Bio-Based Plastics: Which Become Compost and How?

Not all bio-based plastics are created the same way. Find out how differences in production affect each plastic's ability to decompose.

 

May 20, 2020

By Emily Kichler

Let's say you're looking at two types of PET polyester. One is made from plants, and the other is made from petroleum and coal. Which one breaks down faster?

The answer is: they take the same amount of time to decompose.

Plastics may take several generations to decompose, or may not decompose at all, depending on their final resting environment.(1So, bio-based plastics have been developed to help us get away from fossil fuels and to create plastics with life cycles that are more sustainable. Some bio-based plastics eliminate the need for fossil fuels, some shorten the material's life span, and some do both.

Below are different types of bioplastics that are made from either plant- or fossil-based materials and are either biodegradable or non-biodegradable.

- Kirk-Othmer Encyclopedia of Chemical Technology

So, what makes some bio-based plastics non-biodegradable?

Some plastics that are not single-use may need to maintain a high level of durability throughout their lifespan. If put in the position, companies may choose a bio-based plastic with high structural integrity over one that can be composted.

Toray Plastics, which manufactures plastics used inside airplanes, behind phone screens, and for a number of miscellaneous applications, is one company that works with bio-based plastic. The company is able to use sugarcane byproduct and break it down into the same molecules that create the structure for polyethylene terephthalate (PET or PETE). PET is the polymer most often used to create plastic bottles and polyester clothing.
 

So, the same exact molecules that are generally derived from petroleum and coal can also be derived from plants. The same exact molecules from bio-based materials can create the same exact compounds as those made from fossil fuels, which can ultimately create the same exact material. As a result, both materials will break down exactly the same in a landfill when made in this way.

The pervasiveness and the deep integration of plastics into today's most widely used tools make up, perhaps, the messiest part of the plastic industry. Choosing materials based on the desired life span of the end product is important. If it is not possible to coordinate the life span of the product and the materials used to make it, then designing for resourceful recycling and replacement of parts needs to be a top priority.

When it comes to plastics, which can only ever be down-cycled and not truly re-cycled, considerations for all possible material alternatives need to be taken.

What do we do about single-use plastics?

Plastic makes up one fifth of municipal solid waste being sent to landfill(2), and 40% of plastic produced is used just once and then thrown away(3). Landfills and oceans cannot reasonably and sustainably maintain the levels of trash that we create, and creating more sustainable, ecological habits and products to replace our current ones is absolutely vital to the health of the planet.

Biodegradable plastics are ultimately a small step in helping to lower our impact, especially when they're not the most helpful--and in some cases, are even harmful--to soil. Reusable items are best. When reusable is not an option, bioplastics are one option.

Compostable vs. Biodegradable
 

When understanding bioplastics, there is a difference between compostable and biodegradable. Something that is compostable returns nutrients to the soil through naturally occurring micro-organisms and within a specified time frame. Something that is biodegradable will break down without any specified time line and often needs the heat or environment of a compost pile in order to help it break down.

The largest certification program for compostable products in North America is Biodegradable Products Institute (BPI), which bases their lab tests on ASTM International standards.

The UK’s Association for Organics Recycling is another example of a certification program, which makes labeling clear and lets people know whether compostable products are suitable for at-home composting or industrial composting only.

What’s next?

Certifications and labeling are helpful in a society that is generally not accustomed to composting, and greater research is helpful for improving our understanding, use, and labeling of biodegradable plastics.

A recent study coming out of the Netherlands followed a range of compostable products through different industrial facilities, tracking how different materials decomposed when using different methods of sifting, shredding, turning, etc. Due to the size of the compost piles at industrial facilities, the compost materials heat up to an average of 58°C or 136.4°F.

Home composting solutions will not reach this level of heat, and so compostable plastics will not break down in the same way. Wider spread knowledge of compostable materials in the home setting are helpful, and perhaps a push toward the UK's standards of labeling would be beneficial.

 

Some industrial facilities will not take compostable plastics, because they take longer to compost than the facility’s set production time. Through studies like the one mentioned, industrial composting facilities are better able to understand what kinds of plastics they can handle and how.

Bioplastics do not only play a role in disposable culture but in medical fields and directly in agriculture. Currently, farms use polyethylene mulch to help balance the moisture and aeration of the soil with relative ease. As polyethylene will leave toxic residue in the soil, biodegradable mulches are a preferable alternative to this kind of agriculture practice. Longer-term testing is needed, however, in order to know the true effects of plastics in soil.

If plastics are to be kept a part of society, thorough knowledge of the materials is needed, and any accompanying systems for processing such materials are also of great use.

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Gratitude to the water and soil used to grow our food.

Gratitude to those who cultivate the earth to share in its abundance, and

gratitude to anyone inspired to contribute to this cultivation and sharing.

© 2020

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