Water Based Coatings
Additives - Defoamers
Mr. Veeramani shares his knowledge in this Column devoted exclusively to Water Based Coatings
HAVING dealt with in detail about Additives in a Paint Formulation, as mentioned earlier, it is very essential to know the function of each Additive, in presence of the other additives, Pigments, Extenders, Binders etc. So now we take up all the important Additives one by one and analyse their properties and its function in a Paint Formulation. It is necessary for the end user or the manufacturer to know this, rather than just mixing all the ingredients mentioned in the Formulation without knowing anything. This way one cannot make paint in the real meaning.
Additive that is used to eliminate or reduce foam in the Production Process. Defoamers are surface active agents that break the foam bubble and stop the foaming once it is formed. Some believe in saying it as “Foam Control Agent” in aqueous formulations. The correct chosen additive will eliminate foam more particularly in the dried film to avoid foam related film defects. Foam will occur during milling stage hence Defoamers are introduced in the beginning of the process. More so with some Surfactants with air/water interface foaming will occur. It is also important to choose the correct manufacturing equipment and correct stirring conditions to avoid air entrapment, which causes foaming. In latex paints elimination of foam in the manufacturing stage, for storage and good application.
High foaming can be observed during the milling/dispersion stage, so Defoamers are added earlier in the process. Foam slows down the Dispersion process and affects the moisture resistance of the coating adversely. Foaming also occurs during application to some extent, which depends on the method of application. Spray application of Paint at low humidity or in high temperature conditions, can increase the chances of foam entrapment. Latex Paints that are stabilised with Surfactants can generate foam easily during agitation; hence elimination of foam is essential for the manufacturing process, storage and application.
During Dispersion large volume of gases thus generated with low volume of liquid, eventually dissolves in the liquid depending upon the temperature. As the film starts drying the dissolved gases try to escape in the form of bubbles. These bubbles pack side by side on the surface as densely as possible. When the bubbles start breaking tiny pores, pinholes are formed, this is a surface defect. We have seen from the above what harm the bubbles and foam can create on the Paint film and hence they are to be prevented and let us see how to avoid or prevent them during the manufacture of Paints.
Good Defoamers need to be good bubble breakers and also need to keep the action sustained and maintain good defoaming. They should not be soluble in the Defoaming system otherwise it will increase the foaming. Also they should have excellent dispersibility in the system and also have spreadability, which will help the product to spread uniformly on the surface, coating the bubble particles and eliminating them. They work by lowering the surface tension around the bubble and coalesce to larger bubbles and eventually break. In general, defoamers consist of several compounds, which in total are not compatible with the coatings medium. Problems arise with defoamers that are too compatible with the system, which means that they are inefficient. Otherwise, if they are too incompatible in the system, these defoamers might be highly efficient but they will cause problems in the applied film. The ideal solution in the application of defoamers is to find the best balance between compatibility and incompatibility of the Defoamer for a maximum of efficiency.
Most Foam Control Agents have a carrier, actives (hydrophobic materials) that increase the spreadability, compatibility and stability of the Paint System. To illustrate some of them, Mineral Oils, Vegetable Oils, Glycol Ethers, Alcohols, Silicone Oils and water. Defoamers vary from each manufacturer and the Vendors can help to choose the right product. Defoamers are multi-component systems with different groups of functional ingredients. There are three principal classes of components: carriers, with a share of 75 - 90 %, hydrophobic ingredients, with a content of 5 - 10 % and other special substances (e.g. biocides, thickeners, emulsifiers) with 0 - 20 % in the formulation.
On a general basis, defoamers have to fulfill the three following conditions:
They must be almost insoluble in the medium which is to be defoamed
They must have a positive spreading coefficient to be surface active
They must have a positive entering coefficient
With this the function of Defoamer in a Paint Formulation is well understood. But the question arises as to what percentage of Defoamer should be used while Paint manufacturing and at what stage or stages they are to be added. As indicated earlier foam is generated at the time of dispersion and hence before this stage along with other additives, a part of Defoamer specified in a Formulation is added to arrest the foaming. Next at the let down stage, when the Emulsion is added and mixed the remaining part of the Defoamer is added. We will deal this in detail when we come to the chapter on Formulation of Emulsion Paints.
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To be continued.
A post-graduate in Chemistry from University of Madras, after a brief stint of teaching worked in a Food Industry for three years. Then shifted to Corrosion-Engineering in an Indo German joint venture and worked for 16 years. During this period was trained in Germany in the manufacture of Corrosion Resistant Mortars and Coatings. Subsequently shifted to Paint Industry in a well known all India brand and worked in various capacities viz: Projects, Operations and R & D. After superannuation started working as a free lance Paint Consultant and now attached to M/S VENLAC PAINTs, in Namakkal Tamil Nadu. Specialized in Formulating Water Based Paints to suit manufacturers’ needs.”
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