To recycle or not to recycle, that is the question (for plant-based plastics)

// Some background: In search of environmental responsibility

The science research story covered here at Science Daily Dose last week was about a new strain of bacteria discovered that can possibly digest components of a popular type of plastic, PET. Part of the environmental impact of PET comes from the fact that generally less than half of all PET bottles are recycled, and they take 450 years to degrade in a landfill. Another part of the impact comes from the fact that PET requires petroleum-based products, so there is a great energy cost associated with PET bottles.

Because of this, researchers have thought to design plastics that could use other materials. Either by developing plastics that are able to biodegrade, or plastics that do not require petroleum, these could be methods of reducing the environmental impact of plastics.

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Corn-based plastic cups, from ECO-PRODUCTS. Image via Amazon.

Maybe you’ve seen biodegradable cups and silverware made from corn products. Or maybe you heard (several years ago) about the Sun Chips experiment with compostable plastics for their chip bags. (They decided to pull and re-design the product, though, because customers claimed that the bags were too loud.)

So how does it work? Certain compounds (like lactic acid, in the case of a plastic called PLA, or poly-lactic acid) and use them as the basis for the plastic resin. These can be formed into various products, including cups and bottles.

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You can even make your own bioplastic at home!

// The experiment, or: Are bioplastics making a difference?

One of the key motivations for using bioplastics is an environmental benefit. Switching from petroleum-based materials could reduce the reliance on oil for the production of plastic (to the tune of 200,000 barrels of oil per day), which could have a major impact. Also, these plastics could (in principle) break down in the environment, reducing the burden on landfills. If the idea for using plant-based plastics is to reduce the environmental burden, is it working? This is what researchers sought to investigate.

Corn-based plastics

Consider using corn as the plant source for bioplastics, as is common today. This has a big impact on a few things, like food production, since crops specifically for plastics are needed. Corn-based plastic uses 65% less energy to produce than petroleum-based plastics, and reportedly produce fewer greenhouse gases.

Corn-based poly-lactic acid plastics (PLA) can be composted, and they will biodegrade to form carbon dioxide and water.

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Some facts about bioplastics, from SelfEco.

 

// The results: Are bioplastics all we thought they were?

Uncertainty in the Life Cycle Greenhouse Gas Emissions from U.S. Production of Three Biobased Polymer Families

Research article published in the journal, “Environmental Science & Technology”

In the article, the researchers looked not only at the production process, but also at the end-of-life analysis. In other words, what happens to a product after a consumer has used it. If it gets recycled, what impact does that have? If it gets thrown away and gets into a landfill, what is the corresponding environmental impact? This more complete analysis allows us to look not only at the environmental impact of making the plant-based plastics, but also about the impact after we, as consumers, have used these products.

The researchers looked at two plants that can be used for making plastics: corn and switchgrass.

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Growing switchgrass at Michigan State University. Image via Dennis Pennington.

The researchers found two pretty important things.

First, the choice of crop makes a big difference in the amount of carbon dioxide emissions during plastic production. In fact, using switchgrass saved between 2.9-3.4 kg in emissions per kg of plastic produced compared to petroleum-based methods, while corn-based plastics saved only 0.6-1.4 kg of carbon dioxide emissions.

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Maybe in bioplastics, corn is no longer king… Image via Peabody Awards.

The second big finding was a really interesting result of looking at end-of-life greenhouse emissions. The researchers found that a type of plastic called bioethylene made from switchgrass had the lowest greenhouse gas emissions of all the plastics. But this wasn’t the big finding. The big result was that switchgrass-based PLA was runner-up for lowest greenhouse gas emissions — but only if the plastic was buried in a landfill so that the carbon dioxide from degradation didn’t get released into the atmosphere.

// The context: Does that mean I should stop recycling?

In a word… no!

It is true that burying plastic bottles will prevent them from releasing carbon dioxide into the atmosphere, but it doesn’t mean that we should stop recycling. In fact, plant-based plastics still make up a small fraction of plastics in consumer products, so it is even more important to make sure that you are recycling!

But wait. If petroleum-based plastics don’t decompose, (so they shouldn’t release a lot of carbon dioxide into the environment in a fill), doesn’t it mean that plant-based plastics are not important? Think about this: Plant-based plastics still use considerably less energy to manufacture, and decrease (or possibly even eliminate) the need to use petroleum in their production. While bioplastics do have the ability to decompose under certain conditions in a way that petroleum-based plastics cannot (at least not yet, anyways), this is a benefit because of the burden that plastics pose on landfills. Plus, no doubt scientists are working on ever-better methods to make bioplastics and consider this end-of-life impact on the environment.

// tl;dr

Plastic bottles are bad for the environment. What you might not have thought of is that they can continue to contribute to greenhouse gas emissions even after they are thrown away (in the case of plant-based plastics). Does that mean we should abandon using bioplastics? Or is the benefit derived from using renewable resources (instead of oil) greater than the drawback of end-of-life (after a consumer has used a product) greenhouse gas emissions?

 

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