Structure and mobility of lignin in relation to pulp washing and bleaching

Abstract

The roles of molecular size and structure were investigated in relation to the extent of peroxide bleaching and lignin adsorption on pulp. Structural analysis of components from effluents generated by peroxide bleaching was accomplished using UV-visible spectrophotometry and 1H and 13C NMR spectroscopy. These results provide evidence for a 1:1 sugar : aromatic ratio of components in the effluent mixture. Results are consistent with the presence of both hydroquinone and benzylic acid aromatic structures. Both 13C NMR and UV analysis support the hypothesis that about 12% of the aromatic structures are of a benzylic acid form. During peroxide brightening, low-mass components with a higher fraction of phenolic structures are released into solution most rapidly while carbohydrate degradation appears to be more important over extended periods. Changes in the UV spectra of the effluent during bleaching appear to be dominated by changes resulting from a reduction of pH arising from carbohydrate degradation. The principle pK. values, determined by titration with UV spectrophotometry, for effluents generated from alka- line hydrogen peroxide bleaching are at 4.64, 8.01 and 10.01. These pK. values correspond to the dissociation of carboxylic acid, p-hydroxybenzylic acid (phenolic proton), and phenolic hydroxyl groups. In contrast, the pK, of kraft lignin was estimated at -7.5 and 10.3 corresponding to the ionization of phenolic groups with and without conjugated a-carbonyl structures. The acid dissociation constants decrease between 0.2 and 0.7 pK. units with increasing ionic strength as do model compounds. The combined effects of decreasing the pK. and ionic strength screening of ion-ion interactions leads to increased adsorption of lignin on cellulose with increasing ionic strength. Around the effective pK, values the protonation of phenolic groups dominates the extent of interaction of lignin with cellulose. Pulsed Field Gradient NMR (PFGNMR) was utilized to evaluate the structure-size relationships for lignin preparations. Difficulties with the PFGNMR technique, including interaction of the gradient with r.f. and the superconducting magnet are discussed in detail. Results from PFGNMR indicate that high molecular weight kraft lignin has a lower frequency of ionizable phenolic groups. PFGNMR is compared to other NMR relaxation methods as a means to investigate size-structure relationships.