Colloidal Properties of Dissolved Kraft Lignin, 1992 Papermakers Conference Proceedings

Dissolved kraft lignin has been identified as a major interference to the efficiency of wet end chemicals in papermaking. In this research, a model lignin was studied under various conditions in order to improve our understanding of the behavior of lignin in the papermaking system. Dissolved lignin concentration were determined by a method based on the ultraviolet absorption by lignin solutions. Using this approach, it was possible to measure lignin concentrations in the 1 to 100 parts per million range. (Figures 1 and 2).

Lignin sols were stable at pH levels above 4.0 They became unstable when the pH was reduced below 4.0. The presence of CaC12 and/or centrifugation accelerated particle aggregation and precipitation. The removal of impurities and low molecular weight lignin fractions by dialysis increased the stability of the sols. (Figures 3-5). A colloid titration method was used to determine lignin particle surface charge. Polyvinyl sulfate and polydadmac were the anionic and cationic polymers employed, respectively. (Figure 6). Orthotoluidene blue dye was the indicator in the titration. Lignin surface charge increased significantly when the pH was increased from 4 to 11. When cationic polymer was added to the anionic lignin particles, their surface charge became more positive until the isoelectric point was reached. As the net charge approached zero. The lignin sol became less stable and precipitation occurred. The subsequent addition of more cationic polymer resulted in charge reversal and redispersal of the lignin sol. As expected, less cationic polymer was needed to neutralize the lignin at lower pH. Interestingly, the range of polymer addition levels corresponding to isoelectric conditions was much smaller at low pH than at high pH. (Figures 7 and 8). Lignin precipitation experiments in the presence of bleached kraft fibers revealed that strong interaction occurred between the two entities. Any lignin that adsorbed onto the fibers during precipitation could be easily removed by stirring. It was concluded that there is little, if any, inherent attraction exists between fibers and kraft lignin under papermaking conditions unless cationic polymers or salts are present to provide some kind of attraction and bonding between the two. (Figures 9 and 10).