Analysis of Differences in Coating Structure Induced Through Variable Calendering Conditions, 1998 Coating Conference Proceedings

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A laboratory-scale hot soft-nip calender has been used to generate a series of finished clay-coated papers with a range of nip pressures and temperatures. Conventional optical, physical and print testing has been performed to provide gross comparisons between the papers. The alignment of the platey clay particles in the bulk of the coatings has been determined by X-ray diffraction and the results have been correlated with those from gloss goniophotometry measurements.

The same sheet gloss level could be achieved through various combinations of pressure and temperature, from high temperature/low pressure to low temperature/high pressure. These different calendering regimes produced measurably different sheet and print properties.

The use of higher calendering temperatures produces a coating layer which has a more deformable surface. This results in the clay particles nearer to the surface being, on average, more aligned with that surface, whereas those in the bulk of the coating layer are more randomly orientated. The well-aligned platey clay particles on the surface give the sheet a relatively low degree of microroughness. The lower pressure used means that the sheet retains more macro-scale roughness from the underlying base.

Lower calendering temperatures reduce the amount of coating deformation possible and the whole sheet is compressed more evenly. This results in a sheet that has good macro-smoothness. However, the particles on the surface are less well-aligned than those when higher temperatures are used, so the surface exhibits a relatively high degree of microroughness.