Bertaud F, Lichère H, Serrano L, Mortha G, Marlin N
In order to promote the recovery and use of industrial lignins from black liquor in pulp and paper industry, the reactivity and physico-chemical properties of several industrial lignins were evaluated. The recent development of a new conductometric/pH metric method was demonstrated to be fast and reliable for the determination of hydroxyl and carboxyl unit contents in kraft, soda and organosolv lignins, as well as in lignosulfonates. Comparing with the classical methods (31P NMR and differential UV, mainly), the fast method showed similar results, between 1 to 4.5 mmol/g, i.e. 0.3 to 0.9 mmol / C9 unit. Sulfonic acid groups of lignosulfonates were evaluated by total sulfur ICP-OES analysis, and by a more specific potentiometric dosage. Both methods were applied on six different commercial lignosulfonate. Potentiometric dosages showed 5% to 8% (by weight) sulfur eq. of sulfonic acid groups in lignosulfonates samples, meaning 1.5 to 2.5 mmol / g (i.e. 0.4 to 0.9 mol / C9 unit). Results from total sulfur determination were usually similar to the potentiometric results or slightly higher in case of sodium lignosulfonates. Weak acid functions (OH and COOH) could also be determined by this technique. Therefore, the fast titrations of functional groups could be used to characterize the lignin reactivity driven by hydroxyl and carboxyl groups, as well as water solubility given by sulfonic and weak acidic groups. Complementary analyses proved that contaminants, such as sugar residues and minerals considered as potential disturbing components for lignin application, were present at different levels in all industrial lignin samples.