EFFECT OF MOISTENING ON CROSS-SECTIONAL DETAILS OF CALENDERED PAPER CONTAINING MECHANICAL PULP

Smooth surface is a key quality factor for graphic paper and processes like calendering and pigment coating are applied to mise surface smoo thness. Converting processes like coating and offset printing involve the addition of moisture to a paper surface. The moisture will release frozen-in stresses in the paper imparted by the surface treatment. Th is is especially the case for paper containing mechanical pulp fibres. A consequence of this is a pronounced ronghening of the surface when moistened. The roughening has a negative effect on print quality and c oating colour distribution. One important mechanism behind the roughen ing phenomenon appears to be the changes in the cross-sectional shape of the individual mechanical pulp fibres. To quantify, the effect of m oistening on commercial paper containing mechanical pulp, supercalende red (SC) paper samples were moistened at differ ent and well-defined a ir humidities, and by wetting with water By image analysis of scanning electron microscope (SEM) electron micrographs, cross-sectional detai ls of paper were studied, and the class-sectional shape of fibres in p aper was analyzed. During the moistening of SC paper; intact mechanica l pulp fibres tend to recover the cross-sectional shape they possessed before the calendering The tendency was quantified and related to the degree of moistening. A strong relationship between fibre wall thickn ess and the fibre shape recovery is shown.