The colloidal behaviour of kraft lignin III. Swelling behaviour and mechanical properties of kraft lignin gels

Summary Kraft lignin gels have been synthesized by cross-linking kraft lignin (Indulin ATR) in water with varying amounts of epichlorohydrin under alkaline conditions. The effects of pH and salt concentration on the swelling behaviour and on the mechanical properties of these gels have been investigated.Swelling was determined gravimetrically and the mechanical properties of the gels were tested under uni-axial compressive creep in the time interval 1–900 s. The gels show a linear viscoelastic behaviour without viscous flow.The kraft lignin gels exhibit a swelling behaviour typical for polyelectrolytic networks, i. e. the degree of swelling increases with an increasing number of ionized groups and decreases with increasing salt concentration. The swelling behaviour and ion exchange capacity were found to be nonspecific towards type of alkaline solution (LiOH, NaOH, and KOH).The ability of the carboxylic groups to form intermolecular hydrogen bonds has a strong influence on the mechanical properties of the gels. Creep compliance and creep rate increase greatly when the carboxyl groups are dissociated. It is therefore concluded that the effective number of crosslinks in the networks in addition to chemical cross-linking is also dependent on the state of dissociation of the carboxylic groups.With 11 figures and 1 table