The influence of application system on the structure of coating layer

Extensive experimental studies have been carried out to determine the effect of coating application system on the structure of a coating layer. All-clay and all-CaCO₃ coating colors have been coated at high speed and at equal coat weight and machine conditions on woodfree base paper, and the surface and porous structure of the coating layer have been characterized with advanced analytical techniques. The automated serial sectioning technique that combines microtomy, microscopy and image analysis reveals that curtain coating in general provides a higher coating thickness and lower smoothness than blade and filmcoating. The results furthermore show that the clay formulation provides a significantly higher coating thickness than the CaCO₃ coating in curtain and filmcoating but that this effect is eliminated in blade coating. This is believed to be due to the random orientation of plate-like clay particles resulting in the high coating bulk in curtain and filmcoating whereas the parallel particle orientation under the blade perhaps explains the lack of positive bulk contribution of clay in blade coating. The conclusions from the serial sectioning based coating layer analysis are supported by FIB cross-section imaging, ATR particle orientation spectroscopy as well as mercury porosimetry results.