Introduction of biodegradable plastics

At present, there are three types of important biodegradable plastics in foreign countries. 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 the high production of fermentation processes. Polymer based. The first category has been reported in a large number of papers. This article only reviews the manufacture, characteristics, applications, development status and prospects of the second and third types of biodegradable plastics.

Also in the 1970s, it became clear that when plastics were discarded in the environment as waste, some of the advantages of making plastics so widely available became a drawback and a burden to humans. Because such products are durable and do not degrade under environmental conditions for a long time, causing a threat to the environment. Therefore, people want to use plastics that can be degraded by organic microbes in the environment and biodegradable plastics that can be made from renewable resources. As a result, new biodegradable plastic technology has emerged.

Many biopolymers that exist in nature, as well as biopolymers produced from biological processes and polymerized from natural monomers, all have biodegradability and are important resources for the manufacture of biodegradable plastics. The biopolymers in biodegradable plastics are inherently degradable, but biodegradable plastics also contain additives that are added to improve their properties. The degradability of these additives varies greatly.

Some biopolymers are thermoplastic and they can be processed by processes that process synthetic polymers. Some biopolymers are non-thermoplastic and they can be processed into plastic sheets by casting. There are also some naturally occurring low molecular weight biomolecules that can polymerize to form thermoplastic materials during processing, and may also crosslink to form thermosets. Biodegradable plastics are usually water-based because many biopolymers (or after proper treatment) are soluble or dispersible in water.

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. Others, despite being feasible in the production process, have not yet been able to attract sufficient investment. Industrial production, but some of them are expected to be successful in the near future. People are excited to wait for the birth of more new biodegradable plastics, and expect more of these new technology products to enter the market.

1. Biodegradable plastics based on polymers derived from polymerization of natural monomers

This class of biodegradable plastics is based on polymers derived from the natural polymerization of low molecular weight biomolecules. They are thermoplastic and thermosetting. An important representative of such polymers are polylactic acid and triglyceride polymers.

1,1 Polylactic acid (2-hydroxypropionic acid)

Polylactic acid (PLA) is a polyester, and lactic acid (LA) is a naturally occurring biomonomer. PLA is thermoplastic and can be processed into plastics by processing plastics into films, sheets and fibers. PLA is soft and rigid. It is transparent in itself, but it can also be made opaque and fillers can be added. Due to its high strength, very thin sheets can be made. PLA is insoluble in water and has good water resistance and oil resistance. The mechanical properties and other properties of PLA can be improved by altering the relative molecular mass and crystallinity, or by copolymerizing lactic acid with glycolic acid or caprolactone.

The United States Cargill Corporation began research and development of PLA around 1987, pilot production in 1992, and sold under the trade name EcoPLA. At the end of 1997, Cargill and Dow Chemicals established a joint company, Cargill Dow Polymer, which specializes in the production and sale of PLA. The company has built a 125kt per year PLA unit. Due to the increase in production, the selling price has dropped to about $1.0/kg.

PLA can be used as a plastic film and recycled and biodegradable packaging materials. It can also be used as disposable tableware, beverage containers and sporting goods. PLA also has a series of uses in biomedicine, such as drug release materials, bone growth and repair materials. Lactic acid and glycolic acid copolymers can support the growth and attachment of new cells. It can be made into a porous material, so it can provide a large area of ​​continuous surface, so that the cells can reproduce in the entire matrix. The implants of PLA in the human body can be completely degraded within a certain period of time, which depends on the shape and size of the implant and usually does not exceed 2 years.

Another use of PLA for growth factors is as a coating for human body metal implants to stabilize bone shape. 80% of the growth factor was released within 42 days. Compared to uncoated implants, wound acceleration was improved when PLA coating was used. Moreover, even if the PLA coating does not contain growth factors, the healing of the wound can be improved.

LA can be manufactured by chemical synthesis or fermentation. Currently, it is mainly produced by fermentation, and its raw material is glucose. The yield of LA produced by the fermentation process can be greater than 90%, and its production cost mainly comes from its multi-step purification process.

When LA is polycondensed, usually a low molecular weight polymer is first obtained, and the latter is treated with a coupling agent (chain extender) to obtain a high molecular weight PLA. Recent studies have shown that, for example, polycondensation of LA in a high-boiling solvent and in the presence of reduced pressure and a catalyst, high molecular weight PLA can be directly obtained. PLA has recently become one of the most important industrial biodegradable plastics.

PLA has many excellent properties, a wide range of uses and attractive prospects. Since LA is now ready for mass production, it is possible to supply PLA in large quantities. Despite the abundance of raw materials for the manufacture of PLA, it is doubtful whether PLA can meet the needs of the world, but even if it is satisfied, it will inevitably lead to competition between plastics production and food production, because the raw materials for manufacturing PLA are all food products.

1,2 triglyceride polymers

Triglyceride is a rich source of biomolecules that has recently attracted great attention because its polymers can be used to make new biodegradable plastics.

The triglyceride is first converted into a highly reactive intermediate, and the latter is polymerized by epoxidation to a low molecular weight liquid polymer, which is then mixed with a catalyst and an accelerator to facilitate the cross-linking reaction. Finally, the mixture is injected into a mold filled with reinforcing fibers and heat-cured in the mold to form a rigid thermosetting bioplastic.

Glass fiber-reinforced soybean oleoresin is a durable thermosetting material that can be used in fields such as agricultural machinery, automotive, and construction. This resin is entirely made from renewable resources, but it is high in strength and does not contain formaldehyde. Using other vegetable oils and reinforcing materials other than glass fiber (such as plant fibers, even straw and hay), thermosetting materials similar to the above can also be produced, and the soybean-based resin has a strong affinity for natural fibers. If straw can be used instead of wood fiber to make compression-molded composites (such as fiberboards currently widely used in the construction industry), a new use can be found for rice, a source rich in resources and with a short regeneration cycle, saving on regeneration cycles. Long wood fiber. In the future, it is also possible to use nanocellulose instead of glass fibers. It is expected that due to the development of enhanced vegetable oil thermosetting composites, it is possible to make a biodegradable material that is inexpensive and durable.

In addition, epoxidized soybean oil can be polymerized with citric acid to form a polyester coating on kraft paper for the production of biodegradable agricultural films. This coating can increase the wet strength of the paper and reduce the rate of degradation of the paper, thus allowing the agricultural film to inhibit weed growth for up to 10 weeks.

2. Biopolymers based on polymers produced during the fermentation process

With the fermentation process, biopolymers can be produced on a large scale, and the raw materials for the fermentation process are generally natural resources derived from plants.

The recently developed biopolymer polyhydroxyalkanoates (PHA) is a kind of polyester that can be obtained by fermentation and plays an important role in the industrialization of bioplastics. British ICI began manufacturing polyhydroxybutyrate (PHB) around 1978. In 1987, ICI produced PHB and its copolymers and sold them under the trade name Biopol in Europe. In the following years, ICI's biodegradable polyester production grew rapidly. Since 1991, a great deal of research has been conducted on the manufacture of biodegradable polyesters, particularly in the use of DNA recombination. In the mid-1990s, the United States used DNA recombination to produce PHBV, a copolymer of hydroxybutyrate (HB) and hydroxyvaleric acid (HV), in plants such as soybeans.

PHB plastics are brittle, but PHBV can adjust its brittleness, strength and other properties according to its composition, and the composition of PHBV depends on the ratio of raw materials that produce it. PHBV is a white granular material, and its physical properties and processing properties may be similar to those of PE or PP, and may be brittle plastics or elastomers, depending on its HV content. The use of additives can further improve the performance of PHBV.

PHBV is thermoplastic and can be blown, extruded, or injection molded to form films, fibers, coatings, and laminates. It can be used to make a variety of packaging vessels (bottles, plates, cups, etc.) and many other appliances. PHBV was used to make shampoo bottles. Current shampoos are generally biodegradable, so biodegradable PHBV bottles are very suitable.

PHBV is much more water resistant than most glycans and proteins, so PHBV bottles can be stored in a humid atmosphere and are quite stable. For coated paper products, PHBV can replace PE. PHBV coating can make starch foam products heat and cold water resistant. PHBV is biodegradable in soil, river water, seawater, aerobic, and anaerobic sewer sludge. For example, in anaerobic sewer sludge, nearly 80% of PHBV degrades to CO2 and methane in 30 days. In urban compost farms, PHBV compression molded products lost about 60% in 6 months. The rate of degradation of PHBV is also related to its composition, molecular weight, crystallinity, surface area, and whether it contains biodegradable additives. At high temperatures, the hydrolysis of PHBV also contributes to its degradation. PHBV formula should be able to recycle and clean incineration.

PHBV has also been tested for use in biomedical applications such as drug delivery systems, human implants, and the like. Degradation of PHBV in the human body is completely caused by hydrolysis, so the degradation rate is slow, and complete degradation takes several months to one year or even longer. The descending (aqueous) solution product of PHBV is hydroxybutyric acid.

Starch-polyester blends (including blends of starch with PHBV and PHA) are being actively studied and are aimed at producing biodegradable plastics with excellent physical properties of polyester but at a lower price. Whether or not PHBV can be produced in large scale in the future and market prospects is still unknown, but its excellent performance may make it a place in the future of biodegradable plastics.

PHA can now be produced on an industrial scale by fermentation. In the bioreactor, organic microbes and a carbon source matrix are added (glucose or sucrose in the production of PHB, propionic acid in the production of PHV, propionic acid can be obtained by wood pulp waste or petroleum fermentation), and are manufactured by a two-stage fermentation method. The first segment is cell growth and the second segment is polymer accumulation. The obtained PHA reached 80% to 90% of the dry weight of the cells and was purified into a product. PHA is biocompatible and biodegradable and is the raw material for biodegradable plastics.

At present, the PHA fermentation process is relatively inefficient and therefore expensive. Now, people are studying the use of genetic engineering to modify rapeseed to produce seeds containing PHA. If the yield of such seeds is high enough, the price of PHA may compete with synthetic polymers. However, the planting of this kind of rapeseed still requires a lot of land. According to a rough estimate, even if the rapeseed is grown with 10% of the global land used, the PHA produced can only meet 7% of the packaging material consumed in the United States.

Biodegradable plastics produced in the fermentation process have excellent physical properties, but the current prices are higher, but the fermentation process is easy to expand the scale of production, the price