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FLEXO Magazine : March 2011
Technologies & Techniques As you might imagine, water-based resin technology has also progressed through the years. Much of the innovation has moved toward the development of resins that provide better printability while retaining some of the water resistance and alkali resistance that customers may require. It seems contradictory to have to use a water-based ink to produce a water-resistant print. However, this is common through the use of fast film-forming resins, cross-linking resins or catalysts. But remember, if the ink is designed to form a fast water-resistant film on the substrate, it is likely to form that same water-resistant film on the plates and anilox--especially if the ink supplier ’s instructions are disregarded and the ink is not properly maintained through the length of the print run. RESOLUBILITY Resolubility is the chief characteristic required for printing cleaner colors, and it is greatly affected by the resin choice. As defined, resolubility is the ability of an ink, when partially dried, to be redissolved by fresh ink. Solution resins provide for significant resolubility, but this usually comes at the cost of water and alkali sensitivity. Emulsion polymers that rely on a coalescent solvent for film formation are poor for resolubility because the coalescent will generally lack the power to resol- vate the ink film once it has formed. Resolubility is the main reason why pH-dependent inks have worked in flexographic inks for so long. The right pH adjuster will provide enough neutralizing power to keep the colloidal dispersions open for longer times on press. It may also allow for better resolubility on press, but require longer cure times for the ink film off press before creating optimum resistance properties. With the growth of pH-independent technologies, more advanced resin chemistries have enabled ink makers to pro- duce inks with better resolubilities without relying heavily on pH adjusters. Resolubility, in a fashion, is also a leading factor in the growth of UV and EB curable inks. Such inks remain fluid without needing pH adjusters or other additives until the ink film is exposed to the appropriate energy source. SURFACE TENSION Surface tension is often overlooked. Water has a surface tension greater than 70 mN/m (or 70 dyn/cm), meaning that it is necessary to add surfactants or solvents to a water-based ink to get the ink to wet and adhere to the surface of a poly- mer film properly. This is especially important when the film has not been properly treated. Poorly treated substrates, and some recycled substrates, require special surfactants or solvents to obtain good cover- age. However, surfactants can create foam issues, and as you know, foam does not print very well. Adding solvents to water-based inks is often impossible because of environmen- tal restrictions on volatile organic compounds. So, when the ink supplier asks the printer what the sub- strate’s dyne level is and whether the printer checked the corona treater, the questions are asked for reasons. Ink sup- pliers know what can be done to improve their inks, but the improvements may create further complications. RHEOLOGY Rheology is a key characteristic of an ink’s drying speed, transfer properties and tonal properties. Rheology must be tightly controlled to achieve consistent and cleaner printing, including process printing. This flow characteristic of the ink is governed by the choice of resins, solvents (including water) and pigments used in the formulation. Without getting into too much detail about the differences between Newtonian, pseudoplastic, viscoplastic and thixo- tropic rheological behaviors, it is important to note that how the ink flows affects how the ink prints. Watery inks tend not to transfer from anilox roll to plate, and plate to substrate, as would heavier inks. However, if the ink is too heavy, it may not flow out of the anilox as well, and can lead to mottling or anilox starvation. So, theoretically, there is an optimal ink rheology for each press and set of printing conditions a printer might have. This is the main reason why ink suppliers ask to be involved in the fingerprinting process, or at least put so much effort into the characterization process to determine a proper profile for matching inks to the press. LUBRICITY Lubricity is often affected by additives used by ink formula- tors. Waxes and silicone derivatives are added to increase lubricity, or slip, whereas frictionizing agents are added to decrease lubricity. Printers or end-users often speak of this in terms of the coefficient of friction (COF) or slide angle of the print. Waxes and silicones added to improve an ink’s rub resistance or abrasion resistance may lead to print defects because of the surface tension or particle size of the wax. These additives will also greatly affect the over-printability of the ink, or the ability of one ink to trap on top of another. Frictionizing agents are often mineral fillers or tacky poly- mers added to the ink to increase the COF or slide angle, but these can be problematic in terms of plugging aniloxes, promoting blade wear and creating subtle increases in ink opacity. If a print job has a particular COF requirement, it is best that the ink supplier knows this before matching any colors for the job. PIGMENTS Choice of pigments and the degree of dispersion, or fine- ness of grind, can also affect an ink’s printing performance. Certain pigments, such as methyl violet, alkali blue, barium reds and rhodamine reds, produce bright, clean colors, but these pigments can have significant disadvantages. Some of them have very poor light-fastness, others may have poor alkali resistance and still others may bleed in certain solvents or have environmental baggage that make them difficult to manage in any ink system. If you cannot control the color on press or you do not have the resources to document the use of regulated pigments accurately, ask yourself if it is worthwhile to use the brighter pigments. Higher quality pigments might not be quite as bright as those they are meant to replace, but their improved stability in terms of viscosity or color-fastness may be well worth the slight reduction in chroma. 30 FLeXO march 2011 www.flexography.org FLX_March11.indd 30 3/18/11 1:32 PM