Adsorption of lignin and lignin/cationic polymer complexes on cellulose fibres and their effect on sheet properties

Abstract

The objective of this dissertation was to deepen the understanding of lignin uptake by cellulose fibres by systematic studies under well-controlled conditions, and to clarify its effect on the papermaking properties of the fibres. It was possible to gain some direct insight into lignin sorption mechanisms by a comprehensive approach including studies of lignin behaviour in the solution, adsorption measurements and surface chemistry examination, studies of lignin distribution in the fibres, as well as analysis of sheet technical properties. It was found that van der Waals' interactions between lignin and fibres in aqueous dispersion are very weak and do not overcome the mutual electrostatic repulsion. No lignin is adsorbed from aqueous lignin solutions onto pure cellulose fibres: lignin loosely attaches to bleached fibres by imbibition with solution and filtration effect, and can be washed away with water. However, in sheets lignin binds to the fibres: some lignin is distributed throughout the cell wall, but the major part is located in the layers formed between the fibres. Restrained drying is responsible for binding of trapped lignin to the fibrous mat.

Irreversible adsorption may occur in the presence of cationic polymers. The course of adsorption depended on the sequence of polymer addition. Kraft lignin is adsorbed as a monolayer onto cationised surfaces. In the aqueous solutions lignin and cationic polymers (PDADMAC and cationic starch (CS)) form both soluble and insoluble complexes, which are cationic or anionic, depending on the lignin/cationic polymer charge ratio. The complexes adsorb on kraft pulp fibres (and mica), especially at low lignin/polyelectrolyte ratios, when the complexes are cationic, covering the surface with patches, films or blobs. The driving force for adsorption of lignin/cationic polymer complexes on chemical pulp fibres is primarily electrostatic: adsorption takes place as long as the fibre surface and the lignin are oppositely charged. Adsorption of complexes is highly dependent on the sequence of addition of lignin and cationic polymer. If the complexes are formed in solution prior to adsorption on the fibres there is a relatively sharp maximum in adsorption at cationic polymer/lignin ratios below the isoelectric point. If complexes are formed directly on the fibre surface (adsorption of lignin on fibres treated with cationic polymer) the isotherm reaches a plateau value. Adsorbed complexes are located on the outermost surface of the fibres, which leads to brightness losses. Papers made from fibers with adsorbed lignin complexes exhibit improved strength and water resistance. This could be utilized in the manufacture of packaging grades of paper. Lignin particles in the solution are elongated and are about 1 nm thick and 2.5 nm wide. They adsorb onto cationised substrates in the form of individual granules, consisting of about 1200 primary lignin particles that are held together by strong cohesive forces.