Water retention value (WRV) and apparent density of pulp sheet are the valuable parameters of paper making, and are widely used in pulp evaluation. In order to elucidate the characteristics of Na0C1·peroxide bleaching, the composition and the various properties of bleached TMP, such as WRV, apparent density and specific surface area ware investigated. Changes of the morphological structure of bleached long-fibers in TMP were also studied by scanning electron microscopy.
Relationship between WRVof never-dried TMP bleached by Na0C1, Na0C1·H2020 and NaOCl·O3 systems and apparent density of the pulp sheet was almost a linear function. WRV of never-dried TMP increased gradually by treating with increase in concentration of NaOCl solution under mild conditions. WRV of NaOC1-treated TMP further increased by H2O2 or O3 bleaching. WRV of fines in bleached TMP was higher than that of the long-fiber fraction.
The influence of pH on WRV of bleached TMP was attributed to the difference of the amount and the dissociation of acidic groups, particulary uronic acids, in the pulp and the subseqent swelling.
Relationship between WRV and specific surface area of bleached long-fibers separated from TMP indicated that, at higher yield, specific surface area of the fibers increase more quickly during the mild bleaching described above, whereas the increase of WRV is less. On the other hand, WRV of the bleached fibers at low yield was higher.
From the results, it is concluded that WRV of bleached TMP depends on the chemical composition and the physical state of the pulp. WRV increased along with increasing hydrophilicity of TMP by removing lignin and hemicellulose. The dissolution of both components from the fiber surface by mild NaOCI and Na0Cl·peroxede bleaching caused the rapid increase of specific surface area of TMP.
Changes of the morphological structure of long-fibers showed that the fibrillation on the fiber surface related to the specific surface area occurs by mild bleaching with Na0C1speroxide, whereas the delamination of the cell wall of the fibers is caused by lignin removal during violent bleaching.