This paper deals with an experimental investigation aimed at determining the extent of and the nature of the aggregation behaviour of kraft lignin (Indulin AT), in aqueous solutions. Gel permeation chromatography (GPC) in dimethylformamide (DMF) revealed a bimodal molecular weight distribution of kraft lignin. The high molecular weight fraction (sol fraction) was isolated by dialysis and used in this investigation. Stable lignin sols in aqueous solutions, which were not light scattering as determined by the naked eye, were prepared. The extent of aggregation of these sols was followed viscometrically under different pH-conditions. Aggregation was found when the pH was lowered. The extent of aggregation as determined from viscosity measurements was found to increase irreversibly with storage time and storage temperature. Prolonged heating induced the formation of a 3-dimensional network (gelation). Although both association and gelation were thermally irreversible, it was found that the cohesive interactions could be broken by deprotonization of the carboxylic groups. The aggregation is discussed with reference to previous publications in the field. It is suggested that the long-range van der Waals forces play an important role together with intermolecular and intramolecular associations of the carboxylic groups. © 1979 Dr. Dietrich Steinkopff Verlag.
