This work explores the use of lignin as a raw material for developing bio-based foams. Lignin is a phenolic polymer obtained as a by-product from lignocellulosic biorefineries, with 80 million of tonnes passing through industrial processes each year. Despite this potential raw material supply, lignin is primarily combusted for energy recovery, with only a small fraction utilized for high-value applications.4 Among the different types of technical lignins, lignosulfonates isolated through sulfite pulping, dominate the technical lignin production. The presence of sulfonate groups renders lignosulfonates soluble in water at neutral pH, distinguishing them from other technical lignins.5 However, brittleness and low mechanical strength of lignosulfonates still pose challenges in structural applications. The ability to valorize this underutilized by-product represents a key step towards sustainability and a circular economy.
To overcome the structural limitations of lignin, in the present work chitosan was added to provide structural integrity and support to the foam structures. Chitosan is a long-chain polysaccharide comprised of glucosamine moieties, and is the only cationic biopolymer available in mass production. The polyelectrolyte complexation between lignosulfonate and chitosan in acidic aqueous solutions allows the formation of a physically crosslinked hydrogel between the oppositely charged polymers,6 eliminating the need for covalent modifications. Lyophilization of these hydrogels results in lightweight, self-standing foams with successful incorporation of up to 50 wt.% of lignin, and densities of approximately 20 kg/m3.
The complexes were evaluated by electrophoretic mobility measurements and conductometric titration. Thereafter, the obtained foams were characterized by gas sorption analysis to determine surface area and porosity. In addition to this, the mechanical properties were assessed by evaluating the compression behaviour of the material. Morphological studies using electron microscopy and X-ray tomography revealed a macroporous open-cell structure. Moreover, thermal conductivity measurements were performed using the transient hot disk technique showing initial values around 50 mW/mK, showcasing the potential of these foams as insulators. Finally, moisture uptake tests were conducted to evaluate the humidity resistance relative to the intended thermal insulation application. Overall, this study highlights the potential of lignin-chitosan foams as a fully bio-based alternative for thermal insulation.
1. International Energy Agency. Developing a Global Energy Efficiency Workforce in the Buildings Sector. IEA (2023).
2. Apostolopoulou-Kalkavoura, V., Munier, P. & Bergström, L. Thermally Insulating Nanocellulose-Based Materials. Advanced Materials 33 (2021).
3. Mort, R., Vorst, K., Curtzwiler, G. & Jiang, S. Biobased foams for thermal insulation: material selection, processing, modelling, and performance. RSC Advances 11, 4375 (2021)
4. Tardy, B. L., Lizundia, E., Guizani, C., Hakkarainen, M. & Sipponen, M. H. Prospects for the integration of lignin materials into the circular economy. Materials Today 65, 122–132 (2023).
5. Pylypchuk, I. & Sipponen, M. H. Organic solvent-free production of colloidally stable spherical lignin nanoparticles at high mass concentrations. Green Chemistry 24, 8705-8715 (2022).
6. Fredheim, G. E. & Christensen, B. E. Polyelectrolyte complexes: Interactions between lignosulfonate and chitosan. Biomacromolecules 4, 232–239 (2003).