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FLEXO Magazine : May 2012
Technologies & Techniques Testing Surface Treatment IQ By Rory A. Wolf Do you think you know everything there is to know about surface treatment? Well, let me pose a few questions to test your IQ on this subject: 1. Low Density Polyethylene (LDPE) film is corona treated. One portion of the film is left exposed to air, and another portion is protected from exposure to light, moisture, air and dust. Printability is evaluated for each based on print density, dot gain and mottling. Which corona treated films had the better overall print quality? 2. Paperboard is coated with SB co-polymers to improve flexo printability and barrier properties. Corona and atmospheric plasma treatments were applied sepa- rately and flexo print quality evaluated. Which surface treatment process provided the best print quality without sacrificing barrier properties? Before you search for the answers within this article, let’s explore some other surface treatment tenets you might find revealing. First, when we talk about surface treatment, what are we considering by definition? Surface treatment is the act of modifying the surface of a material to change the physical, chemical or biological char- acteristics originally resident on the surface of a material. This kind of treatment has been effective for many years in modifying substrate surface characteristics to promote the adhesion of flexographic inks, films and papers. For papers, the partial absorbency of water-borne and solvent-borne inks into the substrate (and partial vaporization) is key for adhesion. However, the printing of water-based inks on a non-porous film or foil cannot rely on absorbency and partial vaporization. Unlike solvent-based inks where binders are dissolved in the ink solvents, flexographic inks use binders, which are not soluble in water. Specifically, these inks are primarily composed of a styrene acrylic alkali-soluble binder and a resin emulsion. The alkali-soluble binder dissolves in alkaline water, which contains ammonium hydroxide. This resin also is used to disperse the pigment. This binder has a large number of carboxylic acid sites and a relatively low molecular weight. The emulsion is a high-molecular-weight resin. As the ink dries, ammonia and volatile amines leave the ink to allow the emulsion particles to “touch” and combine to form a continu- ous surface ink film. Waxes trapped in this continuous ink film will migrate to the surface. The movement of these waxes, plasticizers, and other ingredients takes about 24 hours after the ink film is dried. Higher temperatures increase the rate of migration. High humidity will cause the water-based inks to “dry ” more slowly. When the surface tension between the non-porous sub- strate and flexo ink is too great, modification of the substrate surface tension is required to aid ink transfer and adhesion. Specifically, if the substrate surface is not approximately 10 dynes/cm higher than the surface energy of the flexographic TREATING SURFACES INTELLIGENTLY • Surface treatment is the act of modifying the surface of a material to change the physical, chemical or biological characteristics originally resident on the surface of a material • Unlike solvent-based inks where binders are dis- solved in the ink solvents, flexographic inks use bind- ers, which are not soluble in water • It is typical for the addition of ink system vehicles to generate higher viscosity in water to therefore allow for lower resin solids • Plasma treatment can produce higher and longer lasting results than corona treatment 40 FLeXO mAy 2012 www.flexography.org