1. Direct polycondensation method

The polycondensation method is to directly condense the lactic acid monomer, also known as the one-step polymerization method. In the presence of a dehydrating agent, the hydroxyl and carboxyl groups in the lactic acid molecule are dehydrated by heat and directly polycondensate into oligomers. Add the catalyst, continue to heat up, low molecular weight polylactic acid polymerizes into higher molecular weight polylactic acid.

2. Two-step method

The lactic acid is formed into cyclic dimer lactide, which is then ring-opened and polycondensed into polylactic acid. This technology is relatively mature, and the process used by NatureWorks in the United States to produce polylactic acid is the process. The polylactic acid production technology jointly developed by China's Hisun and the Chinese Academy of Sciences is also similar. The main process is that the raw materials are fermented by microorganisms to produce lactic acid, and then refined, dehydrated and oligomerized, and then polymerized into polylactic acid.

3. Reactive extrusion to prepare high molecular weight polylactic acid

By combining a batch-type stirred reactor and a twin-screw extruder to conduct continuous melt polymerization experiments, polylactic acid with a molecular weight of 150,000 can be obtained by continuous melt polycondensation of lactic acid. A twin-screw extruder is used to further polycondense the low molar mass lactic acid prepolymer on the extruder to prepare a higher molar mass polylactic acid. When the reaction temperature is 150°C, the amount of catalyst is 0.5%, and the screw speed is 75 r/min, the molar mass of polylactic acid can be quickly and effectively improved by the twin-screw reactive extrusion polycondensation method, and the dispersion coefficient of the reaction extrusion product is reduced. The uniformity becomes better. Through the comparison of DSC curves, it is found that the crystallinity of the polylactic acid prepared by the reactive extrusion polycondensation method is reduced, which is beneficial to improving the brittleness of the polylactic acid material during use.