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FLEXO Magazine : May 2012
ink, the ink resin and binder will not sufficiently spread. For example, flexo inks typically have a surface tension of 35-38 dynes/cm. It is recommended therefore that the substrate sur- face tension be treated to 45-48 dynes/cm. To further optimize adhesion, chemical bonding can be improved by introducing more alcohol (COH) groups to the substrate surface-by-sur- face treatment. Additional barriers to adhesion, particularly to films, include high-load levels of slip agents (> 500 ppm) and other components, which provide the film the ability to glide more easily over conveying rolls during processing. However, these additives are, by design, formulated to be incompatible with the film. As such, they migrate in all directions from the interi- or of the film to the surface, creating a “ weak boundary layer” to ink adhesion. This boundary layer of fatty acids needs to be removed so more of the native substrate surface is exposed. OTHER CONSIDERATIONS It is typical for the addition of ink system vehicles to gener- ate higher viscosity in water to therefore allow for lower resin solids. Higher viscosity will also form issues related to dot gain and dot bridging. However, too high a viscosity will im- pede wettability and reduce surface anchorage. Reducing ink viscosity in concert with increased surface tension by surface treatment will allow the ink to adhere better to the substrate. Also, paying close attention to the pH of the flexo ink will con- tribute greatly to ink transfer from the anilox cells and wet-out on the substrate surface. Consider the trapping of flexo inks for a moment. It is well known that print trapping is essential with certain graphic designs in order to compensate for minute misalignments be- tween print stations. This is particularly true when challenging presses to impart high-resolution graphics at high processing speeds. During trapping, if the wet-out of the first ink down is adequate, but the ink to be trapped does not wet-out properly, then it is highly probable that there will be insufficient ink dry- ing and poor ink adhesion. These issues can manifest them- selves with failed in-field performance, such as ink rub-off. Did you know that there is a commonly applied surface treatment methodology that can contribute to the successful trapping of inks? This is known as the “step technique. ” The key principle of the technique is recognition of sub- strate-to-ink and ink-to-ink surface tension dynamics. For example, if a polypropylene base substrate is corona treated from 29 mN/m to 42-45 mN/m to properly wet-out a flexo ink but the surface energy of the “skin layer” of the overprinted ink is not modified from its native state to wet to the base ink layer, there can be a high potential for ink picking. The step technique applies steps of descending surface tensions from the substrate to the first ink layer, and from this one to the next, etc. In this way, proper adhesion of each ink layer to its support (or trap) is achieved. Insufficient trapping, i.e. inad- equate ink adhesion because of incorrect surface tension, is therefore avoided. To successfully surface-modify for optimal wetting, trapping and adhesion, accurate measurement of substrate surface energy is key. The use of dyne solutions (or dyne pens) is currently the predominant technique. The dyne solution technique is transferable to many materials. However, did you know that it is critical that the dyne test fluid does not alter the surface properties of the substrate? For example, if the test fluid permeates a porous sub- strate, such as paper and causes swelling, surface energy results will indicate unrealistically easy wetting. Just as non- qualifying, a chemical reaction between the test fluid and the substrate invalidates results altogether. To ensure test replicability, material preparation and test technique must be standardized. ASTM Standard D618 documents the suggest- ed conditioning methods. However, this standard is untenable for treated film testing, since the material conditioning times range from 24 to 96 hours. These conditioning times may be of value for research and development purposes, but for normal quality control testing, much shorter conditioning times are commonly used. In this vein of thought, standardization of ambient, sub- strate, and test solution temperatures is critical—as is the inspection methodology. It is recommended that one trainer be identified to instruct all surface energy testers to minimize measurement variability. Also, relative humidity should not be excessive because high relative humidity will increase data variability. Finally, the elapsed time between extrusion and coating-to-surface energy test (or from this test to printing, etc.) must be controlled. Other advisable precautions with this test procedure include the following: • Avoid touching or contaminating the surface to be tested, as dirty surfaces lose their wettability • Avoid using contaminated test fluid; discard every six months • Do not retest the same location on a sample • Store and use all test fluids at room temperature • Use fresh cotton swab applicators for each test Did you know that aging of the substrate could affect surface characteristics and therefore dyne level testing? If the constraints of your process preclude good standardization of test timing, designed experimentation should be used to measure the effect of aging on your substrates. Substrate suppliers will typically set material specifica- tions to conservative levels to compensate for treatment loss. Although surface energy is critically important to many converting operations, the topography of the substrate, coat- ing rheology, and chemical incompatibility are just as critical. Other important factors include the type of resin used for film or coating, the particular ink or adhesive to be used, surface roughness, and the interaction between the media and the reactive sites. A starting point is established from which to resolve adhe- sion problems by systematically measuring substrate surface energy. TREATMENT TECHNOLOGIES Corona treatment is the most widely used surface treat- ment technology deployed by flexographic printers to control surface energy for successful ink adhesion. Bare and ceramic covered ground rolls are used in conjunction with high-pow- ered ceramic electrodes to generate the corona for these ap- www.flexography.org may 2012 FLEXO 41