Yellowing of ECF (Elementally Chlorine Free) or TCF (Totally Chlorine Free) bleached pulp has became a significant problem since 1990 s. It has been shown that hexenuronic acid (HexA) is a main cause, but the mechanism of the yellowing is not clear. In order to suppress the yellowing, the removal of HexA from pulp has been conducted by acid treatment, hot chlorine dioxide bleaching, ozone bleaching and others. However, they need a large amount of energy, and decrease pulp yield and strength.
In this report, mechanism of the yellowing was investigated. The results suggest that the yellowing consists of two-stage reactions. The first reaction is the hydrolysis of HexA, and the second one is the reaction of 5-formyl-2-furancarboxylic acid (FFA) , which is one of hydrolytic products of HexA. Therefore, it is suggested that the yellowing can be depressed by inhibition of the reaction of FFA.
We found that low molecular weight carboxylic acids (for example, citric acid, tartaric acid and others) and polycarboxylic acid (for example, polyacrylic acid) are useful as a yellowing inhibitor. Size press coating test was carried out as one of the practical ways of adding the inhibitor to the paper. This method was effective to depress the yellowing, while maintaining physical paper properties.
View full abstract