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PLA synthesis method
Source: | Author:xinxinghuanbao | Published time: 2018-05-24 | 1442 Views | Share:
In general, polylactic acid (PLA) is synthesized from lactic acid. At present, there are many synthetic methods, the more mature is the direct condensation polymerization of lactic acid, the other is the synthesis of propyl ester from lactic acid, and then the ring-opening polymerization under the action of catalyst. There is also a kind of solid - binding law.


1) direct polymerization of lactic acid direct polymerization as early as 30 ~ 40 s of the 20th century has begun to study, but because the key technology such as involves the reaction of water removal can't get very good solution, so the product of low molecular weight (under 4000), the intensity is extremely low, easy to decompose, not practical.
Showa polymer company USES to lactic acid in inert gas heating temperature and slow decompression, slowly make lactic acid dehydration condensation directly, and make the reactants in 220 ~ 260 ℃, 133 pa further polycondensation, get the relative molecular mass polylactic acid in more than 4000. However, the reaction time of this method is long, and the product will decompose, change color and be uneven at the later high temperature. Japan mitsui pressure chemical co., LTD. USES solution polymerization to make lactic acid polymerize directly to polylactic acid.
The main characteristics of the direct method is the synthesis of poly (lactic acid does not contain catalyst, so the polycondensation reaction to a certain degree of system will be the equilibrium state, need to heat up pressure to break the balance reaction, reaction conditions are relatively strict. In recent years, through technical innovation and improvement, direct polymerization has made certain progress, should be in the near future as the technology matures, can be used in the industrial production.


2) open-loop polymerization
Open-loop polymerization is one of the most popular production methods in the world. As early as the mid-20th century, dupont researchers used open-loop polymerization to obtain high molecular weight polylactic acid. In recent years, studies on polylactic acid synthesis abroad have focused on the open-loop polymerization of propyl ester.
The poly-lactic acid series produced by German company Boeheringer Zngelhelm by this method appear on the market as commodities. The polylactic acid produced by Cargill of the United States is processed by melt spraying and spun-bonding, and the medical metathesis fabric product is developed. However, there are only a few high - molecular - molecular - research institute of sun yat-sen university. Open-loop polymerization is usually initiated with tin octylate as the initiator, with a molecular weight of up to millions, high mechanical strength and two steps of polymerization and separation:
The first step is the preparation of propionate by dehydration cyclization of polylactic acid.
The second step is to prepare polypropylene ester by means of ring opening polymerization.
But the ring opening polymerization on the purity, the catalyst in polymerization monomer purity requirements is extremely high, even trace amounts of impurities can also make the PLA molecular weight less than 100000, and polymerization conditions such as temperature, pressure, the variety and dosage of catalyst, reaction time and so on will also greatly affect the PLA's molecular weight, so the synthesis of high molecular weight PLA is a technical difficulty.


3) fixed together this method is to directly polymerization were low molecular weight of resin in vacuum vacuum, temperature between Tg, Tm under the conditions of polymerization reaction, in order to improve the degree of polymerization, increase the molecular weight, thus improve the strength of materials and processing performance.
1.2. Preparation process
We mainly talk about the more common open-loop polymerization method, and its manufacturing process is roughly as follows:
1) after crushing corn and other shell crops, starch is extracted from them and then made into unrefined glucose. Many high technologies have now overcome the process of subtracting crushing and extracting raw materials directly from a large number of crops.
2) fermentation ferments glucose in a manner similar to the production of beer or alcohol, and the fermentation of glucose turns into lactic acid similar to the addition of food to human muscle tissue.
3) the intermediate product converts the lactic acid monomer into the intermediate product -- water-reducing lactic acid, or propyl crossate, through a special concentration process.
4) after the polyacrylate monomer has been vacuumed and purified, the polyacrylate monomer can be opened by a solvent free dissolution process to make the monomer polymerize.
5) polymer modification
Due to the difference in molecular weight and crystallinity of the polymer, there is a large space for material properties to change. Therefore, PLA is modified differently for different products in different applications.