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Preparation of PLA polylactic acid

2020-12-21

1.1. Synthesis method

In general, the preparation of polylactic acid (PLA) is synthesized with lactic acid as a raw material. At present, there are many synthetic methods. The more mature method is direct polycondensation of lactic acid, and the other is to synthesize lactide from lactic acid first, and then polymerize it by ring-opening under the action of a catalyst. There is also a solid phase polymerization method.

1) Direct polymerization of lactic acid

The direct polymerization method has been studied as early as the 1930s and 1940s, but because the key technologies involved in the water removal in the reaction cannot be solved well, the molecular weight of its products is low (all below 4000). Very low strength, easy to decompose, and not practical.

Showa Polymer Corporation of Japan uses slowly heating up lactic acid in an inert gas and slowly reducing pressure to directly dehydrate and condense the lactic acid, and further polycondensate the reactants at 220~260℃ and 133Pa to obtain a molecular weight above 4000. Polylactic acid. However, this method has a long reaction time, and the product will be decomposed, discolored, and uneven in the later high temperature. Japan's Mitsui Akira Chemical Company 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 equilibrium, and the reaction equilibrium needs to be raised and pressurized to break the reaction balance, and the reaction conditions are relatively harsh. In recent years, through technological innovation and improvement, the direct polymerization method has made certain progress. It should be used in large-scale industrial production in the near future as the technology continues to mature.

2) Ring-opening polymerization method

The ring-opening polymerization method is currently the most used production method in the world. As early as the middle of the 20th century, researchers at DuPont used ring-opening polymerization to obtain high molecular weight polylactic acid. 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 Boeheringer Zngelhelm in Germany using this method appeared on the market under the trade name; the polylactic acid produced by Cargill in the United States was processed by meltblown and spunbond 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 Polymer Research Institute of Sun Yat-sen University. 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 by dehydration and cyclization of polylactic acid;

The second step is to prepare polylactide by ring-opening polymerization of lactide;

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

3) Solid phase polymerization

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

1.2. Preparation process

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

1) Take materials

After crushing corn and other shell crops, starch is extracted from it, and then the starch is made into unrefined glucose. Now many high technologies have overcome the process of subtracting the crushing and directly extract raw materials from a large number of crops.

2) Fermentation

Glucose is fermented in a manner similar to the production of beer or alcohol, and the glucose is fermented into lactic acid similar to food added to human muscle tissue.

3) Intermediate products

The lactic acid monomer is converted into an intermediate product-water-reduced lactic acid, or lactide, through a special concentration process.

4) Aggregation

After the lactide monomer is vacuum-purified, a dissolution process without solvent is used to complete the ring-opening action to polymerize the monomer.

5) Polymer modification

Because the molecular weight and crystallinity of the polymer are different, the material characteristics can vary greatly. Therefore, the PLA is modified differently for different applications.

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Next: Polylactic acid has good thermal stability, processing temperature 170~230℃

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