Paper coating properties as affected by pigment blending and calendering

To gain insight into how and why pigment blending affects coating pore and surface structures and how these structures are affected by calendering, we prepared coating colors with different blending ratios of ground calcium carbonate (GCC) and clay with a similar particle size distribution using a constant level of binder and co-binder. We applied the coatings on plastic films and calendered them using a paper sheet backing to account for the part of compression absorbed by the base paper. The coating layer structures were characterized by mercury porosimetry, atomic force microscopy, gloss, and spectrophotometric measurements. The results showed that synergy occurs for pore volume, pore size, and light-scattering coefficient when blending GCC and clay; that is, the property from a pigment blend can be higher than what is predicted from simple additivity. For example, the pore volume and pore size reached a maximum value of approximately 70% clay and 30% GCC. However, no synergy was observed for smoothness and gloss. Calendering significantly compressed the coating layers and reduced some of the synergy in pore volume and pore size created by the blending. Although the 100% clay coating was twice as much compressed by the calendering as the 100% GCC coating, the blending of clay and GCC at a wide range of ratios showed a similar extent of compression (calculated as reduction in pore volume). We proposed a mechanism to explain the synergies by blending and the compression caused by calendering.