A non-contact spray coating technique with color application directly onto a surface has recently been introduced for surface treatment of paper and board [1]. The technique opens possibilities to improve coating process efficiency with less web breaks over conventional techniques with a direct mechanical contact. The reduced strength requirement for the base paper also allows reduction of the Kraft content, which then contributes to lower furnish costs. Additionally, there is also a color formulation saving to be gained over other coating methods. The simultaneous (C2S) paper treatment and the possibility to increase base paper moisture also add savings in investment costs due to less space required. The application method is based on a spray fan producing color droplets, which levels at the surface impact and produces contour-type coverage. The spray application, however, puts new requirements on the color formulation design to achieve optimal atomization and on the base sheet properties to get optimal leveling of the droplets. Here, the atomization requires low flow resistance of the colors, which is mainly connected to pigment properties and their flow characteristics. A 5-10 % lower gloss for spray coated than MSP and blade coated paper at constant calendering conditions is balanced by the extreme contour-type coverage enhancing optical evenness. An even binder distribution in the coating layer is currently a challenge, which is needed to reach a required surface strength for offset (HSWO) printing.
展开▼
