Stable plastics in particular often survive in the environment for centuries without being biologically degraded. Researchers have now developed a new variant of the versatile plastic polyester, which is almost as stable as the plastic material previously used, but is more degradable. This is made possible by the incorporation of special predetermined breaking points in the polymer chains, which offer starting points for microbial enzymes. In initial laboratory tests, enzymes decomposed the new polyester in a few days, and microbes need around two months to do this in a composting facility. This plastic could thus be a more environmentally friendly alternative - for example for plastic films made of polyester.
Plastics consist of long chains of one or a few basic chemical building blocks. Depending on their structure, these polymers can have a wide range of properties – from hard and resilient to rubbery soft. The plastic variants, whose chains form a crystal-like structure through hydrogen bridges and which are also water-repellent, are particularly durable and stable. An example of this is high-density polyethylene (HDPE), which is used to make plastic bottles for detergents, rigid films, plastic pipes and injection-moulded household goods and packaging. But the great stability also means that these plastics are hardly degradable - they remain in the environment for decades to centuries without being decomposed. "The crystallinity in combination with the water-repellent character usually slows down the biodegradability of the materials because it makes it difficult for microorganisms to access the predetermined breaking points," explains senior author Stefan Mecking from the University of Konstanz.
A new polyester variant
Mecking's team is therefore looking for plastics that are strong and durable in use, but are still easy to recycle and degrade. With plastic such as HDPE, it is often very energy-intensive and inefficient if you want to recover the basic building blocks during recycling and thus make them usable for new plastic. One strategy for circumventing this supposed incompatibility between the durability and degradability of plastics is to incorporate chemical “predetermined breaking points” into the polymer chains. The chemists have already been able to show that this significantly improves the recyclability of polyethylene-type plastics. However, this plastic was not yet biodegradable. Now they have also succeeded in taking this second important step: they have developed a new polyester that combines material properties that are in demand in industry and good environmental compatibility in one plastic.
The new plastic material, polyester-2,18, consists of two basic building blocks: the 18-carbon dicarboxylic acid 1,18-octadecanedicarboxylic acid and the two-carbon atom ethylene glycol. As the team explains, both of these building blocks can be easily obtained from sustainable sources of raw materials. For example, the starting material for the dicarboxylic acid is of vegetable origin. The chaining of these basic units creates a polymer that, like high-density polyethylene, has a crystalline structure and therefore has high mechanical stability and temperature resistance. In tests, the PE-2.18 showed a similarly high compressive and tensile strength as HDPE.
Dismantling in a few days to weeks
More importantly, despite this high crystallinity and strength, the polyester is surprisingly biodegradable, as demonstrated by laboratory testing using naturally occurring enzymes and testing at an industrial composting facility. "We were amazed at how quickly it degraded," says Mecking. In this way, the plastic could be broken down by the enzymes in a laboratory test within a few days. The microorganisms in the composting facility needed about two months. "Of course, the results from the composting plant cannot be transferred one-to-one to every conceivable environmental situation," admits the chemist. "Nevertheless, they demonstrate the biodegradability of the material and suggest that it is many times less persistent than plastics such as HDPE should it accidentally end up in the environment."
Both the recyclability of the new polyester and its biodegradability under various environmental conditions are now to be further investigated. Mecking sees possible applications for the new material in 3D printing or in the production of packaging films, for example. There are also other fields in which crystallinity in combination with recyclability and degradability from abrasion or similar material losses is desirable.
Source: Marcel Eck (University of Konstanz) et al., Angewandte Chemie International Edition, doi: 10.1002/anie.202213438