Will biodegradable plastic degrade in the air or in use? Can biodegradable plastics only be composted? Knowledge Science(Part III)

Are biodegradable materials completely degraded in different environments? Is it harmful to the environment?Knowledge Science(Part II)
2020-07-20
What are the application fields of biodegradable plastics?Knowledge Science(Part Ⅳ)
2020-08-18

Will biodegradable plastic degrade in the air or in use? Can biodegradable plastics only be composted? Knowledge Science(Part III)

Q: Can polymers with benzene rings in the main chain be degraded by microorganisms?

A: Because the main chain of polymers containing benzene rings is relatively stable and high, many people think that biodegradable plastics containing benzene rings cannot be degraded by microorganisms. However, because of the special chemical structure of biodegradable aromatic polymers, natural There are also microorganisms that can degrade benzene rings, so they can also be biodegraded, and the toxicity of the products after degradation will not harm the environment. The figure below shows the changes in organic carbon and microorganisms (hyphae) during the degradation of PBAT:

Fig.2 Mineralization and surface colonization of films of three PBAT variantsduring a 6-week soil incubation

Fig.4 NanoSIMS images acquired after extended sputter erosion of the PBAT film

(Image Source:Biodegradation of synthetic polymers in soils: Tracking carbon into CO2and microbial biomass,Zumsteinet al., SCIENCE ADVANCES,4: eaas9024,2018,July)

The organic carbon in each structural unit of the PBAT chemical structure is decomposed by microorganisms in the soil, including filamentous fungi, etc., to form new substances. Studies have shown that the organic carbon has no effect on plants after degradation.

Q:What are the shortcomings of the public’s understanding of degradable plastics?

A:1. Think that biodegradable plastics will degrade in air or in use

Some consumers will mistakenly believe that biodegradable plastics will be degraded during use or in the air, because the degradation of biodegradable plastics requires temperature, humidity and microorganisms to degrade under suitable conditions. Therefore, it is used or stored daily. It will not be biodegraded during the period.

2. Biodegradable will degrade in any environment

Biodegradable plastics have different degradation behaviors under different conditions due to different varieties and chemical structures. In addition, degradation requires certain external environmental conditions as described above. Therefore, biodegradable plastics cannot be degraded in any environment. Look, most biodegradable plastics will be degraded in soil, seawater, compost and other environments under proper temperature and humidity conditions and will be decomposed by microorganisms.

Therefore, it is recommended that biodegradable plastics are like traditional plastics. After being used and discarded, the ones that can be recycled for physical reuse should be recycled and reused. For those that are difficult to recycle or difficult to recycle, biological recycling or chemical recycling is recommended.

3. Biodegradable plastic can only be composted

The degradation conditions and environments of biodegradable plastics can be various. In compost, soil, river water, sea water and other environments, it can be degraded under suitable conditions of temperature, humidity and microorganisms, so it can not only be degraded under composting conditions, It can also be biodegradable under soil and other conditions. But because composting is a controlled condition, relatively speaking, the biodegradation process is easier to control.

Q: Can biodegradable plastics be recycled? How to recycle?

A: Biodegradable plastic is actually a special kind of plastic. Its recycling and reuse is not much different from traditional plastics. It can be physically recycled and reused, that is, melting regeneration and reprocessing.
Just because it has the characteristics of biodegradation, it has more ways to biorecycle (such as composting) than traditional plastics, and it can no longer be recycled in the application of mulch.In addition, the chemical structure of biodegradable plastics is mainly based on ester bonds, which are easily degraded by alkalis, acids or alcohols, so that they can be chemically recycled compared to transmission plastics. The monomer recovery method is used for material recycling use.

Q: Will biodegradable plastics affect the recycling of ordinary plastics?

A: There are currently more than 160 types of traditional plastics commonly used. Biodegradable plastic is one of them, and the amount is currently small. After it enters the recycling system, even if it is not subjected to composting biological recycling or chemical recycling, it will not be biodegraded. The mixing of plastic affects the recycling of traditional plastics. The complexity of the traditional plastic hybrid system will not be very different because of the additional type of degradable plastic. In some recycling systems, such as the PET bottle recycling system, the additional PLA material will increase the difficulty. It is possible, but PET bottles In the recycling system, the use of new non-degradable polyester bottles such as traditional plastic PBT and PEN will also cause difficulties. From the modern sorting system, through the infrared sorting method, it is completely possible to realize separate recycling. So this question is just a subjective view of some people without considering the improvement of the original recycling system technology.

Q: What are the roles of biodegradable plastics in garbage classification and recycling? In order to fully demonstrate the significance of biodegradable plastic degradation, what else can the waste sorting and disposal system do?

A: From the perspective of its design and use significance, biodegradable plastics are used when disposable products and their use are mixed with organic waste for biochemical disposal, or when they are difficult to recycle after use, such as plastic film products. The biodegradation function can be more fully manifested.
However, because the global waste classification and waste disposal are not reasonable, even in developed countries such as Europe and the United States where waste classification and disposal are very standardized, some plastic packaging will always be unintentionally or intentionally released into the natural environment. This part of the products can be replaced with biodegradable plastics, which can also reduce the risk of environmental pollution. Therefore, the use of biodegradable plastics can also be considered as a preventive measure to prevent plastic waste from being unintentionally released outside the closed waste system to cause environmental pollution.
In addition, with the improvement of China’s garbage classification and disposal system, the formulation of biodegradable plastic garbage bags has been adjusted to make them more suitable for the biochemical treatment of urban organic domestic garbage, which can solve the problem of daily kitchen waste in residential communities. When I leave the trash can, I need to take the initiative to break the bag and bring personal hygiene problems.