Adsorption of lignin-cationic starch complexes on cellulose fibres and their effect on sheet properties

Kraft lignin and cationic starch (CS) form both soluble and insoluble complexes in aqueous solutions. It was found that the complexes are cationic or anionic, depending on the lignin/CS charge ratio. The complexes adsorb on kraft pulp fibres, especially when the complexes are cationic. The presented adsorption isotherms give indications of acceptable levels of dissolved lignin e.g. under conditions of water circulation closure. Paper properties are significantly affected by the adsorption of lignin/CS complexes. Sheet brightness decreases but different strength properties of paper generally increase when lignin/CS complexes are adsorbed. Moreover, the increase brought by the lignin/CS complex is greater than the increase due to the addition of CS alone. Papers made from fibres with adsorbed lignin/CS complexes exhibit lower brightness, but improved strength and water resistance. This could be employed in the manufacture of packaging grades of paper.