Introduction: With the increasing popularity of environmentally friendly packaging, the demand for degradable materials is gradually increasing. People have begun to look for and develop some environmentally friendly materials that are sustainable or ecologically recyclable. Biodegradable plastics are one of the most environmentally friendly and promising green materials. This article briefly introduces several common plastics of biodegradable plastics. Purchasing friends for reference:
Under natural conditions such as soil and/or sandy soil, and/or under specific conditions such as composting conditions or anaerobic digestion conditions or aqueous culture solutions, degradation is caused by the action of microorganisms existing in nature such as bacteria, molds and algae, etc., And finally completely degrade into carbon dioxide (CO2) or/and methane (CH4), water (H2O) and its mineralized inorganic salts of the elements it contains, as well as new biomass plastics. Also known as biodegradable plastics.
Classification of biodegradable plastics: According to the composition of raw materials and manufacturing processes, it can be divided into the following three types: natural polymers and their modified materials, microbial synthetic polymer materials and chemically synthesized polymer materials. Currently commonly used biodegradable plastics are: poly3-hydroxyalkanoate (PHA), polylactic acid (PLA), polyε-caprolactone (PCL) and polybutylene succinate (PBS).
Poly 3-hydroxyalkanoate (PHA)
Polyhydroxyalkanoates are aliphatic copolyesters with different structures synthesized by microorganisms through fermentation of various carbon sources. Among them, the most common ones are poly-3-hydroxybutyrate (PHB), polyhydroxyvalerate (PHV) and the copolymer of PHB and PHV (PHBV).
PHB is a thermoplastic polyester that is ubiquitous in nature, especially found between bacterial cells. Many physical and mechanical properties of PHB are close to polypropylene plastics, but it is biodegradable and biocompatible, and can be completely degraded into β-hydroxybutyric acid, carbon dioxide and water in vivo.
Materials made from this bioplastic can be used in drug-delivery systems, implants, and some devices that decompose harmlessly in the human body after healing, but PHB is harder and more brittle than polypropylene. Through the copolymerization of PHB and PHV (PHBV), the weak point of high crystallinity and brittleness of PHB can be improved, and its mechanical properties, heat resistance and water resistance can be improved.
Polylactic acid (PLA)
Polylactic acid (PLA) is a polyester chemically synthesized from microbial fermentation product-lactic acid as a monomer.
The production of polylactic acid is based on lactic acid. Traditional lactic acid fermentation mostly uses starchy raw materials. At present, the United States, France, Japan and other countries have developed and used agricultural and sideline products such as corn, sugar cane, sugar beet, and potatoes as raw materials to ferment lactic acid, and then produce polylactic acid. Corn is the preferred raw material for the biodegradable plastic polylactic acid. The process of making biodegradable plastic polylactic acid is as follows: first, the corn is ground into powder, the starch is separated, the original glucose is extracted from the starch, and finally the glucose is converted into lactic acid by a fermentation process similar to beer, and then the extracted Lactic acid makes the final polymer, polylactic acid.
Polylactic acid is a biodegradable polymer produced from renewable resources such as grains. In the polylactic acid production route, the lactic acid monomer is first prepared by hydrolyzing grain starch into glucose, and the glucose is converted into sodium lactate by the fermentation process. Lactic acid is further concentrated and then polymerized in the order of polycondensation (to form prepolymer), thermal depolymerization (to form dilactide), ring-opening polymerization, and depolymerization. The molecular weight of the obtained polylactic acid is as high as 75000 g/mol.
By carrying out the lactic acid polycondensation reaction by a general method, only a lactic acid oligomer can be obtained. The most studied method for preparing high molecular weight PLA is through the ring-opening polymerization of lactide, which is synthesized from lactic acid oligomers through high-temperature cracking. There are detailed reports on the mechanism and reaction conditions of the ring-opening polymerization of lactide. Recently, Japan’s Mitsui Chemicals Co., Ltd. proposed a new technology for preparing polylactic acid directly by polycondensation of lactic acid without lactide. This technology uses a highly active catalyst to obtain high molecular weight polylactic acid through solution polycondensation. Because lactic acid and lactide contain asymmetric carbon atoms, PLA with different stereoregularities, such as L-PLA, D-PLA and DL-PLA, can be obtained by polymerization.
Polylactic acid has good moisture resistance, grease resistance and airtightness, and its performance is stable at room temperature, but it will automatically degrade when the temperature is higher than 55 ° C or under the action of oxygen enrichment and microorganisms. After use, it can be completely degraded by microorganisms in nature, eventually generating carbon dioxide and water, without polluting the environment, which is very beneficial to protecting the environment.
The degradation of PLA is divided into two stages: 1) First, it is pure chemical hydrolysis to lactic acid monomer; 2) The lactic acid monomer is degraded into carbon dioxide and water under the action of microorganisms. The food cup made of polylactic acid can be completely degraded in only 60 days, which truly achieves the dual effect of ecology and economy.
Polyε-caprolactone (PCL) is a low-melting polymer obtained by ring-opening polymerization of ε-caprolactone, and its melting point is only 62°C. The degradability of PCL has been studied since 1976. In both anaerobic and aerobic environments, PCL can be completely decomposed by microorganisms. Compared with PLA, PCL has better hydrophobicity, but the degradation rate is slower; meanwhile, its synthesis process is simple and the cost is lower.
PCL has excellent processing performance, and can be made into films and other products by ordinary plastic processing equipment. At the same time, PCL has good compatibility with various polymers, such as PE, PP, PVA, ABS, rubber, cellulose and starch, etc. Through blending and copolymerization, materials with excellent properties can be obtained. In particular, its blending or copolymerization with starch can not only maintain its biodegradability, but also reduce costs, so it has attracted much attention. By blending PCL and starch, a degradable plastic with good water resistance can be obtained, and its price is similar to that of paper; by using the in-situ polymerization method, ε-caprolactone can be grafted with starch to obtain a thermoplastic polymer with excellent performance.
Compared with similar products, the advantages of polyester biodegradable plastics:
1) One of the fatal weaknesses of the above-mentioned biodegradable plastics (polylactic acid, polyε-caprolactone, polyhydroxyalkanoate) is poor heat resistance, which affects its application and promotion in the field of catering.
2) The processing conditions of the above-mentioned biodegradable plastics (polylactic acid, polyε-caprolactone, polyhydroxyalkanoate) are harsh, and there are some unavoidable difficulties in industrialization.
3) Polylactic acid is a water-degradable bioplastic, which cannot accept water molecules during storage, and its performance cannot be guaranteed during ordinary storage and normal use.
Polybutylene succinate (PBS) is a typical polyester biodegradable plastic. It is precisely because of the above weaknesses that it has become a leader in biodegradable plastic materials. It has a wide range of uses and can be used in packaging, tableware, cosmetics. Bottles and medicine bottles, disposable medical supplies, agricultural films, pesticide and fertilizer slow-release materials, biomedical polymer materials, etc. PBS has excellent comprehensive performance, reasonable cost performance, and has good application and promotion prospects. Compared with PCL, PHB, PHA and other degradable plastics, the price of PBS is basically the same, and there is no advantage; compared with other biodegradable plastics, PBS has excellent mechanical properties, close to PP and ABS plastics; good heat resistance, the heat distortion temperature is close to 100 ℃, the use temperature after modification can exceed 100℃, it can be used to prepare hot and cold beverage packaging and lunch boxes, overcoming the shortcomings of low heat-resistant temperature of other biodegradable plastics; the processing performance is very good, and it can be used on existing general-purpose plastic processing equipment It has the best processing performance of degradable plastics and can be mixed with a large amount of calcium carbonate, starch and other fillers to obtain low-cost products; PBS can be produced by slightly modifying the existing general-purpose polyester production equipment. At present, the domestic polyester equipment has a serious overcapacity, and the transformation of the production of PBS provides new opportunities for the excess polyester equipment.
In addition, PBS only degrades when exposed to specific microorganisms such as compost, and its performance is very stable during normal storage and use.
PBS uses aliphatic succinic acid and butanediol as the main raw materials for production, which can not only meet the demand through petrochemical products, but also can be produced through biological fermentation through natural renewable crop products such as starch, cellulose, and glucose. Green circular production that comes from nature and returns to nature. Moreover, the raw materials produced by the biological fermentation process can also greatly reduce the cost of raw materials, thereby further reducing the cost of PBS.