AKD sizing of TCF and ECF bleached birch pulp characterized by peroxide edge wicking index

摘要

To study the impact of extractable materials and xylan on the sizing efficiency of totally chlorine-free (TCF) bleached birch pulp and elemental-chlorine-free (ECF) bleached birch pulp, several pulps with different amounts of extractable material were studied. The content of extractable material in the pulp was shown to be detrimental for sizing efficiency, measured both as Cobb_(60) values and edge penetration in a model liquid packaging board, and a higher content of extractable material causes a reduced sizing efficiency for a given pulp. No significant difference was observed between extracted TCF and ECF bleached birch pulps, while unextracted pulps containing the same amount of extractable material yielded different sizing efficiencies. The TCF bleached birch pulp had a lower sizing efficiency than did the ECF bleached birch, probably because of differences in the fatty acid composition. A greater amount of extractable material also resulted in a smaller fraction of non-extractable alkyl ketene dimers (AKD) relative to the total amount of AKD in the sheet. Enzymatic removal of xylan and thus the reduction of the surface charge had no measurable effect on sizing efficiency. Extracted TCF and ECF bleached pulps only need a content of 0.05 kg/ton non-extractable AKD or 0.08 kg/ton total AKD in the sheets produced to achieve a good sizing effect.