A novel method to improve wet strength of paper

We applied catalytic oxidation using TEMPO (2,2,6,6 tetramethylpiperidine-1-oxy radical), sodium bromide, and sodium hypochlonte to aqueous kraft pulp slurries at pH 10.5 under various conditions at room temperature to chemically modify pulp fiber surfaces. This TEMPO mediated oxidation can effectively introduce aldehyde and carboxylate groups into accessible regions of the pulp fibers. When the amount of sodium hypochlorite (NaCIO) added in the oxidation was optimum (about 0.3 mmol/g pulp in this study), we observed almost no loss in pulp yield, and the hand sheets prepared from the oxidized pulp had maximum wet and dry tensile strengths. Interfiber covalent bond formation through hemiacetal linkages between hydroxyl groups of cellulose/hemicellulose and aldehyde groups brings about the clear wet tensile strength improvement. These linkages form at the C6 primary hydroxyl groups of cellulose through TEMPO-mediated oxidation. Moreover, wet tensile strength of the handsheets was further improved when aluminum sulfate was used in handsheet-making in combination with TEMPO oxidized pulps, while dry tensile strength decreased to some extent. Formation of aluminum carboxylate linkages between pulp fibers in the dried handsheets may have an additive effect on the wet tensile strength improvement. Application: The combination of TEMPO mediated oxication of bleached kraft pulp under suitable conditions and alum addition in papermaking can give sulficinnt wet strength to paper without adding conventional wet strength agents.