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FLEXO Magazine : May 2012
based upon the viscosity, percent solids, and opacity of the ink. It is crucial that any comparisons between inks made in this manner be done at equal viscosities. Regardless of what type of ink is being used, coverage is also impacted by the surface energies of the inks and substrates. There are too many test methods available to measure surface energies to mention in this article. Here are two that are often mentioned: • ASTM D2578-09 Standard Test Method for Wetting Ten- sion of Polyethylene • Polypropylene films and ASTM D7490-08 Standard Test Method For Measurement of the Surface Tension of Solid Coatings, Substrates and Pigments using Contact Angle Measurements Many printers will also use dyne pens to quickly establish the surface energy of a substrate in order to determine print- ing suitability. The important thing to remember from an ink fitness standpoint is that the surface energy of the ink (see chart) needs to be less than that of the substrate in order to achieve optimal wetting and coverage. Solvent inks will generally have lower surface energies than water-based inks, so there are some substrates that will be more difficult for water-based inks to cover well. Work is continually being done to try and find ways to replace solvent inks due to VOC and other regulatory concerns, but these often lead to seri- ous impacts upon print performance. DOES IT STICK? Adhesion is probably the most important criteria for any ink system and substrate combination. If the ink does not have good adhesion to the substrate, there could be significant issues with rub, delamination, and contamination. Consider- ations of rub must be considered. No one wants to get the call from an angry consumer with ink transfer on new upholstery. Adhesion can be tested in a number of ways. The simplest would be via some type of tape adhesion test. Good refer- ences are: • ASTM D3359-09e2 Standard Test Methods for Measuring Adhesion by Tape Test • ASTM F2252-03(2008) Standard Practice for Evaluating Ink or Coating Adhesion to Flexible Packaging Materials Using Tape • ASTM F2452-04 Standard Practice for Determining the Adhesion of Print Media Utilizing Mechanical Stress • Two Different Test Methods x2014; Score/Tape and Cross Hatch If you can pull the ink off with a tape test, either the ink needs to be improved or the substrate needs to be refined. Resins used for film substrates are often softer than those used for paper substrates to improve the adhesion. The ink supplier uses other additives to improve adhesion, but these often come with sacrifices in toughness, blocking perfor- mance and resolubility. However, a big impact on these types of tests can be with the age of the substrate. Many surface energy treatments on a substrate will diminish with time. Plasticizer migration can affect adhesion on some films. For these reasons, it is important to test the ink on fresh substrate treated the same way that it will be treated on press. RESISTANCE IS NOT FUTILE When it comes to choosing an ink system, it is important to understand the conditions in which the dried ink will come into contact. If the print will be exposed to high moisture conditions, it will be important for the ink to have good water resistance. If the print needs to withstand high temperatures, it is important for the ink to have good heat resistance, and so on. There are all sorts of resistance consideration that inks may be required to withstand. These could be chemical (water, solvent, ammonia, gasoline, oil, etc.); thermal (heat, cold); physical (abrasion, scratch, curl, cracking, etc.); or conditional (fade, yellowing). Common test methods used to test resistance properties of inks and coatings are: 46 FLEXO may 2012 www.flexography.org Surface Energies Substrates Liquids Aluminum 40 Water 73 Polyester 44 Propylene Glycol 35 Polyethylene 34 Toluene 29 Polypropylene 32 Isopropanol 22 Paraffin 26 Siloxane surfactant 25 PTFE 20 Silicone Fluid 20 Coated papers 24-67 Fluorosurfactant 17