PFFC - June 2008 - (Page 14)

PROCESS MANAGEMENT Wettting Dyne Levels— Part 1 urface energy is a fundamental property of solids and varies from low in plastics such as PE to high for glass and metal surfaces. Surface tension is the analogous property of liquids. For a liquid to wet the surface of a solid, its surface tension must be lower than the surface energy of the solid. The equilibrium value is known as critical surface tension (CST). Contact angles of the liquid/solid interface often are used as a measure of wetting, with lower values indicating better wetting. The phenomenon of wetting in our industry is the property of a liquid-solid system, typically water-based ink, primer, lacquer, and adhesive as applied to plastic films. Organic solvents used in solvent-based products assure good wetting of commonly used plastic films. They have surface tensions of 20–24 dynes/ cm, which is well below the surface energy of packaging films such as LDPE, OPP, and PET. Almost without exception, solvent-based products lay down smoothly on packaging films—a required attribute in graphic reproduction. �� S �� By David Argent Contributing Editor �� water-based formulations to achieve the same effect of depressing surface tension. Side effects of such additions can be foaming, loss of water resistance, and loss of adhesion. This tenuous formulating balance has limited the acceptance of water-based products—particularly inks—for film printing. In addition, the different rates of water and alcohol evaporation are a fundamental problem. On low coverage decks, the ink is replenished slowly, and this leads to print defects due to dropping alcohol levels. The Solids Liquids make-up solvent may have to be Surface Energy Dynes/cm Surface Tension Dynes/cm 50:50 water:alcohol to keep a Untreated Treated 100% Water 72 90:10 balance in the press ink during the run, resulting in one 22 LDPE 31 38–42 100% Ethanol more variable to manage. 31 38–42 90:10 Water:Ethanol 47 OPP In a converting operation, PET 42 50+ 75:25 Water:Ethanol 33 water-based products are delivered in ready-to-use form, Many packaging films present wetting hurdles and there is little room for dyne level adjustment. for water-based inks and coatings. There are two Measurement devices for dyne levels of inks and fundamental ways of attacking this issue. One is to coatings are expensive, delicate, and more suited lower the surface tension of the water-based prodto lab conditions. For film substrates, “dyne soluuct, and the other is to raise the surface energy of tions” are employed widely in operations to verify the film by various treatment methods. the surface energy level. The dyne solutions are The table on liquids illustrates that adding alco- stepped mixtures of ethyl cellosolve and formamide hol to a water-based ink will lower the surface covering a dyne range of 30–60. Dyne solutension significantly. At the 10% alcohol tions are a simple “go, no go” test. The level, 47-dyne/cm ink will not spread dyne level of the film is determined evenly on most films, even when by the lowest dyne level mixture that treated. Increasing the quantity of stays wetted on the surface. These alcohol to 25% would lower the sursolutions can be applied by a felt tip face tension to 33 dynes/cm, assurcotton swab, or wire wound SPECIFICS OF FILM pen,applicator. The rod application ing good wetting of treated films. rod TREATMENT However, in most cases these is the least likely to be subject to Part 2 of 2 levels of alcohol are not permitted, contamination problems, while the so wetting agents are incorporated into felt tip pen is easiest to use. COMING NEXT MONTH Process improvement expert David Argent has 30+ years of experience in process analysis with particular emphasis on ink and coating design and performance. Contact him at 636-391-8180; djvargent@sbcglobal.net. 14 | JUNE 2008 WWW.PFFC-ONLINE.COM http://WWW.PFFC-ONLINE.COM

Table of Contents Feed for the Digital Edition of PFFC - June 2008

PFFC - June 2008 Contents First Glance Calendar of Events From the Editor Web Lines Process Management News Clips Narrow Web & Label Reporter Narrow Web & Label What’s New Products Carton & Box Reporter Carton & Box What’s New Products Paperboard Packaging Cartons/Boxes Cover: New Materials Offset Printing TLMI 75th Diamond Anniversary What’s New Products Services Directory Classified Marketplace Equipment Buyers & Sellers Advertisers Index Experience Speaks

PFFC - June 2008

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