Residual Lignin Structures

The kappa number in unbleached and also bleached kraft pulps is made up of lignin

and other oxidizable structures denoted as false lignin. The latter contains

contributions from hexenuronic acid groups (HexA) and “non-lignin” structures

[67](one kappa number unit corresponds to 11.6 lmol HexA [68]). The amount of

true residual lignin can be measured directly using the Ox-Dem kappa number

method [69]. The contribution to the kappa number of the “non-lignin” structures

can then be calculated as the difference between the standard pulp kappa number

and the sum of the contributions of the residual lignin and HexA. It was shown

that about 24% of the kappa number in spruce kraft pulp and about 41% of the

kappa number in birch kraft pulp can be attributed to false lignin structures. In

absolute values, the false lignin accounts to approximately 5–6 kappa number

units for both softwood and hardwood kraft pulps [67]. In the softwood kraft pulp

the “non-lignin” structures dominate the false lignin fraction, whereas in the hardwood

kraft pulp HexA constitutes the main part of the false lignin (Tab. 4.30).

4.2 Kraft Pulping Processes 257

Tab. 4.30 Fractionation of pulp kappa number of spruce and

birch kraft pulps before and alter oxygen delignification

(according to [67]).

Pulp Kappa

Number

Ox-Dem

Kappa

Number