Polycarbonate (Polycarbonate)

There are two methods for the synthesis of polycarbonate, one is the phosgene method and the other is the transesterification method.

Phosgene Method: Phosgene Method is also called solvent method and Interfacial Polycondensation Method. It is a polycarbonate prepared by Interfacial Polycondensation reaction between sodium salt of bisphenol A and phosgene at normal temperature and pressure. The production process of phosgene is mainly divided into the following steps:

light gasification. The prepared sodium bisphenol salt is added to the light gasification kettle, then dichloromethane is added to the light gasification kettle, and stirring is started. When the temperature in the kettle drops to 20℃, the light gas is introduced into the light gasification reaction at a constant speed. When the pH value of the reaction medium is 7-8, the light gas is stopped, and the light gasification reaction ends.

Polycondensation. Add light gasification, 25% alkali liquor, Catalyst, phenol, molecular weight regulator, etc. to the reaction kettle. Under the condition of stirring, continue the reaction at 25-30 ℃ for 3-4 hours. The reaction is exothermic reaction, cold water should be used to cool down. When the reaction is terminated, the upper layer base saline solution is removed by standing in layers, and then 5% formic acid is added to neutralize the material until it is slightly acidic, and the upper layer acid water phase is separated. After neutralization, the reaction solution is post-processed.

Post-processing. On the one hand, the post-treatment is to remove the salt in the resin; On the other hand, it is to remove the low molecular substances and unreacted phenol A in the resin. For the removal of salt, the water washing method is generally adopted, that is, the neutralized resin solution is stirred with water, and the salt in the resin is washed away until the washing water does not contain chloride ions. Low molecular substances generally adopt precipitation method. That is, the resin solution after washing is added with precipitation agent to precipitate polycarbonate in powder or granular form. Precipitation agent alcohol, ketone and petroleum ether, toluene or xylene can be used. The conversion efficiency of phosgene method is high, generally above 90%, and the relative molecular mass can be as high as (15-20)× 104, but the molecular mass is too high, making it difficult to form. Therefore, molecular weight regulators are commonly used to control the molecular weight below 104.

Transesterification method: transesterification method is also called Melt Polycondensation Method. No solvent is used during polymerization and no solvent recovery equipment is required. The product can be directly discharged and pelletized, but the reaction time is relatively long, and it needs to be carried out under high vacuum and high temperature. Due to the high viscosity of the reaction materials, it is difficult to exchange heat and evenly mix the materials during the reaction, so it is difficult to produce high molecular weight products. Transesterification is carried out in the presence of alkaline catalyst under high temperature, high vacuum and melting conditions of phenol A and diphenyl carbonate.

Take phenol A: diphenyl carbonate = 1:(1.05-1.1), the reaction temperature is high, the transesterification reaction speed is fast, but over 180 ℃, phenol A is easy to decompose, therefore, the initial stage of transesterification reaction can only take 180℃ as the upper temperature limit.

The catalysts for transesterification include sodium benzoate, lithium acetate, chromic acetate, cobalt acetate, Potassium hydride, etc. Its dosage is 0.0025% of that of bisphenol A.

Transesterification process has three steps:

transesterification after adding phenol A and diphenyl carbonate to the transesterification kettle and melting, adding catalyst, transesterification is carried out at about 175℃ under 6650PA vacuum for the first stage reaction. When a large amount of phenol in the reaction system is distilled off, the temperature is increased to 200-230 ℃, and the byproduct phenol is continuously distilled under 1330-5320PA vacuum. When the steaming amount is 80-90% of the theoretical amount, the transesterification ends. The reactant was pressed to the polycondensation kettle with chlorine gas.

Polycondensation reaction under stirring and vacuum (133Pa), the reactant after transesterification is controlled to conduct polycondensation reaction at 295-300 ℃, and phenol and diphenyl carbonate are removed. As the reaction proceeds, the melt viscosity of the product increases and the molecular weight increases until the desired index.

After the post-treatment polycondensation reaction is finished, heat it in an inert gas at 300℃ for 2-5H, and drain the melt and soak the pellets with 10% hydrochloric acid solution to remove the alkali metal ions of the alkaline catalyst, wash with water without ions until the pH value is equal to 7, and package after drying.