Adsorption of xylans on cellulosic fibres -Influence of xylan composition on adsorption characteristics and kraft pulp properties

Doctoral thesis, 2010

Xylans are polysaccharides present in large amounts in cell walls of vascular plants. They possess an inherent affinity to cellulose and will thus adsorb irreversibly on cellulose surfaces. This propensity, along with the great abundance in nature, provides great opportunities for the use of xylans as renewable and biodegradable cellulose fibre modifying agents. The research presented in this thesis provides further knowledge of how the composition of xylan fractions influences solution properties and characteristics of xylan adsorption on cellulosic fibres. The study also describes how xylan adsorption can be used to improve the strength properties of bleached softwood kraft pulp. Furthermore, the work investigates the potential of utilizing xylan isolated from agricultural waste materials in cellulose fibre modification. Results show that lignin has a great influence on the aggregation behaviour of glucuronoxylan (GX) from birch. As lignin affects the solubility of xylan, it most likely also influences the driving force for xylan adsorption and deposition on cellulosic surfaces. Adsorption of GX on bleached softwood kraft pulp before drying is shown to preserve a considerable part of the fibre-swelling, fibre wall porosity, specific fibre surface area and wet fibre flexibility observed in the never-dried state. It is therefore suggested that GX adsorption reduces the degree of drying-induced fibre wall cross-linking. The use of GX as a hornification inhibitor provides once-dried fibres with increased tensile strength and beatability. Water-soluble (glucurono)arabinoxylan (GAX) isolated from barley husks is shown to interact with cellulose fibres. GAX with a low degree of substitution has an enhanced propensity to form aggregates in aqueous solutions and adsorb on cellulosic surfaces. In order to increase the driving force of adsorption of the more highly substituted xylan, GAX was cationized. This chemical modification increases the rate and magnitude of GAX adsorption extensively, due to the induced electrostatic interaction between anionic pulp fibres and cationic xylan. Furthermore, it is shown that adsorption of GAX on never-dried fibres may be beneficial in the production of market pulps with increased tensile strength and beatability.