- April 01, 2003, Dr. Richard M. Podhajny, Ph.D., Contributing Editor
Most flexible packaging plants can measure surface tension of their films by using dyne solutions. The dyne solutions are chemical mixtures that have a specific dyne number.
Ink and coatings, like dyne solutions, have a surface tension associated with them. Typically, water-based systems have higher surface tension numbers than solvent-based inks and coatings. However, unlike the static number we associate with substrate measurements, inks and coatings are complex mixtures and behave differently depending on the press speed.
The surface tension of inks and coatings on press do not necessarily stay constant. The values can change during the printing process as ink splits from one surface and then wets down another. The primary reason for this is the ink or coating is a mixture of various components, and during the dynamic printing process these components rearrange themselves every time the ink is split and creates a new surface.
Surface tension describes the cohesive behavior of the surface of the ink or coating. For instance, if water at a meniscus takes a concave shape, this suggests the surface in contact is at a higher energy. However, if the meniscus shows a convex shape, the surface in contact with the water has a low surface energy.
Most inks fall into the range of 25-40 dynes/cm. Obviously the lower the surface tension number, the better the surface wetting. However, since the ink is a mixture of components, during the printing process these components change position due to ink splitting and need to reposition themselves at the new ink/substrate interface. As the ink is split, a new ink surface is created, and the ink's components must re-establish their positions with respect to the surface in contact.
Since the components of the ink or coating come in different sizes and shapes, they don't re-establish their positions at the same speed. So, if a given surfactant is in the ink, its ability to reduce the surface tension depends on how quickly it can reposition itself to reduce the surface tension. Not all surfactants behave equally when it comes to this task. In general, the larger the molecule, the slower it is to respond and the higher its dynamic surface tension.
Therefore, an ink can have a surface tension of 25 dynes/cm, but when it is subject to a dynamic condition, such as a printing press, its “dynamic” surface tension behaves like it has a value of 40 dynes/cm. The faster the press speed, the more demanding is this effect on wettability and print quality.
Technologists have been aware of this dynamic effect and can measure this property. One of the instruments available to measure the dynamic surface tension of an ink or coating is based on measuring the maximum bubble pressure.
This method measures the dynamic surface tension of a liquid by using two probes having orifices of different diameters below the liquid surface. The pressurized gas forms bubbles in the liquid, and the differential pressure of the bubbles is measured and equated to the liquid's surface tension.
In practice the actual interface formation time (bubble rate) is measured and compared. The method is described in ASTM Method D3825-90.
The application of “dynamic surface tension” measurement finds a place not only in quality control but also in production processing and product design. The use of dynamic surface tension methods has assisted technologists in designing better inks and coatings for higher press speed applications.
Dynamic surface tension measurements have become a useful tool in our industry to further the understanding of surface and rheological behavior of liquid inks and coatings.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314; firstname.lastname@example.org.