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FLEXO Magazine : May 2012
but the majority of the pigment in paint is clay or other fill- ers. These are low cost and do not provide much opacity. Although fillers do not add much to opacity, but they do add bulk and lower the cost per gallon. Since paint is applied at a heavy coating weight—about 25 times more than ink—opacity is not the highest concern. This is because some opacity can be gained through the higher coating weight. The use of fillers has a negative impact on opacity since ink is applied at a much lower coating weight. The key factor for opacity in ink is the refractive index of the pigment used. Each pigment has a different refractive index; the higher the index the more light is refracted and the higher the opacity. There are a number of white pigments that could be used to make white ink, but TiO2 has the highest refractive index (see chart). Any amount of other pigment will lower the opacity of ink. So, reducing the TiO2 content of a white ink, or adding other pigments, is not a feasible approach to cost mitigation, due to the resulting loss of opacity. Printers desire increased opacity for two reasons. The first is to give better mileage through higher coverage per pound of wet ink (economics). Secondly, printers long to provide more hiding power (graphics). Simply put, opacity is mainly related to the pigment content of the applied ink. If the end goal is increased mileage, then the increase of pigment to resin ratio is a viable approach. But there is a limit because a minimum pigment-to-binder ratio is required to provide other properties related to printing (transfer) and end use perfor- mance (resistance). However, if the end goal is an increase in opacity or a combination of both opacity and coverage, then little will be achieved through a simple increase in pigment, as the resulting viscosity increase and the flatter viscosity curve will result in a loss, or at best, equal opacity. www.flexography.org may 2012 FLEXO 31 White ink—the finished product next to its raw element.