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Knowledge of defoamers: Types of defoamers. Immiscibility, high spreading ability, and low surface tension are important properties that every defoamer must possess. The defoamers commonly used in water-based wood paints fall into two main categories: mineral oils and silicone compounds. Mineral oil-based defoamers are inexpensive but have weak defoaming capabilities; they consist of approximately 85% carrier oil and 15% hydrophobic particles. These hydrophobic particles are usually made of hydrophobic fumed silica, as well as metal stearates and fatty acid derivatives. More recent versions of such particles use polyurea compounds. These defoamers can cause the paint film to turn black, and they are suitable for plasters and primers that do not require a high level of gloss. A representative example of this type of defoamer is Deqian Company’s 082 defoamer ； Silicone resin-based defoamers are currently the preferred choice for defoaming agents in water-based wood paints. They are mainly composed of emulsions of highly hydrophobic silicons such as polydimethylsiloxane and polyether-modified polydimethylsiloxane. These defoamers have no effect on gloss or transparency; therefore, they should be used as much as possible in high-gloss, transparent topcoats. Representative products include Degussa’s 805 and 902 defoamers. Evaluation and Use of Defoamers The role of defoamers in water-based wood paints is manifested in two main aspects: first, ensuring smooth production processes; second, preventing the formation of foam during application, thereby resulting in a high-quality paint film. Improper use of defoamers in coatings can have adverse effects; in addition to causing shrinkage cavities, it can also lead to a decrease in the gloss of the paint film, as well as reduced adhesion and water resistance. Therefore, it is essential to pay attention to the evaluation of comprehensive performance, as well as to the selection of defoamers and the control of their addition amounts. The following methods can be used to select an antifoaming agent. Experimental conditions: 88% water-based resin (polyurethane emulsion), 5% ethylene glycol butyl ether, 0.2% wetting agent; an appropriate amount of antifoam agent was added, and high-speed dispersion was carried out at 2000 r/min and 60°C. Evaluation method: The antifoam agent’s foam-suppressing ability was assessed based on the amount of foam generated in the paint sample in the container. Additionally, a film was prepared on glass using a 75m wet film former to evaluate the antifoam agent’s compatibility, the gloss of the paint film, and any defects present ； The defoaming ability of the defoamer is evaluated by applying it to wood boards. Table 1 shows a comparison of the performance evaluations of different defoamers. Therefore, for water-based wood coatings based on polyurethane emulsion systems, the use of silicone resin-based antifoam agents can achieve good antifoaming effects. A dosage of 0.2% is appropriate. Additionally, defoamers tend to separate into layers; even if they do not separate, they need to be mixed thoroughly before use. If necessary, they can be diluted with water or a co-solvent (since diluted defoamers have poor storage stability, it is recommended to dilute them just before use). It should be added slowly while stirring; if necessary, add it in stages to achieve satisfactory results. For example, strong defoamers such as 805 defoamer can be added during grinding, while defoamers with weaker defoaming capabilities such as 902 defoamer can be used when mixing the paint. This helps to prevent defects such as shrinkage pores, pinholes, and fish-eye defects in the paint film during application of water-based wood paints. The shear force during mixing is used to ensure a proper distribution of the particle size of the defoamer, but excessive shear force results in particles that are too small, thereby reducing the defoaming effect ； Defoamers that are not fully emulsified can also be harmful; the effects produced initially due to the relatively low shear force when they are added may be good, but in most cases this is not sufficient ； During the storage of the paint, the defoamer particles form larger droplets, increasing the tendency for foaming and surface defects in the coating. Selected from the book Painting Techniques