Can degradable plastics solve the problem of disposable packaging| Source: pexels.com
"Degradable" plastics have become a new trend of environmental protection. However, from production and use to sorting and processing, the problems caused by "degradable" are far more than solved. Not only the cost is high, but also not as durable as traditional plastics; After turning into waste, it still needs very detailed classified collection and industrial composting environment to truly realize "degradation". However, these two conditions are difficult to provide in most areas, so we have reason to question whether it should go out of the laboratory and move towards large-scale application.
Written by Yang Jinqi
Editor in charge: Feng Hao
●●● in order to welcome the upcoming 2022 Beijing Winter Olympic Games, Sinopec announced that its affiliated Beijing Petroleum will donate 100000 degradable plastic bags to Zhangshanying Town, where the Yanqing competition area of the Winter Olympic Games is located, so as to reduce plastic pollution during the operation of the games. The material of these plastic bags is PBAT (copolymer of butanediol adipate and butanediol terephthalate), which can be degraded under composting conditions. With the improvement of public awareness of plastic pollution, degradable plastics are regarded as the golden key to solve white pollution.
However, more and more studies begin to question the effectiveness of degradable plastics as a solution to plastic pollution. For example, under what circumstances can the so-called degradable plastics be degraded? What impact will it have on the current waste treatment facility system?
Before answering these questions, we might as well take a look at the concept, principle, conditions and current practical application of "degradable plastics".
What is degradable plastic?
Talking about degradation without conditions is like talking about toxicity without dose.
The conditions of degradability include many aspects - temperature, humidity, oxygen, microbial population, etc. In addition, the time factor is crucial. Ordinary fossil based plastics, such as common plastic bottles, can also be degraded in the natural environment after 450-500 years, but this is meaningless for the environmental protection we are talking about .
From the perspective of material molecular structure, the degradation process actually interferes with the chemical structure of general plastics, that is, the long-chain carbon bond of polyester is decomposed into short chains, and then into carbon dioxide, water and biomass, so as to safely return to the material cycle of nature.
Most of the common plastics on the market are non degradable plastics, such as polypropylene (PP), ethylene terephthalate (PET), polyvinyl chloride (PVC), etc. They can be recycled, but if discarded in landfills, it generally takes hundreds of years to degrade.
There are generally two ways of plastic degradation, photooxidative degradation and biodegradation. In view of the great controversy over photo oxidative degradation (as shown in the table below), this paper focuses on biodegradation.
Photo oxidative degradation (oxo degradable) generally accelerates the fragmentation of traditional fossil based plastics under oxygen, light or high temperature by adding additives. Although photo oxidative degradable plastics can be fragmented or even invisible to the naked eye within a few months or years, the fragmented plastics will gradually become micro plastics (usually plastic particles with particle size less than 5mm) in the environment. At present, there is no evidence that micro plastics can be completely degraded in a short time. Therefore, photo oxygen degradation is controversial and suspected of "washing green".
In addition, photo oxidative degradable plastics are not durable in practice, and can neither be recycled nor composted after use. On the contrary, they will interfere with both treatment methods - reduce the performance of recycled plastics and pollute compost products. Major brands around the world (such as Unilever, Pepsi Cola), research institutions and public welfare institutions are now proposing to ban the production of this plastic , until there is evidence that it can be completely degraded in a short time .
Biodegradation refers to the complete conversion of materials into carbon dioxide and water through microbial action in a specific environment. Microorganisms that can convert polyester short chains into carbon dioxide include bacteria, fungi and protozoa, and an enzyme secreted by them can decompose polyester bonds .
At present, there are more than 20 kinds of biodegradable plastics on the market . People often confuse it with bio based plastics (components from renewable biological resources). In fact, biodegradable plastics can be either bio based or fossil based (components from non renewable fossil resources).
Bio based plastics often use cassava, corn and sugarcane as raw materials . It should be clear that not all bio based plastics are biodegradable. For example, bio PE produced on a large scale with sugarcane as raw material in Brazil is not biodegradable. Most fossil based plastics are not biodegradable. Of course, there are exceptions, such as polycaprolactone (PCL), polybutylene succinate (PBS) and the copolymer of butylene adipate and butylene terephthalate (PBAT).
Three types of biodegradable plastics that can be produced on a large scale
According to the European bioplastics market data report, in 2019, the global total production capacity of biodegradable plastics reached 1.17 million tons, accounting for 0.3% of the global annual output of plastics (360 million tons). At present, there are three main types of large-scale commercial production .
One is starch plastics and polylactic acid (PLA) with plant starch as raw materials . Starch is cheap, high yield and simple process. The disadvantage is that it is not waterproof and depends on food crops as raw materials, occupying a large amount of cultivated land. At present, the industry is also studying the extraction of raw materials from agricultural and industrial wastes (such as corn cobs and cellulose) .
Second, dibasic acid diol copolyesters (PBS, PBSA, PBAT, hereinafter referred to as PBS plastics) with petroleum by-products or biological raw materials, and the production capacity of such plastics is also increasing year by year.
The third is polyhydroxyfatty acid esters (PHA), which are synthesized by microorganisms with sugars or oils as raw materials during fermentation. At present, the production capacity is still small, only 25000 tons in the world. However, because it is similar to PLA and has the potential to use agricultural by-products and other organic wastes as raw materials, the prospect of PHA is very promising.
At present, the price of biodegradable plastics is generally higher than that of traditional plastics, but with the scale effect brought by the increase of production capacity and the impact of oil price fluctuations on traditional plastics, the price of biodegradable plastics is becoming more and more competitive in the market.
Jia Xinnan, a doctor of polar marine ecology at the University of Tasmania in Australia, told intellectuals that the most common form of degradable plastics in China is film, especially dominated by the test and promotion of degradable plastic film. In urban areas, it is most widely used in supermarkets, shopping bags and food packaging.
Degradable plastic, is it really easy to degrade?
If plastic waste cannot be effectively collected and scattered in the natural environment, it will cause extremely serious negative effects. The common treatment methods of collected plastic waste include landfill, incineration, recycling and degradation.
Landfill is the least ideal treatment method, which not only pollutes the environment and threatens public health, but also releases methane together with other organic wastes after biodegradable plastics are landfilled. The contribution of the same amount of methane to the greenhouse effect is 25 times that of carbon dioxide, exacerbating climate change. Due to the lack of temperature, humidity and other conditions required for plastic degradation, there is virtually no difference in the "degradability" between biodegradable plastics and non degradable plastics in landfills.
Wang Jun, technical vice president of plastic recycling branch of China synthetic resin Association, told intellectuals, "even if it is assumed that degradable plastics only need 20 years to degrade in nature, which is much shorter than non degradable plastics, however, in the 20 years when they have not been completely degraded, the harm to natural ecology and biology is no different from other plastics."
Incineration for power generation or heating is better than landfill, especially considering that the calorific value of fossil based plastics is higher than that of coal. If only from the perspective of energy utilization, burning used plastics is more efficient than burning other non renewable energy sources (coal, oil and natural gas).
However, the research team of Tongji University found difficult micro plastics in the slag of the incinerator ]. The research estimated that 360000 ~ 102000 micro plastic particles could be produced per ton of waste put into the incinerator. In other words, even incineration can not solve the problem of plastic pollution once and for all. Moreover, from the perspective of resources, plastic incineration is also very wasteful. Both non degradable plastics and degradable plastics are forced to shorten their service life. Compared with turning into recycled plastics through the recycling link mentioned below, they lose the possibility of recycling.
Compared with the simple and crude treatment of resources by incineration, recycling is a widely advocated solution for plastic waste. Fossil based non degradable plastics (such as PET) can be recycled on a large scale and put into the production of plastics or textile products. Chang Xinjie, vice president of circular economy business of taolang group, a Norwegian based sorting and recycling solution provider, told intellectuals, "PET bottle recycling has become a large scale in China. According to Tao Lang's estimation, the collection rate has reached 85%; at present, degradable plastics are mainly replaced by materials in one-time application scenarios, without consideration of post-use collection and recycling, so they are not necessarily better than traditional recyclable plastics."
Chang Xinjie introduced that at present, the optical sorting equipment of many post-processing companies can identify biodegradable materials such as PBAT and PLA from PP and PE. However, the problem is that many products are not single materials. Different manufacturers may mix different materials such as PLA, PBAT and starch in their products, and the proportion is different. The current equipment cannot be targeted Identification, which brings more trouble to the later sorting.
Chang Xinjie said: "at present, the amount of degradable plastics entering the recycling system in China is small, but if degradable plastics are widely used in the future, and the front-end product standards and waste classification are not strict and fine enough, then degradable plastics are likely to mix with recycled plastics and even disrupt the whole recycling process."
According to the similar principle, the general paper food packaging (such as disposable paper cup) is composed of composite materials. There is a layer of plastic film for waterproof and oil proof. If it cannot be separated effectively, the paper will also affect the plastic recycling.
In practice, biodegradable plastics after use must enter supporting degradation channels to have the opportunity to complete their degradation mission. However, Chang Xinjie revealed that at present, there are still few such supporting facilities in China.
Degradable plastics currently cannot form a material closed loop
Wang Jun believes that biodegradable plastics are very immature, or a pseudo concept most of the time.
The reason is that the concept of "degradation" in people's mind comes from the observation of nature. For example, organisms such as wood can be completely degraded by naturally existing microorganisms to become the nutrition of other organisms and form a material closed loop. At present, it is "biodegradable" Artificial polymers such as plastics are polyester polymers, which need specific hydrolysis conditions for the first step of chemical degradation before they can be digested by microorganisms.
"This is different from natural degradation. Before nature evolved microorganisms with synthetic plastics as the direct food source, synthetic plastics took the natural closed-loop Road, that is, throwing plastics to nature for degradation and absorption is not a responsible approach to the environment," Wang Jun said.
The degradable plastics studied in the laboratory can enjoy appropriate temperature, humidity and rich microbial population to help the degradation process. However, to get out of the laboratory, the degradable plastics face a real "mountain pass" -- classification, collection and treatment -- all linked and indispensable.
At present, the garbage classification system in domestic cities can not realize the separate storage and transportation of degradable plastics. If all kinds of plastics enter the incineration plant by the same way, consumers will lose the significance of environmental protection by paying high prices for biodegradable plastics .
To put it another way, assuming that the waste separation system is strictly implemented, the back-end degradation facilities also need to meet the conditions of biodegradation (oxygen is generally required for efficient degradation, high temperature of 50 ℃ and humidity of 55%). At present, most domestic anaerobic digestion facilities for kitchen waste can not effectively meet the needs of biodegradation, such as lack of oxygen.
Composting is the most common way of degradation.
Finally, polyester can be transformed into carbon dioxide, water, mineral salts and biomass through biological process, and no substances toxic to the natural environment (water, soil) or animals and plants will be produced. According to EU standard, under the condition of industrial composting, compostable materials must be completely decomposed by naturally existing microorganisms and meet four conditions:(1)The ingredients must contain more than 50% organic matter, and the content of heavy metals does not exceed the standard;(2)Under composting conditions, 90% of the materials need to be completely degraded within six months;(3)Under composting conditions, it needs to be fragmented to a size indistinguishable to the naked eye within 12 weeks.(4)Compost products are harmless to plant growth and germination (and earthworms).
At present, it is difficult to verify the authenticity of so-called compostable products, and the global standard industrial composting system is far less than the output of such plastics. These "green" packages with degradable labels will still flow into the natural environment, such as water or soil.
Wang Jun believes that "degradable plastics are easier to fragment than ordinary plastics. If they enter the environment on a large scale, it is almost impossible to be collected again, and the problem of plastic pollution will be more serious."Even if the problem of back-end treatment is temporarily ignored, degradable plastics have limitations in the use stage.Wang Jun explained that the immaturity of degradable plastics compared with natural degradable materials is that the degradation of natural materials has a "switch" mechanism. "For example, leaves do not degrade when they are on trees, and degradation occurs only after they fall, that is, after the end of the life cycle." he believes that "biodegradable" plastics do not have this mechanism, and it faces a "use" and "Degradable plastics are not as durable as traditional non degradable plastics, nor as degradable as all natural materials," Wang Jun said.
Human beings have lived on the earth for millions of years, and the use and popularization of plastics has only been in the last century. Before plastics were used on a large scale, the relationship between people and product packaging was not the same as now. Food with short shelf life was often sold in bulk near the origin; people took their own oil pots and rice bags to grain and oil stores to buy rations; exquisite tea and biscuits with longer shelf life were packaged in exquisite metal boxes, most of which were left by the owner Other uses.
Plastic has undoubtedly brought us a lot of convenience, but not all plastic packaging is necessary.
"Plastic is the product of human beings. We should not rely on nature to solve the problems caused by human beings. Since we make plastic, we should try our best to make it form a closed loop and stay in the human system cycle, rather than enter the natural system cycle," Wang Jun said.
Policy is changing
With the increasing challenges of degradable plastics in practice, on September 8, 2021, the national development and Reform Commission and the Ministry of ecological environment issued the action plan for plastic pollution control in the 14th five year plan (the "plan") - improve the whole chain control system of plastic pollution, promote the reduction of plastic production and use at the source, and promote plastic substitutes scientifically and steadily .
In the "scheme", it is proposed to fully consider the resource and environmental impact of degradable plastic products in the whole life cycle, study the mechanism and impact of different types of degradable plastics, and scientifically evaluate their environmental safety and controllability.
Wang Wang, executive vice president of plastic recycling branch and recycled PET branch of China synthetic resin Association, analyzed intellectuals and said that this "scheme" It shows the policy level's attitude towards degradable plastics, that is, the full life cycle environmental impact, degradation mechanism and safety controllability of degradable plastics are not clear, and there are disorderly development and blind expansion of production capacity, which should be corrected.
For example, it is proposed in the "plan" that the degradable plastics industry should develop in an orderly and rational layout, its application fields need to be standardized, and the degradation conditions and disposal methods also need to be clear. The policy signal can be understood as: "it is not suitable for the large-scale promotion and application of degradable plastics at present", Wang Wang concluded.
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