Calendering Response of Pigments, 1999 International Paper Physics Conference Proceedings
Dr. A.G. Hiorns, Mr. L. Coggon, Mr. D.J. Parsons
A wide variety of pigments is available to the papermaker for use in coatings. An appropriate choice of pigments can provide a basis for the production of a high quality coated paper. Pigment morphology and formulation will determine both the rheological properties of the coating colour and the final structure of the coating layer.
The majority of coated papers are finished using a calender. The calender settings, such as temperature, nip pressure and speed, will determine the magnitude and timescale over which the thermal and mechanical forces act on the paper. The response of the paper to these forces will depend on the structure in the coating layer.
A range of pigments, based on both US and UK kaolins and ground calcium carbonate, have been coated onto both a precoated woodfree and an LWC base paper under standard conditions to provide a set of test papers suitable for a calendering study. The coated papers were calendered on a laboratory scale supercalender and a hot soft-nip calender.
The changes in paper glossing and print properties in response to calendering has been measured. The effect of pigment size and shape on the coating structure and hence the response of that structure to the forces of calendering are investigated.
“Calenderability” is loosely defined word that means different things to different people. It may mean either how easy a coating can achieve a given target gloss level or the maximum gloss level to which a given coating can be calendered. When coated papers are compared, these properties may sometimes give the same answer, but not always.