A. Henn: Scalable Surface Chemistry for Lignin Modification - Creating Value for a Forest-Based Society

Abstract: 

Lignins is a highly abundant biomaterial produced as a side-stream in chemical pulping. Kraft lignin is the most common technical lignin, but it is difficult to utilize. Kraft lignin's properties can be improved, but chemical modification processes often struggle with balancing costs, performance, and environmental burden. The recent emergence of technologies to prepare and apply colloidal lignin nanoparticles (LNPs) has however increased lignin's potential, as it allows lignin to be used in multiple water-based applications. This thesis aims to demonstrate the usefulness of lignin and LNPs by presenting new scalable ways to modify lignin, in native and colloidal form, for both high-value and high-volume applications.  

LNPs are formed through a solvent shifting process, where an anti-solvent is added to a lignin solution. Lignin's hydroxyl group content and overall chemical structure determine how the particles are assembled during solvent shifting. Reducing lignin's hydroxyl group content through acetylation allowed smaller particles to be prepared at higher concentrations than what is possible with non-modified lignin. The acetylated particles' small size made them optically clear, which allowed them to be used for both antifogging coating and photonic films. 

The LNPs' inner structure could be changed by including hydrophobic fatty acids in the solvent shifting process. This resulted in hybrid nanocapsules whose cores consisted mainly of fatty acids and the shell mainly of lignin. Many fatty acids can be used as latent heat storage, but their meltingand solidification temperatures are prone to shift. However, the fatty acids' melting and solidification temperatures were effectively stabilized when encapsulated by lignin.   

LNPs outer structure could be hydrophobized for water-repellent surface coatings by adding a hydrophobic glycerol-based epoxy cross-linker to LNP dispersions. The LNPs allowed the epoxy to fuse with the aqueous dispersion phase despite its poor water solubility, and simultaneously acted as a curing agent. By combining LNPs with epoxidized lignin, strong and thermally stable adhesives with exceptionally high lignin contents could be prepared.   

The thesis also presents new methods for performing lignin epoxidation and acetylation. Both methods were designed to be scalable and material efficient, and therefore utilized short reaction times, moderate temperatures, and recycling of all excess chemicals.   

Overall, this thesis demonstrates how lignin's utility is improved in colloidal form, and by doing so provides examples of how lignin could be successfully valorized and applied.

Follow the remote defence: https://aalto.zoom.us/j/63093470298#success (this link should work!)

Opponent: Professor Lars Wågberg, KTH Royal Institute of Technology, Sweden

Supervisor: Professor Monika Österberg,  Aalto University, Department of Bioproducts and Biosystems

Link to electronic thesis: Scalable Surface Chemistry for Lignin Modification - Creating Value for a Forest-Based Society