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FLEXO Magazine : March 2011
Technologies & Techniques Ink & Its Influence How to Print Faster with Cleaner, Truer Colors By James Ford Printing inks have often been compared to gasoline: Use the wrong kind and you won’t get the best perfor- mance from your finely tuned machine. Choosing the correct ink is increasingly important as the industry switches to ever-finer aniloxes in presses running at ever-faster speeds. But as with gasoline, ink’s effectiveness and value come from the materials used in its creation. INK CHARACTERISTICS Several ink characteristics influence the quality of a print job, including drying speed, resolubility, pH dependence, sur- face tension, foam control, rheology, solids, fineness of grind, lubricity, chemical resistance and cost. Each of these charac- teristics may be adjusted to fit the needs of a particular print job. Unfortunately, sometimes optimizing one characteristic detracts from another. For example, optimizing resolubility will affect water resistance in a water-based ink. Water-based inks that provide a water-resistant film have a tendency to be less resoluble unless the drying speed is also affected. Skid resistance and rub resistance comprise another common example. In the end, selecting the best ink for a particular job amounts to a compromise among several characteristics. RESINS If we look at each characteristic individually, our choices seem simple. As press speeds increase, the inks need to set or dry faster. This becomes possible by selecting the proper resins, increasing the solids of an ink or, for a conventional pH-dependent ink, changing the pH adjuster. Choosing a resin is usually the starting point for develop- ing new ink systems, and there are many types of resins from which to choose. Acrylics and styrenated acrylics are most common, but polyurethanes, polyvinyls, epoxies and a whole host of bio-derived polymers are also available. Resins can take the form of colloidal dispersions, resin emulsions, resin solutions or even something more complicated. Subtle differ- ences between resins mean a lot to the ink maker, but may not be that well-known to the printer. In a nutshell: • Colloidal dispersions are water-based dispersions of resins that can be made water-soluble by the addition of a basic neutralizing agent. • Resin emulsions are dispersions of insoluble resins that usually require some type of coalescent to form a film. • Solution resins are resin salts already made soluble in water. • Other resins include self-crosslinking resins, core-shell morphologies, etc. 28 FLeXO marCH 2011 www.flexography.org Ink Flow...Chemistry, Art, Adventure • As press speeds increase, the inks may need to set or dry faster. • Resins provide for better printability while retain- ing the water resistance and alkali resistance that customers require. • Resolubility is the chief characteristic required for printing cleaner colors and is greatly affected by the resin choice. • Surfactants or solvents must be added to a water- based ink to get it to wet and adhere to the surface of a polymer film properly. • Rheology must be tightly controlled to achieve consistent and cleaner printing, including process printing. • Choice of pigments and the degree of dispersion, or fineness of grind, can also affect an ink’s printing performance. FLX_March11.indd 28 3/18/11 1:32 PM