Water-based Flexographic Printing on Polymer-coated Board

Sammanfattning: This doctoral thesis comprises three studies dealing with water-based flexography on polymer-coated boards, which have been presented in the five papers included at the end of the thesis. The first study (covered in three papers) presents results from characterisation of a matrix of water-based inks with respect to their rheology, surface tension, wetting, and fulltone printing performance on PE-coated board. The commercial water-based inks used vary in type of pigment and acrylic vehicle, and in pigment/vehicle mixing proportions. For all mixing proportions, the inks were shear thinning, with viscosity increasing strongly with content of solution (versus emulsion) polymer in the vehicle. Increasing amount of solution polymer also gave the indirect consequence of lower surface tension values. The wet ink amount transferred to the board and the resulting print uniformity responded sensitively to ink formulation and could be correlated to the corresponding changes in these two ink properties. Transferred amount correlated well with ink plastic viscosity and static surface tension, exhibiting a maximum at intermediate values of both. Print mottle decreased with increasing content of solution polymer, principally due to the associated increase in low-shear viscosity and decreased surface tension, both serving to reduce substrate dewetting.In the second study, the effect of corona discharge treatment of PE-coated board on its surface chemistry and print resistance properties was evaluated. The increase in total surface energy, due to its polar component, with increasing corona level was strongly correlated to surface oxidation fraction from XPS. AFM revealed oxidised material nano-mounds, which increased in size and substrate coverage with corona level. Wet rub resistance (with water) first increased for lower corona dosages before decreasing at higher dosages. This worsened water resistance at higher dosages is presumably due to the oxidised material dissolved and dispersed in the ink film.The third study presents results from another set of water-based acrylic inks printed on boards coated with PE, OPP and PP. Print quality and resistance properties, though dependent on the polymer type, were strongly influenced by the choice of emulsion polymer and presence of silicone additive in the vehicle, with corona treatment level playing a lesser role. The emulsion polymer giving best resistance performance was generally found to give poorest press performance and visual appearance. Again the trends could be understood in terms of wet ink surface tension (versus substrate surface energy), dictating print uniformity, and dry print surface energy (and swellability), dictating resistance properties.