AGITAN® Defoamers from Münzing Chemie for Coatings, Adhesives and Construction Chemicals

Blog Archive | 12 minutes  | Author: Ian Brown , BSc.

In this Technical Article:

What is foam and why is it bad?

When air is introduced into pure water, the air bubbles rise to the top and burst. This is because the interfacial tension between the air and the water is too high and the air bubbles cannot be stabilized.  However, in a waterborne coatings formulation, there are surface-active components present.  These are present through the resin stabilization system, the pigment dispersions or tinting system and other additives such as wetting and levelling agents. 

When air bubbles are created in a coating, the bubble rises to the surface, where a layer of surfactant molecules are present between the air and liquid interface (Figure 1). A stable double layer is formed including the surfactant layers of the bubble and the surface. This layer consists of the monolayer of surfactant molecules at the air-liquid interface of the bubble and the monolayer of surfactant molecules that cover the liquid-air interface.

 

Mechanism for the stabilisation of foam by surfactants

Figure 1. Mechanism of foam stabilisation by a surfactant double layer in waterborne systems.  

 

If this foam creation is allowed to develop unchecked the consequences are numerous. Foam creation in coatings gives rise to poor visual appearance of the coating once applied, difficulties in manufacturing, difficulties in filling packaging and also problems during application of the coating itself.

How is foam formed in the manufacture and application of waterborne coatings?

During the manufacture and the use of a waterborne coating, the coating is subjected to multiple physical forces that lead to foam generation.

In general, these are:

  • Foam formation during production -  Vigorous stirring or grinding of pigments and fillers leads to the incorporation of air and stabilization due to the presence of surface-active components. At this stage, there is often an exchange of air entrained in pigments and fillers with water due to the presence of wetting agents.
  • Foam formation during pumping and filling - The shear forces imparted upon a coating during pumping and filling can also introduce air via the air-liquid interface.  Therefore, the defoamer used in the formulation must destroy this foam. Even after filling containers, the defoamer must persist long enough until the coating is used. Even then there will be further demands upon the defoamer as the coating is moved from its packaging to coating machines.
  • Foam formation during application - The method of application and the substrate itself can also lead to foam generation and put further demands on the defoamer.  Porous substrates can be problematic as entrained air within the substrate can migrate to the coating surface to give undesirable visual effects.  Application methods such as brushing, rollers and spraying also give opportunities for air to be introduced leading to microfoam formation on the substrate surface.

 

For these reasons a defoamer is an integral part of any coating formulation and Münzing Chemie has a full suite of technologies in their AGITAN®, DEE FO®, FOAM BAN® and FOAMTROL® products to help coatings formulators develop optimized products.

What is the composition of a defoamer and how do they work?

As mentioned above, the inclusion of foam stabilizing components is unavoidable in any waterborne coating, ink, adhesive or sealant. Therefore, one must start with the expectation of foam generation and the inclusion of a defoamer to prevent this from happening.  How do defoamers work and what are they made of?

As a general rule, the role of a defoamer is to introduce a controlled incompatibility into a coating. Since defoamers are themselves multi-component mixtures getting the balance of compatibility and incompatibility in a coating system is key.  If a defoamer is too compatible, it is inefficient. If it is too incompatible, defoaming efficiency may be high but surface/visual defects in the dry film are more likely.

 

Mechanism of foam destruction by a defoamer

Figure 2. Mechanism of foam destruction by a defoamer

 

In terms of composition, the main constituents of a defoamer are a carrier, hydrophobic particles and an emulsifier (Figure 2). Each serves a role in the efficacy of a defoamer and is explained below:

  • Carriers – These are typically oils such as mineral oil, vegetable oils, medical oils, silicones or polysiloxanes.  The function of the carrier is two-fold. Firstly, it spreads on the coating surface to remove surfactant molecules and secondly it transports hydrophobic particles in between the surfactant double layer of the air bubble to release the air.  Therefore, the carriers must be incompatible and insoluble with water to rise to the surface.
  • Hydrophobic Materials – These have an important task in defoaming. Their primary role is to remove surfactants from the double layer surrounding the air bubble to increase the surface tension and burst the bubble.  The hydrophobic component of a defoamer can be a liquid or a solid. If it is a liquid it will be present as emulsified droplets.  In both cases, particle size is important and a particle size range of between 0.1 to 20 microns is usually effective to allow the particle or droplet to enter in between the surfactant double-layer stabilizing an air bubble to burst it.  Typically, hydrophobic materials used are hydrophobically coated silicas, waxes or polypropylene glycols.
  • Emulsifiers – Earlier we said that finding the balance between compatibility and incompatibility was critical to defoamer performance.  In finding that balance, the emulsifier used is crucial. It is the emulsifier and the amount used which controls the extent to which the defoamer is dispersed in the coating and the droplet size of the defoamer.  It is important to note that emulsifiers themselves are surface-active materials so it is important that in their own right they have a low foaming tendency.

Influences of Defoamer Properties, Formulation and Manufacturing Processes on Defoamer Efficiency

The manufacturing process and shear forces

The efficiency of a defoamer depends on the balance of compatibility and incompatibility and this is influenced by the emulsifier type. The emulsifier and how much of it is used in a defoamer will dictate how easy it is to emulsify in water.  Defoamers are generally classified as easy to emulsify, emulsifiable (with some effort) and difficult to emulsify. Why have this selection?

Well, let’s say we are trying to defoam a highly pigmented coating.  Such a formulation is likely to experience significant shear forces to achieve the necessary level of pigment and filler dispersion. In this case, a defoamer which is difficult to emulsify may be a good choice as it is likely to have a greater resistance to particle size reduction due to the shear forces of the dispersion process. 

A defoamer with a high level of emulsifier is likely to generate small defoamer particles. Under the influence of shear stresses exerted upon the coating during dispersion, the particles will decrease in size. Thus by the time they penetrate between the surfactant double layer of the air bubbles they are too small to have an effect.  Therefore, with a lower emulsifier content, the defoamer particle size will be larger and will remain large enough to disrupt the surfactant double layer and burst the bubbles.

In contrast to a highly pigmented coating, a clear, unpigmented lacquer may be better served with an easy to emulsify defoamer. In the absence of fillers and pigments to disperse, the coating viscosity will be lower and the time required to mix all the components will be shorter. Therefore, the defoamer particle size, though small, is unlikely to diminish any further and can still be effective in penetrating the foam surfactant double layer to burst the air bubbles.

Does resin chemistry make a difference?

The short answer is “yes”.  The resin technologies available to the coatings formulator are numerous; from vinyl acetate copolymers, to styrene-acrylics, pure acrylics, alkyd emulsions and polyurethane dispersions.  If, for example, you have a wood stain formulation based on a styrene-acrylic resin and you wish to reformulate to a vinyl acetate co- or terpolymer, it is unpredictable whether or not your chosen defoamer will be effective in the new formulation. 

Taking an example from Münzing Chemie's portfolio, AGITAN® 295 (mineral oil-based defoamer with silica) generally works superbly in pure acrylic resins but in PU-acrylate copolymers it is not so efficient. Conversely, AGITAN® 230 (also a mineral oil-based defoamer containing silica) works very well with PU-acrylate resins but not so efficiently with pure acrylics. The difference between the two? The answer is the ease of incorporation and emulsification. AGITAN® 230 is hard to incorporate whilst AGITAN® 295 is easy to incorporate. The likely reason for this is that a PU-Acrylate hybrid resin could have a higher emulsifier content to stabilise it but this in turn renders AGITAN® 295 ineffective as the defoamer particle size is reduced beyond its effective threshold.

Is the resin emulsification system influential?

As seen in the above example of AGITAN® 295 and AGITAN® 230, we can already say “yes”. In addition, a common scenario is to evaluate nominal countertypes to a resin system you are already using.  The generic composition may look almost identical, the Tg (Glass transition temperature) is similar and the MFFT (Minimum Film Forming Temperature) is also similar. However, be prepared for the fact that your current defoamer is not equally effective in both resins with all other components being identical. The emulsifier stabilisation system from nominally similar resins will differ and that can in turn dictate a change in defoamer.

My resin supplier has recommended a defoamer for the resin itself.  Will this work in a formulated product?

It is common that some resin manufacturers may have suggestions for which defoamers work well with their resins. This can be a good starting point. However, as the complexity of the formulation increases, it becomes increasingly likely that this recommendation becomes less valid. As we have discussed earlier, coating formulations will contain other surface active components and the presence of fillers and pigments can create extra shear forces during dispersing. Therefore, it is critical to conduct the necessary tests to find the optimal solution for your formulation and process.

How can Münzing Chemie and Lawrence Industries help?

At this point, finding the optimal defoamer for a formulation especially when starting from scratch may seem overwhelming bearing in mind all the different influences of defoamer chemistry, formulation composition and processing.

As a result of years of experience, Münzing Chemie and the team at Lawrence Industries can easily direct you to a selection of defoamers that are likely to work as a starting point for your defoamer evaluations.  Alternatively, Münzing Chemie can assist with customer formulations in their own laboratories.

Laboratory-Based Technical Service at Münzing Chemie

Münzing Chemie have invested heavily in a state-of-the-art technical service facility in Abstatt in Germany.  The prime function of the laboratory and the technical staff is to provide support customers to find solutions to their additive problems.   

Common tests conducted for defoamer projects include Red Devil and dissolver testing to screen candidate defoamers. Further tests to evaluate the impact of the defoamer on other coating properties such as levelling and wetting can be also carried out along with coating application tests such as flexo proofing for inks and spray application for industrial coatings.

An overview of the customer support facilities is given in our previous blog article.

This is support is freely available to customers and we would encourage customers to contact your account manager at Lawrence Industries to discuss your particular project.

Author: Ian Brown , BSc.

Ian has a technical background in Chemistry and Materials Science, having studied at the University of Greenwich and then having worked in a technical capacity for both the MOD and Synthomer. Since then he has held various sales positions and is now one of the longest serving members of the Lawrence Industries team. Ian's specialities are in the coatings, adhesives, construction and graphic arts sectors.