The heterogeneity of two Canadian kraft lignins

Two kraft lignins, one precipitated with carbon dioxide and the other with sulphuric acid, have been fractionated into three distinct fractions by sequential extraction using organic solvents of increasing hydrogen-bonding capacity. The lignins and fractions were comparatively characterized in terms of yield, methoxyl group content, molecular weight distribution, and 13C NMR spectroscopy. For a given lignin, the fractions showed differences in yield, composition, and chemical structure. A significant portion (21%) of the carbon dioxide precipitated kraft lignin is composed of ethyl acetate soluble material of low molecular weight. This fraction, which is richer in guaiacyl moieties than the other fractions, is virtually absent in the sulphuric acid precipitated kraft lignin. The occurrence of "tails" in the high molecular weight fraction, especially from the carbon dioxide precipitated kraft lignin, suggests the presence of material of very high molecular weight. Data from 13C NMR spectroscopy does little to elucidate the nature or origin of the material causing these tails. Keywords: kraft lignins, heterogeneity, fractionation, characterization.