Self-healing plastic from Konstanz
Plastics are a ubiquitous part of our everyday lives. There is almost no manufactured product on the market that does not contain plastic, due to the fact that it is comparatively cheap to produce and easy to process. But there are also disadvantages: A high percentage of plastics produced globally are simply thrown away, and, because they are so durable, these plastics are becoming a growing ecological problem worldwide. What sets the mineral plastic created by Helmut Cölfen's team at the University of Konstanz apart is that it is highly recyclable. The Cölfen research team and its partners are currently collaborating on projects that aim to either speed up the decomposition of plastics in nature or to develop plastics that are manufactured entirely from renewable resources.
Traditional hard plastics fall into one of two categories: thermoplastics and duroplastics. Thermoplastics can be melted and then re-shaped. In principle, this can take place repeatedly, as, for example, in the case of plastic bottles made of polyethylene terephthalate (PET). The process comes, however, at a high energy cost since so much heat is needed. Duroplastics like resins, however, cannot be re-shaped after they have finished hardening. They are thus hardly ever recyclable.
The novel mineral plastic, on the other hand, is completely different from the two traditional types of plastics in how it is produced and processed. "The production of our plastic is different from everything that has come before. We make it at room temperature and use water as a solvent. If you wanted to, you could stir some together at home in a glass of water," says Cölfen.
Shapeable like chewing gum, self-healing and easy to recycle
In the production process three basic ingredients are mixed together – polyacrylic acid, calcium chloride and soda (sodium carbonate) – to form a hydrogel with the colour and consistency of soft chewing gum. The plastic hydrogel is white and can be pulled or pushed into any shape. Once it has dried out and hardened, the material becomes as transparent as glass, many times harder than traditional acrylic glass and yet remains extremely flexible.
And while these characteristics are interesting enough on their own, it is also possible to transform the mineral plastic back into a hydrogel using water. Without adding any heat at all, the plastic can be re-shaped and hardened any number of times – it is truly recyclable. The plastic can also be completely dissolved using acids like citric acid, and then, by raising the pH value with soda, a new hydrogel can be produced. The hydrogel is also self-healing:
Cuts in the hydrogel repair themselves within a few seconds
If it is cut, the damaged spot repairs itself within a few seconds. "Imagine having your phone's protective film scratched. If it was made of mineral plastic, you could just drip a little water on the scratch. Then the plastic in that spot would turn into self-healing hydrogel, and the scratch would disappear on its own. Once it dries, you would have an undamaged protective film again," Cölfen describes a possible application for the mineral plastic.
Mineral plastic foam as fire-resistant insulation
Another decisive difference is that, unlike the thermoplastics, the mineral plastic does not melt when exposed to heat. On the contrary: It only gets harder. It takes temperatures of 400-450°C before this plastic breaks down, which is what also happens to every other type of plastic. This heat-resistant quality in addition to its poor flammability make the mineral plastic an interesting candidate for use in insulation. This is due to the high proportion of minerals it contains. The crux, however, is being able to create the necessary plastic foam form, which was a technical challenge for Cölfen and his team. "A hydrogel is easy to shape, but not to make into a foam. With the help of our colleagues at the University of Stuttgart, however, we were able to solve this problem," Cölfen states.
The full synthesis process for mineral plastic foam was recently described by Professor Cosima Stubenrauch and her doctoral researcher Philipp Menold from the University of Stuttgart's Institute of Physical Chemistry as well as by Helmut Cölfen in the journal Materials Horizons. "The secret of producing the foam lies in foaming the plastic not in its hydrogel form, but in a liquid precursor. Only afterwards is the resulting foam transferred to the hydrogel form and finally cured," Cölfen explains.
The mineral plastic is much less flammable than other plastics
High interest from the industry
Cölfen and his collaboration partners have since applied for a patent for their mineral plastic foam and are looking for ways to produce the insulation at an industrial scale. The new material has been met with great interest, especially in the construction and automotive industries, where it can be used to insulate buildings and automobiles. But there have also been requests for the second end product, the solid form of the plastic.
Inspiration from shellfish
The impetus for developing the mineral plastic, says Cölfen, came from one of his postdoctoral researchers, Dr Shengton Sun. Sun had taken a close look at the structure of crab and shrimp shells and taken inspiration from them. "Just like our mineral plastic, shrimps store tiny particles of calcium carbonate (or "lime") in their chitin shells. This makes their shells extremely hard, even though they are so thin," Cölfen explains Sun's rationale. Since shrimp shells cannot grow along with the animals, they have to molt repeatedly throughout their lives. This is where the mineral plastic's recyclability has an advantage over the model taken from nature.
But there is also a clear disadvantage: Unlike the shellfish, whose shells can quickly and effectively be decomposed by bacteria, the current form of the mineral plastic, just as other plastics, poses an ecological risk to the environment. "Even though our mineral plastic has exceptional characteristics and can be recycled, it still contains polyacrylic acid, which is not biodegradable," Cölfen states.
The search for sustainable alternatives
In their current projects, Cölfen and his team are thus working hard to find possible alternatives. For example, they are testing how to produce a mineral plastic using a biodegradable polyacid instead of polyacrylic acid. Some of the potential polyacids are available in industrial amounts, are manufactured partially from renewable resources and can be completely decomposed by bacteria. "If we were able to replace the polyacrylic acid in our mineral plastic with a natural polymer, we would have an entirely renewable and biodegradable material that still has all the great characteristics of our current mineral plastic," Cölfen concludes.
Title image: Colourized electron microscope image of the mineral plastic foam © Helmut Cölfen