The possibility to use xylan from agro waste in order to improve the strength properties of spruce kraft pulp was examined. Xylan was extracted from wheat straw and rice husks, by subjecting the agro waste to kraft cooking. The extracted xylan was subsequently used at the end of a kraft cooking process of spruce, by replacing half of the black liquor with dissolved spruce xylan with white liquor containing agro waste xylan. The strength tests performed on the obtained softwood kraft pulps showed that even a small amount of rice husk or wheat straw xylan deposited on softwood fibers had a positive effect on the tensile index of the paper at given sheet density.
The aim of the study was to investigate the possibility to use spruce xylan more efficiently by possible relocation of dissolved xylan with certain characteristics from the first part of the kraft cooking to the later part, when precipitation occur. The characteristics of re-located xylan was controlled by replacing half the black liquor (BL) at a late stage of a kraft cook, with the same amount of black liquor containing spruce xylan with known molecular weight and content of uronic acid (UA). Pulp with addition of xylan with high amount of UA groups responded strongly on beating, resulting in improved tensile strength. It is proposed that the best effect of xylan on tensile strength occurs when the xylan penetrates some distance into the subsurface of the fiber wall. Both low molecular weight (M w )and a high degree of substitution decreases the tendency of xylan to aggregate, which enables the dissolved xylan to penetrate some distance into the exposed fiber surface. Upon beating, this xylan will be exposed thus facilitating improved fiber-fiber joint formation, which leads to increased tensile strength.
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