1. Direct polycondensation(PLA mask)
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. Add catalyst and continue to heat up. Polylactic acid with low relative molecular weight is polymerized to polylactic acid with higher relative molecular weight.
2. Two step method(PLA mask)
Lactic acid is formed 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 the raw materials are fermented by microorganisms to produce lactic acid, 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 mask)
The continuous melt polymerization experiment was carried out by the combination of batch stirring reactor and twin-screw extruder. The polylactic acid with molecular weight of 150000 can be obtained by continuous melt polycondensation of lactic acid. The lactic acid prepolymer with low molar mass was further polycondensated on the extruder by twin-screw extruder to prepare polylactic acid with high molar mass. 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.