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FLEXO Magazine : May 2012
LEAFING OR NON-LEAFING Metallic inks have been used for decades in printing and packaging applications because they offer a unique eye- catching appeal. The luster of such inks helps symbolize luxury and elegance or something futuristic or technical. These effects are actually derived from metal particles, such as brass, which creates a gold hue and aluminum, which cre- ates a silver color. The metals are ball milled to form coarse particles down to finer platelet or corn flake particles that can be used in printing applications. During the course of milling the metal particles, a lubricant is used to help maintain the integrity of the finer particles. Generally, a fatty acid lubricant is used and this imparts a special property to the metal particle or pigment. The prop- erty imparted is referred to as either leafing or non-leafing. Leafing means that the metallic particles float and cluster at the surface of the ink film; non-leafing pigments have a random orientation (see Figure 1). CURE METHODS MAKE A DIFFERENCE Not only does the choice of metallic pigment influence the behavior, but the curing process used to print shrink sleeve labels also has a major impact. The most common techniques used to cure inks in these print processes are to thermally cure to evaporate solvent or water or a process to use UV energy to polymerize the ink. Thermally curing metal- lic inks produces a desirable metallic effect compared to UV curing since the resulting printed film has an optimal metallic pigment-to-binder ratio. (See Charts 1 & 2) The resulting pigment-to-binder ratio is significant in how it impacts the luster or brilliance. In general, a metallic ink film with a high pigment to binder ratio will produce a more brilliant effect. As previously described, metallic pigments can be leafing or non-leafing, this also influences the behavior of optimizing metallic hues. For shrink sleeves the inks are usually reverse printed on the film. Obtaining a good metallic effect requires that a high degree of pigment orientation at the ink/ film inter- face rather than the ink/ air interface. It is necessary to use me- tallic pigments with non-leafing behavior to achieve this. These types of pigments are readily available for solvent-borne inks; but not so for UV systems. Most pigments for UV ink formula- tions have a surface that provides leafing behavior. While there are pigments for UV systems that are non-leafing, most are too coarse to be used in graphic arts applications. OVERCOME THE CHALLENGE Today, advanced milling technology has created a new me- tallic pigment that has non-leafing-like behavior and can be used in UV ink formulations. One of the unique characteristics of this pigment that makes it ideal for shrink sleeves is the size and shape, allowing it to maintain brilliance during the shrink- ing process compared to traditional flakes. www.flexography.org may 2012 FLEXO 37 FIGURE - 1 Comparison of pigment to binder ratios of flexo metallic printed ink