Effects of Repeated Drying-and-rewetting and Disintegration Cycles on Fundamental Properties of Kraft Pulp Fibres and Paper Made from Them

In order to study sheet strength reduction and the net effect of simple drying-rewetting cycles, handsheets from softwood and hardwood bleached kraft pulps were repeatedly subjected to only drying-and-rewetting cycles (up to 30 times) and properties were compared with those recycled through the drying-and-rewetting and disintegration processes. The decrease in coarseness of softwood pulp fibres both with and without disintegration was unexpectedly large. Decreases in density and tensile index of the sheets at early recycling stages were large but the, decrease per cycle, beyond about five times recycling, decreased in all cases except for hardwood pulps without disintegration (where the density of the recycled sheets .hardly altered with increased recycle count.) Although the decreases in density and tensile index of the sheets from recycled softwood pulp at early recycling stages were smaller than those for recycled hardwood pulp, the decrease in WRV (Water Retention Value) of softwood pulp at early recycling stages was larger than that from hardwood pulp. Furthermore, although the decreases in density and tensile index of the recycled sheets without disintegration at early recycling stages were smaller than those from recycled pulp with disintegration, the WRV of the former was larger than that of the latter. These results throw doubt on the hypothesis that the hornification, indicated by the decrease in WRV, is the main mechanism for the tensile strength reduction on recycling. Observation of the freeze-dried wet sheets using scanning electron microscopy suggest that a decrease in external (surface) fibrillation with increasing recycle number could have an effect on the changes in the CSF and WRV and further could partly cause the larger decrease in tensile strength of the sheet from recycled pulp with disintegration. Qualitatively, the loss of external (surface) fibrillation on recycling of hardwood pulp was more significant than that of softwood pulp.