In the production of architectural coatings, the presence of emulsifiers, dispersants, thickeners and other additives in the formula not only generates a large number of bubbles, but also stabilizes them, resulting in a large number of bubbles appearing in architectural coatings. During the film formation process, if the bubbles cannot disappear, it will cause defects such as shrinkage and needle holes in the coating. Therefore, adding defoamers to eliminate these bubbles is necessary to achieve satisfactory results.
1. Reasons and prevention of bubble formation
Bubbles are dispersion systems in which gases are dispersed in liquids. In architectural coatings, due to the presence of surfactants, a large number of bubbles can be generated and stabilized. These bubbles rise towards the air interface, but they are still surrounded by a surface facial mask with surfactant, forming a double molecular layer, namely bubble interlayer. The main reasons for the generation of bubbles in the production of architectural coatings are: ① The pigments and fillers in the coatings have a large specific surface area, and the pigments and fillers added during the production of coatings can adsorb air into the coatings to generate bubbles; ② Due to the addition of various surface active substances such as emulsifiers, dispersants, wetting agents, thickeners, etc. in the production and application of coatings, bubbles are easily generated during production and construction Due to mixing, dispersion, grinding, construction, etc., bubbles are easily formed through the intervention of air in the system; ④ Due to certain side reactions producing CO2, bubbles are generated.
2. Defoaming mechanism
For architectural coatings, defoamers must be combined with a stable surfactant layer and penetrate into the foam bilayer, rapidly spreading upon contact with bubbles to form a very thin bilayer layer. With the further spread and invasion of defoamer, the film wall of the original foam will be replaced. Due to the fact that low surface tension liquids always flow towards high surface tension liquids, the surface tension of defoamers is lower than that of bubble liquids. Therefore, defoamer molecules adhere to the surface of the bubble film, reducing the local surface tension of the bubble film surface, while the rest of the film surface still maintains a large surface tension. This difference in surface gravity on the bubble film causes strong tension to pull the weaker tension part, resulting in stress imbalance throughout the bubble and leading to bubble rupture [2-5].
3. Selection of defoamers and their application in architectural coatings
Defoamers are highly targeted, often able to defoam in one bubble system but have poor or ineffective effects in another system, and even assist in foaming. The selection of defoamers for water-based architectural coatings is a complex issue. There are many types of additives in the coating system, and how to combine them with defoamers to produce a good overall effect requires experimentation to determine. How to choose defoamers? In theory, the main considerations are surface tension and miscibility. When choosing, various factors should be taken into account: ① Due to different types of coatings, main film-forming agents, and additives, different defoamers are used, resulting in different defoaming effects, which require a thorough analysis. ② Conduct screening experiments on several types of defoamers and select the best defoamer.
4. Issues to be noted when using defoamers
(1) Before use, it must be thoroughly stirred and generally does not need to be diluted with water. It can be added directly. If dilution is required, it can be used as needed.
(2) Add the defoamer into the system while stirring the paint.
(3) In order to achieve good defoaming effect, it is best to add it in two times. The first time, half of the total amount is added when preparing the coating, and the other half is added after grinding.
(4) The dosage of defoamer should be appropriate. Excessive addition can cause side effects such as shrinkage of the coating film. The amount added is too small to achieve the defoaming effect.
(5) After adding defoamers, it takes at least 24 hours to test the performance of the coating, as defoamers have durability, otherwise product quality problems such as shrinkage may occur.
5. Conclusion
The use of defoamers is to prevent the formation of bubbles or to eliminate already formed bubbles as soon as possible, ensuring product quality. The selection and dosage of defoamers have a significant impact on the performance of coatings. If not chosen properly, not only will the defoaming effect not be achieved, but the quality of the coating will also be affected. In the experiment of determining the formula, careful screening is carried out based on specific circumstances to select the best defoamer and dosage to meet product quality requirements.
