Coatings are widely used as a protective layer on many of our everyday products. Protective, but also passive. However, that doesn’t have to stay this way. In theory, we could make coatings a lot ‘smarter’. By tuning the material’s structure, it should be possible to develop coatings that change colour upon a change in temperature. Or coatings that simply heal themselves when they get scratched. ‘You can, for instance, think of coatings that respond to light or chemicals’, says Ben Feringa, professor of Organic Synthetic Chemistry at the University of Groningen. ‘Together with the chemical industry, we are exploring ways to make this happen.’

Alternative to acrylate

The Groningen-approach builds on previous research efforts that resulted in the development of a sustainable molecule from woody waste streams. ‘At present, acrylate is one of the main ingredients in coatings, especially in paint, but it is still made from oil. We developed an acrylate replacement by producing furfural from wood waste and reacting that further to make butenolides’, Feringa explains. ‘This alternative to acrylate works really well, and we filed several patents on the process together with AkzoNobel. But now we also want to provide these molecules with more functionality.’

‘It would be great if we could coat a car in a dirt-repelling and self-healing coating’

Ben Feringa

To do so, the team first reviewed the manufacturing process. ‘We used volatile organic compounds while making these molecules, but we are now working to apply a more sustainable water-based procedure’, says PhD student Andries Jensma. He is currently considering emulsion polymerisation, but his molecules turned out to be unstable in water. ‘We can adjust the long carbon chain tail of the butenolides to make them more or less hydrophobic, but if you make them too hydrophilic, they will hydrolyse. It is a delicate balance, especially when we want to add functionality.’

Adding the desired functionalities is where PhD student Jelmer Meijer comes in. His research focuses on adjusting the monomers and adding structures to see what will work. ‘I made a whole range of monomers with molecular switches and other functional groups, just to see what was possible’, Meijer explains. ‘Some of the polymers I produced from these monomers can, for instance, crosslink under the influence of light.’ The main challenge is the polymerisation of the adapted monomers. Meijer: ‘The coating needs to be stable. It shouldn’t immediately switch while polymerising or while applying it to your surface. It should only respond and adapt at the right moment, when you want it to. That is quite hard to design.’

Industrial expertise

Meijer just started his project, so he hasn’t found the definitive answer to this conundrum yet. Lucky for him, he is not alone. Within ARC CBBC, there are researchers who are trying to tackle similar problems. In the group of Bert Weckhuysen at Utrecht University, for example, where they are working on coatings that can crosslink under the influence of blue light. This could be useful for the automotive industry, where heating is now required for curing paint. Using light instead could save a lot of energy. The Weckhuysen lab also collaborates with the team of Guido Mul at the University of Twente to study coatings that can break down harmful molecules under the influence of light, and as such, filter the air. And at Eindhoven University of Technology, Caterina Esteves and her group aim to incorporate liquid crystals in coatings to create colour without using pigments that degrade over time.

‘I created a whole range of monomers with molecular switches and other functional groups, just to see what is possible’

Jelmer Meijer

Even though all their projects and approaches are different, the researchers can learn a lot from each other. ‘We come together once in a while with the ARC CBBC smart coatings subgroup, and exchange ideas’, Meijer says. ‘This is very useful.’ Besides discussing science with their academic colleagues, the PhD students also have the opportunity to interact with the industrial partners on a regular basis. ‘I have a monthly call with experts from AkzoNobel’, Jensma says. ‘And I was also invited to spend a week in one of their factories, so I can learn from their techniques. That will help me a lot.’

Sci-fi car coating

Feringa also cherishes the strong link with industry. ‘We are mainly focused on fundamental research, but our industrial counterparts also ensure that we keep our eyes open to applications.’ For the smart coatings project however, they don’t have specific applications in mind. Meijer: ‘We don’t want to limit ourselves, but first explore what we can achieve and then consider potential applications where our materials could be useful’, Meijer says. Feringa agrees, but of course he already has some ideas in mind. ‘Wouldn’t it be great if we could coat a car with a dirt-repelling and self-healing material. This may sound like science fiction, but I think it is a real possibility in the future.’

‘We can adjust the long carbon chain tail of the butenolides to make them more or less hydrophobic’

Andries Jensma

How long we have to wait for such a coating is still a big question. ‘The step from lab to industry is quite large’, Feringa says. ‘We can make a smart coating work in the lab, but before you can actually spray it on a car, it needs to be really robust. To take that step, we really need industry to help us, and luckily they are more than willing to do so. Together, we can make it work in the end.’