Anti-foaming agents are used in waterborne coatings systems to reduce foam formation during production, transportation, storage and application. Several test methods are used for determining the efficiency of anti-foaming agents. Depending on the cause of the foam formation, the best corresponding laboratory test method is chosen.

This article describes a new generation of ultra stable anti-foaming agents. Depending on the specific criteria for compatibility and usage, a selection of proper anti-foaming agents can be made.

Excessive foam formation is one problem that can surface during the formulation of waterborne paint systems. Foam can develop, and is a major concern, during all stages of a waterborne paint: production, filling, transportation and application.

Foam formation should be avoided by all means. During the production and filling stages, foam leads to inefficiency, overflow in tanks, instability and other problems. Foam formation during paint application can lead to properties like cratering, loss of opacity and protection.

To fight foam formation, anti-foaming agents are used. Almost every waterborne paint system contains an anti-foaming agent, typically in the concentration range of 0.05-0.5% by weight.

There are many anti-foaming agents on the market. Selecting the proper type for a certain application is quite a task. The efficiency of anti-foaming agents strongly depends upon the specific paint system and process used. This article presents some test procedures and properties of a new generation of anti-foaming agents.

Foam

Foam is a stable dispersion of a gas in a liquid or solid phase; for purposes of this article, it is formed from air distributed in the water phase of the paint formulation. The air is stabilized by the surface active components in the paint, such as emulsifiers, wetting agents and thickeners.

Air is added during several stages, such as during mixing, dispersing, filling, transportation and application. Besides the incorporation of air, other gases can develop as a result of chemical reactions in the paint. For example, in two-component systems such as those based on isocyanates, carbon dioxide can be formed when the isocyanate groups react with water.

Pure liquids don't form foam. The entrapped air will be released spontaneously as a result of differences in specific gravity (Stokes Law). In the presence of surface active agents, air dispersions will be stabilized.

Defoaming

Optimizing the paint formulation and the production and application process can reduce the entrapment of air. However, air entrapment cannot be avoided completely. Anti-foaming agents are needed in almost every waterborne system.

The efficiency of anti-foaming agents depends on their ability to spread themselves throughout the media and the ability to penetrate into the foam.

The following parameters can be described as follows1,2:

    Penetration coefficient E:
    E = sf - sd + sint. (1)

    Spreading coefficient S:
    S = sf - sd - sint. (2)

    In which:
    sf = surface tension of liquid to be defoamed
    sd = surface tension of anti-foaming agent
    sint. = interfacial tension between defoamer and the liquid to be defoamed.

A defoamer can penetrate a foam containing medium if E >0 and a defoamer can also spread itself spontaneously within the medium if S>0.

A positive value for S is of great importance for a defoamer since E-S=2sint. (see Figure 1).

The efficiency of anti-foaming agents is increased if surface tensions and/or interfacial tensions are decreased; anti-foaming agents, therefore, contain surface active components.

Also, the defoamer's viscosity and compatibility with the medium to defoam will play an important role in its efficiency. A low viscosity contributes to efficient penetration and spreading. Incompatibility ensures a defoamer concentration build-up at the interface liquid/air; that is, if the specific gravity of the defoamer is lower than the liquid to defoam.

Defoamer Composition

The composition of defoamers is extremely diverse. However, characteristic components of defoamers include the following.

  • One or more hydrophobic compounds. The hydrophobic component destabilizes the foam dispersion because it displaces the stabilizer. Hydrophobic components are considered among the most active ingredients in defoamers.

    Also, hydrophobic components prevent the formation of stable interfacial surfaces between air/liquid. Consequently, the air bubble can penetrate the interface and release itself or it can form a bigger, less stable, air bubble by coalescing with another air bubble. Typical hydrophobic components are mostly solids, such as silicas, polyamides and waxes.

  • Mineral oil. The mineral oil acts as the carrier for the hydrophobic components.

  • Surface active dispersing agents/emulsifiers. The dispersing agent ensures an optimum distribution of the hydrophobic component in the oil while the emulsifier eases the spreading of the defoamer throughout the medium to defoam. The type and quantity of emulsifier to be used depend upon the application of the defoamer and should be selected carefully. The minimum possible quantity of emulsifier should be used since emulsifiers themselves can contribute to foam formation. Also, the very fine distribution and stability of the defoamer hinders its efficiency since the working mechanism is based on a surface activity, partly caused by a certain incompatibility and strong activity on the interface air-water.

    The amount of emulsifier added will depend on the final application of the anti-foaming agent: for applications in which the defoamer is added with high shear forces or for applications of defoamers in media with high emulsifying properties, the quantity of emulsifier in the defoamer should be limited.

    Together with the dispersant - and in many formulations also the oil - the emulsifier determines to a great extent the interface activity of the defoamer. Also contributing to this are silicones. Silicones, however, can cause negative side effects such as crater building and adhesion problems.

    New-Generation Defoamers

    Traditionally, hydrophobic components are dispersed in the oil or emulsified in the melt stage. The end products are meta-stable. Most conventional anti-foaming agents will struggle with phase-separation in time and, therefore, they need to be homogenized before use.

    When not homogenized, there is a risk that only the top layer of the defoamer is used while most of the active ingredients remain in the bottom layer. This results in non-efficient defoaming and surface problems in the paint film. This is a phenomenon seen all too often in practice.

    By using a new procedure - in this study referred to as ultra dispersion process (UDP) - the hydrophobic component can be dispersed into the carrier, resulting in extremely stable anti-foaming agents. Above all, the combination dispersing agent/emulsifier is optimized. Defoamers made by the UDP process show spreading efficiency and stability of the hydrophobic component (see Table 1).

    In this test series, we intentionally chose a selection of defoamers with different emulsifying properties in water. SERDAS 7010 and 7015 are made following the UDP process, resulting in an extremely fine dispersion of the hydrophobic component in the oil. SERDAS 7540 and 7580 are composed of just one component: the hydrophobic component is chemically anchored within the ester component. The traditional, manufactured according to the conventional method, is based on a silica-mineral oil system and is used as a reference.

    Procedure

    The efficiency of the anti-foaming agents is demonstrated by using the following laboratory test procedures.

    Shelf Life Stability Test
    The anti-foaming agents are stored for three months at room temperature in a 100-mL glass bottle. Afterwards, separation will be judged.

    Shake Test
    Paint with anti-foaming agent is shaken on a Red Devil shaker for three minutes (or multiple three minute time periods): dosage is 150 grams of paint in a 300-mL bottle. Immediately after shaking, the specific gravity is determined with a 50-mL pycnometer. The percentage of foam can be expressed as:

      A de-aired paint is obtained by centrifuging a paint sample until a constant specific gravity is obtained. This shake test has proven to give a good indication about the defoamer efficiency in practice, specifically for dispersion paints, and is used for systems with a viscosity between 750-3,000 mPa.s. This test can be repeated after a certain shelf time, depending on the application.
    High-Speed Mixer Test
    This test method is extremely useful for defoamer evaluations in low-viscosity systems (50-750 mPa.s) as well as for pigment pastes. The percentage of foam is determined immediately after stirring 150 grams of test medium for three minutes in a 1,000-mL plastic beaker. The foam breakdown is noted, for example, one minute after stirring.

    Application Test
    For this test, a typical application method is used: roller, brush or spray application. For instance, in evaluating a wall paint, a roller with coarse pores is used. The foam development is evaluated in both the wet stage and dry stage of the paint and noted as: 10 = excellent; no foam or other defects; 1 = poor; foam and other defects like craters and coagulates. The defects will be described in each case.

    Also, changes in properties like viscosity, pH, color and adhesion will be determined. All test procedures for anti-foaming agents should reflect the 'real-life situation' as close as possible.

    For this study, the efficiency of the defoamers mentioned in Table 1 will be demonstrated in the following systems.

    System 1: Interior/exterior acrylic dispersion paint, PVC 55%

    System 2: High loaded styrene-acrylic dispersion paint, PVC 73%

    System 3: Acrylic dispersion gloss paint, PVC 16%

    System 4: Water based alkyd paint, PVC approx. 18%

    System 5: Pigment paste, based on TiO2, associative HEUR thickener, acrylic based dispersing agent and water

    System 6: Furniture coating, acrylic copolymer-based

    The dosage of anti-foaming agents in all systems is kept to a minimum of 0.2% by weight, based on total formulation, so that differences in defoamer efficiency can best be highlighted.

    Results and Discussion

    Many oil-based defoamers will show some form of phase separation upon storage. Shelf-life stability tests of the products listed in Table 1 indicate that indeed the traditional defoamer showed phase separation as seen in Figure 2.

    The defoamers, produced according to the UDP process, are stable and no separation is observed after three months. This obviously is the case for the clear defoamers SERDAS 7540 and 7580 as well (see Table 2).

    For System 1, the best results are obtained with SERDAS 7010 (see Table 3). The defoaming efficiency as well as the film appearance is perfect. Remarkable is the efficiency after six months of paint storage and the almost nonexistent relation of foam development and 'dispersing energy.' Typical for SERDAS 7010 is the weak emulsification rate. However, the paint itself shows sufficient emulsifying properties, allowing the defoamer to distribute itself homogeneously in the system.

    Although SERDAS 7580, a strong emulsifying grade, gives excellent defoaming properties, the wet paint film contains foam resulting in craters in the dry paint. It is assumed that the compatibility of this defoamer with this paint system is too good, leaving a too-low concentration of the SERDAS 7580 at the interface liquid/air. A higher dosage of the defoamer in the paint formulation might result in better application test results.

    For System 2, a dispersion paint, best results are obtained with SERDAS 7010: good efficiency and good shelf life storage properties (see Table 4).

    The other defoamers, all with better emulsifying properties, show a higher loss in efficiency upon storage. This can possibly be explained by the fact that probably the total interfacial surface defoamer/medium in these fine dispersions is larger/more intense, resulting in an increased change of physical or chemical interactions, like adsorption or absorption onto the pigment.

    System 3, the gloss dispersion paint shown in Table 5, shows that best results are obtained with SERDAS 7580, the type with the best water-emulsifying properties. Besides efficient anti-foaming properties the efficiency is maintained upon storage. A paint film without surface defects is obtained, as opposed to the system with SERDAS 7010.

    Hard-to-emulsify defoamers, like SERDAS 7010, show poor spreading in this system, resulting in concentrates of the defoamer on the surface of the paint film. This results in surface defects and in a loss of gloss.

    Table 6 shows that for a waterborne alkyd, SERDAS 7540 is a good defoamer. This product shows good efficiency and no surface defects. Also SERDAS 7580 performs well. The paint with SERDAS 7015 shows surface defects, presumably caused by the silicones present in the defoamer.

    In the system shown in Table 7, SERDAS 7540 shows the best results. 7540 is one of the better emulsifying defoamer types. Remarkably, the defoamers with weak emulsifying properties and silicones are less efficient in this system. There is no good explanation for these results, and this shows once again that selecting the optimum defoamer for a specific system requires testing of a range of defoamers.

    In an acrylic copolymer-based finish, good results are obtained with SERDAS 7015 (see Table 8). This wood finish is formulated with an acrylic dispersion with good emulsifying properties. Again, we see that the silicone-free defoamers, showing good emulsifying properties by nature, do not give the best results. Can this phenomenon be explained by the fact that these products are 'silicone free' or 'good emulsifiers'? Both are assumed. The silicone-based products yield a low interfacial tension. This relates to a positive effect for the penetration and spreading coefficients. Sufficient emulsifying properties of the medium to be defoamed will ensure a strong interfacial activity and, therefore, a low interfacial tension between liquid/air. Yielding higher interfacial tension, the silicone-free products cannot concentrate themselves in the interface liquid/air in favor of the foam-stabilizing components, and can therefore not act as efficient anti-foaming agents.

    SERDAS 7540 and 7580, with good emulsifying properties, are well distributed throughout the total medium. At the low concentrations added to this medium in this study, most likely an insufficient defoamer concentration ends up in the interface area liquid/air. In another similar wood finishing system, SERDAS 7580 turns out to be the most efficient defoamer: the resin in this system shows poor emulsifying properties.

    Conclusion

    A new production technique, the ultra dispersion process, allows production of anti-foaming agents with excellent shelf life stability. The overall efficiency of anti-foaming agents is extremely system dependent. However, by using a selected series of anti-foaming agents with different properties, it is possible to select the most efficient defoamer for each individual paint system. The selection of defoamers is chosen based on emulsifying properties and silicone-based or silicone-free types.

    Also, the sensitivity of the paint film to generate surface defects is important when choosing the right defoamer. In highly pigmented systems, a silicone-based and weak emulsifying defoamer, produced according to the ultra dispersion process, tends to be the best choice.

    References
    1 J.H. Bieleman; W. Heilen; S, Silber, "Additives for Coatings," Wiley- VCH, (2000), p. 100.
    2 Wallhorn, Heilen, Silber, Farbe und Lack 102, 12/96, p. 30
    3 Verkholantsev, European Coating J., 05/99, p.61