Researchers have succeeded in developing new types of coatings from lignin in trees. The invention has many potential uses, for example in anti-fogging coatings, where it can replace highly toxic chemicals.
Chemical engineers at Aalto University have developed a way to make lignin, one of the most abundant wood components, transparent. This ground-braking invention allows lignin to be used in optical applications, such as colour-altering films and antifogging coatings. The discovery is based on a newly developed method where thin lignin nanoparticle films remain transparent when deposited on glass.
Lignin is a polymer found in the cell walls of almost all plants. Trees are 20-30% lignin, which is a very strong binder. Lignin is known for being difficult to use and apply due to, for example, its structural heterogeneity and strong brown colour.
Industrial applications of lignin are rare except for heat production. This research demonstrates that even highly advanced inventions can be achieved for this underutilised resource.
Anti-fogging sprays that are used to prevent condensation on eyeglasses and car windows contain several highly toxic ingredients one of them being for example PFAS (Per- and Polyfluorinated Substances). They 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.
Prior to this research, it was known that lignin nanoparticles could be an option for antifogging coatings if the particles’ light scattering was more homogeneous and their appearance would be less opaque.
"Optical coatings need to be transparent, but thus far, even rather thin lignin particle films have been visible. We knew that small particles appear less turbid, so I wanted to see if I could make invisible particle films by pushing the particle size to a minimum, says doctoral researcher Alexander Henn. "We used acetylated lignin and demonstrated an improved method to acetylate lignin in minutes at only 60 °C with a high yield".
The possibility to make photonic films came as surprise
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.
"The lignin particles I made from the acetylated lignin had rather surprising properties, which made the rest of this study very interesting. The possibility to make photonic films, for example, came as a total surprise".
Both the very short reaction time and high possible lignin concentration make the acetylation process industrially feasible.
"Lignin-based products could be commercially valuable and simultaneously act as carbon sinks, therefore contributing to relieving the current fossil-dependence and reducing carbon dioxide emissions", says Professor Monika Österberg. "High value-added applications like demonstrated here are important to drive lignin valorisation and take a step forward from using lignin only as a fuel".
"Teamwork was an important part of making this study impactful", says Alexander Henn. "Together with Professor Pekka Oinas and his team we were able to include techno-economical perspectives to this study". In particular, Henn would like to give credit to doctoral researcher Susanna Forssell in Oinas's research group, who provided the techno-economic perspectives. Doctoral researcher Sahar Babaeipour's efforts were key to controlling the particles' photonic properties. Staff Scientists Paula Nousiainen's and Kristoffer Meinander's respective expertise on lignin chemistry and photonic phenomena helped us make sense of our results and use them effectively. He adds, "It is not difficult to find talented team players at Aalto University when you look around."
This work was a part of the Academy of Finland Flagship programme FinnCERES (competence center for materials bioeconomy). This research was published in September 2023 in the Chemical Engineering Journal.
