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If plastic comes from oil and gas that originally came from plants, why isn’t it biodegradable? – Nirupama, 11 years old, Delhi, India
To better understand why plastics do not biodegrade, let’s start with how plastics are made and how biodegradation works.
Oil, also known as oil, is a fossil fuel. This means that it is made from the remains of many old living organisms, such as algae, bacteria and plants. These organisms have been buried deep underground for millions of years. There, heat and pressure turned them into fossil fuels.
Oil contains a lot of chemicals called propylene. To make plastic, refiners heat propylene together with a catalyst, a substance that accelerates chemical reactions. This causes the individual propylene molecules to stick together like string beads.
The chain is called a polymer – a large molecule made up of very small molecules strung together. Its name, polypropylene, literally means “very propylene”. And the bonds between these molecules are super strong.
When something that is biodegradable, such as a cardboard box, breaks down, the microorganisms present in nature break down and absorb the polymers in it. They do this with the help of enzymes – proteins that help speed up the breakdown of compounds such as lignin, a natural polymer found in plant tissues.
If oxygen is present, which usually means that the microbes and what they break down are exposed to air, the polymers will break down completely. In the end, all that will be left will be carbon dioxide, water and other biological material.
Oxygen is essential because it helps the microorganisms that break down the material to live longer. Biodegradation is usually fastest in a hot, humid environment where there are enough microorganisms – such as moist leaves on the ground in a warm rainforest.
But polymers like polypropylene are not abundant in nature. Enzymes in microorganisms that break down biodegradable materials do not recognize the bonds that hold polymers together.
The story continues
Eventually, polymers in plastic waste can disintegrate, perhaps in hundreds of thousands of years. But when it takes so long, the damage is already done to the environment. Plastic waste can release harmful chemicals into the soil and water or break into small pieces that animals, fish and birds eat.
In my lab, we are developing what we hope will be the plastic of the future – materials that work like ordinary plastic but do not spoil the environment because they can degrade when people run out of them.
We work with bioplastics – materials that are made of small live bacteria. Bacteria produce these substances for uses such as energy storage or protection from the environment. They can do this over and over again, so we have a lot of bioplastics to work with.
We mix these polymers with natural rubber, a rich resource that comes from rubber plants, and with oil removed from the waste left over from making coffee. Rubber makes our bioplastics flexible, and we chemically modify ground coffee oil to help the material flow into the industrial machines we use to shape it.
The production of bioplastics is not cheap, as there are currently not enough different ingredients that go into the production of these materials, and it costs a lot of money to set up the equipment for their production. But when enough people want them, the price will go down. I hope that one day these new biodegradable materials will replace plastics made from fossil fuels.
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This article was republished by The Conversation, a non-profit news site dedicated to sharing ideas from academic experts. Written by: Yael Vodovotz, The Ohio State University.
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Yael Vodovotz receives funding from PepsiCo, Coca Cola, Kellogg’s, the Center for Innovative Food Technology and the Center for Advanced Processing and Packaging.
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