Chemical modification of pulp fibers by TEMPO-mediated oxidation

A bleached hardwood kraft pulp was oxidized with TEMPO (2,2,6,6-tetrametyl-1-piperidinyloxy radical), NaBr and NaClO in pulp suspensions at pH 10.5 and 20 degrees C for 2h. The carboxyl content of the oxidized pulp increased up to 0.47 mmol/g by the TEMPO-mediated oxidation, which was about eight times as much as that of the original pulp. Viscosity measurement of the oxidized pulps showed that some depolymerization occurred on the kraft pulp during the oxidation. Water retention values of the oxidized pulps were almost equal to that of the original pulp. When handsheets were prepared from the oxidized pulps, apparent density, tearing index and folding endurance of the handsheets were roughly constant in the range of carboxyl content from 0.06 to 0.47 mmol/g. On the other hand, the introduction of carboxyl groups had some influences on dry tensile index, elongation at the breaking point, Young's modulus and opacity of the handsheets prepared thereof. When a polyamideamine-epichlorohydrin (PAE) resin was added to pulp suspensions, higher retention of PAE was achieved by using the oxidized pulp and the maximum wet strength was obtained for the oxidized pulp with carboxyl content of 0.12 mmol/g. Probably, the introduction of carboxyl groups into the pulp increases opportunity of electrostatic interactions between anionic pulp fibers and cationic PAE molecules in pulp suspensions. When the oxidized pulp had carboxyl content greater than 0.2 mmol/g, however, wet strength of the handsheets decreased.