- June 01, 2008, By David Argent Contributing Editor
Surface 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.
Many packaging films present wetting hurdles for water-based inks and coatings. There are two fundamental ways of attacking this issue. One is to lower the surface tension of the water-based product, and the other is to raise the surface energy of the film by various treatment methods.
The table on liquids illustrates that adding alcohol to a water-based ink will lower the surface tension significantly. At the 10% alcohol level, 47-dyne/cm ink will not spread evenly on most films, even when treated. Increasing the quantity of alcohol to 25% would lower the surface tension to 33 dynes/cm, assuring good wetting of treated films.
However, in most cases these levels of alcohol are not permitted, so wetting agents are incorporated into 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 make-up solvent may have to be 50:50 water:alcohol to keep a 90:10 balance in the press ink during the run, resulting in one more variable to manage.
In a converting operation, water-based products are delivered in ready-to-use form, and there is little room for dyne level adjustment. Measurement devices for dyne levels of inks and coatings are expensive, delicate, and more suited to lab conditions. For film substrates, “dyne solutions” are employed widely in operations to verify the surface energy level. The dyne solutions are stepped mixtures of ethyl cellosolve and formamide covering a dyne range of 30-60. Dyne solutions are a simple “go, no go” test. The dyne level of the film is determined by the lowest dyne level mixture that stays wetted on the surface. These solutions can be applied by a felt tip pen, cotton swab, or wire wound rod applicator. The rod application is the least likely to be subject to contamination problems, while the felt tip pen is easiest to use.
|Surface Energy Dynes/cm|
|Surface Tension Dynes/cm|
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; firstname.lastname@example.org.