Improving cellulose fibre properties by controlled adsorption of xylans

Licentiate thesis, 2007

Hemicelluloses are, together with cellulose and pectins, the polysaccharide constituents in the cell walls of higher plants. Xylans are the most common hemicelluloses, and are considered to be the second most abundant biopolymer in the plant kingdom. Xylans have an inherent affinity to cellulose and will thus adsorb on cellulose surfaces; a propensity providing great opportunities for modification of cellulose fibres. The molecular structure of xylan is proposed to affect the solubility and the level of cellulose interaction. This thesis investigates the influence of xylan composition on solution properties in aqueous systems, and its effect on cellulose fibre adsorption. In addition, the effects of xylan adsorption on softwood pulp properties and the possibility of utilising xylans from agricultural residues, i.e. barley husks, in cellulose fibre modification are studied. Lignin has been shown to have a great influence on the self-assembly behaviour of birch xylan. It is proposed that the agglomeration starts with the nucleation of lignin polymer molecules that are complexed with water-soluble lignin-containing xylan molecules via secondary bonds. As lignin affects the solubility of xylan, it most probably also influences the driving force for xylan accumulation and adsorption on cellulose surfaces. Xylans isolated from barley husks have been shown to interact with cellulose. Temperature and initial concentration of xylan has been shown to be important parameters in controlling the level of adsorption on cellulose fibres, but the adsorption appears to be largely dependent on xylan molecular structure. Arabinoxylans with a low degree of substitution have an enhanced capacity to interact with each other and with other polysaccharides; a feature that explains the observed propensity of low substituted xylan to form agglomerated structures and adsorb on cellulose surfaces. Furthermore, barley husk xylan has shown great potential as a dry strengthening agent for bleached once-dried softwood pulp. Adsorption of birch xylan on never-dried bleached softwood kraft pulp has been shown to improve tensile strength and beatability, but tensile strength at a given °SR, density and light scattering coefficient is more or less unaffected. The effects are more pronounced for once-dried pulp than for never-dried pulp. It is hypothesised that adsorbed xylan has a stabilising effect on cellulose fibrils, preventing aggregation upon drying, and thus reducing the effect of hornification.