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The growing environmental awareness of the global paint and coatings market, as well as the need to reduce CO2 emissions and use renewable resources and green energy are also present in waterborne paint formulations. Artur Palasz, Ph.D., Spektrochem, discusses how until recently, replacing solventborne formulations with waterborne paints to reduce VOC was a sufficient step to being greener but today it is not enough, with a special focus on defoamers
On the one hand, it can be said that the green transformation is a necessity resulting from care for our planet; on the other hand, there is a fear of replacing conventional raw materials; and on another hand, not very honest communication of this care through greenwashing.
All this makes sustainable raw materials an object of interest today, but also of close observation before they are taken for testing, let alone used to replace fossil-based raw materials. In all the ecological criteria, (reducing the carbon footprint, using raw materials from renewable sources with a high content of bio-components), we cannot mention what seems to be most important – their performance in formulations and that their effectiveness is at least no worse compared to the raw materials traditionally used.
Nowadays, renewable raw materials in the waterborne paint industry can be found in almost every folder of raw material producers. From 100% bio-based to those partially produced with raw materials from sustainable renewable resources, you can find polymer dispersions, dispersing agents, defoamers, coalescing agents and even more recycled raw materials, e.g. recycled calcium carbonate fillers. One of the raw materials that is relatively difficult to replace, even between conventional ones, are defoamers and this article will focus on that topic.
Foam and defoamers in latex paints
Waterborne paints require the use of various surfactants to prepare their formulations. They are needed to stabilise latex binder particles, fillers and other ingredients that have no affinity for water. A surfactant chain containing different polarity and hydrophobicity on both sides connects the particles together, enabling the emulsification of monomer droplet particles during the emulsion polymerisation process in the case of a latex binder, or as an additive dispersing, it allows the stabilisation of pigment particles and fillers in suspension. A side effect of this stabilisation is also the stabilisation of foam that is created as a result of mixing, pumping or pouring, which cannot be avoided during unloading from tankers, the production process, pouring into packaging or finally painting with a brush, roller or spray.
It is mainly surfactants, such as dispersants used in the process of grinding pigments and fillers, as well as surfactants present in the polymer dispersion after the synesis process, that are responsible for stabilising the foam in the prepared waterborne paints. However, not every surfactant stabilises the foam, and even if it contributes to its formation and stabilisation, the volume of the resulting foam may vary greatly.
The foaming tendency of different raw materials can vary greatly and these are not extreme cases, as is the reality when comparing random polymer dispersions [Figure 1] or dispersant additives [Figure 2]. The subsequent process of selecting the defoamer additive depends on the tendency to create foam and its stabilisation by the raw materials introduced into the formulation of the prepared latex paint. Figure 1 shows the foam stabilisation tendency of two acrylic polymer dispersions with air introduced to their 1:1 dilutions with water. In the graduated cylinder on the left, you can see a much smaller volume of foam generated, as well as a trace on the walls of the cylinder from the collapse of the foam after aeration was completed. In the cylinder on the right you can see that the foam occupies a much larger volume and is stable even after aeration is completed.
Figure 1. Two polymer dispersions and foam generation as a result of aeration
Figure 2 shows photos after testing the foaming properties of surface active agents, which in this case are commercial dispersants (0.5% of active ingredients in DI water). As you can see, dispersant A does not generate foam at all, unlike the stable foam generated in various volumes by other dispersants.
Figure 2. RMFA test results ASTM D1173 test for various dispersant additives
Foam created during paint production, mixing before application or during the application itself is completely undesirable. The presence of foam causes aesthetic problems such as pinholes and craters in the coating, loss of opacity, and decreased durability of the coating. The thin walls created in the paint between the air trapped and stabilised by the surfactants weaken the coating in every respect, and the presence of surface defects also disturbs the gloss.
Special additives come to the rescue, their role is to avoid the formation of foam at every stage, from the formation of paint during various stages of production, through its storage, shaking out after tinting, to painting using various application techniques. These additives are defoamers, i.e. paint ingredients whose role is to release air in order to reduce or eliminate foam during each paint operation. Defoamers are often also called antifoaming agents and these definitions are generally used interchangeably, although in professional terminology there is a difference between these additives. The role of antifoaming agents is to prevent the formation of foam, unlike defoamers, whose role is to collapse the foam practically at the moment of its formation.
The task of defoamers is to burst the walls of the foam stabilised by surfactants, allowing the formed bubbles to be emptied and the air to be removed at the interface between the liquid and gas phases (air) In order for this process of penetration, spreading over the surface of the air bubble and finally its bursting to occur, the defoamers must be of a specific type of surface substances that are insoluble in the medium they defoam, but are effective at the liquid-air interface, causing agglomeration and entrainment of larger bubbles towards the liquid surface where they destabilize their walls [Figure 3].
Figure 3. Model of foam destruction by defoamers
Defoamers used in water-borne paints have different compositions and strengths. This article only presents basic information about defoamers, but this description does not exhaust the full landscape of defoamers. The basic defoamers are:
- Oil-based defoamers, among the most effective defoaming agents, in which the carrier is mineral or vegetable oil insoluble in the medium to be defoamed. Oil-based defoamers also contain hydrophobic silica and waxes, e.g. paraffin wax, ethylene bis stearamide, fatty alcohol waxes to increase effectiveness. They may also contain a small amount of surfactants to improve spreading.
- Silicone oil-based defoamers are silicone oil or water-borne emulsion in which hydrophobic silica has been dispersed, usually in the presence of surfactants to improve spreading in the defoaming paint. They are also efficient and strong defoamers, but they are often responsible for various coating defects.
- Polydimethylsiloxane, also silicon-based, is quite effective defoamers, usually not causing defects as often as silicone oil-based defoamers.
- Polymer-based defoamers which are hydrophobic organic polymers, usually silicone-free with additional hydrophobic particles, e.g. waxes or silica, often also with the addition of surfactants to improve spreading
Defoamers ingredients are increasingly replaced with components from renewable resources, vegetable oils, also glycols and surfactants from sustainable sources are used. These are usually different chemical compounds than those used in conventional defoamers that do not contain renewable resources. This means that their effectiveness will be different, the way of incorporation in the paint during production, and the addition stage will also be different. Therefore, this article draws attention to very important aspects of replacing conventional defoamers with new bio-based defoamers. It should also be emphasised that regardless of whether the defoamer is from conventional sources (e.g. fossil-based) or from bio-based renewable resources, the efficiency is expected to be at the same level so that its use in the formulation also brings performance benefits.
READ MORE BY ARTUR PALASZ:
Application studies and transfer of starting point formulations to the R&D stage and full-scale production
Recommended use of defoamers
Due to their often strong hydrophobic nature and almost always immiscibility in the medium to be defoamed, defoamers are difficult to introduce into systems, even waterborne paints. They create drops [Figure 4] whose size depends on their effectiveness, but this does not mean that the smaller they are, the more effective they are. Emulsification of defoamers in the process of adding them to paint production requires appropriate knowledge of their introduction into the system. Poor emulsification leads to strong defoaming, but causes various defects, such as craters or poor substrate wetting. Over emulsification causes poor defoaming, but no craters and good wetting of the substrate. Therefore, it is very important to use defoamers appropriately in accordance with the recommendations, however, these must be created on the basis of research, application studies and case studies delivered together with the defoamer sample to the R&D department of the paint manufacturer. It should be remembered that the defoamer works at the liquid-air interface, which does not only apply to liquid paint in the packaging or vessel during production. The liquid-air interface is also the applied coating, where the defoamer migrates to the surface, usually remaining on the surface.
Figure 4. Droplets on a liquid-air surface showing the immiscibility of the defoamer with water
In addition to the problems with emulsification mentioned earlier, there may also be problems that are particularly visible on the surface, related to the impact on gloss, defects such as wrinkling of the coating, leaching (especially mineral oil defoamers), or even the usual incompatibility. In technical data sheets, you can often find statements that one defoamer is recommended to be introduced in the grinding process (to the mill-base), another in the let-down process, another at any stage, and the vast majority of them are recommended to be added in a dose of 2/3 to mill-base and the remaining 1/3 dose to let-down. There are often recommendations to add a defoamer to the mill-base and then supplement its operation with another defoamer at the let-down stage.
Which guidelines are right? There is one answer – those whose recommendations were based on application studies and it was shown why this dosing method is recommended. Are they set correctly in each recommendation case? This will be revealed later in the article.
In order to demonstrate the effectiveness of defoamers in the formulation of waterborne paints, an experimental part was prepared in which a conventional defoamer and two new bio-based defoamers were compared. An outline of basic case studies that we perform in the Spektrochem laboratory is also presented, showing at what stage a given defoamer should be added and why. The following defoamers were used for the experimental part – see Table 1.
|Table 1. Defoaming agents used in the project
|Conventional – control sample
|Polymer-based with hydrophobic solids, VOC-free and silicone-free
|Blend of oils with polyalkene glycols and surface active components, VOC-free
Defoamer 100% based on renewable resources
|Hydrophobic organic polymer with hydrophobic particles, VOC-free and silicone free
Defoamer 100% based on renewable resources
Performance tests of the presented defoamers were carried out in wood &trim PVC 15 semi-gloss paint formulation based on self crosslinking acrylic polymer latex. The dosage of defoamers was assumed to be 0.5% of active content based on total formulation. This dose was added as:
- 100% in the grinding process (mill-base, grinding by cowles dissolver)
- 2/3 in the grinding (as above) + 1/3 in the let-down process
|Table 2. PVC 15 paint formulations for defoamers tests
|The stage of adding defoamers
|Mill-base – grinding by cowles dissolver
Wetting and dispersing
Defoaming – grinding
White prime pigment
|Let-down – mixing by anchor stirrer
|Self crosslinking acrylic latex
Mill-base – slurry
Defoamer (dose 1/3)
Carrier of TiO2 and additives
Defoaming – let-down
Film forming aid
|Formulation constants (calculated):
Q (PVC/CPVC): 0.27
Volume solids: 40%
Density: 1.21 g/mL
The prepared paints were tested as follows:
Stir test involves foaming the paint using a high-speed mixer. Air is introduced during mixing, which is trapped in the paint, and then the paint is subjected to density determination using weight per gallon cup (ASTM D1475) and compared with the density determined initial (after paint preparation) and 24 hours after paint preparation. The result shows how much the paint density decreases for particular defoamers and their incorporation methods.
Figure 5. Density comparison
Density measurements after the stir test showed that none of the tested defoamers maintained the initial density of the paints. The control defoamer was the most effective in this case, as the density decreased to 1.1 g/mL, while the tested bio-based defoamers after the stir test allowed for a density reduction from 1.03 g/mL to even 0.97 g/mL.
Analysing the initial and overnight density results, as well as the effectiveness of maintaining density after the stir test, no significant differences were noticed between the method of dosing 100% for grinding and dividing 2/3 + 1/3. There was a very slight difference in density for the bio-based defoamer #1, in favor of initial and overnight densities for dosing divided into 2/3 for grinding and 1/3 for let-down, however the difference is within the error limits of the density measurement.
This test involves applying paints with defoamers onto test charts using a porous sponge roller. This test allows air to be introduced during application using a porous sponge roller, and then the appearance of the wet and dry coating is assessed. The test also allows you to assess the effectiveness of wetting the substrate and the appearance of the coating in terms of craters, pinholes and other defects.
Figure 6 shows the results of the roller test evaluation. The control sample and the sample with bio-based defoamer #1 show no foam. Paint sample with bio-based defoamer #3 unfortunately shows significant foam on the wet film after incorporation of the defoamer during grinding, as well as moderate foam on the wet film when incorporating the defoamer during grinding + let-down. Craters also remain visible on dry coatings.
It should also be noted that when assessing the quality of the appearance of the coatings, the control sample shows the smallest defects in the form of orange peel in the case of 100% defoamer dosing during grinding. The sample obtained from the paint in which the defoamer was dosed with a division of 2/3 for grinding and 1/3 for let-down shows much larger defects in the form of wrinkles and orange peel. In the case of coatings with bio-based defoamer #1, no difference in the appearance of the coatings was noticed, as both have an unattractive appearance with an orange peel and problems with substrate wetting.
Figure 6. Comparison of wet coatings after roller test application
This test already shows the first indication of the effectiveness of defoamers in terms of practical application. The roller test proved to be valid with a low rating for the bio-based defoamer #2 for foam occurrence. To some extent, the foaming tendency was shown by the foaming test using Stir Test, in which this defoamer obtained a very low paint density (a lot of air in the paint). Bio-based defoamer #2 showed good effectiveness in reducing foam formation, however, the appearance of the coating is not entirely satisfactory and there are problems with wetting the substrate. Interesting results were also obtained for the control sample, which showed better defoamer compatibility when introduced into the paint in the grinding only process, as the coating with the dose divided into 2/3 for grinding and 1/3 for let-down showed a worse appearance of the coating, which clearly indicates that the recommendation for use should be limited to the use of this defoamer in the grinding process only.
Figure 7 shows the appearance of the roller test coatings after they have dried. To better observe the defects, the assessment was made under 10x magnification. There are clear differences between adding the defoamer at 100% during grinding, and in a split dosage of 2/3 during grinding and 1/3 during let-down. These differences are especially visible in the control sample and bio-based defoamer #1. Both cases show that in the tested defoamers, both control and bio-based #1, a much better appearance of the coatings is achieved with 100% dosing during grinding. This is because defoamers with high hydrophobicity, especially oil-based and even polymer-based ones but with a high content of hydrophobic particles, require higher shear forces to be introduced in order to form droplets and distribute them in the system. In the case of bio-based defoamer #3, coating defects are visible after drying in both dosage cases.
Figure 7. The appearance of dried coatings after the roller test under 10x magnification
The compatibility test shows how its distribution in the coating affects its appearance and texture. This parameter is illustrated graphically in Figure 8, which shows the appearance of the coating by intense light passing through a glass panel with a wet coating.
Figure 8. Compatibility in PVC 15 semi-gloss formulation
As you can see, the control and bio-based #2 defoamer paint coatings are acceptable in terms of compatibility, although some incompatibilities are visible in the control coating up close. Bio-based defoamer coating #1 as seen in the image shows incompatibility. This test shows how important it is to differentiate the tests performed to verify the results, which show that obtaining a relatively good defoaming result, e.g. in a roller test, means that the defoamer is working properly. The example of such incompatibility of the bio-based defoamer #2 shows that it should be focused on in the further part of the research work to check whether it may not show better compatibility during incorporation at higher shear rates, over a longer period of exposure to these forces, etc. In turn, the example of bio-based defoamer #3 clearly shows that good compatibility does not necessarily mean good effectiveness, as shown by stir tests and roller tests. Figure 8 shows coatings only for paints with defoamers added at the grinding stage, while no differences in improved compatibility were observed for paint samples in which the defoamer was divided 2/3 for grinding and 1/3 for let-down.
The last test presented in this article is the assessment of the impact of defoamers and the method of its incorporation on the gloss of the coatings. Semi-gloss formulations were intentionally prepared to better observe the effect on gloss. Of course, the results with other polymer dispersions present in the formulations will look different, therefore this test should be extended to other binders to make appropriate comprehensive recommendations regarding the impact of a given defoamer on the gloss of coatings.
Gloss ratings were performed for paints applied by automatic drawdown using a 7 mils gap-applicator and the coatings were conditioned at 73.5 °F ± 3.5 °F (23 °C ± 2 °C) and relative humidity for 3 days. Gloss measurement was performed in accordance with ASTM D523 using a triangular glossmeter, however, due to the gloss characteristics of the semi-gloss coating, the measurement was performed at 60°. The results are shown in Figure 9.
Figure 9. Influence of defoamer and incorporation method on gloss
As you can see, the differences between individual results are small, however, there is a clear tendency of the bio-based defoamer #1 added at 100% at the grinding stage to show the highest coating gloss in the test formulation. Bio-based defoamer #3 clearly shows a tendency to achieve a slightly lower gloss, but these values are still extremely close to each other. It can therefore be assumed that in the tested formulation and the dose used, differences in gloss are conditionally negligible. An interesting result could be the shine for the bio-based defoamer #3 at an increased dose, which could affect the shine much more (and perhaps show better defoaming).
The gloss of coatings with defoamers is also tested after storage stability, in which liquid paints are exposed to elevated temperatures in tightly closed containers to demonstrate the effect of defoamers on reducing their effectiveness and decomposing into individual components that may reduce the gloss (e.g. waxes, silica, etc.). Due to the articles being edited while these tests were being conducted, they have not been included in this article.
The presented case study results clearly show that bio-based defoamers can demonstrate good properties in waterborne paint formulations, however, technical data on their performance in formulations must be extensively characterised and compared to conventional defoamers currently used on the market. A good way is to present the results in a summary form as in Table 3.
Summary results presented in this way allow the formulator to understand which defoamer may be of interest for obtaining priority properties in a given formulation. Such data from studies on the effectiveness of use in formulations are key to facilitating the selection of the appropriate sample for testing, and the breadth of types of formulations and the scope of tests in which the effectiveness of new bio-based defoamers is presented are the door to eliminating concerns about using bio-based additives in paint formulations. waterborne.
Author: Artur Palasz, Ph.D.
e-mail: [email protected]