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FLEXO Magazine : September 2009
www.flexography.org SEPTEMBER 2009 FLEXO 75 TECHNOLOGIES & TECHNIQUES HISTORY METALLIC PIGMENTS As early as 2,500 B.C., gold was used as decorative coating in building structures and ceramics. In 1830, George Benda in Nuremburg, Germany invented a bronze grinding mill, and in the 1930s E.J. Halls developed the process for safe indus- trial production of aluminum pigments. Copper, zinc-copper alloy and aluminum have been used ever since through these mass production processes and have made possible applica- tions such as metallic effect for cosmetic, plastic, and building structures in the form of paints and inks. Metallic effect pigments for surface printing come in both granules and platelet-shaped forms dispersed in vari- ous binder systems. In different manufacturing processes, lubricants are used to prevent cold welding. The chemical composition of the lubricant influences the wetting behavior of the metal pigment in a characteristic way. With aluminum pigments, there are leafing and non-leafing types, while with gold bronze and zinc pigments only leafing types are pro- duced by the grinding process. Leafing pigments have higher surface tension. As a result, they are not wetted by the binder matrix and, therefore, will float in the wet film and orientate themselves on top the surface. Because of their higher reflective/gloss nature, leafing pigments are used in decorative paints and chrome effect inks. However, the rub- and scratch-resistance are only moderate because these flakes are marginally surrounded by binder on the top surface. Non-leafing pigments, on the other hand, are fully wetted by the binder and, therefore, distributed uniformly in the matrix. The appearance of a non-leafing pig- ment system is not as metallic as that of a leafing system but does have good rub- and scratch-resistant properties. ADVANCES IN ALUMINUM PIGMENTS Requirements of the ink industries have always called for brilliance, strong flop (angle-dependent lightness), hiding powder, and mirror-like effect. The leafing nature of aluminum and the correct polarity range of the binder system will deliver more reflective characteristic of the final film. First-generation silver inks are based on cornflake-like, spherical aluminum powders and give more metallic finishes than regular gray inks. Second generation silver inks are based on so-called "silver dollar" aluminum pigments, which are created by flattening the spherical/granule pigments. Brightness and flop effects were achieved with first- and second-generation silver inks. The reflectivity is improved with silver-dollar type aluminum. However, the mirror-like brilliance effect is still missing compared to those of the foil laminated boards and metallized films. As a result, the graphics look dull compared to their metalized counterparts. The latest breakthrough to make foil-like finishes pos- sible was the production of microscopically thin, mirror-like aluminum particles (platelets) created in a physical vapor deposition (PVD) process. This foil-like mirror effect is shown in Figure 1. Due to the thinner thickness of the PVD aluminum particles (30-50nm versus 100-500nm of the first and second generations), better packing and less edge effect of these thin platelets can be achieved. These special parameters lead to more light reflection, plus less scattering at the surface and produce mirror-like finishes. UV CURABLE METALLICS The newest type of innovation in metallics is a a single- component, 100-percent solid, UV-curable, silver-color coating utilizing the PVD aluminum particles (platelets). This shelf- stable, press-ready silver UV coating can be cured at the same line speed as regular UV topcoats in standard printing presses and can create foil like finishes rivaling that of foil boards. The term silver coating is based on the fact that it can be applied by, or designed to be used in, almost all available coating methods. Optimum brilliancy is also obtained through the coating unit. The UV-curable approach is preferable to solvent- or water-based systems because of the curing speed, the fact that no additional oven and incineration units are required, the small equipment footprint and, and finally, the environ- mental-friendly technology. Shelf stability of the liquid is unmatched when compared to alternative aluminum-containing UV chemistry. Aluminum normally causes pre-mature-polymerization of UV chemistry, which shortens the shelf life of aluminum-containing UV coat- ings and inks. For this reason, many aluminum-containing UV products are either offered in two-part systems to avoid such reactions or partially filled in small containers to allow airborne oxygen in the headspace to slow the polymerization reaction via oxygen inhibition. With proprietary technology, these new coatings negate this effect and offer a standard six-month shelf life, similar to regular UV coatings. As a result, it maintains brilliance with age under recommended stan- dard storage conditions for most UV products. IN-LINE CAPABILITY By itself, UV-curable metallic coatings exhibit foil-like bril- liant silver finishes. However, they are also designed to be over-printable and over-coatable with other inks and protec- tive coatings. Therefore, besides the inherent brilliant silver color, a broad range of metallic colored effects---including all shades of gold---can be achieved by printing transparent or opaque inks over cured silver finishes, similar to those used for the foil boards, but with some variation in techniques. It is important to fully cure the coating and freeze the silver platelets in their shinny state before any other ink is applied to create other metallic colors. To assist graphic designers in artwork creation, the manufacturer provides a metallic color selection guide using Pantone and process colors (Figure 2). Protective topcoats, including UV topcoats for maximum bril- liancy, and water-based topcoats, should be applied over the entire design. The best way to take the maximum advantage---especially for cost savings---of these coatings is to execute the entire job in a single in-line printing step. A printing press with a front- end coater is the best way to utilize this in-line foil-on-demand feature. A foil-like silver coating is pattern applied in the front-end coater, followed by the desired color sequence (both transparent and opaque colors), then finished off with the top- coat in the back-end coater. An in-line processing schematic FIGURE 1. A typical cross-section of the conventional silver (left) and PVD silver (right) layers.