Researchers at Aalto University in Finland have developed new types of coatings made of lignin from wood, amongst them colour-alterning films and anti-fogging coatings.

Lignin is one of the main components of plants, a polymer found in the cell walls that acts as a strong binder. Trees consist of 20 to 30 per cent lignin. Lignin is known for being difficult to use and apply due to, for example, its structural heterogeneity and strong brown colour. It is also a waste material of the paper industry.

The Aalto chemical researchers found a way to make this substance transparent so it can be used in optical applications, such as colour-altering films and anti-fogging coatings.

Traditional anti-fogging sprays that are used to prevent condensation on eyeglasses and car windows contain several highly toxic ingredients that have a variety of health effects to humans as well as wildlife. They are also persistent synthetic chemicals as they hardly degrade in the natural environment.

It was already known that lignin nanoparticles could be an option for anti-fogging coatings. However, because this substance is so notoriously difficult to work with, this wasn’t viable, until now. The researchers used acetylated lignin and demonstrated an improved method to acetylate lignin in minutes at only 60 °C with a high yield. Because they managed to push the particle size to a minimum, they were able to make optically transparent films.

The research also showed a method of using the dispersions as photonic coatings by precisely controlling the coating thickness and thus absorbance of light at different wavelengths and consequently the colour. The nanoparticles could therefore be used to prepare multi-layered films with bright structural colours. This technology could be used in multiple valuable applications, such as anti-reflection coatings, sensoring materials, and perhaps even on vehicles.

Photos: Alexander Henn / Aalto University