This article is from WeChat official account:Science University (ID: kexuedayuan), author: Hsieh Yu-Man, planning: Li, Lv Xuefeng, author: Qingdao Institute of Bioenergy and manufacturing Engineering Center, Chinese Academy of microbial processes, from the title figure: vision China
Are you cooking or ordering takeaway today? Are you still stuck in the entanglement of neither washing dishes nor using disposable tableware?
What if I want to use disposable tableware but are afraid of polluting the environment? do not be afraid! Learn about biodegradable plastic PHA.
Just imagine, if the materials of disposable tableware are replaced with biodegradable plastics, and they are thrown away after eating, there is no need to wash the dishes or worry about polluting the environment, is it too happy? Up!
the “injury” of plastic
Since the advent of plastic, people have been unanimously sought after for its convenience and durability, and they have become increasingly dependent on it. However, plastics are durable because they are not biodegradable, which has led to the accumulation of large amounts of plastic waste on land and oceans. According to reports, plastic garbage accounts for about 80% of the total marine garbage. There are more than 600 types of marine organisms with marine plastic garbage ingestion records. The natural capital cost of ocean pollution for one year is conservatively estimated to be about US$3,300 per ton of ocean plastic. $33,000.
As of 2017, global plastic production has reached approximately 348 million tons. In addition, plasticA large amount of greenhouse gases are produced during the production and combustion process, which makes the environment on the earth worse. The plastic problem has become one of the global crises threatening the ecosystem and human health.
What is biodegradable plastic?
What exactly is biodegradable plastic? As the name implies, biodegradable plastics refer to materials that have plastic properties and can be degraded by microorganisms in the process of biochemical action or in the natural environment, including some chemically synthesized polymers, starch-based biodegradable plastics and microbial synthesis Polyester compounds, etc.
Representatives of chemically synthesized polymers are polyethylene glycol acid, polylactic acid, poly-caprolactone, polyvinyl alcohol, polyethylene oxide and so on. This type of material currently has certain applications, but it still cannot compete with all the properties of plastics.
Starch-based biodegradable plastics are mainly used to add starch as a filler and crosslinking agent to conventional plastics to produce a mixture of starch and plastics(for example, starch polyethylene), reusing soil microorganisms can easily degrade the properties of starch to decompose polymers, which will significantly reduce the degradation time of plastics. However, the fragments left by this kind of plastic after the starch is removed are not easily degraded and will stay in the environment for a long time.
The polyester compounds synthesized and accumulated by microorganisms, mainly polyhydroxyalkanoate(polyhydroxyalkanoate, PHA) , with similar properties to various synthetic thermoplastics (such as polypropylene). Many microorganisms in nature contain PHA hydrolase, which can hydrolyze PHA into water-soluble oligomers and monomers, and then use these products as nutrients in cells, which is very environmentally friendly and efficient. Therefore, PHA can be said to be the most promising kind of biodegradable plastics.
How was PHA discovered step by step? At the beginning of the 20th century, scientists first observed a lipid substance in bacteria, and subsequently identified similar ones in Bacillus megaterium(Bacillus megaterium) Lipid substance poly(3-hydroxybutyrate) [PHB], which is the first member of PHA. In the decades that followed, scientists continued to discover PHA with different compounds such as 3-hydroxyvaleric acid (3HV) and 3-hydroxyhexanoic acid (3HHx) from microorganisms.
So far, more than 160 PHA monomers have been identified. These natural PHAs can be divided into two main types, short-chain length (SCL)-PHA and medium-chain long (MCL)-PHA. SCL-PHA is composed of monomers containing 3-5 carbon atoms. Typical representatives are PHB and PHBV [poly(3HB-co-3HV)]. MCL-PHA is usually found in Pseudomonas, and its monomer is composed of 6-14 carbon atoms.
The structural changes of PHA are almost limitless. Different monomers can form different copolymers, homopolymers, random copolymers and block copolymers. Kind of structure. The more diverse the structure, the more plastic properties that can be achieved.
The general structure of PHAs (Picture source: Sang Yup Lee, 1995)