Influence of oxidation and cationization on the properties of thermomechanical pulp fibers

Thermomechanical pulp (TMP) fibers were separately oxidized using 4-acetamido-2,2,6,6-tetramethylpiperidyl-1-oxyl (4-acetamido TEMPO) and cationized by 2,3-epoxy-propyl-tri-methyl ammonium chloride. The goal was to evaluate the influences of the interaction of oxidized, cationized, and untreated fiber components on paper properties. TEMPO-mediated oxidation can enhance interfiber bonding potential of mechanical long fibers. Cationization can improve the interfiber bonding only when the cationic charge density is lower than 400 mmol/kg; the bonding capacity drops off with increasing cationic charge density. However, as these modified long fibers were mixed with untreated long fibers and short fibers, the interaction between these fibers affected the paper sheet strength. The addition of oxidized fibers improved the sheet density remarkably, but it only had apparent effort on tensile strength when the carboxyl group content was less than 500 mmol/kg; higher carboxyl content did not enhance the sheet properties. The addition of cationized fibers had no positive effort on sheet density, and when they were mixed with other fiber fractions the interaction between positively charged and negatively charged fibers resulted in poor paper structure with rough surface and decreased tensile strength. Both oxidation and cationization had adverse influence on intrinsic fiber strength and tear index; either high carboxyl content or high cationic charge density could lead to remarkable decrease in these two properties. The side reactions, such as beta-elimination and alkoxy fragmentation during TEMPO-mediated oxidation and alkaline degradation during cationization, which degraded the cellulose, were responsible for the drastic decrease in zero span tensile strength and tear strength.