Calendering Thermal & Moisture Effects on LWC Paper with Precipitated Calcium Carbonate Coating Applied by Metered Size Press, 2002 Metered Size Press Forum

The effects of calender roll temperature, calender line load, and paper moisture on the finished properties of coated paper were studied at pilot scale. The paper was coated with a precipitated calcium carbonate (PCC)-containing formulation applied by a metered size press (MSP). The finishing was carried out with a multinip temperature gradient calender stack. Compared to low temperature and low moisture, high temperature and high moisture calendering generated much better paper gloss and smoothness, and produced lower brightness and opacity. A synergy between PCC and temperature gradient calendering was found. The ratio of gloss gain to brightness loss increased rapidly with calender roll temperature. This indicates that temperature gradient calendering can be used to increase gloss while PCC can preserve high brightness. Smoothness was most influenced by paper moisture, suggesting that smoothness is increased by interior as well as by surface compression. Mercury intrusion porosimetry was used to determine the effect of the calendering on the PCC coating layer itself. At constant paper gloss, higher calender roll temperature resulted in higher coating porosity. The higher temperature produced the target gloss at lower pressure, thereby causing less crushing of the coating layer and less loss of optical scattering.