Degradable plastics refer to a type of plastic whose products have various properties that can meet the use requirements, and whose properties remain unchanged during the shelf life, and which can be degraded into environmentally friendly substances under natural environmental conditions after use. Therefore, they are also called environmentally degradable plastics.
There are many new types of plastics: photodegradable plastics, biodegradable plastics, photo/oxidation/biodegradable plastics, carbon dioxide -based biodegradable plastics, and thermoplastic starch resin degradable plastics.
Concept
The degradation of polymers refers to the process of breaking the polymer macromolecular chains caused by chemical and physical factors. The degradation process of polymer macromolecular chains breaking under environmental conditions such as exposure to oxygen, water, radiation, chemicals, pollutants, mechanical forces, insects and other animals, and microorganisms is called environmental degradation. Degradation causes the molecular weight of the polymer to decrease, and the physical properties of the polymer material to decrease until the polymer material loses its usability. This phenomenon is also called aging degradation of polymer materials.
The aging and degradation of polymers are directly related to the stability of polymers. The aging and degradation of polymers shortens the service life of plastics. For this reason, since the advent of plastics, scientists have been committed to the anti-aging, that is, stabilization research of such materials in order to produce highly stable polymer materials. Scientists from various countries are also competing to develop environmentally degradable plastics by utilizing the aging and degradation behavior of polymers.
The main application areas of degradable plastics include: agricultural mulch, various plastic packaging bags, garbage bags, shopping bags in shopping malls, and disposable tableware, etc.
Degradation Principle
The degradation process of environmentally degradable plastics mainly involves biodegradation, photodegradation and chemical degradation. These three main degradation processes have synergistic, synergistic and coherent effects on each other. For example, photodegradation and oxidative degradation often proceed simultaneously and promote each other; biodegradation is more likely to occur after the photodegradation process.
Trend
The world market for degradable plastics was forecast to be about 20 million pounds in 1997, with sales of $23 million; it is expected to grow by 35% by 2004. Degradable plastics available in the U.S. and Japanese markets include biodegradable polyesters such as polylactic acid, biodegradable PET, polybutylene succinate /caproate, polyhydroxybutyrate / valerate, polyethylene succinate, polyester amines and polycaprolactone /mixtures; starch and its mixtures; and other plastics with additives that enhance degradation. The reason for the growth is the increase in the use of environmentally friendly plastics, and another reason is the reduction in production costs. By expanding production and economies of scale, manufacturers have reduced their prices. However, the high cost of degradable resins and the fact that various existing plastics have firmly occupied their markets make it difficult for biodegradable plastics to enter the market.
Environmentally degradable plastics are a new type of plastic, and the development of environmentally degradable plastics began in the 1970s. At that time, the main development was photodegradable plastics, with the aim of solving the environmental pollution problems caused by plastic waste, especially disposable plastic packaging products. In the 1980s, the development and research shifted to biodegradable plastics. In addition, biodegradable plastics produced from renewable resources such as plant starch and cellulose, animal chitin, etc. instead of petroleum have also appeared. In addition, biodegradable plastics produced by microbial fermentation have also been developed. A type of medical plastics that can be biodegraded, such as polylactic acid, has also attracted people’s attention. It is hoped that it can be used to solve the environmental pollution problem of plastics. However, there are different opinions on whether this type of plastic should be classified as environmentally degradable plastics. The opinion of the Japanese Degradable Plastics Research Association is that it cannot be classified as environmentally degradable plastics. However, from the perspective that degradable plastics are a new type of plastic, biodegradable plastics should also be included, and degradable plastics can be classified according to their uses into environmental (natural) degradable plastics and bio (environmental) degradable plastics. The latter has been used in medicine for surgical sutures, artificial bones, etc.
The development and research of degradable plastics in China is basically synchronized with the world. However, the research and development of degradable plastics in China began with agricultural mulch films. China is a large agricultural country, and the consumption of mulch films ranks first in the world. In order to solve the problem of residual mulch films accumulated in farmland causing harm to plant root development and affecting crop yields,In order to solve the problem of residual film hindering the operation of agricultural machinery, the development of photodegradable plastic mulch began in the 1970s. Around 1990, biodegradable plastics filled with starch in general plastics appeared. At the same time, on the basis of photodegradable plastics, mulch filled with starch with both photodegradable and biodegradable functions was developed. Various types of degradable mulch are under development and are still in the stage of application demonstration and promotion. With the improvement of the living standards of the Chinese people, the environmental pollution caused by disposable plastic packaging products is becoming more and more serious. For this reason, degradable plastic products used for packaging, mainly disposable packaging, such as garbage bags, shopping bags, lunch boxes, etc., are also being actively developed.
Development
1. Further deepening of research according to different uses and environmental conditions, improving formulations through molecular design research, and developing timely and controllable environmentally degradable plastics have become key research topics in many countries.
2. Actively research and develop highly effective and low-cost photosensitizers, oxidants, biological inducers, degradation accelerators, stabilizers, etc. to further improve on-time controllability, rapid degradation after use, and complete degradation.
3. Accelerating the development of biodegradable plastics or ordinary plastics with starch, cellulose or inorganic materials filling, blending or alloying technology, as well as fully biodegradable plastics and natural materials coating and lamination technology are the hot spots among the hot spots.
4. Hydrolyzable plastics and edible plastics have attracted worldwide attention due to their special functions and uses, thus becoming another hot spot for environmentally friendly materials.
5. In order to accelerate the development of degradable plastics, countries are committed to accelerating research and establishing a unified definition, degradation mechanism, evaluation methods and standards for degradable plastics.
6. Explore and cultivate strains that can degrade common plastics, so that widely used common plastics can be easily degraded after use to meet environmental protection requirements. At the same time, attach great importance to cultivating biological plants that can produce polyester, etc., to reduce the cost of biodegradable plastics, which is conducive to the promotion and application.
Main Applications
There are two main areas of use for degradable plastics: one is the areas where ordinary plastics were originally used. In these areas, plastic products after use or consumption are difficult to collect and will cause harm to the environment, such as agricultural mulch and disposable plastic packaging; the other is the area where plastics replace other materials. The use of degradable plastics in these areas can bring convenience, such as golf course spikes and tropical rain forest afforestation seedlings fixing materials. Specific applications are:
1. Agriculture, forestry and fishery, mulch film, water-retaining materials, seedling pots, seedbeds, rope nets, pesticides and fertilizer slow-release materials.
2. Packaging industry, shopping bags, garbage bags, compost bags, disposable lunch boxes, instant noodle bowls, cushioning packaging materials.
3. Sports equipment, golf course tees and golf tees
4. Hygiene products, women’s hygiene products, baby diapers, medical mattresses, disposable razors.
5. Medical materials, bandages, clips, small sticks for cotton swabs, gloves, drug sustained-release materials, as well as surgical sutures and fracture fixation materials.
Research Status
Wide range of applications
Synthetic polymer materials have been widely used in various fields. However, plastic waste has become a public hazard to the environment and society. Some developed countries have successively formulated regulations to restrict or prohibit the use of non-degradable plastics in certain occasions and require the use of degradable plastics. For this reason, governments and the plastics industry have attached great importance to the research and development of degradable plastics while formulating effective measures to deal with and recycle waste plastics. With the coordination and support of the government, degradable plastics have become a research hotspot in the international plastics industry.
Environmental hygiene
Degradable plastics are generally considered to be plastics that can be decomposed into low molecular weight substances by sunlight radiation or microorganisms in the soil. In addition to being degradable, they should also be easy to process and meet the requirements of use. The harmful effect of sunlight on polymer materials is the combined effect of ultraviolet light and oxygen, so it is called photo -oxidative degradation. Taking polyolefins as an example, photo-oxidation often causes chain scission or cross-linking of polymers, accompanied by the formation of some oxygen-containing functional groups, such as ketones, carboxylic acids, peroxides and alcohols. Its degradation mainly comes from catalyst residues in the polymer, as well as the initiation of peroxides and carbonyl groups introduced during processing.
Biochemical reactions
The cracking of micro-photopolymers mainly comes from the effects of biophysics, biochemistry and mold. Its sensitivity to polymers depends on the structure of the polymer itself and the surrounding environment such as water, temperature, pH value and oxygen. According to the degradation mechanism, degradable plastics can be divided into photodegradable plastics, biodegradable plastics and photo/biodegradable plastics.
Hazards of non-degradable plastics
Plastics are widely developed in many fields due to their advantages such as light weight, high strength, stable chemical properties and low cost. The plastic industry is developing rapidly, but there is no proper disposal method for used plastics, and plastic waste brings serious pollution to the natural environment. Most non-degradable plastics are developed and produced from low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE), followed by high-density polyethylene (HDPE), polypropylene (PP), polystyrene (PS) and polyvinyl chloride (PVC). These plastics are generally eventually disposed of as solid waste, resulting in the formation of pollutants such as acid rain in the air, which is harmful to our lives.
Therefore, it is inevitable to study degradable plastics! The discarding of solid waste such as plastics will pollute the environment, deep burial will occupy the land, and burning will pollute the air. These are not the fundamental ways to solve the problem. The fundamental way to solve the problem is to develop degradable plastics to replace non-degradable plastics.
Development of biodegradable plastics
Degradable plastics are a general term for photodegradable plastics and biodegradable plastics. Developed countries began to study photodegradable plastics in the 1970s, and the concept is relatively mature. Biodegradable plastics began in the mid-1980s and have developed rapidly, and already have industrial products.
China started the research of photodegradable plastics in the 1980s, and only started the research of biodegradable plastics in recent years. There are dozens of units engaged in this project in my country, but the application is not large, and the promotion should be just starting. China’s biodegradable plastics are mainly concentrated in starch-filled types. Its products have reached the same level as foreign products, but there is still some distance from industrial production.
The prospects of biodegradable plastics
First of all, degradable plastics are developed to protect the environment. It is of great significance. The excellent performance of degradable plastics is obvious and has broad development prospects. The research, development and production of degradable plastic masterbatch have a great role in promoting the development and promotion of degradable plastics. Because the production of degradable plastics by blending degradable plastic masterbatch with corresponding polymers does not need to change the original plastic molding process. It has wide practicality.
Sustainability
The research on degradable plastics is just right for China’s sustainable development strategy and can adapt to the development of society. The main direction of our research and development is to use polymer materials to synthesize light/biodegradable plastics by chemical and biological methods. We can use these polymer synthesis methods to synthesize the materials we need – degradable plastics.
Current situation in various countries
China
On June 1, 2022, the two recommended national standards, “ Degradability Performance and Labeling Requirements for Biodegradable Plastics and Products ” and “ Biodegradable Drinking Straws ”, were officially implemented, which means that the promotion and application of biodegradable materials products will be greatly accelerated. By the end of 2022, biodegradable plastics will be promoted in all prefecture-level cities across the country.
Classification
Photodegradable plastics
Refers to materials whose polymer chains decompose in an orderly manner under the influence of ultraviolet rays. Most polymers do not absorb light energy with wavelengths above 285NM. However, if light-sensitive groups or chemical additives with light-sensitive effects are added to the polymer, the photo-oxidation reaction process can be accelerated, causing it to degrade rapidly. According to the molecular design principle and manufacturing method of photodegradable polymers, they can be divided into synthetic photodegradable plastics and additive photodegradable plastics.
Copolymer photodegradable plastics were invented by DuPont in the United States. They are made of polyethylene (PE) and carbon monoxide copolymers, namely E-CO copolymers, or polyethylene and vinyl copper copolymers, namely GUILLET copolymers. The purpose is to make PE carry carbonyl groups to enhance the degradability of PE plastics. By changing the carbonyl content in PE, the degradation period of this plastic can be controlled to about 60 to 600 days. Later, carbonyl copolymers such as polystyrene (PS), polypropylene (PP), polyvinyl chloride and polyamide ( PA ) were developed. In some developed countries in Western countries, PE photodegradable films have been used as ground films, food bags and garbage bags, and PP degradable films are also used in food packaging and cigarette production.
Additive photodegradable plastics are made by adding a small amount of photoinitiators and other additives to polymers. Typical photoinitiators or photosensitizers include aromatic ketones, aromatic amines, ferric acetylacetonate, 2-hydroxy-4 -methylacetophenone oxime iron, iron stearate, etc. It is feasible to add appropriate amounts of these photosensitizers to polymers such as PE, PP, PVC and PS.
In recent years, the research on photodegradable polyethylene made from long-chain alkyl ferrocene derivatives has been completed, as well as the photodegradable PE plastic film using iron compounds as photosensitizers successfully developed by the Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences. The photodegradable PE film using metals as photosensitizers was developed by the Dalian Institute of Plastics.
Biodegradable plastics
From the perspective of biodegradation process, plastics can be divided into two categories: fully biodegradable plastics and biodegradable plastics; from the perspective of preparation methods, they can be divided into four categories: biological fermentation synthesis, chemical synthesis, and the use of natural polymers or minerals from animals and plants.
When fully biodegradable plastics are synthesized by chemical methods, they are easily degraded by using aliphatic polyesters, polyvinyl alcohol (PVA) and polyethylene glycol. The biodegradable plastics are researched and developed by using the biodegradable properties of these polymers, among which the research on aliphatic polyesters is particularly outstanding. Among the many aliphatic polyesters, polycaprolactone ( PCL ) is widely used. It is a thermoplastic crystalline polyester that can be hydrolyzed into small molecules by lipase and then further assimilated by microorganisms. UCC in the United States has carried out mass production and has been used in surgical supplies, adhesive membranes, demolding agents and other products. PCL can also be blended with PHB to prepare biodegradable plastics. Aliphatic polyesters and nylon undergo an amine exchange reaction to synthesize polyacrylamide copolymers (CPAE), which is a new type of biodegradable plastic.
When synthesized using natural polymers from animals and plants, valuable biodegradable plastics can be produced, such as cellulose, starch, chitosan, polyglucosamine, animal glue, and algae from marine organisms.
It is also possible to use chemical methods and natural polymer blending technology to synthesize degradable plastics, the main varieties of which are PHB/PCL, gelatinized starch/PCL and other products. Their main feature is that they can be completely degraded, and at the same time, their heat resistance and water resistance can be improved through blending, and their costs can be reduced, making them universal degradable plastics.
Biodegradable plastics are incompletely biodegradable plastics. They are made by mixing biodegradable substances into general-purpose olefin plastics, causing the material to lose its mechanical properties and shape. Through composting, the same effect as biodegradability is produced. Due to the low cost of this type of plastic, this method has been adopted in many countries.
Aliphatic polyester biodegradable plastics are very fine fibers of general-purpose plastics that are evenly dispersed in biodegradable polyester to make the copolymer biodegradable. Aliphatic general-purpose plastics such as PE, PP, PS, PVC, etc. are blended to control their phase structure and dispersion state to obtain biodegradable plastics with excellent physical properties; natural mineral biodegradable plastics are similar to calcium carbonate -filled modified polyolefin plastics. In order to adapt to environmental needs, high-filled calcium carbonate masterbatches and special materials have been developed to make packaging materials such as films, sheets, and boxes. Jilin Research Institute has studied PE/calcium carbonate-based degradable materials. This type of material has the advantages of low plastic consumption, low energy consumption, and low cost. However, it has the disadvantages of high density, low air tightness, poor control of degradation induction period, and poor mechanical properties. Therefore, it can only be used as a disposable packaging material, and its degradability needs further study.
The development direction of biodegradable plastics is:
- To use polymer materials such as cellulose, starch, chitin, etc. to produce biodegradable plastics, and further develop and improve the functions and technologies of natural polymers.
- To use polymer design and fine synthesis technology to synthesize biodegradable plastics. By analyzing the biodegradation mechanism of synthetic polymers with biodegradability, biodegradable plastics are produced; at the same time, research and development are carried out on the segmented copolymerization of such polymers with existing general polymers, natural polymers, microbial polymers, etc.
- To improve the biodegradability of biodegradable plastics, reduce their costs, and expand their applications.
- Research on the control of degradation rate. In short, with the needs of society, biodegradable plastics will receive more and more attention and become a major research topic in the future.
PHA degradable plastics have the best performance among biodegradable plastics. However, due to their high cost and complex production process, they are still in the initial stage of the market. In 2010, the global PHA production capacity was less than 80,000 tons, and Metabolix in the United States had a production capacity of about 50,000 tons, accounting for more than 60% of the market. Chinese companies are also relatively advanced in the production process and research and development of PHA. Tianjin Guoyun Biomaterials Co., Ltd. has a PHA production capacity of 10,000 tons, Ningbo Tianan has a production capacity of 2,000 tons, and Shenzhen Yikeman Biotechnology Co., Ltd. has a production capacity of 5,000 tons. Kaneka in Japan and PHBIndustrial in Brazil are also typical representatives of the PHA industry. These companies are all promoters of the PHA industry. Although the application of PHA is relatively limited at present, resulting in the actual annual sales of Metabolix not exceeding 100 tons, but with the gradual expansion of downstream applications in the future, especially in the film packaging, agricultural film, edible tableware, non-woven fabrics and other industries, the market potential of PHA is huge.
Light, oxidation/biodegradable plastics
It combines photodegradation, oxidative degradation and biodegradation to achieve complete degradation. It is one of the main research and development directions of degradable plastics in the world. This kind of plastic has achieved good results in the United States, but it is still one of the more difficult research topics in China.
Thermoplastic starch resin degradable plastics
The starch molecules are transformed and disordered to form a thermoplastic starch resin, and then a very small amount of plasticizers and other additives are added to form the so-called full starch plastic. The starch content is more than 90%, and the small amount of other substances added is non-toxic and can be completely degraded, so full starch is a truly completely degradable plastic. Almost all plastic processing methods can be applied to the processing of full starch plastics. Full starch plastic is considered to be the most promising fully biodegradable plastic in many countries. Sumitomo Corporation of Japan, Wanler Lambert Company of the United States and Ferruzzi Company of Italy have claimed to have successfully developed full starch plastics with a starch mass fraction of 90% to 100%. The product can be completely biodegraded within 1 year without leaving any traces and pollution. It can be used to make various containers, films and garbage bags. Battelle Research Institute in Germany has developed a degradable plastic using modified green pea starch with a high linear content. It can be processed and formed by traditional methods. As a substitute for PVC, it can be completely degraded in a humid natural environment.
CO2-based biodegradable plastics
Inoue Shohei et al. of Japan found that carbon dioxide can be bonded and ring-opened with epoxide to form aliphatic polycarbonate ( APC ), which is the most promising carbon dioxide copolymer to date. Takanashi et al. used a terpolymer of carbon dioxide, propylene oxide and epoxide containing ester bonds as a drug sustained-release agent. Masahiro et al. prepared PPC microspheres as a carrier of the drug sustained-release system by evaporating solvents, and studied the factors affecting the drug release rate of the system, such as the molecular weight of PPC and the drug content. The results showed that the drug release rate increased with the decrease in the diameter of the microspheres or the increase in the concentration of loaded drugs, but the drug release rate and biodegradability were not related to the molecular weight of the copolymer. The morphology of the microspheres before and after drug release was observed by SEM, confirming that PPC microspheres supported the long-term and uniform release of drugs. American experts adopted a new technology, using a special zinc catalyst, to mix and copolymerize carbon dioxide and ethylene oxide (or propylene oxide) in a certain proportion to produce a plastic packaging material with new properties. The carbon dioxide-based fully biodegradable plastic project jointly implemented by China Jilin Oil Group Corporation and the Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences has been included in the National 863 Scientific Research Program. It is a new high-tech environmentally friendly material research and development project with broad development prospects.
Advantages Analysis
Practicality: It has application performance and sanitary properties equivalent to or similar to those of common plastics of the same type.
Degradability: After completing its function, it can degrade quickly under natural environmental conditions and become fragments or scraps that are easily utilized by the environment and eventually return to nature.
Safety: Substances produced during the degradation process and remaining after degradation are harmless or have no potential harm to the environment.
Economical: The price is the same as or slightly higher than that of similar ordinary plastics.
Evaluation Criteria
The test standards published by ASTM in the United States are as follows: Evaluation test standards for biodegradable plastics, ASTMD5209-92, ASTMD5210-92, ASTMD5247-92, ASTM D5271-92, ASTM D5338-92. Evaluation test standards for photodegradable plastics, ASTM D5071-91, ASTM D5208-91, ASTM D3826-92. Evaluation test standards for environmental safety, ASTMD5152-91. The relevant test standards published by China are as follows: GB 18006.1-1999 “General technical conditions for disposable degradable tableware “, GB/T 18006.2-1999 ” Test method for degradation performance of disposable degradable tableware “, HJBZ 12-2000 ” Technical requirements for environmental labeling products for packaging products “, HBC 1-2001 “Technical requirements for environmental labeling products for disposable tableware”, QB/T 2461-1999 ” Degradable polyethylene film for packaging “. In the future, we will still refer to the relevant test standards of developed countries and China, and combine the market promotion of various types of degradable plastic products to conduct in-depth research on the definition and evaluation methods of degradable plastics, and continuously improve and perfect the relevant test standards. This is the key to the promotion of degradable plastic products.The foundation and premise of industrialization.
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