Chemical Modification of Cellulose Fibres and Fibrils for Design of New Materials

Abstract: Due to the growing interest in biobased materials in today’s society, where the need for a cyclic economy is obvious, there has been a huge increase in the  interest for using cellulose due to its excellent mechanical and chemical properties. However, the properties of cellulose have to be modified and improved in order to satisfy advanced material applications where the cellulose properties can be tuned to fit the properties of other components in composite mixtures. This thesis explores the heterogeneous chemical modification of cellulose for improved material properties of cellulose-based materials and the use of cellulose fibres and fibrils in novel applications.In the first part of the work described in this thesis, a fundamental study was performed to clarify how the chemical composition and the fibre/fibril structure of the cellulose following chemical modification affect the material properties. The second part of the work was aimed at exploring the potential for using the chemically modified fibres/fibrils in novel material applications. Lignocellulosic fibres with different chemical compositions were modified by periodate oxidation and borohydride reduction, and it was found that the most important factor was the amount of holocellulose present in the fibres, since lignin-rich fibres were less reactive and less responsive to the treatments. Despite the lower reactivity of lignin-rich fibres, it was however possible to improve their mechanical properties and to achieve a significant increase in the compressive strength of papers prepared from modified unbleached kraft fibres.The chemical modifications were then expanded to nine different molecular structures and two different degrees of modification. Fibres modified at low degrees of modification were used to prepare handsheets, followed by mechanical and physical characterization. Highly modified fibres were also used to prepare cellulose nanofibrils (CNFs). It was found that the properties of handsheets and films prepared from modified fibres/fibrils are highly dependent on the chemical structure of the modified cellulose and, as an example, the ductility was greatly improved by converting secondary alcohols to primary alcohols. A detailed analysis of the modified fibres and fibrils also showed that, due to the heterogeneous chemical reaction used, the modified fibrils had a core-shell structure with a shell of modified cellulose with a lower crystalline order surrounding a core of crystalline cellulose. The results also showed that the chemical structure of the modified shell dramatically affects the interaction with moisture. Materials from fibrils containing covalent crosslinks have shown to be less sensitive to moisture at the cost of being more brittle. In a different application, modified CNFs were used as paper strength additives. Three differently modified CNFs were used: carboxymethylated CNFs, periodate-oxidised carboxymethylated CNFs and dopamine-grafted carboxymethylated CNFs. The properties of these CNFs were compared with that of a microfibrillated cellulose from unbleached kraft fibres. In general, a great improvement in the dry mechanical properties of handsheets was observed with the addition of the periodate-oxidised oxidised and dopamine-grafted modified fibrils, whereas only the periodate-oxidised carboxymethylated CNFs improved the wet strength.Finally, it was found that the chemically modified fibres could be used to prepare a novel low-density material with good mechanical strength, both wet and dry, and excellent shape recovery capacity in the wet state after mechanical compression. The fibre networks were produced by solvent exchange from water to acetone followed by air drying at room temperature. The properties of the fibre networks could also fairly easily be tuned in terms of porosity, density and strength.