, good optical performance, high transparency, suitable for optical applications, has been successfully applied to CD, CD-R and DVD discs
.
The disadvantages of the non-phosgene method are the high temperature resistance, high vacuum resistance of the production equipment, and the wide molecular weight distribution range of the polymer due to the high melt viscosity, which limits its end use
in some ranges.
During the production process, phenol can be recycled for the preparation of diphenyl carbonate, reducing the cost of raw materials, especially avoiding the use of the highly toxic chemical phosgene
.
The process is a "green process"
that meets environmental requirements.
It is the development direction of PC synthesis process in the future, and will gradually occupy a dominant position
in PC production in the future.
Technology patentees with this technology include GE, Bayer and Asahi Kasei
.

New process development and application

●In
recent years, with the increasing awareness of environmental protection and the increasingly strict restrictions on the use of phosgene by environmental protection agencies in various countries, the world's major PC manufacturers who originally used phosgene interface polycondensation process have successively developed environmentally friendly clean production processes - non-phosgene processes
.

(1) Mitsubishi Gas' non-phosgene process technology - urea-methanol method
In order to avoid the formation of azeotropes in the reaction system, Mitsubishi Gas Company has developed a technical route
for the synthesis of di-n-butyl carbonate or diisoamyl carbonate from urea alcohololysis, and then exchanged with phenol esters to synthesize diphenyl carbonate.
Compared with dimethyl carbonate, the boiling point of di-n-butyl carbonate is closer to the reaction temperature of synthetic diphenyl carbonate, and no azeotrope is formed in the reaction system, and the product is easy to separate and has strong competitiveness
.
If the overall conversion rate and selectivity can be improved, and the recovery and reuse of ammonia gas can be done, the method is very promising
.

(2) LG Chem non-phosgene process technology LG Chem of South Korea has also developed a process technology

for making PC by non-phosgene method.
Compared to other alternatives, this method requires few steps and very little equipment
.
The process uses a new catalyst and a combination of polymerization and crystallization processes, which are estimated to reduce investment costs by 70%.


LG Chemical process uses DMC and phenol reaction distillation to generate DPC, and then uses a special catalyst in a single reactor to melt and condense DPC and bisphenol A and crystallize
.

LG Chem has validated the new process in a 2kg/h unit, producing colorless PCs with 98% transparency and similar processability
to PCs manufactured by other methods.
The company estimates that the investment cost of the 60,000 t/a plant will be less than $100 million, while the installation using other routes will require $250 million
.
Savings are also expected on operating costs
.
LG has issued 6 patents covering processes, products and catalysts
.
The company has considered technology transfer or joint ventures to commercialize
it.

(3) The biggest advantage of non-phosgene process technology is still constantly improving and improving
non-phosgene process is: methanol and phenol can be recycled, reducing the cost of raw materials, and do not require washing and drying processes, reducing investment; The operation is simpler than the interface process, no by-products, does not use toxic raw materials such as phosgene and dichloromethane, which is conducive to environmental protection; The product has high purity, good optical performance and high transparency, which is more suitable for the application of high value-added optical discs and other products, and has been successfully used in the preparation of CD, CD-R and DVD discs
.

The drawback of the non-phosgene process is that the polymer tends to rearrange under reaction conditions and produce branched aryl ketones
.
The concentration of this branched substance in PC is as high as 2500~3000ppm, which leads to a decrease in product ductility and poor
rheology.
Recently, GE patents mention that the use of sulfites instead of commonly used basic metal hydroxides as polymerization catalysts can significantly reduce the content
of branched-chain aryl ketones.
When sodium metabisulfite is used instead of sodium hydroxide and sodium methoxide composition, the concentration of branched-chain species can be reduced from 2763ppm to 484ppm, the average molecular weight is also increased from 8252 to 8459, and the pH of the system is also reduced from 11.
6 to 4.
3
.
In addition, in non-phosgene production processes, polymers have a wide molecular weight distribution range due to high melt viscosity, limiting their end use
in some ranges.
Therefore, major companies are focusing on the technical problems
of low molecular weight and high hydroxyl capping of non-phosgene fusion polymerization processes.
At present, GE has patented technology that can increase the molecular weight of PC from 10,000~35,000 to 50,000~80,000, and LG can reach 45,000~60,000, which can basically expand its application field to plates
.

● China
has also made certain progress
in PC technology in recent years.
Zhonglan Chenguang Chemical Research Institute, East China University of Science and Technology and Tianjin University Petrochemical Technology Development Center have carried out research and development work on non-phosgene PC synthesis technology, and Zhonglan Chenguang Chemical Research Institute has carried out 1000t/aPC industrialization test and device design
.

Zhonglan Chenguang Chemical Research Institute undertakes the "Ninth Five-Year Plan" national science and technology research project "500~1000t/a transesterification method PC continuous polycondensation new process industrial technology development"
。 On the basis of small tests, the 100-ton intermediate test was completed in 1998, and the 1,000-ton continuous test device was established in 1999, and after testing, the key technologies such as polycondensation reactor and conveying viscous materials and mass and heat transfer under high temperature and high vacuum were solved, and the continuous and stable operation of the device was realized, qualified PC products were produced, and the technical parameters required for the design of the 10,000-ton production device were obtained, laying the foundation
for the design of the 10,000-ton production device.
The research results were appraised
in April 2000 by the State Petroleum and Chemical Industry Administration.

The design of
the 10,000 t/aPC production device developed by Zhonglan Chenguang Chemical Research Institute and China Textile Industry Design Institute.
The transesterification production process is adopted, which mainly includes three units
: feeding, transesterification reaction and polycondensation reaction.
The process flow is as follows: the raw materials bisphenol A and diphenyl carbonate are fully mixed with ingredients, and after adding the catalyst, they enter the pre-reactor to carry out the transesterification reaction between small molecules, and remove phenol, the pre-reaction melt enters the tower first transesterification reactor, and the melt obtained by transesterification enters the second transesterification reactor through a low-viscosity transfer pump, and under full stirring, the transesterification reaction is completed and accompanied by polycondensation reaction
.
The top of the first and diglyceride exchange reactors is equipped with a phenol recovery cooling system and a water ring jet vacuum system
.
After the melt is released from the second transesterification reactor, it enters the vertical precondensation reactor with a disc with round holes and a stirring slurry, and the molecular chain continues to grow and the viscosity increases
.
The pre-condensed polymelt enters the horizontal first polycondensation reactor with spiral propellant, and the molecular weight of the polycondensation reaction melt rises to 15,000~18,000, the terminator is added to the melt after the reaction, and it is transported by the medium viscosity transfer pump, and the high-precision filter is filtered into the pelletizer, which can produce disc-level slices
.
The medium viscosity melt can enter the second polycondensation reactor through the transfer pump to continue the polycondensation reaction, and after the high viscosity transfer pump, the terminator is added and the granulation is obtained to obtain a section with a
molecular weight of 24,000~27,000.

In addition, the Chunying Institute of Chinese Science and Technology carried out the "Research on PC Technology for Phosgene Interface Method", which broke through a series of technical keys to the production of PC by phosgene method, such as raw material ratio, catalyst dosage and molecular weight control, and reaction pH control, and obtained independent intellectual property rights technology
.
Compared with the existing domestic technology, this technology does not need to prepare an aqueous solution of bisphenol A before the reaction, and does not need to use antioxidants, which greatly simplifies the process and makes the reaction flow control have good stability
.
On this basis, the kilogram-level PC resin with excellent quality was successfully prepared, and its mechanical properties reached or exceeded the performance indicators
of imported engineering grade PC.
The technology reacts at room temperature and pressure, the equipment is simple, and the existing conditions in China can be fully achieved
.
The reaction is easy to control, the requirements for monomers are low, it is easy to industrialize, and it has broad development prospects
.
And has cooperated with Gansu Yinguang Group to build a 500t/a PC device
.

Future trends

Non-phosgene PC process is a kind of "green process" that meets environmental requirements, and has less investment, low operating costs, large economic benefits and high product quality, which has become the development direction of PC process technology in the future and occupies an important position
in future PC production.
And its technology has made new and important progress
.
Several technical routes for the preparation of PC by non-phosgene method can be summarized as follows:
among them, 1-methanol oxidative carbonylation synthesis of dimethyl carbonate 2-dimethyl carbonate transtitution synthesis of diphenyl carbonate
3-phenol oxidative carbonylation synthesis of diphenyl carbonate
4-bisphenol A oxidative carbonylation synthesis PC
At present, route 1 and route
2 technologies have achieved
。 Route 3 and Route 4 are hot technologies
with broad prospects developed by major companies.
Phenol oxidative carbonylation to synthesize diphenyl carbonate, the method of raw materials from a wide range of sources, inexpensive, no phosgene, less three waste, the yield of diphenyl carbonate can reach up to 29.
3%.

With obvious competitiveness, it will become the development direction of diphenyl carbonate synthesis technology; The direct synthesis of PC by bisphenol A oxidative carbonylation method has the advantages of low toxicity, no pollution, high product quality and short process flow, which is a hot spot for research in all countries in the world and a technology
with great development potential.