HP Scitex FB6100 Printing Technologies - Page 20

The surface of an uncoated paper printed with dyes and pigments is seen in the figure at left in two microscopic views.57 In the image labeled "dye", dye molecules (too small to be seen) have coated the surface fibers and been carried into the fiber matrix by the ink vehicle.58 In the image labeled "pigment", a polymer film containing pigment particles has coated the surface fibers. Dyes are chemical molecules that are small enough to be carried by the ink vehicle deep into absorbent media, and this can reduce black optical density and color saturation. Printing with higher ink limits in an attempt to leave more dye at the surface can cause color bleed and feathering, where ink wicks along surface fibers and reduces edge sharpness, and strikethrough. Dye molecules can be damaged by exposure to atmospheric pollutants (e.g., ozone) and UV light, and dyes may redissolve when the print is exposed to water. For these reasons, dye-based prints on absorbent media generally have lower display permanence and water resistance compared to prints on coated media. Print permanence and durability can be improved by laminating prints with a thin plastic film that blocks UV and protects the print from gases, water, and abrasion. Lamination improves the durability of prints made with pigment inks on absorbent media by providing a mechanical barrier against water and abrasion. Lamination is especially useful for making prints more durable on paper-based and other media that may not be very water- or abrasion-resistant. Pigments are too large to penetrate deeply into most absorbent or porous media. Pigment penetration can be reduced by causing pigments to separate from the ink vehicle at or near the surface of the printing material. For this purpose, the ink vehicle can contain binding agents or polymers59 or pigment particles can be coated by resins.60 In addition, chemical treatment of the surface and changes in ink composition as some ink vehicle components penetrate more rapidly than others can cause the pigments to crash, meaning that they are no longer dispersed by the ink vehicle and carried with it. Once separated from the vehicle, pigments remain at or near the surface of the print medium as the ink vehicle is absorbed into the substrate. Nonabsorbent Media Nonabsorbent media do not allow the ink vehicle to penetrate into the bulk of the printing material. Plastic materials, such as backlit films and vinyl, are examples of nonabsorbent media. Although inks designed for nonabsorbent, uncoated media (such as vinyl) contain solvents that soften or dissolve the surface, the ink penetrates only a few microns and the print dries by natural or forced evaporation. This figure shows a schematic cross-section of vinyl printed with a solvent ink. The solvent dissolves a thin surface layer, pigments diffuse into this layer, and the solvent evaporates leaving the pigments encapsulated by the substrate material. HP's low-solvent inks soften the surface to improve adhesion to a binder in the ink vehicle that forms a polymer film encapsulating the pigments. HP Latex Inks use co-solvents to soften vinyl for adhesion to the filmforming latex polymers that encapsulate the pigments. A surface treatment can be used to improve adhesion of inks that do not dissolve or soften the surface of nonabsorbent materials. For example, the HP Scitex FB6700 Printer applies a liquid pretreatment to the substrate before printing with (water-based) HP Scitex WB300 Supreme inks. 57 Electron microscope images, source: HP R&D. The white bar represents 10 microns. 58 The small, white globules around the fibers are sizing agents and other additives. 59 For example, the latex particles in HP Latex Inks. 60 For example, Vivera pigment inks with Electrostatic Encapsulation technology. 20