Conventional waterborne ink formulations are composed of many ingredients including resins, pigments and additives. These additives include waxes, surfactants and defoamers. Surfactants are critical to lowering the surface tension of the ink and to wetting out low energy substrates like films and foils.
However, these surfactants can cause foam problems in the formulation during manufacture and application. The defoamers used to remove the foam or to prevent the foam from forming can often cause surface defects, such as craters or pinholes, in the final print. This may require the formulator to use additional surfactant to overcome the defects creating a negative feedback loop. Therefore, it is critical to select the best wetting agent and defoamer combination that provides a balance of foam control and wetting. This paper explores fundamental and practical defoamer selection criteria and illustrates how choosing the proper defoamer can eliminate foam from waterborne printing inks without causing detrimental effects to the finished print.
Foam can be introduced into a system in a variety of ways.1 Agitation, shearing and re-circulating of the ink are ways in which foam can be incorporated into the system. While most processing involves incorporating air into a system while it is being prepared, application techniques can also play a role in adding foam to the system.
The re-circulating trough of ink on a printing press is an ideal place for foam to be generated. During the printing process, ink is continuously being emptied out of the anilox cells. As the anilox roll turns and the cells are re-filled with ink, air is introduced into the system as microfoam, or minute bubbles not visible to the naked eye that aggregate in the bulk liquid, and is transferred to the substrate upon printing. This microfoam can be trapped in the cured ink film causing aesthetic, gloss and adhesion issues. As shown
Figure 1: Ink Samples Prepared Without and With Defoamer
Foam is simply a dispersion of a gas, usually air, within a liquid. In a pure liquid, foam is inherently unstable and will eliminate itself from the liquid phase due to the lack of stabilizing materials in the liquid.2 In a simple air-liquid system, foam will not form spontaneously and will require input of mechanical energy such as mixing to disperse the air into the liquid. Gas bubbles may also be introduced into the liquid through other means such as displacement from a porous surface (when wetting out unsealed paper) or through direct injection.
In a low viscosity, pure liquid, as the mechanical energy is released, small bubbles of the dispersed gas coalesce into larger bubbles and quickly rise to the liquid-gas interface to be released rapidly back into
Figure 2.Foam Release in a Pure Liquid
Stable foam forms when gas bubbles become stabilized within the system and are prevented from breaking. This will occur when a surface active material is present in the formulation and the foam film exhibits surface elasticity.3 Comparing a fully formulated ink to a pure liquid illustrates widely different foam behaviors. Some surfactants used to promote wetting of the low energy substrates used in printing will absorb onto the bubble lamella walls and stabilize foam.
Figure 3. Stabilization of Foam Due to Surfactants
To counter the effects of the foam stabilizing surfactants, defoamers are used to destabilize the bubble walls so that trapped air can escape.
Determining the best defoaming package for an ink formulation requires the formulator to consider the defoaming strength of the defoamer as well as how compatible and/or soluble it will be in the system.
The proper balance between strength and compatibility is critical to quickly controlling foam and optimizing film aesthetics. Highly incompatible defoamers can provide excellent foam control but also may cause surface defects. In contrast, more compatible defoamers may not cause surface defects but may not offer sufficient foam control. It is critical to understand defoamer behavior and function to optimize foam control while minimizing defects.
Defoamers are generally formulated systems that work by disrupting the surface bilayer around the air bubbles, destabilizing the bubble wall so the bubble can break and release the trapped air. Conventional defoamers are formulated with carrier agents used to rapidly spread out in the ink and positively effect defoaming.
The faster the carrier can spread and displace the foam stabilizing species, the more rapidly it can knockdown the foam or prevent it from forming in high shear applications. Typical carrying agents include mineral oils, silicone based materials and other hydrophobic organic compounds. These defoamers may also contain active particles that work to disrupt the bubble wall by both physically interfering with the wall as well as by adsorbing the foam stabilizing surfactants from the bubble surface. These incompatible materials displace the foam stabilizing surfactants and form unstable films at the bubble surface.
Figure 4: Defoaming Mechanism of Conventional Defoamers
Figure 5: Defoaming Mechanism of Molecular Defoamers
In contrast to conventional defoamers, molecular defoamers are silicone-free, oil-free, surfactant-based defoamers and de-aerators that are surface active and break foam on a molecular level by adsorbing at the liquid-gas interface of foam lamella and preferentially displacing the foam stabilizing surfactants.6,7 Being molecules of lower molecular weight by nature, these defoamers can spread rapidly providing excellent knockdown defoaming and foam inhibition.
As they are also surfactants, they often provide excellent compatibility in the ink, thus offering an excellent balance between good defoaming and good compatibility. Molecular defoamers do not suffer the shear stability problems of conventional defoamers and can be used to both control foam and enhance print quality through improved substrate wetting.
Defoamer Selection for Graphic Arts Applications
When selecting a defoamer for an ink, overprint or fountain solution, it is important to select a material that is both effective at controlling foam and also sufficiently compatible to prevent surface defects. A product that is highly efficient at controlling and removing foam might also cause surface irregularities such as pinholes or craters. These defects are caused by surface tension driven flow as the ink is repelled by the hydrophobic defoamer droplets leaving behind a void or depression as the film dries.8
Viscosity of the ink plays a key role as it affects the ability of air to escape and the ink to flow. Entrapped air, or microfoam, can be problematic in high viscosity systems as the bubbles tend to rise slower and become trapped in the dried film. The trapped bubbles cause haze, gloss reduction and pinholes which can affect water resistance of the final print. De-aerators, such as molecular defoamers, remain soluble in the ink and act to prevent stabilization of small bubbles, allowing them to coalesce to larger bubbles that can rise faster to the surface and break.2 Some silicone polyether de-aerators also have sufficient compatibility in the liquid phase to be effective against microfoam.
Lower viscosity systems are generally easier to defoam as the air bubbles can rise more easily and the coating has greater flow. However, due to defoamer incompatibility, these systems may be prone to more surface defects and haze so it is critical to find defoamers with an excellent balance of effectiveness and compatibility. Silicone polyether and molecular defoamers generally give the best performance in these types of formulations.
While most processing involves incorporating air into a system while it is being prepared, application techniques can also play a role in adding foam to the system. A poorly chosen defoamer package may allow a fine microfoam or even a stable, unprintable mousse to form through the high shear forces that are present between the anilox roll and the doctor blade.
Air entrapped in the ink reduces the volume of ink transferred from the anilox cells reducing print quality and color. In addition, compatibility with the ink system is critical in order to prevent phase separation
Figure 6. Foam Density vs. Printability in a water-based ink system.
Figure 7. Results of Defoamer Comparison Study at 0.3 wt%.
In contrast to silicone defoamers B and C, silicone polyether defoamer A imparts wetting capabilities and demonstrates high compatibility, or no phase separation, in this aqueous ink system. Silicone defoamer C does an equivalent job of controlling foam both initially and over time. However, due to the formulated nature of defoamer A, which incorporates both silicone polyether and molecular defoamer technologies, fewer defects are present when it is chosen.
The high shear forces present during printing can also affect defoamer stability, as these forces can breakdown defoamer particles to smaller, ineffective droplets which may lead to emulsification of the defoamer and reduced defoaming effectiveness during printing. The stage when defoamers are added is important, because strongly incompatible defoamers need to be added in the pigment grinding stage to prevent defects such as craters, pinholes and poor ink trapping. More compatible defoamers have to be used in the letdown, but, even here, care must be taken to ensure that the defoamer is sufficiently incorporated in order to prevent defects. Molecular defoamers do not suffer the shear stability problems of conventional defoamers and can be used both to control foam and to enhance print quality through improved substrate wetting.
Table 1.Blue Ink Formulation
The inks tested in Figure 7 were applied to OPP and LDPE film using a flexo handproofer.Photographs were then taken using 10X magnification. As seen in Figure 8, the ink prepared with no defoamer had print defects caused by trapped microfoam. Additionally, when the oil-based defoamer was used, the print had considerable
Figure 8. Prints on OPP Film
Figure 9. Prints on High Slip LDPE Film
Mineral oil defoamers provide excellent knockdown defoaming in both waterborne flexographic and gravure inks and are particularly useful in acrylic and styrene-acrylic based inks. These defoamers are well-suited for eliminating tough surface foam and entrained air generated during the letdown stage of ink production and during application. While silicone-based defoamers also offer similar performance, the organic oil-based defoamers are generally preferred due to their cost and their wide degree of formulating latitude. These oil-based defoamers enable the production of inks that are free of the surface defects often caused by silicone defoamers.
Foam generated during pigment grinding and dispersion can result in inefficient grinds and lower throughput. It may also lead to printability problems due to entrained air. The presence of high levels of solid material in the system can make it difficult for air to escape; therefore, strong defoamers are needed. The defoamer chosen must be effective during the high shear conditions of milling while remaining compatible when the dispersion is letdown.
Figure 10. Defoamer Performance in Pigment Dispersions
Fountain solutions are used for lithographic printing to dampen the printing plate and prevent the non-image area from accepting ink. Properly formulated founts will promote fast spreading of water over the plate surface, lubricate the plate and blanket and control the emulsification of ink and water.9 In the effort to print at faster speeds, new printability problems can be created in the form of excessive foam often caused by the surfactants used to replace alcohols in the founts.
To counter these foamy surfactants, defoamers are used. Solubility is an important factor in selecting defoamers for fountain solutions as many defoamers will be insoluble in these systems and phase separate causing print problems and having limited effectiveness. Molecular defoamers are ideal for this application as they offer a good balance of solubility, compatibility and effectiveness without attacking the polymer plates. Defoamer solubility is also important in ink jet formulations as these systems are often highly dilute.
Foam is a persistent challenge when formulating waterborne inks. From production, where foam can reduce milling efficiency and increase processing time, to application, where foam can reduce the transfer efficiency of the ink or create surface defects in the dry film, foam is a constant presence in a formulator’s work. Many formulators turn to defoamers that have worked for them in the past and spend a considerable amount of time trying to eliminate foam through trial and error.
Defoamer performance is highly influenced by the chemistry of the ink and how it is prepared and applied. By understanding the relationship between defoamer strength and its compatibility within the ink system, formulators can better identify suitable defoamers for their systems, saving themselves both time and money.
1.Chan, S.Y., Galgoci, E.C., Louis, C., Snyder, J.M., “New Innovative Molecular Defoamers for Graphic Arts Applications,” NPIRI Technical Conference, 2005.
2.Reader, C.J., Hegedus, C.R., et al, “Defoaming Theory and Application in Paints and Coatings,” The Waterborne Symposium, February, 2010.
3.Jönsson, Lindman, Holmberg and Kronberg, Surfactants and Polymers in Aqueous Solution, John Wiley and Sons, New York, (1998).
4.Garrett, P.R., ed., Defoaming Theory and Industrial Application, CRC Press, (1992).
5.Chaisalee, Soontravanich, Yanumet and Scamehorn, Journal of Surfactants and Detergents, Vol. 6, No. 4, October 2003.
6.Chan, S.Y. and Louis, C., “New Additives for Waterbased Coatings: A New Class of Defoamers is Born!,” EUROCOAT 2003, Lyon, France, September 23-23, 2003.
7.Breindel, K., “Sugar Coating,” Modern Paint and Coatings, 18-22, May 2000.
8.Hare, C.H., Protective Coatings: Fundamentals of Chemistry and Composition, SSPC, Ch. 32, 458 (1998).
9.Snyder, J. M., Meier, I.K., Whitehead, J., “New Additive Technologies for Fountain Solutions,” NPIRI Technical Conference, 2006.
Jeanine Snyder is a Senior Development Chemist in the Performance Materials Division at Air Products and Chemicals, Inc., where she has worked since 1993. During that time, Ms Snyder has developed and supported numerous commercial raw materials for coatings, inks and adhesives applications including waterborne resins and additives. She has authored numerous papers, including a NPIRI best paper, and holds 3 patents. Ms. Snyder received her bachelors degree in chemistry from Lock Haven University and holds a master of science in materials science and engineering from Lehigh University.
Dr. Charles Hegedus has worked in the Performance Materials Division at Air Products and Chemicals in Allentown, PA since 1993. As a research material scientist, he is responsible for research and applications development of specialty additives and polymers for coatings and other applications. Dr. Hegedus received his bachelor’s degree in chemical engineering and Ph.D. in materials engineering from Drexel University. He has published more than 65 technical papers and has been granted 21 patents. He has received a number of industry awards, including the ACS Roy Tess Award and the EPA Administrator’s Award for Pollution Prevention.
Jim Reader graduated from the University of Warwick (UK) in 1988 with a Ph.D. in Chemistry. He joined Air Products and Chemicals in 1998 in Manchester (UK) as a research chemist and later an application development chemist for the Epoxy Additives business. He joined the Specialty Additives business in 1996 and has worked in Europe and Asia before becoming a lead chemist in Allentown in 2008. His hobbies include soccer, tennis and board games.