1.1. Synthesis method
In general, the preparation of polylactic acid (PLA) is synthesized from lactic acid as a raw material. At present, there are many synthetic methods, the more mature is the direct polycondensation method of lactic acid, and the other is to synthesize lactide from lactic acid first, and then ring-opening polymerization under the action of a catalyst. There is also a solid-phase polymerization method.
1. Lactic acid direct polymerization method
The direct polymerization method has been studied as early as the 1930s and 1940s, but since the key technologies related to water removal in the reaction have not been well solved, the molecular weight of the products is low (all below 4000), Very low strength, easy to decompose, no practicality.
Japan Showa Polymer Co., Ltd. adopts the method of slowly heating lactic acid in inert gas and slowly decompressing it so that the lactic acid is directly dehydrated and condensed, and the reactant is further polycondensed at 220~260 ° C and 133 Pa to obtain a molecular weight of more than 4000. Polylactic acid. However, the reaction time of this method is long, and the product will be aged, decomposed, discolored, and uneven at a high temperature in the later stage. Japan's Mitsui Press Chemical Company uses the solution polymerization method to directly polymerize lactic acid to obtain polylactic acid.
The main feature of the direct method is that the synthesized polylactic acid does not contain a catalyst, so when the polycondensation reaction progresses to a certain extent, the system will appear in an equilibrium state. It needs to be heated and pressurized to break the reaction equilibrium, and the reaction conditions are relatively harsh. In recent years, through the innovation and improvement of technology, the direct polymerization method has made some progress, and it should be applied to large-scale industrial production in the near future as the technology continues to mature.
2. Ring-opening polymerization method
Ring-opening polymerization is the most widely used production method in the world. As early as the middle of the 20th century, DuPont researchers obtained high molecular weight polylactic acid by ring-opening polymerization. In recent years, foreign research on the synthesis of polylactic acid has mainly focused on the ring-opening polymerization of lactide.
The polylactic acid series products produced by Boehringer Zngelhelm in Germany by this method appear on the market under the trade name; the polylactic acid produced by Cargill in the United States is processed by melt blowing and spun bonding to develop medical non-woven products; and There are only a handful of companies in my country that can synthesize polymer polylactic acid, such as the Institute of Polymer Research of Sun Yat-Sen University. Ring-opening polymerization mostly uses stannous octoate as the initiator, the molecular weight can reach one million, the mechanical strength is high, and the polymerization and separation are carried out in two steps:
3. The solid-state polymerization method
This method is obtained by polymerizing the low molecular weight resin obtained by direct polymerization under the conditions of reduced pressure and vacuum and temperature between Tg-Tm, so as to increase its degree of polymerization and increase its molecular weight, thereby improving material strength and processability.
4. Take the material
Shell crops such as corn are crushed to extract the starch, which is then made into unrefined glucose. Now many high technologies have overcome the process of grinding and extracting raw materials directly from a large number of crops.
5. Fermentation
Glucose is fermented in a manner similar to the production of beer or alcohol, which becomes lactic acid similar to a food added for use in human muscle tissue.
6. Intermediate products
The lactic acid monomer is converted into an intermediate product - water-reduced lactic acid, namely lactide, by a special concentration process.
7. Aggregation
After the lactide monomer is vacuum purified, a solvent-free dissolution process is used to complete the ring-opening action to polymerize the monomer.
8. Polymer modification
Due to the difference in the molecular weight and crystallinity of the polymer, the material properties can be greatly changed, so PLA is modified differently for different applications.