There are three production processes for producing polylactic acid from lactic acid:

(1) Direct polycondensation method
The condensation polymerization method is the direct condensation of lactic acid monomers, also known as one-step polymerization. In the presence of dehydrating agents, the hydroxyl and carboxyl groups in lactic acid molecules are heated and dehydrated, leading to direct condensation and polymerization to synthesize oligomers. By adding a catalyst and continuing to heat up, low relative molecular weight polylactic acid is polymerized to synthesize higher relative molecular weight polylactic acid.

(2) Two-step method
Lactic acid is generated into a cyclic dimer of lactide, which is then ring-opened and condensed into polylactic acid. This technology is relatively mature, and the process used by NatureWorks in the United States to produce polylactic acid is called this process. The production technology of polylactic acid jointly developed by China's Haizheng and the Chinese Academy of Sciences is also similar. The main process is to produce lactic acid from raw materials through microbial fermentation, which is then refined, dehydrated, oligomerized, and high-temperature cracked, and then polymerized into polylactic acid.

(3) Preparation of High Molecular Weight Polylactic Acid by Reactive Extrusion
By combining an intermittent stirred reactor and a twin screw extruder for continuous melt polymerization experiments, polylactic acid with a molecular weight of 150000 can be obtained through continuous melt polycondensation of lactic acid. The low Molar mass lactic acid prepolymer was further polycondensated in a twin-screw extruder to prepare high molar mass PLA. When the reaction temperature is 150 ℃, the amount of catalyst is 0.5%, and the screw speed is 75 r/min, the Molar mass of PLA can be rapidly and effectively improved by the twin screw reactive extrusion polycondensation method, and the dispersion coefficient of the reactive extrusion product is reduced, and the uniformity of the product is improved. Through the comparison of DSC curves, it was found that the crystallinity of polylactic acid obtained by reactive extrusion polycondensation method decreased, which is beneficial for improving the brittleness of polylactic acid materials during use.