The effect of molecular composition of xylan extracted from birch on its assembly onto bleached softwood kraft pulp

The effect of the molecular structure of xylan on its assembly onto softwood kraft pulps was investigated. Various xylan-rich fractions were isolated from birch wood chips by different mild treatments using water (H2O), acetic acid (HAc) and sulfur dioxide (SO2). The isolation involved prehydrolysis followed by alkaline extraction, with only the time, temperature and medium in the prehydrolysis step varied. After separation, the fractions were neutralised ( pH 7) and some material agglomerated and became insoluble. Dynamic light scattering measurements revealed differences in aggregate size among the different fractions. The fractions that agglomerated to the greatest extent contained less glucuronic acid and a higher amount of lignin. Such fractions adsorbed to a significantly higher extent (25.5% compared to 5.0% for the lowest soluble fraction) onto bleached softwood kraft pulps in experiments performed in autoclaves. The adsorption was carried out with fixed process parameters (100 degrees C, pH 10, 120 min). The adsorption of various xylan fractions resulted in different surface morphology on cellulose microfibrils, as observed by AFM. Pulps treated with xylans that were soluble at pH 7 showed small differences from the control sample, which was treated in an autoclave without the addition of xylan. Pulps treated with xylans that agglomerated at pH 7 showed a greater difference in the amount of nanosized aggregates covering the surface. ESCA analysis of the chemical surface composition indicated that samples containing more lignin showed a greater shift towards carbon- carbon bonds. A novel sequence for a pulping process is suggested.