Abstract
Yellowing of ECF (Elementally Chlorine Free) or TCF (Totally Chlorine Free) bleached pulp has became a significant problem since 1990 s. This brightness reversion tends to occur around pH 4. 5. Acid-sized paper decreases brightness significantly. Many researchers have studied the causes of brightness reversion of softwood bleached kraft pulp having brightness of around ISO 90%. Affects of cellulose, hemicellulose, hexenuronic acid (HexA) and metal ions were reported. Some factors will probably affect each other. Although the mechanism is still unknown, it has been shown that HexA is a main cause. To remove it, acid treatment, hot chlorine dioxide bleaching, powerful ozone bleaching were introduced. However, they need a huge amount of energy, and decrease pulp yield and strength.
In Japan, brightness of bleached pulp is usually around ISO 86%, and it is rare to produce bleached kraft pulp having brightness of ISO 90%. We have to consider affect of lignin besides HexA. The purpose of this report is making the affect of HexA in Japanese kraft pulp clear, and finding the way to avoid the yellowing by an efficient method.
It was shown that HexA was also a main cause of heat and moisture induced yellowing of Japanese hardwood bleached kraft pulp (LBKP) . Compared with reactivity of chlorine dioxide to lignin, that of HexA was low. Therefore HexA tended to remain in D-ECF bleached pulp. We concluded that reducing HexA and lignin ratio of the pulp entering into D-ECF bleaching is important to decrease HexA content of LBKP. The ratio of unbleached hardwood kraft pulp (LUKP) had a maximum value toward kappa number of the pulp. As one of the ways to avoid the yellowing, the cooking kappa number should be fixed higher or lower than the kappa number that gives the maximum HexA content.
