Six types of pulps that were prepared under acidic or neutral papermaking conditions were investigated about behavior of yellowing or the others after accelerated aging tests. The relationship between the hexenuronic acid (HexA) content of pulps and yellowing was clarified for non-chlorine-bleached pulps (elemental chlorine-free (ECF) and totally chlorine-free (TCF) bleached pulps) prepared from hardwood oxygen-bleached kraft pulp (LOKP). A trial test of enzymatic removal of HexA from LOKP was attempted by treatment with a crude enzyme solution containing a new hexenuronidase.
The results indicated that the brightness reversion of LOKP that was prepared by acidic papermaking and underwent a humid accelerated aging test was greater than that of LOKP that underwent a dry accelerated aging test. On the other hand, the brightness of LOKP increased after sunlight exposure aging test. Under sunlight exposure, the structure of the lignin in bleached thermomechanical pulp (TMP) and bleached chemical thermomechanical pulp (BCTMP),which is similar to the lignin in wood, changed to coloring structures. However, the effect of this change on brightness reversion was minimal in acidic papermaking. Plain paper copier (PPC) paper and news printing (NP) paper, which are prepared by mixing used paper with secondary fibers, were assumed to consist of both chemical and mechanical pulps and therefore showed a relatively stable brightness in spite of accelerated aging conditions and pH in papermaking.
Under conditions of neutral papermaking, LOKP showed the least brightness reversion and the brightness of TCF and ECF pulps were similar to each other. On the other hand, the yellowing of TCF and ECF pulps prepared in acidic papermaking tended to increase with the increase in HexA content of the pulps. In addition, the pulps had the following order of good brightness stability : full-bleached TCF, full-bleached ECF, semi-bleached TCF, and LOKP.
HexA removal from LOKP with crude enzymes solution containing hexenuronidase (activity for Δ-X
3 : 2-O- (α-hexenuronic acid) -D-xylotriose) was optimum under the following treatment conditions : 0.24U⁄g, pH 5.5, 40°C, and 6h. It was shown that 40 mmol⁄kg of xylose was dissolved into the filtrate when 20 mmol⁄kg of HexA was removed from the pulp ; however, the decrease in the pulp yield was minimal.
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