Soft calendering: expériences on a pilot coater

Sized papers, pigmented papers and mat coated papers require, as classical coated papers, good surface properties in order to be printed. In this way, and to maintain the productivity rates, on-line softcalenders have been installed on paper machines. Fewer nips are used than in supercalenders, mostly one nip twice, and the rolls are made of steel (hard rolls) and polymer (soft rolls). Steel rolls can usually be heated using oil or steam. Temperature, pressure and speed are then adjusted in order to reach the required paper properties, taking into account the moisture content of the paper. Calendering is consequently a critical step in the paper production and must be carefully adjusted. Thus, pilots may be very helpful to the papermakers due to their flexibility. A softcalender has therefore been installed on-line on the C.T.P pilot coater. High pressures and temperatures may be reached. In this work, the performance of the C.T.P on-line softcalender is studied. Preliminary trials have been performed under various calendering conditions on two wood-free papers in order to determine the feasibility of trials under high pressure and temperature. Calendering trials were then carried out on a coated paper and a sized paper under various calendering settings. Paper properties, such as gloss, smoothness, brightness and opacity, and the quality of the printed papers were scrutinised. The results obtained show the importance of the dwell time of the paper in the nip, especially under extreme conditions.