Defence of doctoral thesis: Mozhgan Hashemzehi – Tailored Cellulose Modification for Sustainable Papermaking Solutions

Cellulose is a renewable, biodegradable material whose functionality can be enhanced by chemical modification. Conventional cationization relies on organic solvents and yields low substitution, whereas deep eutectic solvents (DESs) offer a greener alternative.

In this thesis, an environmentally friendly DES-based approach was applied to produce cationized dialdehyde cellulose and to evaluate its performance as a wet-end additive and as a paper coating. The modified cellulose significantly improved hand-sheet strength without negatively affecting dewatering and retained its performance when synthesized using recycled DES, demonstrating solvent reusability. As a coating material, cationized dialdehyde cellulose formed dense films with improved mechanical and air barrier properties.

To further enhance additive performance, two pretreatment strategies, enzymatic treatment and cold alkaline dissolution–precipitation, were applied to increase the reactivity of the starting pulp and thereby improve the charge density of the cellulose derivatives.

 In addition, hornification, a phenomenon that negatively affects pulp reactivity, was investigated. Drying experiments on swollen cellulose showed that freeze-drying and glycerol-drying mitigate hornification, whereas air-drying intensifies it. For air-dring method, solvent exchange prior to drying reduced hornification. The results indicate that water-mediated hydrogen bonding dominates the hornification mechanism, which is strongly influenced by solvent polarity and molecular weight.

Overall, this work demonstrates that combining green solvents, targeted pretreatments, and controlled drying strategies can substantially enhance cellulose reactivity and functionalization. These findings broaden cellulose applications from papermaking additives to bio-based barrier coatings and support efforts to replace petroleum-based plastics with renewable materials.