Influence of Precalendering on the Properties of a Coated Woodfree Paper and the Covering Ability of the Coating, 1995 Coating Conference Proceedings

O. Steffner

T. Nylund

M. Rigdahl

STFI

The objective of the work was to investigate the possibility of improving the properties of a coated woodfree paper and the covering ability of the coating layer by calendering the base paper before it was coated. The woodfree uncoat-ed paper was calendered with different line loads at speeds up to 1000 m/min in a pilot calender, either in a steel/steel nip or in a soft polymer/steel nip (one passage). With the steel/steel nip, the temperature of the heated steel roll was 40, 100 or 200 °C and with the soft polymer/steel nip it was 40 or 100 °C. After the precalendering, the papers were blade coated with a conventional clay-based coating colour using a laboratory coater. The coat weight was held constant (about 10 g/m 2 ). A portion of the coated material was subjected to a post-calendering in one soft nip at a speed of 300m/min. The temperature of the heated steel roll was then 80 °C. The precalendering enhanced the gloss and reduced the surface roughness of the coated paper. This was observed for the papers that had been sub-jected to the post-calendering, as well as for those that were only coated. Using a burn-out test and an image analysis technique, it was concluded that the more the paper was densified in the precalendering operation, the more uniform was the mass distribution of the coating layer. Earlier work has indicated that such an improvement in the coating mass uniformity corresponds to a less mottled offset print, i. e. to an improvement in print quality. The changes in the properties of the coated paper and in the mass distribution of the coating layer could be related to the caliper reduction in the precalendering operation. No specific influence of the other calendering conditions (machine speed, roll temperature or roll material j was observed. The results obtained suggest that a precalendering of this type of base paper can have positive effects on the proper-ties and the uniformity of the coated product. A natural continuation of this study would be to ascertain to what extent the improvements due to the precalendering are available in the industrial production of coated woodfree papers.