Published 13 October 2022 by Ruth D. Rittinghaus
The Invisible Danger
For Lindau Alumna Ruth D. Rittinghaus, a successful year continues: After her participation in the 71st Lindau Nobel Laureate Meeting 2022 (Chemistry) in the summer, she has now been awarded the KlarText Award for exemplary science communication in the chemistry category – for the following text. Thanks to the Klaus Tschira Foundation, which has been awarding the prize for many years and is a substantial supporter of the Lindau Meetings, we are glad to publish the English version of the winning article on the Lindau Blog.
Microscopic plastic particles are almost ubiquitous in the environment. This is a problem, but tailor-made bioplastics may offer hope of a solution. The production of such bioplastics depends on environmentally friendly and highly specialised catalysts.
Anybody who is fond of hiking will know the problem: rubbish, in the form of little sweet wrappers and cigarette butts, biscuit wrappers and empty potato chip bags, and even electronic waste. This is not only aesthetically repelling, but the plastics are also generally not biodegradable and remain in the environment for long periods of time. The easiest solution, although sadly not practiced by all, is to dispose of plastic, so that it can be recycled, burned, or at least deposited appropriately.
Beyond the large items of rubbish, which at least theoretically can be collected and removed, the environment also contains so-called microplastics, which are not visible to the naked eye. These microplastics arise through mechanical fragmentation – for example, when a powerful gust of wind throws packaging against a tree. In addition, microplastic also finds its way into the environment through our everyday activities: washing clothes results in the detachment of tiny fibres, which are not all filtered out by sewage plants. The soles of our shoes wear out over time and leave their traces on footpaths. Another major source of microplastics are artificial grass surfaces, which must be regularly renewed because they lose their granules.
The plastic particles may be small and invisible, but they can still cause damage. It has not yet been conclusively established how dangerous microplastics are, but studies show that they accumulate in tissue and in the brain. While the particles have been under-researched, they are now found everywhere: winds blow them onto the highest mountains, and rivers and streams carry them into the deep sea.
It won’t be possible to recover the microplastics that have already found their way into the environment, but we can make sure that their levels don’t increase. All products which use results in the release of tiny plastic particles – for example, shoe soles or car tires – should be made of biodegradable materials. Most plastics are made of crude oil, and although the material itself is stable for several hundred years, it is only used for a few months. Then it becomes a problem.
Bioplastics, in contrast, are either made of biomass or degrade within a matter of months. In the ideal scenario, such bioplastics satisfy both criteria – such as polylactide, PLA for short. This bioplastic is already being produced at large scale and has replaced traditional plastics in a range of applications.
However, plastics can possess a wide range of different properties: from the flexible cling film that we use in the kitchen, up to the hard fittings in cars. The challenge of developing biodegradable plastics that encompass this range of properties is a considerable one.
If you want to understand what determines the properties of a plastic, it is worth taking a look at its inner structure. At the molecular level, plastics are composed of structures that are put together like pearl necklaces. The pearls are known as monomers, and the necklaces are called polymers. Numerous polymer chains together give rise to what we call a plastic. The different properties of the plastic are determined by the different chemical make-up of the pearls and the way in which they are linked together in a necklace.
For the production of a plastic – that is, for the threading of the pearls – a special molecule, often containing a metal, is needed. This molecule, known as a catalyst, activates a dedicated site on the pearl, a functionality that allows the threading of the pearls. This process of the stringing together of pearls by the catalyst is known as polymerisation.
The quality of a catalyst plays an important role in polymerisation, as the functionality of the pearls and the catalysts must match. For this reason, catalysts undergo a long optimisation process before they can be applied to produce plastics. In the case of the highly promising bioplastic PLA, a poisonous heavy metal catalyst is used as it shows the best polymerisation properties. This catalyst is approved for use because it is used in only small quantities. However, as it remains in the plastic, the heavy metal can accumulate in ecosystems. To make a long story short: instead of a non-degradable microplastic, it is now a poisonous heavy metal that ends up in the environment.
In my work, therefore, I wanted to develop a catalyst that works at least as fast as its heavy metal counterpart. Such a catalyst should be iron-based and therefore not harmful to the environment. The catalyst that our team successfully developed is in fact even faster than the heavy metal version and thus a promising candidate for use in production of PLA on a large scale.
Our catalyst is not only fast but also versatile. Catalysts on pearls are usually associated with a specific functionality. Not so with our new catalyst: it can link different functionalities and thus facilitates combinations, and therefore plastic properties, that would otherwise not be conceivable in a reaction.
It’s not only the proportion of individual pearls in the polymer chain that is important; the order in which they appear also determines the chain’s properties. The characteristics of the chain are profoundly affected by whether blue pearls appear first in the chain, followed by yellow, and finally red, or if the pearls appear in a mixed order all along the chain’s length. Catalysts often prefer a particular type of pearl, meaning that they first thread these before then moving on to less-preferred ones. For a colourful, mixed chain, a catalyst is needed that displays no preference for any particular type of pearl. That is very rare.
We could show, however, that there is another way. Our catalyst also prefers particular pearls and incorporates them in a defined order. However, if the temperature is raised, then the catalyst no longer threads pearls in an orderly way based on its preference. Rather, it simply breaks up the already synthesised chain, incorporates a pearl and seals the chain with the end of another chain. This process continues until it has produced colourful, mixed chains. The catalyst can, therefore, depending on temperature, synthesise chains that are sorted by colour or that are mixed, which makes plastic synthesis easier. The environmentally friendly iron catalyst is thus not only suitable for the production of PLA but can also be applied for the synthesis of other plastics of varying compositions and therefore characteristics. The availability of these biodegradable materials will allow traditional plastics to be replaced by everyday objects. In this way, the amount of potentially dangerous microplastics in the environment can be considerably reduced.