Foam formation is a serious problem that can adversely affect the production of radiation-cured printing inks and overprint varnishes. When it occurs during the grind and drum filling steps, it can inhibit efficiency, increasing production time and generating quality problems. This ultimately results in higher costs.
If it is incorporated into pigmented inks during the grind stage, it can interfere with the dispersion process and also inhibit color strength development. Even those mixing procedures that have less shear can create foam that may cause the mixing kettle to overflow, or at the least, reduce production efficiency.
During the filling procedure, the generated foam does not allow proper filling of the drums or containers. Likewise, in pumping the printing ink, a great deal of foam may be created, which will significantly reduce the efficiency of the pump line. Finally, during the printing application itself, foam may cause the ink pans to overflow, or lead to poor ink transfer, adversely affecting quality. While these problems are well known for water-based printing inks and varnishes, UV formulations also display frequent foaming problems, and their production is affected in much the same way. 1,2
Macro and Micro
Close analysis reveals that foam occurs in two basic forms:
One type, known as macro foam (Figure 1), consists of relatively large air bubbles that rise to the liquid’s surface and form a stable crown that is visible on top of vessels, tanks and ink pans. Often caused during the pumping, filling and application of low viscosity flexo and gravure inks, it is especially common in water-based inks and varnishes, but also occurs on low viscosity radiation-cured formulations. A defoaming action is required to prevent or eliminate it, and the corresponding additives are therefore called defoamers.
The second type, called micro foam (Figure 2), has very finely dispersed air bubbles stabilized in the bulk of the liquid. It usually is not readily visible to the naked eye, and is often only recognizable as an increase in viscosity and increase of volume. In extreme cases it may cause varnishes to look like whipped cream. It occurs in radiation-cured as well as in water-based formulations, and is most likely caused by entrained air during long print runs of radiation-cured varnishes, or in chambered doctor blades of flexo printing presses. Silk screen applications are also prone to micro foam generation, especially rotary screen printing. A degassing action is required to prevent or eliminate it, and the corresponding additives are therefore called deaerators.
Macro foam is clearly visible as a crown that forms on the surface of the liquid, while micro foam exists in the body of the liquid, and is usually not apparent to the naked eye.
The general requirements for effective deaerators and defoamers in any kind of formulation are relatively similar:
1. Ideally, they should be liquids in order to facilitate the measured quantity and pumping of the formulation.
2. They also should be surface active, since interfacial processes are very important in both foaming and defoaming actions.
3. They should have a low surface tension and a low surface energy, along with a high spreading ability.
4. Limited solubility in the ink medium is another prerequisite for the effectiveness of defoamers and deaerators. It causes them to migrate and orientate toward interfaces with air and, consequently, toward the foam bubbles.
5. Since limited or partial solubility means incompatibility, which can lead to surface defects, it is vital that defoamers and deaerators be dispersible in the ink or varnish. A well-chosen balance of hydrophilic and hydrophobic properties of the active matter ensures sufficient compatibility.
Water-Based vs. Radiation-Cured
The requirements for defoamers and deaerators used in water-based formulations differ from those employed for radiation-cured formulations in several aspects, and lead to different properties and formulation concepts. In water-based systems, they should be insoluble, and are available in concentrates, solutions and emulsions. For radiation-cured systems, they need limited solubility, and are available as concentrates and solutions only.
To fulfill these requirements, a variety of active matters is used for the formulation of such additives, depending on the ink type and application. These include organically modified polysiloxanes, special silicone-free polymers, silicone oils, aromatic and aliphatic mineral oils and fatty acid derivatives. To offer a tailor-made solution for each application, these active matters can be formulated as concentrates, solutions or emulsions.
Grind vs. Let-Down
It is a common practice to employ defoamers and deaerators in the grind stage that are different from those employed during the let-down stage, especially in water-based inks. This distinction allows the use of a more effective defoamer at the grind stage and a more compatible defoamer at the let-down stage to optimize the overall defoaming and to avoid surface defects.
Emulsions are commonly used in the let-down stage, with the active matter emulsified in water. These can be incorporated at moderate shear rates, and are highly effective and very compatible. In the grind stage, concentrates or highly concentrated solutions are commonly used. While extremely effective, they require high shear rates for good incorporation, which is a requirement for compatibility.
Defoamers and Deaerators
Generally speaking, defoamers and deaerators remain at least partially soluble in radiation-cured formulations. This results in different formulation concepts: Concentrates are commonly used, and the same product often is utilized for both the grinding and let-down stage. Incorporation at high shear rates is required for very effective concentrates, while incorporation at moderate shear rates is possible for solutions and some silicone-free products.
A. Tego Stir Test
There are several quick and easy lab screening tests that help the formulator find the right defoamer or deaerator. These cannot replace the final test run on a printing press, but can significantly narrow the number of potentially suitable additives.
The Tego stir test (Figure 3)has proven to be a fast, easy, reproducible and meaningful method for analyzing the efficiency of defoamers in various ink and varnish applications. It is conducted as follows:
1. 50 grams of ink/varnish are weighed into a 180 ml polyethylene cup.
2. The liquid is foamed by stirring for one minute at 3,000 rpm by a -centimeter dissolver blade. Immediately afterward, 45 grams of the ink/varnish are poured into a 100 ml graduated cylinder and the volume is read. A low reading indicates that the defoamer is effective.
B. Tego Bubble Test
Primarily suited for determining the effectiveness of a product against macro foam, the air-flow, or bubble, test (Figure 4) also demonstrates the rate at which a defoamer or deaerator can reduce existing foam.
In this procedure, a randomly-chosen amount of ink/varnish is poured into a 350 ml graduated cylinder. A tube is inserted into the liquid, and air forced through the tube foams the liquid. After 15 minutes the volume is read: The lower the reading, the more effective the defoamer. Next, the air-flow is stopped, and the time required for the foam to collapse and dissipate is noted.
The shorter the time period, the greater the effectiveness of the defoamer.
C. Draw-Down Test
The compatibility of a defoamer or deaerator can be checked easily by means of a draw-down test, such as that done with a wire bar. When performing this test, it is important to know that its result greatly depends on the additive incorporation, the ink or varnish viscosity, and the substrate used.
Ideally, it is performed at printing viscosity and on the relevant substrate to be printed. It is also possible to use a very critical substrate, such as film, in order to see the trend to surface defects more easily.
Approximately one hour after the stir test, the liquid is applied to untreated polyester film by means of a hand bar coater (film thickness approximately 12 microns). As a substrate for non-pigmented systems, black matte PVC film is preferred. After drying, the ink/varnish surface is judged visually. The following symbol scale is used to indicate its condition:
No defects: +
Few defects: o
Some craters: -
Several craters: --
Extremely damaged: ---
None of the tests described above is really suitable for evaluating defoamers and deaerators for silk screen inks. Their relatively high viscosity makes an air-flow test impossible, and a stir test is difficult to perform because the air incorporation during screen printing cannot be simulated by this kind of evaluation. Therefore, manual screen prints are recommended for making a distinction between the additives tested. (See Appendix A for screen ink test formulations.)
Test After Storage
In water-based as well as in radiation-cured formulations, the compatibility behavior of an additive may change dramatically over time. It is recommended that the foam test and the compatibility tests should be repeated after storage. The ideal additive for a suitable formulation provides persistent performance over a long period of time.
Conclusions and Considerations
Foam problems in water-based and radiation-cured printing inks and varnishes can be solved by tailor-made defoamers and deaerators. Therefore, various kinds of active matters and formulation concepts are used.3 Selecting the appropriate defoamers from the huge and varied range of additives is no easy task. Because of this, recommendation guidance, in the form of printed tables or online data bases, is available. This information, combined with advice from your additive supplier, can help reduce the number of products you need to test. See Appendix B for test ink and varnish formulations and for results that were performed on the inks.
All special requirements must be taken into account prior to testing, because they may either exclude or include certain products. For example, in radiation-cured varnishes, glueability and hot foil stampability are important considerations when using silicone-based deaerators. In this case, the first choice would be a silicone-free deaerator.
Good defoamer or deaerator incorporation is a crucial prerequisite for the screening tests in order to produce meaningful compatibility and effectiveness results. It is ideal to wait one day after incorporation before performing the tests. Finally, it is important to note that simple screening tests are great time savers, but cannot replace final on-press test runs. 4 Your additive supplier will gladly support you in any way to solve the foaming problems of your inks and varnishes.
 Kuschnir, Eley, Floyd: J. Coat. Techn. 59, 75 (1987)
 Ross: Interfacial Phenomena in Apolar Media, 1 (1987)
 Wallhorn, Heilen, Silber: Farbe&Lack 102, 30 (1996)
 J. Bieleman: Lackadditive, 111 (1998). n