Mechanism of Paper Surface Roughening (III)

Abstract

Press-drying, that is, a concurrent process of wet-pressing and drying is potential to restrain surface roughening because it provides strong interfiber bondings. Press-dried handsheets for thermo-mechanical pulp (TMP) and hardwood bleached kraft pulp (HBKP) were compared with calendered handsheets in terms of response to water treatment.It was found that the water absorption rate was much lower for press-dried handsheets than calendered ones. This can be attributed to the structural differences as clearly shown by scanning electron micrographs, where fibers of press-dried sheets appeared intensively densified near the heated-side surface, while those of calendered sheets were transversely uniform in density. The dense structure peculiar to press-drying was considered to obstruct water penetration. Furthermore, press-drying effects on restraining surface roughening was confirmed by the result that even at the same level of water absorption, roughening of the press-dried sheets was much lower than that of the calendered sheets for TMP.In contrast, effects of press-drying on restraining surface roughening were not so striking for HBKP, probably because of some degree of its irreversibility observed even for the calendered sheet. Consequently, it was considered that water was allowed to penetrate through small intraand interfiber pores gradually and that hornification of the fiber walls caused by heat and pressure in press-drying prevented fibers from swelling that would lead to surface roughening.