Degradable plastics based on synthetic biological polymers abroad

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Degradable plastics based on synthetic biological polymers abroad: Currently, there are three main types of biodegradable plastics available abroad. The first type is based on natural polymers, the second type is based on polymers synthesized from natural monomers, and the third type is based on polymers produced through fermentation processes. A large number of papers have been published on the first category; this article only reviews the production, characteristics, applications, as well as current status and future prospects of the second and third categories of biodegradable plastics. As early as the 1970s, it became apparent that when plastic products were discarded as waste in the environment, some of the advantages that made plastics so widely used turned into disadvantages and a burden on humanity. Due to the durability of such products, they do not degrade under environmental conditions, resulting in environmental hazards that pose a threat to the environment. Therefore, people hope to use plastics that can be degraded by organic microorganisms in the environment, as well as biodegradable plastics that can be made from recycled materials. Thus, new biodegradable plastic technologies have also emerged. 　　Many biopolymers found in nature, as well as those produced by biological processes and obtained through the polymerization of natural monomers, are biodegradable and serve as important resources for manufacturing biodegradable plastics. The biopolymers in biodegradable plastics possess inherent degradability, but such plastics also contain various additives added to improve their properties, and the degradability of these additives varies greatly. 　　Some biopolymers are thermoplastic and can be processed using the same techniques as those used for synthetic polymers; other biopolymers are non-thermoplastic and can be turned into plastic sheets through molding processes. There are also some low-molecular-weight biomolecules found in nature that can polymerize during processing to form thermoplastic materials, and they may also cross-link to form thermosetting materials. Biodegradable plastics are usually water-based, as many biopolymers (or when properly treated) dissolve in water or can be dispersed in it. 　　Among the various types of biodegradable plastics, some have entered the consumer goods market on a certain scale, while others have not yet been commercialized due to technical reasons. There are also those that, although feasible from a production perspective, have not yet attracted sufficient investment for industrial production; however, some of them are expected to achieve success in the near future. People are eagerly awaiting the emergence of more new types of biodegradable plastics, and hoping that more such innovative products will enter the market. 　　Currently, there are three major types of biodegradable plastics. The first type is based on polymers obtained directly from natural products; the second type is based on polymers derived from the polymerization of natural monomers; and the third type is based on polymers produced through fermentation processes. Numerous studies have been published on the first type of biodegradable plastics (mainly starch-based), so this text will only provide an overview of the second and third types. 　　1 Biodegradable plastics based on polymers obtained from the polymerization of natural monomers. This type of biodegradable plastic is made from polymers resulting from the polymerization of natural, low-molecular-weight biomolecules; they can be either thermoplastic or thermosetting. Important representatives of such polymers are polylactic acid and triglyceride polymers. 　　1.1 Polylactic acid (2-hydroxypropionic acid): Polylactic acid (PLA) is a polyester, whereas lactic acid (LA) is a biologically occurring monomer. PLA is thermoplastic and can be processed using standard plastic processing techniques to produce films, sheets, and fibers. PLA comes in both soft and hard versions. It is transparent by itself, but can also be made into an opaque material, and fillers can be added to it. Due to its high strength, it can be made into very thin sheets. PLA is insoluble in water and possesses good water resistance and oil resistance. The mechanical and other properties of PLA can be improved by altering the molecular weight and crystallinity, or by copolymerizing lactic acid with glycolic acid or caprolactone. 　　The American company Cargill began developing PLA around 1987, started pilot production in 1992, and sold it under the trade name EcoPLA. At the end of 1997, Cargill and Dow Chemicals established a joint venture called Cargill Dow Polymers, which is dedicated to the production and sale of PLA. This company has built a facility with an annual production capacity of 125kt of PLA. Due to the increased production, the selling price has dropped to around $1.0 per kg. 　　PLA can be used as agricultural film, as well as as recyclable and biodegradable packaging materials. It can also be used for disposable tableware, beverage containers, and sports equipment. PLA also has a range of applications in the biomedical field, such as drug-release materials and materials for bone growth and repair. Lactate and glycolic acid copolymers can support the growth and attachment of new cells; they can be made into porous materials, thereby providing a large area of continuous surface that allows cells to proliferate throughout the matrix. PLA implants in the human body can fully degrade over a certain period of time; this time depends on the shape and size of the implant, but it usually does not exceed 2 years. 　　Another use of PLA containing growth factors is as a coating for metal implants in the human body, to stabilize bone shape. 80% of the growth factors are released within 42 days. Compared to uncoated implants, the wound healed more rapidly when a PLA coating was used. Moreover, even PLA coatings without growth factors can improve wound healing. 　　LA can be produced by chemical synthesis or fermentation; currently, it is mainly manufactured through fermentation using glucose as the raw material. The yield of LA produced by fermentation can exceed 90%, and its production cost mainly stems from the multi-step purification process. 　　During the polycondensation of LA, a low-molecular-weight polymer is usually obtained first; this polymer is then treated with a coupling agent (chain-extending agent) to produce high-molecular-weight PLA. Recent studies have shown that the polycondensation of LA in high-boiling-point solvents, under reduced pressure and in the presence of a catalyst, can directly yield high-molecular-weight PLA. PLA has recently become one of the most important industrially biodegradable plastics. 　　PLA possesses many excellent properties, has a wide range of applications, and holds promising prospects. Since LA can now be produced on a large scale, it is possible to supply PLA in large quantities. Although the raw materials for producing PLA are abundant, it is doubtful whether PLA can meet global demand. Even if it could, it would inevitably lead to competition between plastic production and food production, as the raw materials used for making PLA are food products. 　　1.2 Polymers of triglyceride esters: Triglycerides are abundant biomolecules that have recently attracted considerable attention, as their polymers can be used to produce new biodegradable plastics. 　　First, triglycerides are converted into highly reactive intermediates, which are then polymerized and epoxidized to form low-molecular-weight liquid polymers. These polymers are subsequently mixed with catalysts and accelerators to facilitate the cross-linking reaction. Finally, the mixture is poured into a mold containing reinforcing fibers and heated and cured within the mold to form a rigid thermosetting bioplastic. 　　Fiberglass-reinforced soybean oil resin is a durable thermosetting material that can be used in industries such as agricultural machinery, the automotive industry, and construction. This resin is made entirely from recycled materials, yet it has high strength and contains no formaldehyde. Thermosetting materials similar to those mentioned above can also be produced by using other vegetable oils and reinforcement materials other than glass fibers (such as plant fibers, or even straw and hay), and soybean-based resins have a strong affinity for natural fibers. If straw could be used to replace wood fibers in the production of compression-molded composites (such as fiberboards that are currently widely used in the construction industry), it would enable a new use for straw, a resource that is abundant and has a short regeneration cycle, thereby saving wood fibers, which have a longer regeneration cycle. In the future, it is also possible to use nanocellulose to replace glass fiber. It is foreseeable that, thanks to the development of thermosetting composite materials using vegetable oils, it will be possible to produce inexpensive and durable biodegradable materials. 　　Furthermore, epoxy soybean oil can be polymerized with citric acid to form a polyester coating on kraft paper, which is used to manufacture biodegradable agricultural films. This coating can increase the wet strength of the paper and reduce its rate of degradation, thereby enabling the agricultural film to suppress weed growth for up to 10 weeks. 　　2 Bioplastics based on polymers produced through fermentation processes Biological polymers can be produced on a large scale using fermentation processes, and the raw materials for these processes are generally natural resources derived from plants. 　　Polyhydroxyalkanoates (PHA), biopolymers that have been recently developed, are polyesters that can be produced through fermentation, and they play an important role in the industrialization of bioplastics. The British company ICI began producing polyhydroxybutyrate (PHB) around 1978. In 1987, ICI had already produced PHB and its copolymers, which were sold in Europe under the trade name Biopol. In the following years, ICI’s production of biodegradable polyesters grew rapidly. Starting in 1991, numerous studies were conducted around the world on the production of biodegradable polyesters, with a focus on DNA recombination techniques. In the mid-1990s, the United States used DNA recombination to produce PHBV, a copolymer of hydroxybutyrate (HB) and hydroxyvalerate (HV), in plants such as soybeans. 　　PHB plastic is brittle, but PHBV allows its brittleness, strength, and other properties to be adjusted depending on its composition; the composition of PHBV, in turn, depends on the ratio of raw materials used in its production. PHBV is a white granular material; depending on its HV content, its physical and processing properties can be similar to those of PE or PP, and it can be either a brittle plastic or an elastomer. The use of additives can further improve the properties of PHBV. 　　PHBV is thermoplastic and can be processed by blow molding, extrusion, or injection molding; it can be used to produce films, fibers, coatings, and laminates. It can be utilized to manufacture a variety of packaging items such as bottles, trays, cups, and many other products. PHBV has been used to manufacture shampoo bottles. Today’s shampoos are generally biodegradable, so biodegradable PHBV bottles are very suitable. 　　PHBV has a much higher water resistance than most polysaccharides and proteins; therefore, PHBV bottles can be stored in a humid atmosphere with considerable stability. For coated paper products, PHBV can replace PE. The PHBV coating makes starch foam products heat-resistant and resistant to cold water. PHBV is biodegradable in soil, river water, seawater, and aerobic and anaerobic sewer sludge. For example, in anaerobic sewer sludge, nearly 80% of PHBV is degraded into CO2 and methane within 30 days. In urban composting sites, PH-BV plastic products lost approximately 60% of their weight within 6 months. The degradation rate of PHBV is also related to its composition, molecular weight, crystallinity, surface area, and whether it contains biodegradation additives. At high temperatures, the hydrolysis of PHBV also contributes to its degradation. The formulation of PHBV should allow for recycling and clean incineration. 　　PHBV has also been tested in biomedical applications, such as drug release systems and human implants. The degradation of PHBV in the human body is entirely caused by hydrolysis; as a result, its degradation rate is slow, and it takes several months to a year, or even longer, to be completely degraded. The hydrolysis product of PHBV is hydroxybutyric acid. 　　Starch-polyester blends (including blends of starch with PHBV and with PHA) are being actively studied in an effort to develop biodegradable plastics that possess the excellent physical properties of polymers while being less expensive. It is still unknown whether PHBV can be produced on a large scale in the future and what its market prospects will be; however, its excellent properties may enable it to occupy a significant place among biodegradable plastics in the future. 　　PHA can now be produced on an industrial scale using fermentation. In the bioreactor, organic microorganisms and a carbon source substrate (glucose or sucrose for PHB production, and propionic acid for PHV production, with propionic acid dapat be obtained from wood pulp waste or petroleum fermentation) are added, and it is produced using a two-stage fermentation process. The first stage is cell growth, and the second stage is polymer accumulation. The resulting PHA accounts for 80%–90% of the cell dry weight, and after purification it becomes a product. PHA is biocompatible and biodegradable, and it serves as a raw material for manufacturing biodegradable plastics. 　　Currently, the fermentation process used to produce PHA is inefficient, which results in high product prices. Currently, research is being conducted on using genetic engineering to modify rapeseed in order to produce seeds containing PHA; if the yield of such seeds is high enough, the price of PHA could compete with that of synthetic polymers. However, growing this type of rapeseed still requires a large amount of land. Rough estimates suggest that even if 10% of the world’s existing land were used for growing rapeseed, the PHA produced would only be sufficient to meet 7% of the packaging materials needed in the United States. 　　Biodegradable plastics produced through fermentation processes possess excellent physical properties; however, their current price is high. Yet since the fermentation process allows for easy scaling up of production, the price is expected to drop significantly. 　　3 Conclusion The processes for manufacturing the aforementioned two types of biodegradable plastics are developing rapidly, and some of the industrial products derived from them have already begun to appear on the market. For example, biodegradable bags made from such biodegradable plastics have already been commercialized. PHA, PHB, PH-BV, PLA, and other thermoplastic biodegradable plastics are playing an increasingly important role, as they can all be processed using conventional equipment in the plastic industry. 　　Now, many formulations of biodegradable plastics have been studied, some of which hold great industrial potential, and an industry for biodegradable plastics is emerging. As for the future prospects of biodegradable plastics, it can only be answered after a comprehensive assessment of the advantages and disadvantages of developing the biodegradable plastic industry.