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Surface Tension Phenomenon

By: Pillar Technologies

The purpose of this presentation is to discuss the surface tension phenomenon and give some insight into why many flexographic printers are finding the need to add surface treatment equipment to their presses.

The trend toward converting to waterbased inks has forced the flexographic printer to become familiar with many new sciences, procedures, and processes in order to achieve the final product.

Once the responsibility of the film supplier, flexographic printers are finding that in conjunction with using waterbased inks they must bring the surface treatment process in house in order to produce an acceptable print quality. As a result, it has become necessary for printers to be familiar with the surface treatment process, as well as a basic understanding of surface science and the procedures for testing treat levels.

It has long been recognized that the first step in obtaining good adhesion and print quality is to assure that the ink wets out on the substrate evenly. The more interfacial area between the liquid and the substrate, the greater the possibility to achieve a sufficient bonding.

To achieve this proper wetting of the surface, various surface forces come into play. The primary forces involved are the surface tension of the ink and the surface energy of the film.

Surface tension is described as a phenomenon that results directly from intermolecular forces between molecules of liquids. In other words, molecules at the surface of a drop of liquid experience a net force drawing them to the interior, which creates a tension in the liquid surface, almost as if the liquid were covered with a tight skin. The surface tension of a liquid is measured in Dynes/cm.

The second force that effects the ability of the ink to wet out properly is the surface energy of the film or substrate. Surface energy is a term used to describe the reactivity of the surface of a solid substance. For practical purposes the surface energy of a substrate is expressed in relation to Dynes/cm as well.

In order for the liquid to sufficiently wet out on the surface of the substrate, the material has to have a high enough surface energy, in relation to the surface tension of the liquid being applied. If the tension of the liquid is higher than the surface energy of the substrate, the molecules of the liquid would tend to cling together, forming a bead or drop. As a general rule, the Dyne level of the substrate has to be at least ten points higher than the liquid being applied. The chart below lists the typical dyne levels of various substrates in relation to some of the more commonly used liquids.

As shown on the chart, materials such as polyethylene and polypropylene are more commonly treated substrates, since they have the lowest surface energies, with dyne levels of 31 and 29 respectively. Applying the 10 point rule, it becomes apparent why films have to be treated higher when using waterbased inks instead of solvent based. It should be noted that although water has a dyne level of 72, waterbased inks consist of other solvents and additives, which bring the dyne levels more in the range of 31 to 35 dynes. Solvent based inks are more in the range of 28 to 30 dynes.

The best method to determine the treat level needed is through trial and error. The 10 point rule mentioned above is a good starting point if the printer knows the dyne level of the ink. Although the ink supplier can usually provide the printer with the dyne level of the ink, it will be necessary for the printer to determine the dyne level of the substrate since treat levels may change with time. To determine this, the printer must get involved in the somewhat controversial area of surface tension measurement. This controversy centers on the validity and/or subjectivity of some of the testing methods employed.

How is surface tension phenomenon measured?

One of the more informative methods of measuring surface energies is by measuring the contact angle. This method involves the use of a device known as a goniometer, which magnifies the profile of a liquid drop, placed on the substrate in question, on to a precalibrated protractor.

Early methods of measuring contact angle involved the measurement of a "static" drop. Believed to be at equilibrium, the reading was taken after a few seconds, once the drop was assumed to be stabilized. More recent studies have shown that the measuring of the advancing and receding angles is more accurate and informative of the surface phenomenon that is occurring. These recent studies show that the advancing angle is more representative of the wetting of the substrate, whereas the receding angle is more representative of the adhesion characteristics of the liquid to the substrate.

The most common method of measuring surface energies of the substrate is by employing ASTM D 2578 - 84, which involves the use of a series of different mixtures of Formamide and ethyl Cellosolve. Each of these mixtures represents a liquid with different surface tension. Using a sterile cotton swab, a thin coating of the dyne solution is applied to the substrate. If the solution does not bead up within two seconds, the substrate is believed to at a dyne level equal to or higher than the dyne solution applied.

This process is repeated, progressively increasing or decreasing the dyne level of the solution until the highest dyne solution capable of wetting out is identified. Of course there are several variations to this procedure, such as the use of cotton swabs, felt tipped pens, Mayer rods, etc., in addition to concerns over the somewhat subjective aspect of determining successful wetting. From the printers perspective, this method represents the most practical way of establishing easy to use operating parameters.

As mentioned earlier, the treat levels of plastic substrates tend to change with time. Additives such as slip, which tend to migrate to the surface of the film as the film ages, can mask the treat levels. In addition to slip, physical handling as well as storage temperature can effect treat level.

Printers using solvent based inks were often able to get sufficiently treated films from their supplier, since the required treat levels were not as high. However, with the switch to waterbased inks, many suppliers were faced with having to treat the materials to 50+ dynes/cm, in order to allow for decay in treatment. These higher treat levels frequently resulted in faster decay rates, blocking, and degradation of the film's properties.

It is now common for printers to re-treat material on the press, even though the film supplier has treated on the extruder, already. It is also common for film suppliers to treat to 38-40 dynes/cm, and expect the printer to "bump treat" the material to the necessary 42-45 dynes/cm for waterbased inks.

The conversion to waterbased inks has had a significant impact on the flexographic printer. The use of corona treaters and the need to understand surface tension measurement has become necessary for most waterbased ink applications. The increased use of polyethylenes and polypropylenes with high amounts of slip additives, will not only cause treaters to be necessary, they will become larger, as well.