Paper strength improvement by oxidative modification of sisal cellulose fibers with laccase–TEMPO system: Influence of the process variables

The oxidation of sisal pulp fibers by the laccase–TEMPO system was investigated and the influence of process variables including the laccase and TEMPO doses, and reaction time, on various properties of the oxidized fibers and of handsheets made from them was for the first time assessed using a three-variable statistical plan. The laccase–TEMPO system was found to oxidatively modify cellulose fibers, largely by introducing aldehyde groups and, to a much lesser extent, by introducing carboxyl groups. Based on the mathematical models used, increasing the TEMPO dose and reaction time increases the aldehyde content of the fibers, thereby also increasing their wet strength by effect of inter-fiber covalent bonding via hemiacetal linkages. Although no accurate model for the carboxyl content could be established, this property was found to peak under the specific conditions yielding the highest response in the dry tensile index model. The fact that the oxidative treatment diminished pulp viscosity is indicative of partial depolymerization of cellulose. This was especially marked under the conditions providing the highest contents in aldehyde and carboxyl groups, and the greatest improvements in the dry and wet tensile indices.