Polylactic acid (PLA) is a new biological and biodegradable material made from starch material from renewable plant resources (such as corn). The raw material of the starch is obtained by saccharification to obtain glucose, then glucose and certain bacteria are fermented to make high-purity lactic acid, and then a certain molecular weight polylactic acid is synthesized by chemical synthesis. It has good biodegradation, after use can be completely in nature microbial degradation under certain conditions, the final formation of carbon dioxide and water, do not pollute the environment, which is very beneficial to protect the environment, is recognized as environmentally friendly materials.
PLA Features and Applications
Polylactic acid (polylactide), also known as poly belongs to the family of polyester. Polylactic acid is a polymer obtained with lactic acid as the main raw material. The feedstock is abundant and renewable, mainly corn, cassava and other raw materials. Polylactic acid production process is pollution-free, and the product can be biodegradable, to achieve the cycle in nature, therefore, is an ideal green polymer materials.
Polylactic acid has good thermal stability, processing temperature of 170~230 degrees, good solvent resistance, can be processed in a variety of ways, such as extrusion, spinning, biaxial stretch, injection blow molding. The polylactic acid products made in addition to biodegradation, biocompatibility, gloss, transparency, texture and good heat resistance, but also has a certain bacteria resistance, flame resistance and ultraviolet resistance, so it is widely used, can be used as packaging materials, fiber and non-woven fabric, the mainly used clothing (underwear, coat), industry (construction, agriculture, forestry, paper) and medical and health fields.
PLA was priced at $1000 / kg before it was scaled up. Later, through the industrialization research of Professor Ramani Narayan of Michigan State University, the large-scale production was realized. The technology is now being industrialized by Natureworks.
PLA is the largest manufacturer of American NatureWorks company, followed by the Chinese Hisun their current biology, yield is 100 thousand tons and 10 thousand and 5 tons respectively. PLA has many applications, such as extrusion, injection molding, film drawing, spinning and other fields.
PLA Synthesis Method
In general, polylactic acid (PLA) is prepared from lactic acid as raw material. At present, there are many synthetic methods, the more mature is the direct polycondensation of lactic acid, the other is the synthesis of lactide by lactic acid, and then in the role of catalyst ring opening polymerization. There is also a solid phase polymerization method.
1. Direct Polycondensation of Lactic Acid30~40's law in early twentieth Century has already begun research on direct polymerization, but because of the water removal and key technology in the reaction is not well resolved, so the molecular product quantity is low (below 4000), low intensity, easy decomposition, not practical.
The company will Showa polymer of lactic acid in the inert gas slowly heating and slow decompression, the lactic acid dehydration condensation, and the reaction at 220~260 DEG 133Pa, further polycondensation, relative molecular mass of more than 4000 polylactic acid. However, the method has a long reaction time, and the product will decompose at the later stage of high temperature, change color and be uneven. Mitsui Petrochemical Corporation uses solution polymerization to direct the polymerization of lactic acid to polylactic acid.
The main characteristic of the direct method is that the polylactic acid without synthesis contains no catalyst. Therefore, when the polycondensation reaction reaches a certain extent, the system will appear a balance state, which needs to be heated and pressurized to break the reaction equilibrium, and the reaction conditions are relatively severe. In recent years, through the innovation and improvement of technology, direct polymerization has made some progress. In the near future, with the continuous maturity of technology, it can be applied to the industrialization of large-scale production.
2. Ring Opening Polymerization
Ring opening polymerization is one of the most widely used methods of production in the world. As early as the middle of the twentieth Century, DuPont Co researchers obtained high molecular weight poly (lactic acid) by ring opening polymerization. In recent years, the research on the synthesis of polylactic acid has mainly focused on the ring opening polymerization of lactide.
The German Boeheringer Zngelhelm company for the production of poly lactic acid series products under the brand name appear on the market; the United States Cargill company with the production of polylactic acid by meltblown spunbond and after processing, the development of medical non-woven products; and in our country to study the only Zhongshan University polymer synthetic polymer polylactic acid and a handful of. Ring opening polymerization uses stannous acid as initiator, with molecular weight up to millions, high mechanical strength, and two steps of polymerization and separation:
The first step is the preparation of lactide by dehydration cyclization;
The second step is the ring opening polymerization of lactide prepared poly;
But this kind of ring opening polymerization in polymerization time of catalyst purity, the purity of monomer is extremely demanding, even a little impurity would make PLA the molecular weight of less than 100 thousand, and the polymerization conditions such as temperature, pressure, catalyst types and dosage, reaction time and so on will greatly affect the molecular weight of PLA therefore, the synthesis of high molecular weight PLA is a technical difficulty.
3. Solid Phase Polymerization
This method is the direct polymerization of low molecular weight resin obtained by polymerization method in vacuum and temperature in Tg - Tm between the conditions, in order to improve the degree of polymerization, the molecular weight increased, so as to improve the material strength and processability.
PLA Preparation Process
We mainly talk about the commonly used ring opening polymerization method, and its process is roughly like this:
1. Draw Materials
After the corn and other shell crops are crushed, the starch is extracted, and the starch is refined into glucose. A lot of high technology has now overcome the process of subtracting grinding and extracting material directly from large amounts of crops.
Glucose is fermented in the same way as beer or alcohol is produced, and glucose is fermented into lactic acid, similar to food added to human muscle tissue.
3. Intermediate Product
The lactic acid monomer is changed into intermediate product by special enrichment process, reducing lactic acid, lactide.
The lactide monomer is purified by vacuum and then acts as an open loop in a solvent free solvent process that allows the monomer to be polymerized.
5. Polymer Modification
Because of the difference of molecular weight and crystallinity of polymers, the change of material properties can be greatly changed. Therefore, PLA can be modified differently because of different products.