New arguments and applications of biodegradable materials

In recent years, biodegradable materials have attracted people's attention. Many articles and reports have been published, but many of them are new species development, synthetic manufacturing methods, applications, and degradation mechanisms. An article entitled “Nature or Petrochemistry?—Biologically Degradable Materials” was published on the international edition of Applied Chemistry in Germany (Angew chem.Int.Ed, 43, 1078-1085). Material development background, and with the progress of science and technology, after forming the industry and occupying the market, some issues that should be considered from the economic and social aspects. The article also made predictions on the development prospects of several types of varieties with market prospects. Although the article is still based on polymer materials, there are many new ideas that are now included for reference.

The progress and achievements made by the chemical industry since the second half of the 19th century are largely due to the use of mineral raw materials as the basis for synthesis. Synthetic dyes prepared from coal have replaced natural dyes, and this light-stable colorant has for the first time entered the lives of the general public. Nowadays, mineral raw materials represented by oil and gas are the most important raw materials for the chemical industry, exceeding 90%, ranking second only to energy and transportation. According to the statistics of OECD member countries, energy accounts for 54%, transportation accounts for 35%, and the chemical industry accounts for 12% (half each for raw materials and processing). In the chemical industry, oil and gas resources used as raw materials are mainly converted into polymers. Over the past 50 years, General Plastics has achieved tremendous success, providing a reliable raw material base and a variety of applicable properties. By melting, a large number of items (such as films and moldings) can be manufactured. The processing method is not only inexpensive but also environmentally friendly. Very small.

After the energy crisis of 1973, alternative energy sources and resources, such as biomass problems, attracted attention and strengthened research. With the drop of crude oil prices, public interest in it has declined again. However, in terms of geopolitics and economic development, over-reliance on oil and its limited availability make people think about alternative energy sources and resources. According to the proved oil reserves, using modern mining technology, it can only be adopted for 40 years. This forecast is more optimistic. This is based on the continuous increase of crude oil reserves in the Middle East. Now that the greenhouse gas CO2 is completely generated from mineral raw materials, this has become the unpredictable and irreversible cause of global climate change. Traditional plastic waste is buried underground because degradation is slow and will occupy valuable land for a long time. Therefore, it is envisaged that recycling applications based on renewable resources will be very attractive, especially for applications. Natural products.
The biodegradable materials that people research and develop nowadays are mostly based on natural products, some are polyesters synthesized by microorganisms, and some are made from renewable resources and then polymerized into materials such as polylactic acid (PLA). In fact, some Monomers can also be prepared using petrochemical routes. Therefore, the biodegradability of the research materials should include both renewable resource base materials and petrochemical base materials, and their ecological potential (eclogical pontmfids) should be compared.

1. About biodegradability

Biodegradable materials are now receiving attention. Biodegradability and preparation from renewable resources are two different concepts. Naturally occurring polymers, such as cellulose or natural rubber, are biodegradable, but biodegradability is related to the chemical structure of the material, regardless of whether the structure is made from renewable resources or mineral resources.

Germany has a biodegradability clause in the 1998 standard test method as a measure of the decomposability of plastics. In the analysis, in addition to the chemical composition (such as the presence of a certain heavy metal), it is also necessary to test the possibility of complete degradation under laboratory conditions, to test the degradation and decomposition properties under actual living conditions, and to determine the decomposition of the large bird, Earthworms and other ecotoxicities. The definition for biodegradability is: Under laboratory conditions, 60% of the organic carbon must be completely converted within 6 months. In actual conditions, 90% of the plastic should be able to degrade into fragments smaller than 2mm. In addition to natural polymers, biodegradable polyesters prepared by microorganisms or chemical methods have also become the center of attention. Degradation generally occurs in two steps: First, it is enzymatically or chemically hydrolyzed into low-molecular-weight fragments, and sometimes it can be decomposed into original monomers, which can be reabsorbed by cells and eventually become CO2 and water. The amorphous regions in the polymer erode much faster than the crystalline regions. The crystallinity and grain size of the polymer have a great influence on the degradation rate. Traditional polyesters and polyamines have a high degree of crystallinity, and this structure plays a decisive role in their main mechanical properties. Therefore, molded parts and fibers can be made. However, it has resulted in a hard-to-degrade condition that has remained stable over the useful life and under the influence of the environment.

2. About natural polymers

Each year, 1×1011t of biomass is produced through photosynthesis, most of which are cellulose, starch, various polysaccharides, and lignin. Paper came out for more than 2,000 years. Now the world produces 320 x 106 tons of paper and board every year, which is higher than the annual output of petrochemical plastics 200 x 106 tons. However, its hydrophilic, mechanical and mechanical properties are very sensitive to water, limiting its use as a material. Soaked paper bags are useless. Moreover, unlike cellulose, which is a general-purpose plastic such as polyolefins, cellulose cannot be processed by thermoplastic methods. Therefore, cellulosic fibers (viscose fibers) or cellulose plates (celluloid) are all prepared by decomposing cellulose xanthogenate by the solution method. If they are derivatized to obtain a material suitable for thermoplastic processing, such as cellulose acetate or cellophane (cellulose nitrocellulose plasticized with rosin), but these require the further synthesis reaction with mineral resources, and the degradability of these derivatives Both are lower than unmodified cellulose.

The main ingredient of pulp is cellulose, which can be used as a chemical raw material in addition to papermaking. It is made after separating cellulose and lignin from wood. The current manufacturing process consumes a lot of energy and water, and releases pollutants (sulfides) into the environment. Therefore, the total impact of raw materials and used garbage on the environment is examined. There is no advantage for paper bags compared with polyethylene bags. . The use of inexpensive pulp and water-stable polyethylene to make composites can be used as a beverage container, which has been used in a large number in the European market under the trade name Tetrapak, which is coated with a very thin polyethylene on the wall of a container made of cardboard. Film protection layer. In post-use recycling, the pulp product can be dissolved and used as a product that does not require high fiber quality, whereas polyethylene is burned to obtain energy.

Starch and cellulose, as long as they contain a certain amount of water, can be processed by thermoplastic methods. Due to its sensitivity to water, the application of various mechanical and mechanical properties is severely limited. By blending with polyethylene or polyester thermoplastics, performance can be greatly improved. Blended with biodegradable polyester, the product can be completely decomposed. Now Noramom sells its Mater-B brand in the market, and it is about 20,000 tons per year.
Some natural polymers show many active functions in living tissues, causing people to attach great importance. Obtaining natural polymers directly from natural renewable resources is a valuable shortcut for material preparation. Therefore, it is necessary to separate from the biomass, and it is necessary to solve the limitation of the application due to poor processability. Therefore, in recent years, the focus of attention has shifted to other biodegradable thermoplastics, such as microbes or chemical synthesis of various polyesters.

3. Combination of microorganisms 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, branded Biopolo, which was marketed as a shampoo bottle made by the German blow molding process; another commercial application is the future of fishing nets, which can degrade when they sink 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 Biomer company in Munich used PHB, a strain of bacteria cultivated by itself, since 1994. It is now producing several tons of carbon per year at a price of 15 to 20 euros/kg. It is mainly used as a fireworks rocket and 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.

Source: National Plastics Processing Industry Information Center