Effect of molecular weight and degree of substitution of quaternary chitosan on its adsorption and flocculation properties for potential retention-aids in alkaline papermaking

N-(2-Hydroxyl-3-trimethylammonio)-propyl chitosan chloride with varying degree of substitution (DS) and molecular weight (MW) were prepared using nucleophilic substitution reaction, the structure of the modified chitosan was discussed using IR, 13C NMR and 1H NMR spectroscopies. The results show that substitution reaction occurred at primary aliphatic amino group rather than hydroxyl group on chitosan under the reaction conditions. DS was influenced by the ratio of epoxide ring to chitosan. The retention-aid properties of quaternary chitosan for natural occurring CaCO3 fillers had been studied using adsorption and flocculation experiments. It was found that quaternary chitosan was almost completely adsorbed onto the surfaces of CaCO3 particles at the experimental dosage level. When DS of chitosan were same (93%), the complete surface coverage, which were determined by zeta potential measurements, of CaCO3 particles used in this work were 0.735 mg g−1, 0.826 mg l−1 and 0.953 mg g−1 for the quaternary salt with MW = 4.8 × 104, 11 × 104 and 19 × 104, respectively. The electrostatic attractive was expected to be the dominant driving forces for the adsorption. Compared with a commercial cationic starch, quaternary chitosan had a lower flocculation concentration and a higher flocculation performance. Further, the optimum flocculation concentration was decreased with the increasing DS, and the flocculation of CaCO3 dispersion was significantly affected by the molecular weight of quaternary chitosan. The optimum flocculation concentrations were about 4, 5 and 6 mg l−1 for the quaternary chitosan with MW = 19 × 104, 11 × 104 and 4.8 × 104, respectively. It was also shown that the CaCO3 flocculation induced by the adsorption of quaternary chitosan was dominated by a charge patch mechanism.