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FLEXO Magazine : March 2008
TECHNOLOGIES & TECHNIQUES 28 FLEXO MARCH 2008 www.flexography.org Each chemistry delivers a specific set of performance characteristics. The decision to choose an ink, and its specific performance charac- teristics, typically is made based on the printer's need to balance per- formance, cost and environmental compliance. Often the choice comes down to performance. To meet the performance requirements a printer may have a very limited choice of ink systems. So choosing an ink of- ten comes down to the selection of an ink that meets the performance requirements on the press, and its end-use performance in the distri- bution chain. The market has, in most cases, moved to water-based inks, where they have the capability to meet the performance requirements. An example is grocery sacks. All grocery sacks are printed with water-based inks. But, bread bags, although they may seem similar, can not be printed with water-based inks. Bread bags have an entirely different set of performance require- ments. All bread bags are printed with solvent-based inks. All inks are made up of four classes of materials; pigments, res- in, solvent, and additives (Figure 2). The pigment is the color por- tion of the formula. Pigments are solid particles. The resin is also a solid, and binds the pigment to the substrate and provides many of the end-use performance requirements. The solvent reduces the formula to a liquid form, allowing the ink to be printed by the flex- ographic printing process, and modifies the drying rate of the ink. And finally, additives are a wide variety of materials which modify the physical properties of the ink to improve its use on the press, and/or its end-use performance. These additives could be slip aids, defoamers, etc. Most of the materials in all of these classes are petroleum-based products. These are highly refined, and processed specialty chemical materials which deliver very unique qualities to the formulas to which they are added. There are some ink raw materials which come from renewable resource feed stocks. However, all pig- ments used in flexo inks come from non-renewable resources. Some are organic, some are inorganic (Figure 3), but all come from a de- pletable feedstock. In the resin class we have nitrocellulose and polyamide resins, where some of their feed stock is derived from cellulose and crude tall oil (pine trees). Nitrocellulose and polyamide are used in solvent based inks and make up the largest volume of resins used in solvent-based flexo inks. In water-based inks we have protein resins and shellac, although very little shellac is used these days. Protein-based resins are based on casein (milk) or soy protein (soy beans). Ink suppliers receive many inquires about soy inks. Most in- quires regarding soy-based inks are generated because a significant amount of soy oil is used in litho (paste) inks. People incorrectly as- sume soy oil can be used in all inks. Vegetable-based oils work well in litho ink, but can not be used in flexo inks. Protein resins have limit- ed application in water inks because they have poor water resistance and poor compatibility with other resins that may be needed to modify specific performance characteristics. Consequently, acrylic resin is the most prevalent resin used in wa- ter-based inks. Acrylic resins are petro-chemically based materials. There are a wide variety of solvents used in flexo inks. The most sustainable is obviously the water used in water-based inks. We also see ethanol derived from grain sources used in solvent-based inks. But, all other solvents are petro-chemical based The additive class of materials is mostly based on petro-chemical feed stocks; although there are a few natural waxes which are used. THE MOST ENVIRONMENTALLY FRIENDLY Almost daily, ink suppliers are asked, "So, which inks are the most environmentally friendly?" As you can see from the infor- mation above, there is no easy answer. Solvent-based inks have resin and some solvents that come from renewable resources. Water-based inks have resins that are petro-chemically based. Radiation-cured inks are 100 percent solids, but also are petro- chemically based. Water-based inks contain some VOCs, which may be exhausted into the atmosphere. Solvent- based inks can be printed in a pressroom fitted with a collection and incinera- tion system that destroys all VOCs before they can be emitted to the atmosphere. It is certainly a very complex, and complicated, subject. So maybe we need to look at the impact ink has on the package rather than looking at the sustainability of the inks itself. Sustainability, biodegrad- ability, and compostability for inks and coatings are all complex subjects. There is little scientifically based lifecycle analysis research in Additiv e s Resins Pigment Solv ents Flexo Ink Components Figure 2. Inorganic: Titanium Dioxide Iron Oxides Metallic Clay Organic: Carbon Diarylide Pyrazolene Disazo Naphthol Pthalocyanine Quinacridone PIGMENT CHEMISTRY Figure 3.