The defoaming effect is the critical selection criterion for a defoamer and there are a variety of test methods used to test this.
To establish differences between the control sample and the defoamer samples we recommend incorporating air, i.e. generating foam in the coating system in a reproducible way. As all methods relate to comparative testing and are not absolute methods, it is primarily a matter of producing as much or as little foam as is needed to make a reasonable differentiation.
Foam can be produced, for example, by shaking in a measuring cylinder, up and down motions of a perforated disc or blowing in air via a glass frit; the volume of foam produced can then be used as a measurement parameter.
In one of the other standard methods, a dissolver is used to stir in air under defined conditions and then the foamed-up material is poured onto an inclined glass plate. During the run-off, you can observe the foam bubbles bursting and assess upon curing, the tendency of the system to foam via the number of remaining foam bubbles. By transmitting light from the back it is possible to see even the smallest foam bubbles.
The test methods described are entirely irrespective of how much later the coating material is processed. Of course, there are also more practical methods; emulsion paints can be easily tested by rolling with a foam roller, whereby using a very large-pored roller can bring the test conditions closer to real-life conditions.
The foaming behavior of architectural coatings can be tested via brush application, whereby you attempt to include as much air in the coating as possible in the handling of the brush, (“stipple test”) to similarly create test conditions that reflect practice.
In highly filled systems (such as plasters) it is most useful to evaluate density: the sample that has been defoamed the most has the highest density. In aqueous dispersions, often only the foam behavior of the (diluted) dispersion binder is examined. Consequently, the test is performed quickly but must be considered to be purely a pretest. The final formulated compound contains numerous components that may also influence the foaming behavior and it is therefore essential to perform foam testing in the finished, formulated coating system. The coating should be tested approx. 24 hours following incorporation of the defoamer but also after storage, as defoamers can lose effectiveness over time.
Frequently, it is beneficial to use defoamers in combination with an (acrylate) leveling additive. Once the foam bubbles have burst, the surface quickly smooths preventing the formation of dents and pinholes. Defoamers are effective if they are insoluble in the medium being defoamed and have a certain level of incompatibility. This results in possible side effects of defoamers:
As individual defoamer products behavior differently in terms of the possible side effects – also dependent upon the coating system – the selection of the correct defoamer must also take this aspect into account. Experiments with various quantities of defoamers should of course be conducted, as only in this way is it possible to adjust the balance between the desired defoaming effect and the unwanted side effects. Whether a coating has foam problems is not just dependent upon the formulation, but also to a large extent dependent upon how it is processed and under which conditions as well as the nature of the substrate. In a specific case, you should strive to include the most important of these application parameters in testing, as only then can you achieve meaningful results that can be used in practice.