This study investigated the effect of cooking and bleaching conditions on the content of hexenuronic acid (HexA) of bleached hardwood kraft pulp (HBKP). The study showed that HexA affects yellowing. It was also shown that HexA tends to remain in pulp bleached with mainly chlorine dioxide compared with chlorine or ozone due to the different selectivity. Chlorine dioxide degrades less HexA than lignin in a fixed ratio. The smaller ratio of HexA/Lignin in unbleached hardwood kraft pulp (HUKP) resulted in a smaller amount of HexA retained in HBKP. In order to depress yellowing through D-ECF, it is important to decrease the HexA/Lignin ratio before chlorine dioxide bleaching. As one of the ways, the kappa number of HUKP should be fixed higher.
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