Effect of Calendering on the Properties of Paper Containing Flexible Calcium Carbonate with a Cellulose Nanofibril Core.

Flexible calcium carbonate (FCC) was prepared by attaching a large amount of calcium carbonate to cellulose nanofibrils (CNFs) by an in situ calcium carbonate formation method. FCC normally consists of CNFs and calcium carbonate at a 1:40 ratio by weight. FCC-containing papers resulted in a higher bulk, higher stiffness, and higher tensile strength than a commercialized ground calcium carbonate (GCC)-containing papers at the same ash content. However, there were speculations that calendering on FCC-containing paper might cause a large drop in bulk and no increase in smoothness due to the larger size of the FCC (avg. dia. 20-30 µm) than the GCC (dia. 2 µm). FCC-containing paper was shown to respond to the calendering process very effectively to increase the Bekk smoothness and to maintain high bulk. Furthermore, FCC-containing paper was so effective in increasing smoothness that it might need less calendering pressure to match the smoothness of GCC-containing paper. If so, there could be potential to increase the bulk and stiffness further in FCC-containing papers at the same smoothness as GCC-containing paper by applying reduced calendering pressure.