Poly l Lactic Acid can be produced by chemical or microbial fermentation. The chemical method uses petroleum-based chemicals as raw materials for synthesis, so it is affected by crude oil supply and price fluctuations (the crude oil price range from 2008 to 2014 is 40 to 145 US dollars / barrel). Another disadvantage of chemical methods is that usually only racemic DL-Poly l Lactic Acid can be synthesized, and its application in food and beverages is limited due to the metabolism of D-lactic acid in the human body. Polylactic acid requires lactic acid monomers with extremely high chemical purity and high optical purity. Usually, the content of L-lactic acid must be greater than 96% to 99% (that is, D-lactic acid is less than 1% to 4%). Therefore, chemically racemic lactic acid is required. It does not meet the requirements of general polylactic acid materials, and can only be used in some chemical fields that do not require optical purity, such as metal plating, detergents, etc., so the market is very small. At present, the only manufacturer of chemically synthesizing lactic acid is Japan’s Musashino. Most of the other manufacturers use microorganisms to anaerobic fermentation of carbohydrates (such as sucrose or starch sugar) to produce acid. One molecule of glucose can be converted into two molecules of lactic acid through glycolysis, and the theoretical conversion rate is 100%, but the actual fermentation yield can only reach about 90%. The difference in the production conditions of D-lactic acid or L-lactic acid is mainly in the bacterial species and fermentation conditions. There is not much difference between the recovery and purification process, but it is necessary to avoid extreme conditions (such as high temperature in the purification process) for D-lactic acid and L-lactic acid. Under the interconversion racemic. During anaerobic fermentation, the pH of the fermentation broth
The pH value will decrease with the formation of lactic acid. Calcium hydroxide or calcium carbonate must be added to neutralize the lactic acid and maintain optimal pH fermentation conditions. After fermentation, the bacteria and impurities are separated by filtration and centrifugation, which can be used as feed additives. After the fermentation broth is cooled, calcium hydroxide and lactic acid are neutralized to form calcium lactate precipitation, which must be dissolved with sulfuric acid to extract lactic acid. This process will generate calcium sulfate precipitation by-products. After filtration and separation, the generated solid waste can be used as gypsum. Construction sector. The crude lactic acid liquid extracted by this process still contains a lot of soluble proteins, sugars, salts, miscellaneous acids, etc., which must be purified and removed. Food-grade Poly l Lactic Acid allows a small amount of soluble impurities, which can be purified by resin adsorption, decolorized by activated carbon, membrane separation or concentrated by evaporation, so that the concentration of lactic acid can reach 80% to 88% of the general commercial level (including 12% to 20% of water). Such as concentrated Poly l Lactic Acid to 92% to 93%, it is more suitable for polymerization application, but its viscosity increases, and it will exist in the form of dimer or trimer at room temperature or low temperature.
Heat-resistant or polymerized lactic acid requires high chemical purity, so the soluble impurities in crude Poly l Lactic Acid must be completely removed by esterification or molecular distillation. The esterification distillation method reacts methanol and lactic acid to form a relatively volatile methyl lactate, and then purifies by distillation. The obtained methyl lactate is hydrolyzed into lactic acid and methanol, and then the methanol is separated by distillation to obtain Poly l Lactic Acid. The energy consumption of molecular distillation method is relatively large, and it can be concentrated by vacuum evaporation or nanofiltration before distillation, and the residual dark waste liquid of distillation can be used as a feed additive. To obtain lactic acid with an optical purity greater than 99.9%, the optical purity can be further improved by crystallization. The purified lactic acid can be neutralized by adding alkali to make a salt solution such as sodium lactate or calcium lactate; or by evaporation, spray drying, crystallization, granulation, drying, to form solid lactate; or by adding methanol or ethanol, etc., esterification to generate Lactate. After dehydration and purification of Poly l Lactic Acid, anhydrous lactide can be generated for polymerization into polylactic acid, or other copolymers. Industrial-grade polylactic acid must use lactic acid with high optical purity to improve crystallinity, heat resistance and physical and mechanical strength.