Preparation of polylactic acid (PLA)

Preparation of polylactic acid (PLA)

Polylactic acid (PLA) is a new type of bio-based and renewable biodegradable material, which is made from starch raw materials proposed by renewable plant resources (such as corn, cassava, etc.). The starch raw material is saccharified to obtain glucose, and then fermented by glucose and certain strains to produce high-purity lactic acid, and then synthesize polylactic acid with a certain molecular weight by chemical synthesis. It has good biodegradability. After use, it can be completely degraded by microorganisms in nature under specific conditions, and finally produces carbon dioxide and water without polluting the environment. This is very beneficial to protecting the environment and is recognized as an environmentally friendly material.
Polylactic acid, also known as polylactide, belongs to the family of polyesters. Polylactic acid is a polymer obtained by polymerizing lactic acid as the main raw material. The source of raw materials is sufficient and can be regenerated. It mainly uses corn and cassava as raw materials. The production process of polylactic acid is non-polluting, and the product can be biodegraded to realize the cycle in nature, so it is an ideal green polymer material.
Polylactic acid has good thermal stability, the processing temperature is 170-230 ° C, and it has good solvent resistance. It can be processed in various ways, such as extrusion, spinning, biaxial stretching, and injection blow molding. In addition to being biodegradable, products made of polylactic acid have good biocompatibility, gloss, transparency, feel and heat resistance, and also have certain bacteria resistance, flame retardancy and UV resistance, so they are very useful. It is widely used as packaging materials, fibers and nonwovens, etc. It is mainly used in clothing (underwear, outerwear), industry (construction, agriculture, forestry, paper) and medical and health fields.

Preparation method of polylactic acid

In general, the preparation of polylactic acid (PLA) is synthesized from lactic acid as raw material. There are many synthetic methods at present, the relatively mature one is the direct polycondensation method of lactic acid, and the other is to first synthesize lactide from lactic acid, and then ring-opening polymerization under the action of a catalyst. There is also a solid state polymerization method.

01 Lactic acid direct polymerization method

The direct polymerization method has been studied as early as the 1930s and 1940s, but because the key technologies related to the removal of water in the reaction have not been well resolved, the molecular weight of the products is low (all below 4000), The strength is extremely low, easy to decompose, and has no practicality.

Showa Polymer Co., Ltd. of Japan adopts the method of slowly heating lactic acid in an inert gas and slowly decompressing it, so that the lactic acid can be directly dehydrated and condensed, and the reactants can be further polycondensed at 220~260°C, 133Pa, and the relative molecular mass is above 4000. polylactic acid. However, the reaction time of this method is long, and the product will age, decompose, change color and be uneven at the high temperature in the later stage. Japan's Mitsui Pressure Chemical Co., Ltd. uses solution polymerization 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, and it needs to be heated and pressurized to break the reaction equilibrium, and the reaction conditions are relatively harsh. In recent years, through technological innovation and improvement, the direct polymerization method has made some progress, and it should be able to be applied to industrialized large-scale production in the near future as the technology continues to mature.

02 Ring-opening polymerization method

Ring-opening polymerization is currently the most 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 studies on the synthesis of polylactic acid have mainly focused on the ring-opening polymerization of lactide.

The polylactic acid series products produced by Germany's Boeheringer Zngelhelm company using this method appear on the market under the trade name; the polylactic acid produced by the American Cargill company has been processed by melt blown and spun bonded to develop medical non-woven products; and In my country, there are only a handful of companies that can synthesize polymer polylactic acid, such as Sun Yat-sen University Polymer Research Institute.

The ring-opening polymerization mostly uses stannous octoate as the initiator, the molecular weight can reach millions, and the mechanical strength is high. The polymerization and separation are carried out in two steps:

The first step is to obtain lactide through dehydration and cyclization of polylactic acid;

The second step is to obtain polylactide through ring-opening polymerization of lactide;

However, this ring-opening polymerization method has extremely high requirements on the purity of the catalyst and the purity of the monomer during polymerization. Even a very small amount of impurities will make the molecular weight of PLA less than 100,000, and the polymerization conditions such as temperature, pressure, catalyst, etc. The type and amount of PLA, reaction time, etc. will also greatly affect the molecular weight of PLA, so the synthesis of high molecular weight PLA is a technical difficulty.

03 Solid state polymerization method

This method is obtained by polymerizing the low-molecular-weight resin obtained by the direct polymerization method under reduced pressure and vacuum at a temperature between Tg and Tm, so as to increase its degree of polymerization and molecular weight, thereby improving material strength and processing performance.

Preparation process of polylactic acid

We mainly talk about the more commonly used ring-opening polymerization method, and its process is roughly as follows:

Source: After crushing shell crops such as corn, starch is extracted from it, and then the starch is made into unrefined glucose. Now a lot of high technology has overcome and subtracted the process of crushing, and directly extracts raw materials from a large number of crops.

Fermentation: Fermentation of glucose in a manner similar to the production of beer or alcohol, which is fermented into lactic acid similar to food added for use in human muscle tissue.

Intermediate product: The lactic acid monomer is transformed into an intermediate product——reduced lactic acid, namely lactide, through a special concentration process.

Polymerization: After the lactide monomer is vacuum purified, a solvent-free dissolution process is used to complete the ring-opening action to polymerize the monomer.

Polymer modification: Due to the difference in the molecular weight and crystallinity of the polymer, the material properties can vary greatly, so PLA can be modified differently for different applications.
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