Contract n° BOS-055-I

Summary

This study was intended to evaluate the effects that the drying of the sheet under Z-direction restraint (press drying) has on the properties of papers made from high-yield, short-fiber pulps, as well as from secondary fibers. The results confirmed the important role of pressure in press drying. The temperature used is quite decisive to obtain a very high wet strength. Press drying was found particularly effective in the case of unbeaten fibers still very lignified. It was also confirmed that press drying can result in sheets with a very high wet strength. To achieve this result it is necessary to use very high platen temperatures (greater than 170–180°C).The presence of lignin seems to play an important role. These elevated levels of wet strength of the sheet can cause problems in paper recycling, which may be more serious that those encountered with normal wet-strength papers. Press drying appears to be tailored for hardwood high-yield pulps. Particularly high levels of strength were obtained in the case of semichemical pulps from hardwoods with thin cell wall fibers. In the case of high-yield wheat straw pulps, press drying was not very effective. The improvements obtained were usually slight and inferior to those obtained with hardwood high-yield pulps. Secondary fibers did not respond very effectively to press drying, even though the improvements obtained were better than those with straw pulps. The improvements in the paper properties that characterize the quality of corrugating medium were rather limited, whereas the increments in the properties important for liners for corrugated board were more marked. The high quantity of chemical fiber present in secondary fibers could be responsible for the poor efficacy of the process. The presence of fines did not have positive effects on the results of press drying, whether their nature was totally parenchymal (straw pulps) or whether they were composed of mineral fillers and more or less lignified fibrous fragments as well as of parenchyma cells (secondary fibers).