Predetermined breaking points in polymer chains

08.03.2023

We encounter high-density polyethylene (HDPE) every day, for example as detergent packaging or films. HDPE is particularly stable and resistant. In terms of sustainability, however, it still has a long way to go, because its high stability is often accompanied by poor degradability.

Professor Stefan Mecking and his team at the University of Konstanz have succeeded in counteracting this property: they build chemical pre-determined breaking points into the polymer chains. This creates starting points for breaking up the structure during recycling without disturbing its stable structure. In an interview, Stefan Mecking explains with which polyester this works particularly well and when the newly researched plastic could be used.

Professor Mecking, what is the "problem" with the bio-based plastics that are currently on the market?

Stefan Mecking: Bio-based plastics certainly don't have a general problem. Establishing new production processes and an alternative raw material base to the existing petrochemical processes that have been optimised over decades is certainly a challenge. Our interest within this broad field is plastics that are crystalline and structurally similar to HDPE, but can be chemically recycled under mild conditions and are non-persistent in the environment.

What solution have you found to this problem and how does this make plastics more sustainable?

Mecking: Building on our work in recent years, we found that this property profile can be achieved with polyester-2,18 which is accessible from commercially available monomers.

Polyethylene was discovered in 1898 by the chemist Hans von Pechmann. But it was not until decades later (1940) that an economically viable manufacturing process could be established. In 1953, the German Karl Ziegler and the Italian Giulio Natta developed the Ziegler-Natta catalyst. Ten years later, they were awarded the Nobel Prize in Chemistry for it.

We were surprised how quickly this material is degraded by naturally occurring enzymes under laboratory conditions and mineralised under the conditions of industrial composting. For us, however, these are indicators of degradability, we are not primarily aiming at applications that end up in industrial compost.

Polyethylene has been used in large quantities since 1957, for example in piping systems for gas and water supply. In space travel, it is suitable for protection against particle radiation because of its high specific radiation absorption capacity. NASA has been using polyethylene for this purpose since the first shuttle missions.

When could "your" newly researched plastic be used?

Mecking: Under the current general conditions, polyolefins are difficult to beat in terms of price. But how the legal requirements regarding real recycling, beyond one-off 'downcycling', develop in the next few years will be interesting for everyone. Beyond that, certain applications in which the material has advantageous properties are already current, for example in the field of 3D printing.

What other steps will follow in your research?

Mecking: We are currently investigating – partly with partners – the suitability for special processing methods and applications. We are also quantifying the degradation in other environments, such as soils or seawater.

What potential do you think bioplastics have in the future?

Mecking: The importance of plastics made from non-fossil raw materials will certainly continue to increase.