The arguments and development analysis of biodegradable packaging materials II

3. Microbes combine into polyester
Poly-β-hydroxybutyrate (PHB) is produced by fermentation of carbohydrates under controlled nutrient conditions by different bacteria, similar to the function of starch and dextrin in other organisms. It is an energy storage repository. It is present in the cytoplasm in approximately 0.5 μm particles. Under appropriate conditions, about 90% of the polymer can accumulate into bacterial bodies. To separate out the PHB, it is necessary to disrupt the cell wall by mechanical shear or by enzyme digestion, followed by extraction of the polymer, extraction in a centrifuge, or use of an organic solvent such as dichloromethane.

In the early 1960s, PHB could only be produced on a kg scale, as it was made from renewable resources and biodegradable, showing potential for commercial applications. In the energy crisis of 1973, interest in PHB was increased. PHBV (3-hydroxybutyrate and 3-hydroxyvalerate copolymer) was successfully prepared from glucose and propionate using a fermentation process. The PHB melting point was 180°C, while PHBV was reduced to 137°C (20 mol% 3-hydroxyl). The valerate unit) significantly improves the thermoplastic processability while increasing mechanical mechanical stability (impact strength) by an order of magnitude. The overall performance can be compared with polypropylene. After the oil price stabilized, the interest in PHB commercial applications declined. In the late 1980s, however, ICI industrialized PHBV under the trade name Bio-polo, which was marketed as a shampoo bottle made by the German blow molding process. Another commercial application is the use of a fishing net, which can be degraded when it sinks to the bottom of the ocean. . Biopol technology was sold to Monsanto in 1996 and the company stepped up research on the direct synthesis of polyhydroxyalkyl esters in transgenic plants and stopped production in 1998. The company's Bio-mer company in Munich has used PHB, a strain of bacteria cultivated by itself, since 1994. It is now producing several tons per year at a price of 15-20 EUR/kg. It is mainly used as a fireworks rocket, and it can be degraded in the environment. . Direct carbohydrate synthesis is also an effective shortcut. Nowadays, the synthesis of polyhydroxyalkanoates is disadvantageous in that the more expensive glucose is used as the matrix, the yield of PHBV is not high (40%), and the resulting polymer needs to be isolated. Designing bacteria with lower substrate requirements or directly producing polyhydroxyalkyl esters in genetically modified plants offers the possibility of future development.

4. About Polylactic Acid

In 1932, the synthesis of high molecular weight polylactic acid (PLA) was reported. Before the 1970s, lactic acid and oxalic acid copolymers were used in medical treatment, such as degradable skeleton materials for sustained-release drugs. Until recently, biodegradable materials have gained wider application. The use of biotechnological methods for the production of lactic acid by fermentation of carbohydrates can provide high purity enantiomers, which are not possible with the chemical route. In common processes, lactic acid is precipitated with Ca(OH)2, calcium lactate is precipitated and then dissolved with sulfuric acid. In this process, 1 kg of gypsum is associated with every 1 kg of lactic acid produced. If a membrane separation process, such as electrodialysis, is used, it can provide a more environmentally friendly route. The polycondensation of lactic acid directly by polycondensation is a typical stepwise growth polymerization: there is an interrelationship between DPn and conversion rate DPn=1/(1-P). To obtain high molecular weight products, it is only possible at very high conversion rates (degree of polymerization 100, conversion >99%). The monofunctional impurities in the fermentation, such as ethanol or acetic acid, limit the increase in molecular weight. To prepare high-molecular-weight PLA (Mw 3 × 10 ^ 5 g/mol), high-purity lactic acid was used, and the generated water was removed azeotropically in the polycondensation. The presence of solvents such as diphenyl ether is disadvantageous, and solvent-free processes are preferred. Japan Mitsui has developed this process and used it for industrial production. Cargil Dow was founded in 1997 by Dow Chemical Company and the agricultural company Cargill. PLA was prepared by the ring opening polymerization of lactic acid dimer. The dimer is an intermediate prepared by the linear oligomerization of lactic acid in the presence of SnII(O2CR)2. Small amounts of impurities such as meso-lactide (formed by racemization during oligomerization and crystallization) can be removed from the levo propiolactone by distillation and crystallization. Because the polymer properties are closely related to its main structure, it is very important to remove impurities. Like chain-growth polymerization, ring-opening polymerization can rapidly produce high molecular weight polymers. For an equilibrium reaction, the appropriate temperature range is limited. On the one hand, the polymerization rate should be kept reasonably fast; on the other hand, the equilibrium position depends to a large extent on the production of the polymer.

Therefore, a small amount of propiopropyl ester often remains in the polymer. The mechanism of the polymer can be cationic, anionic, or coordination polymerization. In the presence of tin octoate, coordination polymerization can occur rapidly and with low racemization. From the standpoint of toxicity, the presence of trace SnII ions in various plastic applications is insignificant. Industrially, polymerization occurs in the molten state or in the solid phase. PLA is a transparent, rigid thermoplastic with a Tg of about 60°C and a melting temperature of Tm170-180°C. Its modulus is higher than PET and cellophane. Controlling meso-β-lactone can reduce its stereoisotropy. Get a softer material. The melt viscosity can be adjusted over a wide range, and various types of products can be manufactured by injection molding, melt spinning, or thermoforming. Hard containers and fibers are the most promising in a variety of potential applications. Clothing made of PLA has a similar wearing comfort to that of PET and cotton, but its dimensional stability is better. Cargil Dow started production at the 140,000-ton-a-year PLA plant in April 2002 (brand Nature Works). With corn starch as raw material, glucose is hydrolyzed. Mitsui's PLA brand is LACEA.
(to be continued)