HP Scitex FB6100 Printing Technologies - Page 19

Swellable Coatings Swellable coatings protect the dyes from atmospheric gases and offer the longest display permanence for dye-based prints. However, swellable coatings for aqueous inks are softened by water and generally offer poor water-resistance and wet-smudge resistance.53 Prints should be protected by displaying them behind glass or by lamination after the print is dry. Swellable coatings consist of several layers over a photobase paper. The imaging layer and undercoat are made from natural and synthetic polymers. The imaging layer swells as it absorbs ink vehicle and dyes. When the ink vehicle evaporates from both layers, the coating shrinks to its original thickness and the imaging layer encapsulates the dyes. Prints dry by evaporation of volatile components in the ink vehicle. While prints may be dry to the touch and handled minutes after printing, drying may take hours or days depending on temperature and humidity.55 A resin coating provides a smooth substrate for the imaging and protection layers. It prevents curl after printing by blocking ink (and water) from entering the photobase paper. Pigments are too large to penetrate swellable coatings, and they remain on the surface as ink vehicle is absorbed by the imaging layer and undercoat. This can lead to gloss variations and bronzing as discussed for pigment inks on nanoporous media. Absorbent Media Absorbent media allow aqueous and solvent ink vehicles to penetrate into the substrate as the print is produced. Plain paper is an example of an absorbent media. The ink vehicle may penetrate completely through the sheet under some circumstances. Strikethrough occurs when the ink vehicle carries dyes far enough through the sheet that a reversed image appears on the back side. Prints on absorbent media can be handled once the ink vehicle is completely absorbed and the colorant binds to the surface. Prints may feel slightly damp until volatile components in the ink vehicle evaporate. This process occurs naturally or may be accelerated by a heater and forced airflow. If UV-curable inks penetrate too deeply into absorbent media, UV light may not reach all the ink. In this case, they will not be completely cured. At the surface, UV-curable ink fills and solidifies within open pores to produce a mechanical bond to the printing material. This process is called "keying". The essential difference between absorbent and other types of printing materials is that the ink vehicle penetrates into and is absorbed by the core material of the sheet. Papers may be multi-layer structures with high-quality fibers in an outer layer covering a core of coarser fibers designed to provide thickness, stiffness, and strength. The schematic drawing at left shows how aqueous and solvent ink vehicles penetrate the porous structure of absorbent media.56 In general, dyes penetrate farther than pigments, which stay at or near the surface. 55 Consult usage information supplied with the printing material. For example, dry time for HP Premium Plus Satin Photo and Photo Gloss Papers is specified as 5 minutes at 23ºC and 50% RH. HP recommends allowing prints on Premium Plus Photo Papers to air dry for one day at less than 70% RH before mounting behind glass or plastic. 56 UV-curable inks also penetrate to form a mechanical bond to the surface, but this penetration is limited by the higher viscosity of these inks and their rapid solidification. 19