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FLEXO Magazine : May 2012
The best approach for increased opacity is to combine an increase in mileage and opacity by maximizing pigmentation at the lowest possible viscosity. This is done through the use of the lowest molecular weight polymers/resins available in the chosen technology. The choice of plasticizer and maximizing the production procedures to increase pigment wetting (dispersing) will also have a great influence on the yield viscosity. Maximizing dis- persing procedures shows enormous benefit where cheaper TiO2 is used. These benefits are not as noticeable when soft, easily dispersed TiO2 is used. Unfortunately, cheaper TiO2 has already been widely incorporated as a cost reduction initiative over the last several years as it has skyrocketed in price. Surfactants can also be used to help disperse these pigments. However, the cost may outweigh the benefit. An effective cost/benefit analysis must be done to understand the total cost impact when using any additives to increase opacity. Some work has been done using the resin to assist with opacity. One method is to use two resins that have critical compatibility levels, such as nitrocellu- lose and ethyl cellulose. These are kept stable by the use of ethyl acetate. But, as the solvent dries, the incompatibility reaches a critical point and one, or all, of the resins kick out and dry opaque. This can enhance opacity. However, inks formulated by this principle can be very difficult to run, especially flexo. The industry has looked into expand- ing on the opaque film principle by using opaque polymers, which are submicron particles of resin and have a very high film forming temperature suspended in inert diluents. As the resin has not been dissolved and does not form a film, it dries opaque. This concept is utilized in water- based inks. However, the opacity gain can be somewhat minimal and these resins tend to be more expensive, which reduces the cost-benefits gained from the increase in opacity. The ink industry has investigated TiO2 replacements and light scattering/refracting techniques. But there has been little success with these for solvent-based inks, probably because the solvent is such a good wetter. The excellent interfacing between the pigment and resin reduces the interference, or scatter. A newer approach is the potential use of nano particles to increase opacity. But high cost of nano materials makes their use counter productive if the intent is to lower the applied cost of an opacifying ink. The secret is “there is no se- cret. ” Opacity is primarily based on the concentration of TiO2 pigment that an ink can carry to the substrate. And there is no replacement for pigment that of- fers the same efficiency of opacity and whiteness. Consequently, the industry must continue to utilize TiO2 and work to minimize the cost impact through effective procure- ment and sourcing. IMPACT White inks are used in nearly all package-printing applications. However, the flexible packag- ing industry uses a much higher proportion of white than any other packaging segment. In paper and board applications, the white background is provided by the white substrate, rather than print- ing white inks. Flexible packaging 32 FLEXO may 2012 www.flexography.org The visible impact of pinholes on opacity. Photo: Flint Group.