PLA (polylactic acid) synthesis methods research

PLA introduction :

Polylactic acid (PLA) is a new type of biobased and renewable biodegradable material.
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 glucose and a certain strain are fermented to produce high-purity lactic acid, and then chemically synthesized into a certain molecular weight polylactic acid.
After using PLA, it can be completely degraded by microorganisms in nature under certain conditions, and finally produces carbon dioxide and water without polluting the environment.
This is very beneficial to the protection of the environment and is recognized as an environmentally friendly material.

Polylactic acid has good thermal stability, processing temperature of 170-230 ° C, good solvent resistance, and can be processed in various ways, such as extrusion, spinning, biaxial stretching, and injection blow molding.
In addition to biodegradability, products made of polylactic acid have good biocompatibility, gloss, transparency, hand and heat resistance, and also have certain bacteria resistance, flame retardancy and UV resistance.
Therefore, it is widely used as packaging materials, fibers and nonwovens, and is currently used mainly in clothing (underwear, outerwear), industry (construction, agriculture, forestry, papermaking) and medical and health fields.

PLA synthesis methods:

1.ring-opening polymerization:
The ring-opening polymerization mostly uses stannous octoate as an initiator, the molecular weight can reach millions, the mechanical strength is high, and the polymerization is separated in two steps:
The first step is to obtain lactide by dehydration and cyclization of polylactic acid;
The second step is a lactide obtained by ring-opening polymerization of lactide;
However, in the ring-opening polymerization method, the purity of the catalyst and the purity of the monomer are extremely high at the time of polymerization, and even a very small amount of impurities causes the molecular weight of the PLA to be less than 100,000, and polymerization conditions such as temperature, pressure, and catalyst.
The type and amount of the reaction, the reaction time, and the like also greatly affect the molecular weight of the PLA, so the synthesis of the high molecular weight PLA is a technical difficulty.
2.solid phase polymerization:
In this method, a low molecular weight resin obtained by direct polymerization is obtained by polymerization under reduced pressure vacuum and a temperature between Tg and Tm to increase the degree of polymerization and increase the molecular weight, thereby improving material strength and processability.

We mainly talk about the more common open-loop polymerization method, and its process is roughly like this:
1) Taking materials
After the shell crop such as corn is crushed, the starch is extracted therefrom, and then the starch is made into unrefined glucose. Nowadays, many high technologies have overcome the process of crushing and extracting raw materials directly from a large number of crops.
2) Fermentation
Glucose is fermented in a manner similar to the production of beer or alcohol, and glucose is fermented to become lactic acid similar to food added for use in human muscle tissue.
3) Intermediate product
The lactic acid monomer is converted into an intermediate product, a water-reduced lactic acid, that is, lactide, in a special concentration process.
4) Aggregation
After the lactide monomer is vacuum-purified, the ring-opening operation is completed in a dissolution process without using a solvent to polymerize the monomer.
5) Polymer modification
Due to the difference in molecular weight and crystallinity of the polymer, the material properties can be changed greatly, so the PLA is modified differently for different applications.

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