Electronic input interfaces are found everywhere such as on a treadmill at home, an infusion pump in a hospital or lighting control console in a theater. One thing they have in common is that the graphics are printed on the back side (“2nd surface”) of polyester film. This film is typically supplied with a UV cured hard coat that provides protection from scratches and chemicals. Most are glossy or anti-glare coatings but they are easily smudged with fingerprints. Even the anti-glare coating is relatively glossy and presents a cold hard appearance. To counter this, texture coats are applied to the non-window areas. These coatings are screen-printed on the front side (“1st surface”) and can be tailored to provide different levels of texture and gloss. They are UV cured to provide equal or better resistance to chemicals and scratching than the original hard coats. When they are applied over keypads, texture coats can provide a warmer tactile feel than the smooth hard coats do.
UV curable texture coats are mainly one of two kinds: those in which the texture is created by the inclusion of particles in the coating and those in which the texture is created by making the coating’s surface wrinkle during cure. Both kinds are formulated with oligomers, monomers, photoinitiators and other additives. The particle type of coating also contains many dispersed particles of varying materials. These can be inorganic like glass or minerals; or organic like beads of acrylic, polyurethane or polyethylene. Particles can be spherical or irregular shaped. They can be of various sizes (i.e., 5 to 50 microns). Even smaller particles (sub-micron) like silica are typically added to reduce gloss. When coatings are screen printed the thickness of the coating is similar to the diameter of the fibers in the screen mesh (there are other factors outside the scope of this article). The maximum particle size is limited by the mesh opening in the screen. A 305-mesh screen might yield a 22 micron thick coating with up to 45 micron sized particles. More realistically, maximum particle size would be less so more than one particle at a time could flow through the screen and so that a greater percentage of each particle could be anchored by the coating. Particles smaller than 22 microns would not add to the texture (they would be submerged) but could affect haze or gloss.
The other type of texture requires a unique combination of processing technique, equipment and chemistry. The coatings may contain a small amount of matting agent but no particles larger than the coating thickness. In general, the texture is created by first UV curing only the surface of the coating but not the bulk below. During UV cure many small molecules combine in a chain reaction to form a few giant molecules. As the molecules come together the overall surface area shrinks. Many chain reactions are initiated so the shrinkage is irregular. Since the bulk of the coating remains uncured and liquid the shrinking skin is unanchored and allowed to form micro-wrinkles. A fraction of a second later the bulk is cured, “freezing” the wrinkles in place. These wrinkles are very small and irregular so they create a fine texture that diffuses light equally in all directions. The trick to this process is partly in the chemistry. The coating needs a photoinitiator that promotes surface cure in the first stage of the overall cure. The coating also needs another photoinitiator that promotes deep cure in the second stage of the overall cure. Working in concert with the photoinitiators, the UV reactor is set up with lamps that emit UV radiation at low power and short wavelengths followed by lamps that emit UV at high power in a broad spectrum, including long wavelengths. Furthermore, since oxygen can inhibit surface cure, ambient air must be displaced with an inert gas. Surface cure has the added benefit of improving scratch and chemical resistance. The texture characteristics can be modified by adjusting cure dose, gas flow and chemistry.
|Particle Texture @ 60X||Wrinkle Texture @ 60X|
The wrinkle texture coats have finer texture, lower gloss, more chemical resistance and greater hardness than the particle texture coats. They are also more brittle and more expensive due to the unique chemistry, equipment and process needed to make them. GM Nameplate is capable of printing both types of coatings in our Seattle plant but prints only the particle textures in San Jose, Monroe, Singapore and China.