Precalendering and its interaction with other unit processes in the manufacturing of woodfree paper and board
During calendering, which is done in order to improve its printing properties, the paper is compressed in one or more roll nips. Paper is a viscoelastic material and this will influence its performance during calendering. At least for woodfree paper, the imposed compressive stress has a much greater influence on the remaining deformation (after unloading) than the duration of the compression. A consequence of this is that a soft calender nip will produce a lower deformation than a hard calender nip at a given line load. Furthermore, a soft nip does not suppress surface topography in a size region corresponding to fibre flocs as much as a hard nip. In this perspective, soft-nip calendering constitutes a more uniform treatment than hard-nip calendering. However, the in-plane density variations increased with both types of calender nips, although less pronounced in the case of soft nip calendering.
Calendering at a high temperature leads to a selective deformation of the surface region. This phenomenon can be used to optimise the relationship between mechanical and surface properties. At a given degree of deformation, properties like stiffness and gloss depends on the conditions at which the deformation was achieved. Calculations of an effective bending modulus can be used to characterise the stratification of the structure induced by the high-temperature calendering. A similar effect can in principle also be attained by establishing a moisture gradient in the thickness direction of the sheet. Moisture is more difficult to use (than temperature) for such a purpose. A water box application directly on a calender was one working strategy employed here in order to obtain moisture gradient calendering.
A positive effect of precalendering a woodfree sheet with regard to surface smoothness could be seen also after coating and final calendering, although it is expected that some of the precalendering effect is offset due to contact with the aqueous phase. Final calendering led to some development of gloss variations. This might indicate an uneven surface porosity, which may have a negative influence on printing properties. However, the precalendering facilitated a more uniform coating layer, which is expected to have a positive effect on printing properties. In total, it seemed that precalendering was on the whole beneficial for the final properties of the paper product. The degree of precalender deformation controlled the final properties. Speed, type of nip or temperature as such in the precalender operation, appeared not to have major influence on the final properties.
Water/paper interactions is a common denominator in many cases of interplay between process steps in papermaking. Minimising these interactions appears in many cases to be a useful route to improve the relation beneficial between mechanical and surface properties, since water exposure can lead to an irreversible deformation of the surface structure. Penetration of water and water-based fluids appeared to be controlled mostly by the openness, or porosity of the surface and on the external pressure used when applying the fluid. Wet-pressing and calendering are processes that lead to closure of the surface, which helps to minimise the water penetration in subsequent process steps, e.g. surface sizing and coating.