Bleachability of alkaline pulps - Part 1. The importance of beta-aryl ether linkages in lignin

Analytical and structural studies were done on different types of alkaline pulps and their isolated residual lignins. Although having the same degree of delignification after cooking, some pulps were easier to bleach than oth ers. All isolated residual lignins were found to contain beta -aryl ether ( beta -O-4) structures in reasonable amounts when analysed by thioacidolysis (greater than or equal to 10% of the native lignin value). At decreasing k appa numbers, there was also a corresponding decrease in the amount of beta -aryl ether structures in the residual lignin. Moreover, a high beta -aryl ether content in the unbleached residual lignin after cooking was found to contribute to a better bleachability of the pulp, especially when hydrogen peroxide stages were involved. At the same degree of delignification, the superior bleachability of alkaline sulfite pulps over kraft pulps was shown to be attributable to a higher content of beta -aryl ether linkages in the further type of pulps.