Certain plastics are easier to recycle based on their internal structure. Researchers find a way to change that structure to increase what we can recycle.
By Adelaide Levenson
We use and dispose of plastics every day. From toothbrushes and water bottles to food wrappers and Band-Aids, plastics are all around us. Although there have been many efforts made to limit the consumption of single-use plastics, there is still a challenge with recycling the plastics that we do use. But thanks to a team of researchers in the United Kingdom, we’re one step closer to giving a new life to plastics that are typically challenging to recycle.
Types of plastics
The plastics we use in our day-to-day lives are made up of thermosets or thermoplastics. Both thermosets and thermoplastics consist of polymers, which are long chains of repeating molecules called monomers.
Thermoplastics, which make up about 80 percent of consumed plastics, are made up of long and short carbon chains. Their structure allows them to become hard or soft, depending on whether they undergo a cooling or heating cycle, respectively. A few examples of thermoplastics include plastic bags, shampoo bottles, and DVDs.
Thermosets are much more durable than thermoplastics and differ from thermoplastics in the way they react to heat. Thermoplastics become soft when heated, but thermosets do not soften or melt in the heat, even when they are exposed to high temperatures.
Thermosets are more resistant to heat due to the presence of crosslinkers in their structure, which makes them stronger and harder to break down. Crosslinkers form links that attach the long, primary polymer chains to one another, creating a 3D, net-like structure. An advantage of this structure is that thermosets are a more durable plastic, so these materials have longer lifetimes.
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Thermoplastics, by contrast, have a structure of long and short chains that are not interconnected, or crosslinked, in any way. This structure is what makes them less durable and easier to manipulate with heat.
How can you recycle thermosets and thermoplastics?
Due to the differences in structure of thermosets and thermoplastics, they cannot be recycled in the same way. Thermoplastics are much easier to recycle; they can be broken down by mechanical or chemical recycling or by incineration. Since thermoplastics become soft when heated, they can be reshaped while they are hot and hardened into a new item when cooled.
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Contrastingly, the structure of a thermoset is much more challenging to break down and therefore recycle. The crosslinkers in the structure of thermosets make them strong and resistant to forms of recycling. In order to more easily break down, materials must contain bonds that are easily broken, called labile bonds, which thermosets do not have.
If thermosets are so difficult to recycle, why do we continue to use them? It is because thermosets have interesting properties. As previously mentioned, they do not change shape when heated, making them a good option for materials used in extreme climates. Additionally, before the material is hardened into its final form, the material is very easy to work with. A popular example of a thermoset is the synthetic rubber used to make car tires. As you can imagine, the inability of the material to break down is an advantage for products like tires, which you do not want to decompose quickly during use.
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A step toward increased recycling
A team of UK researchers led by Dr. Maciej Kopeć has set out to tackle the challenge of recycling thermosets.
To make thermosets more recyclable, the research team knew they needed to add weak bonds into the net-like structure. They also wanted to prevent any changes to the materials’ durability and other properties. To put a weak bond into the structure, the researchers added a degradable bond to either the long, primary polymer strands or to the crosslinkers, both of which can be cleaved by an external force. By varying the location, it can be determined if the placement of the weak bond has an impact on the material’s ability to break down. Here’s what they found.
Cleavable crosslinkers: When adding the weak bond into the crosslinker, the researchers utilized a crosslinker that is commonly used when a material needs to be broken down and then reformed. When testing the degradation of their weakly crosslinked material, they saw full degradation of the material into a solution, meaning that the crosslinks were successfully cleaved. The researchers were able to reform the solution into its previous form, although they did not see a 100 percent return of the crosslinkers. This was an indication that the material could be recycled but it would not completely regain its original properties.
Cleavable strands: The other method the researchers studied was the use of a degradable molecule in the primary polymer strands. With this technique, when the degradable molecules are cleaved, the result is broken primary strands with crosslinkers remaining intact. The researchers were able to successfully degrade the material containing cleavable strands into a solution and subsequently reform the solution into the original material. With the degradable strands, this process can be repeated multiple times successfully. The researchers found that these degradable strand networks were easier to reform than the degradable crosslinker networks and were more similar to the original materials. Additionally, the durability of the thermoset materials was not impacted with the introduction of a weak bond. These materials show promise for making thermoset plastics more recyclable!
While the recycling of thermoset plastics poses a tough challenge, this study offers a new way to overcome these difficulties. Even while advancing the prospects of thermoset recycling, we each can do our part to contribute to a more sustainable future. Pack your groceries in reusable bags, bring a refillable coffee cup to your local café, opt for products with minimal packaging, educate and encourage others. No action is ever too small!
This study was published in the peer-reviewed journal Polymer Chemistry.
Could that next café trip be better for the environment? Read next about Brewing Cell-Derived Coffee in a Lab for More Sustainable Lattes
References
Dawson, F., Kazmi, T., Roth, P. J., & Kopeć, M. (2023). Strands vs. crosslinks: Topology-dependent degradation and regelation of polyacrylate networks synthesised by RAFT polymerisation. Polymer Chemistry, 14(47), 5166–5177. https://doi.org/10.1039/d3py01008b
Kazemi, M., Faisal Kabir, S., & Fini, E. H. (2021). State of the art in recycling waste thermoplastics and thermosets and their applications in construction. Resources, Conservation and Recycling, 174, 105776. https://doi.org/10.1016/j.resconrec.2021.105776
About the Author
Adelaide Levenson is a chemist and baker with a passion for science communication. She recently received her MSc in chemistry with a specific focus on polymers, and currently resides in Germany.
