In Japan, PLA has passed the mildew resistance test and the sanitary safety confirmation of Food Sanitation Law No. 370. Due to the successful introduction of food utensils in the canteen of the Ministry of Agriculture, Forestry and Fisheries and the Ministry of Economy, Trade and Industry (November 2003 to March 2005, and March to April 2004), and the Aichi Expo (2005 Japan International Expo) venue The practicability and reproducibility of the main materials of various food utensils have been confirmed. The future development of PLA in food utensils and foamed sheets for fresh food is very worth looking forward to.
Because polylactic acid (PLA) has asymmetric carbon and is optically active, PLA is also prepared from L-monomers, and then ring-opening polymerization is used to prepare polylactic acid. It is currently the most commonly used test method and industrial preparation method. The specific material source and preparation The method, application field, and cycle of biodegradation are shown in Figure 1.
PLA is synthesized by chemical polymerization method using L-lactic acid obtained by fermentation of starch as raw material. Therefore, it is a high-molecular substance between microbial production and chemical synthesis. The amylase of corn, potato, etc. promotes the hydrolysis to produce D-glucose, which is then fermented by lactic acid to produce L-lactic acid, and then L-lactic acid is heated and polycondensed to form lactic acid oligomers, which are then ring-opened and polymerized to a molecular weight of 300,000 to 900,000. L-lactic acid, this method is called L-lactide hair, is also the most commonly used method. However, recently, the direct dehydration condensation of lactic acid (Mitsui Chemicals method) has successfully prepared PLA with a molecular weight of 100,000 to 200,000.
As shown in the figure, the theoretical yield of each stage when corn is fermented to synthesize PLA. According to this calculation, about 10 grains of corn can synthesize a sheet of PLA A4 film (about 2g). The world’s corn production is 550 million tons, and only a part of the waste corn is used to produce polylactic acid to ensure the output.
The tensile strength and elongation of PLA film are almost the same as those of PE. Taking a biaxially stretched film as an example, the tensile strength and elongation of PLA are 190 MPa and 135%, respectively, while PE is 235 MPa and 130%, respectively. But the problem is that the melting point of PE is 264°C, while that of PLA is only 170°C.
Inferring from the chemical structure, due to the asymmetric carbon atoms on the starting monomer, there are two types of structures: L-body and D-body, as shown in the figure. Previously, it was thought that only L-lactic acid was naturally present, but recently, microorganisms that can synthesize D-lactic acid have also been discovered. When the poly(L-lactic acid) synthesized by L-lactic acid and the poly(D-lactic acid) synthesized by D-lactic acid are mixed in a solution or molten state at a ratio of 1:1, the respective molecular chains are alternately oriented in the crystal to form a three-dimensional Complex.
In this three-dimensional complex, the right-helix 3-turn structure and the left-helix 3-turn structure are alternately oriented in the crystal. The melting point can reach 230°C, far exceeding the melting point of homopolymer (170°C). Therefore, it overcomes the shortcomings of low heat resistance of general polyester and can be used as high strength and high heat resistance materials.
