A review of modified surfaces for high speed inkjet coating

There are many interaction phenomena occurring between inks and the printed surface during the inkjet ink setting and drying process on coated papers. First, the ink wets the surface and then absorption into the capillary network occurs. After milliseconds, the colourant and solvent/diluent parts of the ink separate. Wetting of the coating surface, and the interior of the capillary network, depends on the relative surface energies of the components being brought into contact, and their interaction may include selective adsorption. Additionally, diffusion phenomena occur a few seconds after the ink arrives at the surface, which may be vapour dependent or liquid-liquid and liquid-solute concentration dependent. The final ink drying and polymerization can last for hours. The interactions have a direct connection to the final print quality. The most important properties of the coated surface are the porous structure parameters - pore size, amount and the network they form - and the chemical nature. In the beginning, at the first contact of the ink with the surface, the chemical properties of the coating layer and the ink dominate. As the mobile ink components penetrate deeper into the coating structure, the pore structure becomes more important. In some cases depending on the nature of the binder (water soluble and/or emulsion polymer-based), the ink diluent or solvent may enter the intermolecular structure of the binder of the coating layer by diffusion, and the binder often swells as a result. A more hydrophilic binder in contact with water-based ink swells more than the one that is less hydrophilic. The most important control parameters in designing inkjet print surfaces, from the point of view of ink spreading and penetration, are the porosity, permeability, thickness/bulk, sizing (hydrophobizing), moisture content, temperature, and capillary structure of paper. Depending on whether an ink is dye-based or pigment-based, a main aspect that affects the print quality is the distribution and permanence of the colourant. The aim, in the inkjet ink setting process, apart from providing a dried ink surface, is to fix the colourant part of ink at or in the uppermost layer of the coating whilst allowing the diluent/solvent part of ink to penetrate deeper into the structure. Proper colourant fixation produces high optical print density, bright colour tones, high sharpness, low bleeding, low print-through, and high rub and wet resistance. (C) 2009 Elsevier B.V. All rights reserved.