Predicting Defoamer Performance in Coating Formulations
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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.