The influence of coating color characteristics on board coating with a soft-tip blade*

The soft-tip blade has previously been found to yield improved coating coverage compared to conventional steel and ceramic blades. As the soft-tip blade couples the benefits of contour coating with those of blade metering the increase in coverage has been achieved without substantially decreasing gloss due to higher surface roughness. A study was made to investigate how the relative change in coating quality was dependent on the characteristics of the coating color when changing from a ceramic blade to a soft-tip blade. Eight different coating colors were used for topcoat trials where conventional ceramic blades were compared with soft-tip blades. The coating colors had different water retention and rheological characteristics. Their rheology was measured from low to high shear rates alongside with static water retention. Continuous phase viscosity was measured after phase separation in an ultracentrifuge. Oscillation measurements were made on the coating colors to study the thickener-pigment interaction. The uncalendered board samples were tested for optical properties and for surface roughness with a range of instruments. Coating coverage was quantitatively analyzed by scanning burnout samples with STFI Mottling software. Confocal white light microscopy was used to evaluate the surface roughness alongside with the PPS 10 and Bendtsen instruments. The porosity of the coating layer was evaluated with mercury porosimetry. The results show that the relative difference in the coating quality between the samples coated with the soft-tip blade, compared to the samples coated with the ceramic blade, was dependent on the rheology and dewatering characteristics of the coating color. They also show that the soft-tip blade yielded improved, and reasonably similar, coating coverage with all the coating colors tested. The soft-tip blade appeared to decrease differences in th