Chemical structure of residual lignin from kraft pulp

Structural characteristics of wood lignin and residual lignin in conventional and modified kraft pulps were examined employing elemental and methoxyl analysis and C-13, H-1 and P-31-NMR. The structural analysis revealed that kraft residual lignin differs significantly from wood lignin while differences between residual lignins in conventional and EMCC(R) pulps have similar structural characteristics at kappa numbers corresponding to the point where the selectivity of the cook becomes poor. NMR analysis suggested that residual lignin, compared to the wood lignin, is much less reactive towards pulping chemicals due to the low content of aryl ether linkages and the prevalence of condensed type structures.