PLA is a brief English abbreviation of PLA, all written as: polyactive acid
Polylactide, also known as polylactide, belongs to the polyester family. PLA is a polymer which is polymerized with lactic acid as the main raw material, and its raw material is full of resources and can be regenerated. It mainly uses corn and cassava as raw materials. The production process of PLA is pollution-free, and the products can be biodegraded and recycled in nature. Therefore, it is an ideal green polymer material.
The thermal stability of PLA is good, the processing temperature is 170-230 ℃, and it has good solvent resistance. It can be processed in many ways, such as extrusion, spinning, biaxial stretching and injection blow molding. In addition to biodegradation, products made of PLA have good biocompatibility, glossiness, transparency, hand feel and heat resistance, and have certain bacteria resistance, flame retardant and UV resistance. Therefore, they are widely used as packaging materials, fibers and non-woven fabrics. The products are mainly used in clothing (underwear, coat), industry (construction, agriculture, forestry, etc.) Paper) and medical and health.
PLA was priced at $1000 / kg before scale industrialization. Later, the large-scale production was realized through the industrialization research of Professor Ramani Narayan, Michigan State University. The technology is now industrialised by NatureWorks.
PLA’s largest manufacturer is NatureWorks, followed by China’s Haizheng biology, which currently produces 100000 tons and 15000 tons, respectively. PLA has many applications, which can be used in extrusion, injection molding, film drawing, spinning and other fields.
Preparation of PLA
1.1. synthesis method
In general, PLA is synthesized from lactic acid as raw material. At present, there are many methods of synthesis, the more mature is direct condensation of lactate, the other is to synthesize lactide from lactate, and then open-loop polymerization under the action of catalyst. There is also a solid-phase polymerization method.
1) Direct polymerization of lactate
Direct polymerization has been studied since 1930s-1940s. However, the key technologies such as water removal in the reaction can not be solved well, so the molecular weight of the products is low (all below 4000), the strength is very low, and it is easy to decompose, and it is not practical.
In order to make the reactants further polycondensate at 220-260 ℃ and 133pa, the relative molecular weight of PLA above 4000 is obtained by heating up the lactic acid in inert gas and reducing pressure slowly. However, the reaction time of the method is long, the product will aging and decompose, discolor and uneven at later high temperature. The solution polymerization method was used by Mitsui chemical company to directly polymerize the lactic acid to produce PLA.
The main characteristic of direct method is that the synthetic PLA does not contain catalyst. Therefore, when the polycondensation is carried out to a certain extent, the system will have equilibrium state, which needs to be broken by heating and pressure, and the reaction conditions are relatively harsh. In recent years, through the innovation and improvement of technology, direct polymerization has made some progress. It should be applied in the industrial production in the near future with the continuous maturity of technology.
2) Open loop polymerization
Open loop polymerization is a more widely used production method in the world. As early as the middle of the 20th century, researchers of DuPont company used open-loop polymerization to obtain high molecular weight PLA. In recent years, the research on the synthesis of PLA mainly focused on the ring opening polymerization of lactide.
Boehrenger zngelhelm company in Germany produced the PLA series products in the market under the commercial name; The polylactic acid produced by Cargill company in USA has been processed by melt spraying and spunbond, and the medical nonwoven fabric products have been developed; But only a few of the Institute of polymer of Zhongshan university can synthesize PLA in China. The open-loop polymerization is mainly based on stannous octanoate as initiator, with molecular weight of millions, high mechanical strength and two steps of polymerization and separation
The first step is to prepare lactide by dehydration and cyclization;
The second step is to prepare the poly (lactide) by ring opening polymerization;
However, the purity of catalyst and monomer of this open loop polymerization method is very high when polymerization. Even the very small impurities will make the molecular weight of PLA lower than 100000. Moreover, polymerization conditions such as temperature, pressure, catalyst type and amount, reaction time will greatly affect the molecular weight of PLA. Therefore, the synthesis of high molecular weight PLA is a technical difficulty.
3) Solid state polymerization
This method is to get the low molecular weight resin obtained by direct polymerization under the condition of vacuum and temperature between TG TM to improve its polymerization degree and molecular weight, so as to improve the strength and processing performance of the material.
1.2. preparation process
We mainly talk about the more common open-loop polymerization method, and its process is roughly as follows:
1) Take materials
After crushing corn and other shell crops, starch was extracted from the corn, and then the starch was made into the crude glucose. Now many high-tech have overcome the process of subtracting crushing, and directly extract raw materials from a large number of crops.
Glucose is fermented in a manner similar to beer or alcohol, which becomes a food added lactate in human muscle tissue.
3) Intermediate product
The monomer of lactate was transformed into a kind of intermediate product, water reducing lactate, i.e. lactide.
After vacuum purification, the monomer was polymerized by a solvent free solution process.
5) Polymer modification
Because of the difference of molecular weight and crystallinity of polymer, the change space of material properties is very large. Therefore, PLA is modified differently due to different products.
Post time: Jul-12-2021