Predicting Defoamer Performance in Coating Formulations

C. James Reader and K.T. Griffin Lai Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195-1501 C. James Reader, [email protected], 610-481-7380 K.T. Griffin Lai, [email protected], 610-481-7069

Introduction

What is the usual procedure for finding a defoamer for a new waterborne coating formulation? For many companies, the procedure appears to be as follows: try the defoamers that you currently use and know; if these don’t work then try samples in the lab, ask a colleague or friend or, maybe, ask a supplier. This approach makes good sense when working with formulations that are similar, as defoamers will usually give consistent performance in similar formulations. However, when these tried and trusted defoamers don’t work, the Chemist has the frustrating task of trying to find a suitable product from a large selection of mysterious bottles on his or her lab bench. There is a truism in England that goes “if all else fails, read the instructions”; however, defoamers don’t usually come with instructions. Wouldn’t it be nice if they did?

The difficulties in finding a suitable defoamer exist because the performance of each and every defoamer is affected by the formulation it is used in; change the formulation and the defoamer performance may often change as well. Defoamer selection is also one of the last steps in readying a formulation for final use, so the rest of the formulation is already mostly decided and the defoamer has to work with this. This paper will present a brief review of defoamer chemistry and formulation and describe how defoamer performance is affected by other formulation components in the paint or coating. It will also introduce a new range of defoamers that provide consistent and predictable performance relative to each other so that the results of an unsuccessful test can be used as instructions to logically select the next defoamer for testing with a greater chance of success.

Foam Theory

Foam is a dispersion of gas bubbles, usually air, at the surface of a liquid that can be generated in a number of ways, but most commonly by vigorously mixing. A simple shake test with a bottle of water will show that, in pure liquids, these bubbles are not stable and quickly burst, destroying the foam. Small bubbles coalesce into bigger bubbles that rise to the surface where the bubble expands due to the greater gas pressure inside the bubble. This causes the bubble wall or lamella to thin and ultimately the bubble will break open as liquid in the lamella drains under gravity.

However, waterbased paint and coating formulations are not pure liquids; they are dispersions of many different materials suspended in water and stabilized by surfactants. These surfactants can also stabilize foam so bubbles can accumulate at the liquid surface as foam (Figure 1)1-3. Once foam is present, it can cause many problems including reduced production efficiency and higher energy demand; incorrect raw material dosing due to the lower density of foamed material and incomplete filling of production vessels and product packaging. Foam also affects the application of the paint of coating by reducing the amount of coating applied and bubbles trapped at the surface or inside the dry film will spoil both the surface appearance and protective qualities of the finished coating.

Liquid drains, bubble breaks

Air enters liquid phase from mechanical means Bubbles coalesce Larger bubbles to form larger rise to surface bubbles

Surfactants stabilize bubbles Inhibit Inhibit coalescence drainage Figure 1. Bubble Action in a Pure Liquid (top) and in a Liquid Containing Surface Active Materials (bottom) Defoamers are the most widely use method of removing unwanted foam from a paint or coating. Chemical defoamers work by disrupting and breaking the surfactant stabilized bubble walls to release the trapped air. Most defoamers are complex mixtures of different materials, including:

1. A carrier fluid that can spread across and bridge the bubble wall forming an unstable film that is easily ruptured (Figure 2a); the carrier also facilitates the entry of hydrophobic particles into the bubble wall 2. Hydrophobic particles that bridge the lamella and cause rupture by dewetting (Figure 2b) 3. Non-foamy surfactants that can displace the foam stabilizing surfactants at the lamella surface (Figure 2c) 4. Other components added to improve defoamer stability, incorporation and compatibility.

These mechanisms have been summarized in detail by Garrett4 and it is probable that many of these mechanisms are at work when defoaming formulated paints and coatings. Therefore, all the components in a defoamer may be critical to its performance.

Figure 2a (Left), b (Center) and c (Right). Different Defoamer Mechanisms

Of course, paint formulators are not just concerned with the effectiveness of the defoamer at getting rid of bubbles and foam; it must do so without nasty side effects. The same hydrophobic solids and carrier fluids that break the bubbles can also cause problems with the drying paint film leading to craters and fisheyes. The presence of these components at the surface of a dry (or nearly dry) film can also make it difficult to recoat and create adhesion problems. It is this balance between effective defoaming and avoiding film defects that can make finding the ideal defoamer for a formulation so challenging and frustrating.

Factors Affecting Defoamer Performance

Figure 3 shows how the performance of five commercial defoamers (represented in different colors) changes in four very different paint and coating formulations – a 35% pigment volume concentration (PVC) alkyd primer formulation from Nuplex (circles), a 55% PVC interior paint formulation from Celanese, a polyurethane (PU)-acrylic hybrid clear coat for parquet floor lacquer formulation from DSM and a polyurethane dispersion (PUD) clear coat for plastic formulation from DSM. The defoaming performance and application quality were measured by different methods for each formulation and application and then adjusted to a 1 – 10 scale, where 10 represents perfect performance (no foam and/or perfect film quality) and 1 represents poor performance. The ideal performance is therefore shown in the top right hand corner of the graph.

Defoaming

Application Quality

Figure 3. Performance of Five Commercial Defoamers in Four Different Applications

Hegedus reviewed the many different factors that affect the performance of a defoamer in different formulations and highlighted how this information could be used to more effectively guide defoamer selection.5 Higher viscosity and more highly filled (high filler to carrier ratio) formulations are harder to defoamer, but usually less sensitive to defects. Similarly, fast drying formulations and coatings applied in thick films are also harder to defoam and often less prone to surface defects, whereas low viscosity formulations are generally more sensitive to surface defects but easier to defoam. Craters, fisheyes and other defects are also more visible in high gloss formulations and clear coats and often require more careful defoamer selection. Brush and roller often create more surface foam when the coating is applied, while spray techniques can often leave bubbles trapped below the film surface (microfoam).6 The substrate is also important; porous substrates like wood and concrete can be less sensitive to defects but release air into the coating film as the liquid coating wets and penetrates the substrate. Smooth, low energy surfaces like plastics are harder to wet and more prone to surface defects.

However, even with this understanding, it can still be challenging to find a product that gives acceptable performance from the many defoamer samples that are available. Recently, we have tested many different defoamers in different formulations and observed distinct trends in defoamer performance with different formulation types. These were grouped into four approximate classes (Figure 4) where “D type” defoamers are the strongest and least compatible defoamers and “A type” defoamers are the most compatible products.

C B A D C B D

A

Defoaming Defoaming

An “A-type” system A “B-type” system Very sensitive, prone to defects Less sensitive to defects Easy to disrupt foam Moderately easy to defoam Application Quality Application Quality

D C D

B C Defoaming Defoaming B

A “C-type” system A A “D-type” system Low sensitivity to defects Insensitive to defects A Moderately difficult to defoam Difficult to defoam Application Quality Application Quality Figure 4. Defoamer Performance in Different Formulation Types

New Defoamer Development

A series of eight experimental defoamers (51 – 58), based on polysiloxane chemistry, was developed that could consistently and reproducibly match these profiles in different formulations. Polysiloxane polymers are non-volatile, chemically inert, temperature-stable and highly efficient and they can control almost all types of foams in any media. Due to the flexibility of the Si-O bonds present in these materials,7 all siloxane backbones offer high spreading coefficients and easy orientation at the interface while the methyl groups offer both hydrophobicity and low surface tension.8 These factors make siloxane based defoamers highly efficient because of their low surface tensions and fast spreading on the foam system. Polysiloxane polymers can also be chemically modified to improve the compatibility of the defoamer to minimize surface defects and help incorporation of the defoamer into the coatings formulation. This modification allows a formulator to balance the defoaming power and compatibility of the defoamer within an aqueous system.

A series of eight experimental defoamers (51-58) with the predictable defoaming ability and compatibility balance to fit design targets based on these four different formulation types was developed through understanding these structure-property relationship studies,. Defoamers 51 and 52 are the strongest defoamers that fit the “D type” profile shown in figure 4; similarly, defoamers 53 and 54 are “C type” defoamers; defoamers 55 and 56 are “B type” defoamers; and defoamers 57 and 58 are the most compatible, “A type,” defoamers. We have strategically incorporated a variety of polar organic groups into the polysiloxane backbone and created a series (51-58) of structured siloxane defoamers with a high degree of performance predictability. The goal of this development was to demonstrate that this predictability can be used to aid product selection and improve the efficiency of formulation development by avoiding the trial-and-error approach where data cannot be transferred from each failed experiment.

Figure 5 shows the formulator’s dilemma. This chart shows the defoaming and film quality results from extensive testing of twenty eight industry benchmark defoamers in a simplified water-based pigment grind (dispersing a phthalocyanine blue 15:3 pigment in a styrene acrylic resin) and mass-tone letdown (1:3 dispersion in acrylic emulsion). All the tested defoamers were recommended in product literature for this type of application. The results are shown adjusted to a simple 1-10 scale where 1 represents poor film quality or defoaming performance and 10 is excellent performance. The sample containing a no defoamer or blank is shown with a gray circle and had excellent film quality (rated 10) but was extremely foamy (rated 1). The commercial defoamers varied widely in performance with little to no commonality or consistency. There is a roughly equal spread of defoamers that are either too incompatible (good defoaming, poor film quality), or too compatible (good film quality, poor defoaming). Only two of the twenty eight defoamers tested gave satisfactory overall performance, a less than 10% success rate! This would be a significant time investment and frustration for a formulator trying to choose a suitable defoamer from scratch.

In comparison, the prototype series of defoamers was also tested in the same formulation and standardized to the same scale. While not perfectly linear, a very clear trend is apparent which tracks very favorably to the predicted defoamer performance of a “B-type” system shown in Figure 4. One of the products also meets the acceptable performance standards, a success rate of 20%.

10 10

8 8

6 6

4 4

Defoaming Ability Ability Defoaming Ability Defoaming

2 2

0 0 0 2 4 6 8 10 0 2 4 6 8 10 Film Quality Film Quality

Figure 5. Benchmark Defoamer testing (left) Compared to Prototype Defoamer testing (right)

In order to meet the goal of reduced experimentation and good predictability of defoamer performance, the series of prototype defoamers was tested in a wide variety of different formulation types to determine if they consistently matched the expected trends shown in Figures 4 and 5. The following examples show how this new series of structured siloxane defoamers match the theoretical performance variation and provide a significant level of predictability.

The first two examples show the defoamers performance in two different interior paint formulations, one flat (70% PVC) and the other semi-gloss (45% PVC). The defoamers were added during the preparation of the millbase and their performance was measured as a function of grind density, letdown density, as well as any appearance of foam in the final applied film. The film quality and number of surface defects were also measured subjectively. Both defoaming and film quality results were standardized to a 1-10 scale, with the blank (no defoamer) having the worst defoaming results (rated 1) and best film quality (rated 10). The results are shown in Figure 6 below.

10 10 D

D C 51 52 8 8 52 53 51 54 53 B 56 54

6 C 6

Ability Ability

4 4 A Defoaming Defoaming B 57

2 56 2 57 A Blank Blank 0 0 0 2 4 6 8 10 0 2 4 6 8 10 Film Quality (defect rate) Film Quality (defect rate)

Figure 6. Prototype Defoamer Testing in Flat (left) and Semi-Gloss (right) Interior Paint Formulations

The flat paint formulation is almost completely insensitive to defoamer-related defects, but very difficult to defoam due to the high viscosity and high volume of pigments and fillers relative to the carrier. The results match almost perfectly the “D-type” system performance outlined in Figure 4 and, as expected, the defoamers with the highest degree of incompatibility, 51, 52, and 53, offer the best performance. The more compatible defoamers are not as effective at defoaming. However, the results in the lower viscosity, less filled semi-gloss paint are quite different as the strongest defoamers created surface defects and the best performance is obtained with defoamers with intermediate incompatibility (B and C type); prototypes 53, 54, and 56, giving the best balance of defoaming efficacy and film quality.

This trend is continued in a clear wood coating formulation based on a PU-acrylic hybrid resin. This is formulated without pigments and the defoamers were added to the finished formulation with simple, low shear mixing. The coating was applied in three coats to sanded red oak panels with a foam brush. The first coat was sanded prior to application of the second and third coats. We would expect this formulation to require a much more compatible defoamer (“A-type” or “B-type”) and the results support this prediction exactly, with defoamer 56 giving the best performance. These results are best shown in Figure 7 below. There are no visible defects in the base formulation (no defoamer added) as expected, but the foam trapped in the dried coating is clearly visible. The most compatible “A-type” defoamer, 58 gives improved foam control and no defects, but there is still room for improvement as foam is still present in the finished film. This defoamer type is not incompatible enough for the demands of this formulation; however, in the lower right picture, the more incompatible “C-type” defoamer, 54, eliminates foam but also causes numerous craters and surface defects. This defoamer is too incompatible for this formulation. The “B-type” defoamer, 56 (lower left), has the correct and predictable balance of incompatibility to give optimal defoaming without defects. A quick screening of any one of the three potential defoamer types would allow a formulator to quickly understand the needs of the system and identify which defoamer to try next based on these results, saving time and work.

Blank A (58)

B (56) C (54)

Figure 7. Results for an “A/B-type” Clear Wood Coating Formulation

Formulators of industrial coatings that are applied by continuous methods such as roll coat, curtain coat or even printing methods are also concerned about the persistency of the defoamer during application. Defoamers can often lose performance over time, especially under high shear conditions when the defoamer can become emulsified and broken down into the formulation. In general, stronger defoamers better resist this effect and the high shear mixing can also help reduce the defects caused by these more incompatible materials.

To demonstrate this, the prototype defoamers were tested in a resinated pigment grind (PR22) and then let down into an acrylic ink formulation. Two other silicone defoamers recommended for ink applications were also tested. The ink was then sheared at 1500rpm using a Cowles blade over a period of hours and the foam (measured by density) and application quality (measured by drawdown onto Leneta Chart) tested regularly through the test. The results are shown in Figure 8. As predicted, the strongest defoamers maintained their defoaming properties longest. The surface appearance of the inks formulated with the stronger defoamers also improved during this time. Initially, defoamers 55, 56 and 57 all gave good appearance with 55 giving the best balance of defoaming properties and surface appearance; the stronger defoamers all caused craters or dewetting. After 30 minutes shear, the appearance of the ink formulated with defoamer 54 was also acceptable and this remained the best performer over time.

Defoamer Shear Stability 1.2

1.1

1

0.9

0.8 Foam Density (g/ml)Density Foam

0.7

0.6 0 50 100 150 200 250 300 350 Minutes @ 1500rpm, Type F Cowles

53 54 56 57 Silicone 2 Silicone 1

Figure 8. Shear Stability of Defoamers in Pigmented Acrylic Ink Formulation

Conclusion

Finding a defoamer for a formulation when the well-known and trusted products don’t work can be a time consuming and frustrating task. Defoamer performance is strongly affected by other formulation properties and while these effects are known and understood, they can be very difficult to predict when the defoamers offer little to no guidance. A series of modified siloxane defoamers have been developed that offer a controlled range of defoaming strength and compatibility relative to each other. By understanding the basic formulation demands such as shear, PVC level, sensitivity, and viscosity, a formulator can quickly one product to test first from the series and then use the results of this test as instructions towards the selection of the next defoamer to test, should the first product not give acceptable performance. If the first product shows evidence of defects, then a more compatible defoamer is required. Conversely, if the first product shows inadequate defoaming, then a product in the series that is more incompatible is desired.

References

1 “Foaming of Surfactant Solutions”, In Surfactants and Polymers in Aqueous Solution, Eds. Jönsson B, Lindman B, Holmberg K and Kronberg B, John Wiley & Sons, New York, 1998, 325 – 336. 2 Wasan D T, Koczo K, and Nikolov A D, “Mechanisms of Aqueous Foam Stability and Antifoaming Action with and without Oil,” In Foams: Fundamentals and Applications in the Petroleum Industry, Schramm L L, Ed., American Chemical Society, Washington, D.C., 1994, Advances in Chemistry Series 242, 47-114. 3 Rosen M J, Surfactants and Interfacial Phenomena, 2nd Ed., John Wiley & Sons, New York, 1989. 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 Hegedus C R, Reader J, and Lai K T G, Optimal Defoamer Selection for Coatings: Guidelines and Case Studies, Proceedings of ABRAFATI 2011, São Paulo, Brazil, November 21-23, 2011. 6 Louis C, Reinartz R, Chaigneau W, Reader J, and Lai G, New De-aerators for Airless Spray Applied Waterbased Coatings, Proceedings of the European Coatings Congress, Nürnberg, Germany, March 30, 2011. 7 Grigoras S, In: Computational Modelling of Polymers, Bicerano J, Ed., Marcel Dekker, New York, 1993, 161. 8 Voronkov M G, Mileshkevich V P, and Yuzhelevskii Y A, The Siloxane Bond, Consultants Bureau, New York, 1978.