1. Direct polycondensation(PLA material)
Polycondensation is the direct condensation of lactic acid monomer, also known as one-step polymerization. In the presence of dehydrating agent, hydroxyl and carboxyl groups in lactic acid molecules are dehydrated by heating, and oligomers are synthesized by direct polycondensation. By adding catalyst and heating up, polylactic acid with low relative molecular weight is polymerized to polylactic acid with higher relative molecular weight.
2. Two step method(PLA material)
Lactic acid is made into cyclic dimer lactide, and then ring opening polycondensation is carried out to form polylactic acid. This technology is relatively mature. The main process is that lactic acid is prepared from raw materials by microbial fermentation, then refined, dehydrated oligomerization, high temperature cracking, and finally polymerized into polylactic acid.
3. Preparation of high molecular weight polylactic acid by reactive extrusion(PLA material)
Polylactic acid with molecular weight up to 150000 can be obtained by continuous melt polycondensation of lactic acid. The low molar mass of lactic acid prepolymer was further polycondensated on the twin-screw extruder to prepare high molar mass of polylactic acid. 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 polylactic acid can be rapidly and effectively improved by twin-screw reactive extrusion polycondensation, and the dispersion coefficient of reactive extrusion products decreases and the uniformity becomes better. Through the comparison of DSC curves, it is found that the crystallinity of polylactic acid prepared by reactive extrusion polycondensation is reduced, which is beneficial to improve the brittleness of polylactic acid materials in the process of use.