There are two main methods for the preparation of PLA, namely lactide ring opening polymerization and direct polycondensation. Ring opening polymerization is mainly used in industry, and the technical difficulty of ring opening polymerization lies in the synthesis and purification of lactide
The direct polycondensation method, also known as the one-step method, refers to the process of preparing PLA through dehydration and polycondensation of lactic acid molecules. The disadvantage of this method is that the reaction system is in a dynamic balance of polycondensation and depolymerization. The gradual increase in the viscosity of the system increases the difficulty of removing the by-product water. The water that cannot be removed in time will cause the reaction to proceed in the direction of polymer depolymerization, thereby affecting PLA. Increase in molecular weight.
At the same time, under high temperature and high vacuum reaction conditions, PLA will also undergo depolymerization, coloration and racemization, which will reduce the performance of the product.
The one-step method of producing PLA has a short production process and low cost, but because the molecular weight of PLA is difficult to effectively increase, the product has poor mechanical properties, which limits its industrial application.
Lactide ring-opening polymerization method, also known as two-step method, is a process in which lactic acid is first dehydrated to form oligomers, then depolymerized to form lactide, and then ring-opened polymerization to produce PLA. The core technology is lactide. Synthesis and purification.
The purification of lactide is very important in the entire ring-opening polymerization process. Only high-purity lactide can be used to synthesize PLA with high molecular weight and good physical properties.
The two-step method involves the purification steps of lactide, not only the process is complicated, the cost is also high, but the purity and reaction conditions of lactide can be controlled to realize the production of PLA with high molecular weight, controllable chemical structure and better mechanical properties. , So it is currently the most widely used method in industry.
