Evaluation and use of defoaming agents
The role of defoamer in water-based wood paint is mainly manifested in two aspects: one is to ensure that production can proceed smoothly, and the other is to ensure that no foam is generated during construction and to give a good paint film. Improper use of defoamers in coatings will have side effects. In addition to causing shrinkage holes, it will also cause the gloss of the paint film to decrease, and the adhesion and water resistance to decrease. Therefore, it is necessary to pay attention to the assessment of comprehensive performance, the selection of defoamer varieties and the control of their addition amount. The following methods can be used to select defoamers. Experimental conditions: 88% water-based resin (polyurethane emulsion), 5% ethylene glycol butyl ether, 0.2% wetting agent, add an appropriate amount of defoamer, and disperse at high speed under 2000r/min and 60rain. Evaluation method: According to the amount of foam generated by the paint sample in the container, evaluate the defoaming ability of the defoamer, and use a 75m wet film preparation device to make a film on the glass to evaluate the compatibility of the defoamer, the gloss of the paint film, defects, etc.; brush the test on the wooden board to evaluate the defoaming ability of the defoamer. Table 1 is a comparison of the performance evaluation of different defoamers. Therefore, for water-based wood coatings of polyurethane emulsion system, the use of silicone resin defoamers can achieve better defoaming effects. The addition amount is preferably 0.2%.
In addition, defoamers are often easy to stratify. Even if they are not stratified, they should be stirred evenly before use. If necessary, they should be diluted with water or co-solvents (because the storage stability of the diluted defoamer is poor, it is recommended to dilute it before use). When adding, add it slowly under stirring, and add it in steps if necessary, and you will get a satisfactory effect. For example, when grinding, add a strong defoamer (such as 805 defoamer), and when mixing the paint, you can add a defoamer with weaker defoaming ability (such as 902 defoamer), which can prevent the paint film from producing shrinkage holes, pinholes, fish eyes and other defects during the construction of water-based wood coatings. The shear force during stirring is used to ensure that the particle size of the defoamer is well distributed, but excessive shear force makes the defoamer particles too small and weakens the defoaming effect; insufficiently emulsified defoamers are also harmful. The relatively low shear force when added may produce a good effect initially, but in most cases, this is not enough; during the storage of the paint, the defoamer particles will form larger droplets, resulting in an increased tendency for the paint to foam and have surface defects.
