On the importance of the structural composition of pulp for the selectivity of ozone and chlorine dioxide bleaching

From the literature it is known that both chlorine dioxide and ozone not only degrade lignin but also react with hexenuronic acid (HexA). In the present article, this knowledge is further developed, asking not if but rather under which circumstances these bleaching agents react with the different structures present in the pulp. The results show that, whereas chlorine dioxide prefers to react with lignin, ozone preferentially reacts with HexA. This finding is used to explain several anomalies seen between the actions of the two chemicals, e.g. regarding brightness increase and selectivity. Moreover, it is concluded that, especially for hardwood, a (DZE) stage shows many benefits over a (ZDE) stage, even when all the chlorine dioxide has reacted to completion before ozone addition, i.e. the explanation here is not the normally encountered synergism between chlorine dioxide and ozone.