Interactions of cationized chitosan with components in a chemical pulp suspension

Abstract

N-(2-hydroxy-3-trimethylammonio)-propyl chitosan chloride, carboxymethyl chitosan and cationic starch were tested as wet-end additives for peroxide bleached reed kraft pulp. The adsorption of these polysaccharide samples and their interactions with the main components of pulp suspensions (cellulosic fiber, fines, dissolved and colloidal carbohydrates) were investigated by spectrophotometric, microelectrophoresis, particle size and retention/drainage methods. The results showed that the adsorption and aggregation behaviors which occur in pulp suspension were not only affected by the surface physicochemical characteristics of the cellulosic substrates but were also strongly affected by the nature (charge density, charge type and molar mass) of the polysaccharide additives. That is, N-(2-hydroxy-3-trimethylammonio)-propyl chitosan chloride additives were almost completely adsorbed onto the surfaces of the cellulosic fibers and aggregated the fines at low dosages, corresponding to those used in industrial operation. The optimum polymer concentration was increased with the reducing of the charge density of the quaternary chitosan additives. The adsorption of cationic starch onto the surfaces of cellulosic substrates was weaker than quaternary chitosan and showed higher optimum polymer concentration. The adsorption of carboxymethyl chitosan promoted the stabilization of the fines and colloidal carbohydrates rather than their aggregation. The polyelectrolyte complexes between hemicelluloses and the quaternary chitosans were formed in the adsorption processing, and these complexes then became adsorbed or deposited onto the cellulosic fibers, and this also correlates to the maximum fines retention and drainage. The experimental results also suggested that the dominant interaction between quaternary chitosan additives and cellulosic substrates are of an electrostatic nature and the fine flocculation was a charge patch mechanism.