Recent advances in kraft pulping have extended the degree of practical deli
gnification while improving selectivity. Despite these improvements, little
is known about the nature of the residual lignin in low-kappa-number kraft
pulps. This paper examines the structure of residual lignin for seven conv
entional kraft pulps (kappa no. 33-13) and six Lo-Solids(R) kraft pulps (ka
ppa no. 29-11). The residual lignin in each pulp was isolated and character
ized by proton nuclear magnetic resonance (H-1 NMR). The results of these a
nalyses demonstrate that the overall structure of residual lignin is influe
nced by both the extent of delignification and the nature of the pulping pr
ocess.