Choosing the wrong resin may result in poor application and performance deficiencies of the applied ink. Surfactants used to lower the surface tension of the ink and wet out low energy substrates like films and foils can cause foam problems during ink manufacture and application. The defoamers used to remove the foam or prevent it from forming in the first place can often cause surface defects, such as craters or pinholes, in the final print, requiring the formulator to use additional surfactant to overcome these defects. Identifying the best defoamer can often be challenging, as it consists of finding the right balance between defoamer strength and its compatibility with the rest of the formulation. Often, defoamer selection is the last step in the formulating process and, as such, can be the rate limiting step in commercializing a new formulation.
Formulators tend to use products that have provided good performance in the past. Because of the need to balance defoamer performance with system compatibility, this approach may not always work and when the formulator is presented with a system in which the known products do not perform, the formulator is tasked with finding a suitable defoamer from a large pool of products. To eliminate the time and effort spent in identifying suitable defoamers, a complete, structured siloxane-based defoamer line has been developed to enable the formulator to follow a systematic approach when selecting defoamers for his ink formulation. Using this approach, the formulator will save time and money while simultaneously creating more robust formulations.
Siloxane-based defoamers are often used in waterbased printing inks. Like all traditional defoamers, siloxanes function via an incompatibility mechanism which often results in surface defects in the final film.1-5 To maximize defoamer performance and minimize the potential for incompatibility issues, a new range of siloxane-based defoamers was designed to provide consistent and predictable performance relative to one another. Several studies were performed to illustrate how these defoamers provide optimal performance in graphic arts applications.
When selecting a defoamer for a pigment dispersion, overprint varnish or printing ink, it is important to consider the effectiveness of the defoamer at controlling foam and its compatibility with the system to prevent surface defects like pinholes and craters. Defects are caused by surface tension driven flow as the hydrophobic defoamer repels the ink leaving behind a void or depression as the ink dries.6 Other parameters also play a role in how the defoamer performs in the formulation. The amount of pigment in the formulation has a significant influence on the compatibility of the defoamer because the solid particles can slow or prevent the dewetting process from occurring around the defoamer. Additionally, the viscosity of the dispersion or ink plays a key role as higher viscosity systems can impede the ability of air to escape and the ink to flow. Thus, higher viscosity systems, such as pigment dispersions, are less prone to crater formation than low viscosity systems and can withstand stronger and more incompatible defoamers. In contrast, low viscosity inks are easier to defoam but more susceptible to defect formation; therefore, they require weaker but more compatible defoamers. In addition, craters, fisheyes and other defects tend to be more visible in high gloss systems such as overprint varnishes and, as such, often require strong, yet compatible defoamers to minimize these issues.
Application techniques can play a significant role in determining the foam profile of the formulation. Due to the high shear forces present between the anilox roll and the doctor blade, a poorly chosen defoamer may allow a fine microfoam or a stable, unprintable mousse to form in the feed tank. This microfoam, or entrapped air, can reduce the volume of ink transferred from the anilox cells reducing print quality and color. A highly incompatible defoamer may also phase separate and clog the anilox cells. Because of the high or continuous shear that inks encounter from both the press and the re-circulation tanks, it is critical that the formulator uses shear stable defoamers. The mechanical energy involved in these processes can reduce defoamer droplet size and limit the defoamer’s efficiency. Figure 1 offers guidelines on how to properly choose a defoamer that will provide the right balance of foam control and compatibility by considering all of these factors during formulation.
Development of a Structured Siloxane Defoamer Line
Polysiloxane polymers possess many important properties that are excellent for defoaming efficacy.7 Siloxane-based defoamers are chemically inert, non-volatile, thermally stable and offer a unique ability to control foam in almost any system. Polydimethylsiloxane (PDMS), shown in Figure 2, is one of the simplest polysiloxane polymers; grafting other organic groups onto the backbone results in modifications to the compatibility and performance of the defoamer (Figures 3 and 4). PDMS is characterized by repeating units of Si-O bonds. Because of the flexibility of these bonds,8 the siloxane backbones offer high spreading coefficients and easy orientation at interfaces. The methyl groups offer both hydrophobicity and low surface tension.9 These siloxane-based defoamers are highly efficient due to their low surface tensions and fast spreading rates on the foam present in the system.
PDMS defoamers are generally difficult to incorporate into a formulation; therefore, they can cause surface defects in the coating due to surface tension driven flow of the ink away from the hydrophobic, incompatible PDMS. To improve the compatibility of the PDMS in the formulation, minimize surface defects and improve incorporation of the defoamer, modification of the PDMS can be performed by replacing the hydrophobic methyl groups with other polar groups to increase the overall hydrophilicity of the polysiloxane. In doing so, the compatibility of the defoamer with the aqueous media will improve, giving the formulator a balance of strong defoaming power and compatibility.
A series of seven defoamers, based on polysiloxane chemistry, was developed to consistently and reproducibly match the requirements of a wide range of applications. Several key parameters were identified for the construction of the modified polysiloxane structure-property relationship study including the following: length of the dimethyl siloxane, length of modified siloxane, ratio of length of dimethyl siloxane to length of modified siloxane, total length of dimethyl siloxane and modified siloxane, length of hydrophobic organic groups, length of hydrophilic organic groups, total length of organic groups (hydrophobic and hydrophilic) and ratio of length of hydrophobic organic groups to length of hydrophilic organic groups.10 A Design of Experiments (DOE) response graph of structure elements weight percent (wt.%) of dimethyl silicone (X3), wt. % hydrophobic groups (X2) and wt. % hydrophilic groups (X1) versus the foam density of a packaging ink is shown in Figure 5. Figure 6 illustrates a similar DOE response of defoamer compatibility versus the same three structural elements in Figure 5 by evaluating surface appearance of the coated ink. In both figures, higher values represent higher defoaming strength and better surface appearance, respectively.
Multiple DOEs were conducted to tailor the desired modified polysiloxane structure to its identified performance targets. Once the modified polysiloxane structure was identified, additional defoaming components, including hydrophobic particles, stabilizers and the like, were incorporated in order to create the final defoamer composition. Using structure-property relationship studies, a variety of polar organic groups was incorporated into the polysiloxane backbone, resulting in a series of seven defoamers with a predictable balance of defoaming strength and compatibility covering a wide range of formulation types. The goal of this development program was to demonstrate that this predictability could be used to improve formulation efficiency by eliminating the trial and error approach and narrowing product selection.
Figure 7 depicts the relative positioning of these structured siloxane defoamers. “D type” defoamers, designated by 5100 and 5200, are the strongest of the defoamers and provide the best shear stability. As one moves towards the right, “C type” defoamers, designated by 5300 and 5400, offer strong defoaming with long term persistency while being more compatible in the formulation than the “D type” defoamers. Defoamers 5500 and 5600, the “B type” defoamers, provide an excellent balance of good defoaming and film compatibility, ideal for waterbased inks. On the far side of the spectrum, the “A type” defoamer, 5700, offers the greatest compatibility of the defoamer series.
Red Pigment Dispersion
An evaluation of the structured siloxane defoamers was conducted in a Pigment Red 22 dispersion. The dispersions were prepared using a high speed Cowles disperser and the defoamers were evaluated for shear stability by extracting samples from the grind at regular time intervals and measuring foam density. Figure 8 illustrates the performance of these defoamers over time in the red dispersion. On this graph, the higher the density, the less foam entrained in the system. The data clearly indicates that the “C type” defoamers, 5300 and 5400, are very efficient at defoaming the system and maintain their shear stability even after five hours of grinding at 1500 rpm. Defoamer 5500, a “B type” defoamer, also performs well initially but begins to show reduced defoaming efficiency after about 2 hours of shearing. The other two benchmarked siloxane defoamers show considerable loss in defoaming efficiency over time because they are not shear stable. A clear trend is evident from this graph which shows that, as the formulator moves from the more compatible “B type” defoamer to the stronger “C type” defoamer, foam control improves and defoamer persistency is maintained.
In an effort to understand the effect on compatibility of these defoamers tested, inks were prepared from the grinds. Samples of the dispersions were taken after 5, 30 and 60 minutes of grinding and let down into an acrylic based varnish. The inks were applied using a #4 Meyer rod onto Leneta charts. Figure 9 contains images of the inks containing the red dispersion prepared with the structured siloxane defoamers. The inks prepared with the dispersions ground for 5 minutes show significant surface defects when the 5500 and 5400 defoamers were used. Defoamer 5700, being the most compatible of the defoamers tested, also shows some defects but far fewer than the 5500 and 5400. As grind time increases to 30 and then 60 minutes, it is clear that the defoamers become better incorporated into the system and the surface defects in the ink are eliminated. The “C type” defoamer, 5400, would be the product of choice in this application as it provides excellent defoaming strength that remains constant under high shear and gives excellent surface appearance in the final ink.
Blue Packaging Ink
Further work was done to understand the performance of the structured siloxane defoamers in a phthalocyanine 15:3 blue ink. Dispersions were prepared at a 46% pigment loading using a common industry grind resin and each of the structured siloxane defoamers. The dispersions were prepared using an Eiger mill and upon completion of milling, the density of each dispersion was measured to determine the defoaming capability of each defoamer. Acrylic based inks were then prepared with the dispersions and no further defoamer was added. The inks were separated into two groups. The first group remained at room temperature and was subjected to a Waring blender foam test followed by draw down on a Leneta chart. The second set of inks was placed in an oven at 50°C for two weeks after which the blender test and draw downs were repeated. Figure 10 illustrates the data gathered in this study. The inks follow a nice trend of increasing compatibility with decreasing foam control as the series numbers increase. Additionally, after heat aging for two weeks at 50°C, the defoamers maintain their defoaming power while becoming more compatible in the ink, as shown by the increase in the draw down rankings.
A series of structured siloxane-based defoamers was developed to meet the challenge of persistent foam in ink applications. These defoamers offer a controlled range of defoaming strength and compatibility relative to one another. By modifying the siloxane structure, it was possible to provide flexibility in formulating these defoamers and to provide the formulator with guidance on where to begin when a defoamer is needed. Many formulators reach for defoamers that have performed well for them in the past. This can lead to a significant amount of lost time trying to eliminate foam through trial and error. The chemistry of each ink formulation is different and as such, defoamers will be highly influenced by the other components in the formulation. By understanding how a defoamer’s strength and compatibility relate can save the formulator time and money while producing the perfect ink.
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2. Chan, S.Y., Galgoci, E.C., Louis, C., Snyder, J.M., “New Innovative Molecular Defoamers for Graphic Arts Applications,” NPIRI Technical Conference, 2005.
3. Jönsson, Lindman, Holmberg and Kronberg, Surfactants and Polymers in Aqueous Solution, John Wiley and Sons, New York, (1998).
4. Garrett P R, “The Mode of Action of Antifoams,” In: Defoaming: Theory and Industrial Applications, Garrett P R, Ed., Taylor & Francis Group, LLC, Boca Raton, FL, 1992, Surfactant Science Series 45, 1-117.
5. Snyder, J.M., Hegedus, C.R., Reader, C.J., “The Art of Breaking Bubbles,” Ink World, September 2011.
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. Louis, C., Reinartz, R., Chaigneau, W., Reader, C.J., Lai, K.T.G., New Deaerators for Airless Spray Applied Waterbased Coatings, Proceedings of the European Coatings Congress, Nürnberg, Germany, March 30, 2011.
8. Grigoras S, In: Computational Modelling of Polymers, Bicerano J, Ed., Marcel Dekker, New York, 1993, 161.
9. Voronkov M G, Mileshkevich V P, and Yuzhelevskii Y A, The Siloxane Bond, Consultants Bureau, New York, 1978.
10. Louis, C., Lai, K.T., Reader, C.J., “Predictable Performance with a Novel Defoamer Line,” Proceedings of the European Coatings Congress, Nürnberg, Germany, March 18-21, 2013.
The authors support Air Products’ Specialty Additives business focusing on new and innovative wetting agents, defoamers and dispersants technologies for the coatings, inks and adhesives industries.