SIZE EXCLUSION CHROMATOGRAPHY OF LIGNINS USING LITHIUM CHLORIDE/N,N-DIMETHYLACETAMIDE AS MOBILE PHASE. II. DISSOLVED AND RESIDUAL PINE KRAFT LIGNINS

Abstract

Dissolved and residual pine kraft lignin samples were isolated from different delignification phases of kraft flow–through cooks. The molecular weight distributions (MWDs) were studied by size exclusion chromatography using lithium chloride/N,N-dimethylacetamide (LiCl/DMAc), not previously applied to isolated softwood lignin samples before. The MWDs were compared with results obtained using tetrahydrofuran (THF) or lithium chloride/N,N–dimethylformamide (LiCl/DMF) as mobile phase. Some important characteristics of the isolated lignin samples including the MWDs are reported and compared with a previous study on birch wood kraft lignins. The molecular weight of the lignins was changed in a similar way irrespective of mobile phase used. The molecular weight of the dissolved lignin increased during the cook. By contrast, the change in molecular weight of the residual lignin samples did not show a clear trend with respect to cooking time. One explanation for this irregular change may be the low efficiency of the acid dioxane extraction of the pine kraft pulp obtained early in the cook as this pulp contains a large amount of lignin. Apart from this sample, the size of the residual lignin sample decreases and resembles that of the dissolved lignin at the end of the cook. For all samples, the MWD was positioned at higher molecular weight range when LiCl/DMAc was used as the mobile phase compared with THF. The explanation for this behaviour seems to be that the polystyrene standards elutes later from polystyrene-divinylbenzene columns compared to the lignin samples when a mobile phase of higher polarity is used. The shapes of the distributions were also different between LiCl/DMAc and THF, whereas LiCl/DMAc and LiCl/DMF gave similar distribution profiles. These results emphasize the importance of using the same mobile phase and column packing material when comparing MWD and size of different lignins and also when comparing lignins and carbohydrate polymers in pulp.