Sunday, June 26, 2022

If plastic comes from oil and gas, which originally came from plants, why isn’t it biodegradable?

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If plastic comes from oil and gas, which originally came from plants, why isn’t it biodegradable? – Nirupama, age 11, Delhi, India


To better understand why plastics don’t biodegrade, let’s start with how plastics are made and how biodegradation works.

Oil, also known as petroleum, is a fossil fuel. This means that it is composed of the remains of very old living organisms, such as algae, bacteria and plants. These creatures remained buried deep underground for millions of years. There, heat and pressure turned them into fossil fuels.

Petroleum contains a lot of chemical called propylene. To make plastics, refiners heat propylene with a catalyst—a substance that speeds up chemical reactions. This causes the individual molecules of propylene to clump together like beads on a string.

The chain is called a polymer – a large molecule made up of many smaller molecules held together. Its name, polypropylene, literally means “many propylene.” And the bonds between these molecules are super strong.

Pictures of Polypropylene Molecules and Polymers
This image shows the chemical structure of a molecule of polypropylene (top left), a model of the molecule (upper right) and a chain of polypropylene molecules linked together to form a polymer.
Bacsica.iStock/Getty Images Plus

When something that is biodegradable, such as a carton, breaks down, microorganisms in nature break down and digest the polymers in it. They do this by using enzymes – proteins that help accelerate 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 the thing they are breaking down are exposed to air, the polymers will fully biodegrade. Eventually, all that’s left will be carbon dioxide, water, and other organic material.

Oxygen is essential because it helps the microorganisms that degrade the material and live longer. Biodegradation is usually fastest in warm, wet environments where there are ample microorganisms – for example, moist leaves on the ground in a warm tropical forest.

But polymers like polypropylene are not abundant in nature. The enzymes in microorganisms that break down biodegradable materials do not recognize the bonds that hold polymers together.

Eventually, the polymers in plastic waste can break down, perhaps after hundreds of thousands of years. But when it takes that long, the damage to the environment has already been done. Plastic waste can release harmful chemicals into soil and water, or break down into smaller pieces to be eaten by animals, fish and birds.

Little colored plastic pieces on the beach.
Outside, plastic waste breaks down into smaller and smaller pieces, but it doesn’t fully biodegrade for thousands of years.
Alfonso Di Vincenzo / CONTROLAB / LightRocket via Getty Images

In my lab, we’re developing what we hope will be the plastics of the future – materials that act like regular plastics, but don’t spoil the environment as people do when they’re done with them. can.

We work with bioplastics – materials that are made by tiny living bacteria. Bacteria make these substances for uses such as storing energy or protecting them from their surroundings. 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, an abundant resource that comes from rubber plants, and with the oil removed from the waste grounds that remains from coffee making. Rubber is what makes our bioplastics flexible, and we chemically modify coffee ground oil to help the material flow in the industrial machines we use to shape it.

Making bioplastics is not cheap, as the various materials used to make these materials are not enough, and it costs a lot of money to set up the equipment to make them. But when enough people want them, the price will drop. I hope that one day these new biodegradable materials will replace plastics made from fossil fuels.


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