Researchers from the University of Konstanz are trying to create plastics that maintain their desirable qualities while also being more easily recycled, and are also researching other issues relating to the eco-friendliness of plastics.
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The team presents a novel polyester in their most recent article in the international edition of Angewandte Chemie that demonstrates material qualities that are both appealing to the industry and environmentally friendly.
Normally Hardly Compatible
Long chains of one or more chemical building blocks, or monomers, are used to make plastics. Plastics with a high degree of crystallinity and water resistance are widely used because they are mechanically highly resilient and stable.
High density polyethylene (HDPE), whose fundamental units are made up of non-polar hydrocarbon molecules, is a well-known example.
Even though some qualities may be helpful for applications, they can also have negative consequences: to recycle such plastics and recover the basic modules requires a lot of energy and is inefficient. Such plastics also take a very long time to degrade if they leak into the environment.
To overcome this incompatibility between the stability and biodegradability of plastics, Mecking and his team opted to insert chemical “breaking points” into their materials. They already showed that this greatly improves the recyclability of polyethylene-like plastics. However, good biodegradability is not automatically guaranteed.
Plastics often gain high resilience because they are ordered in densely packed crystalline structures. Crystallinity in combination with water repellency usually strongly decelerates the biodegradation process, as it impairs the microorganisms’ access to the breaking points.
Stefan Mecking, Professor, Chemical Materials Science, Department of Chemistry, University of Konstanz
However, this does not apply to the researchers’ new plastic.
Crystalline and Compostable
A short diol unit with two carbon atoms and a dicarboxylic acid with 18 carbon atoms make up the two fundamental modules of the new plastic, polyester-2,18. Both modules are easily attainable from eco-friendly sources.
For instance, the dicarboxylic acid that serves as the primary building block of plastic is derived from a renewable source. The polyester’s characteristics are similar to those of HDPE; for instance, its crystalline structure results in both mechanical stability and temperature resistance.
At the same time, first experiments for recyclability showed that under comparatively mild conditions, this material’s basic modules can be recovered.
The new plastic also has another, rather unexpected characteristic: despite its high crystallinity, tests at an industrial composting facility and lab experiments with natural enzymes have shown that it is biodegradable. In a lab experiment, enzymes quickly broke down the polyester.
This plastic even complies with ISO composting standards because it took the composting plant’s microorganisms about two months to break it down.
Mecking added, “We too were amazed by this rapid degradation. Of course, we cannot transfer the results of the composting plant one-to-one into any conceivable environmental condition. But they do confirm that this material is indeed biodegradable and indicate that it is much less persistent than plastics like HDPE, if it should unintentionally be released into the environment.”
More research is still needed to determine this polyester’s biodegradability in a variety of environmental settings. Some of the potential applications for this new substance include 3D printing or the creation of packaging foils.
In addition, there are further areas of interest, in which it is desirable to combine crystallinity with recyclability and the degradation of abraded particles or similar loss of material.
The ERC Advanced Grant provided funding for the DEEPCAT (Degradable Polyolefin Materials Enabled by Catalytic Methods) project.
Eck, M., et al. (2022) Biodegradable high density polyethylene-like material. Angewandte Chemie. doi:10.1002/ange.202213438.