1, the formation of bubbles in the paint and the principle of bubble stability:
In the production process of the paint, air will be involved in the formation of bubbles, some raw materials in the paint such as: surfactants, dispersants, etc., to stabilize the bubbles.
Bubbles can also occur during the construction of the paint, which mainly depends on the construction method of the paint. For example, the screen coating can be continuously sucked into the air in the paint, airless spraying is also easy to get sucked into the air, and it is easy to produce bubbles in the spray construction under relatively low humidity conditions or under high temperature conditions.
Principle of bubble stabilization:
Malangani effect
Electrostatic effect: The electrostatic repulsion of the surfactant thickens the liquid layer of the bubble, thus stabilizing the bubble.
2, the action principle of defoamer
Defoamer works during or after the formation of a thin layer of foam:
The uniformly dispersed defoamer penetrates into and distributes in the foam elastic film, and causes the thin layer to break through the reduction of surface tension.
The uniformly dispersed defoamer penetrates into the thin layer of foam and forms a single molecular film, which reduces its adhesion and is easy to break the thin layer.
Defoamer containing hydrophobic particles has a third mechanism. These hydrophobic particles reach the surface of the thin layer and adsorb the surfactant at the top of the thin layer. The thin layer ruptures due to the absence of surfactant.
3. Selection and evaluation method of defoamer:
The defoamer must be able to spread quickly on the surface of the foam and be able to penetrate quickly to make the foam burst quickly. At present, the commonly used types of defoamer are silicone and polyacrylate.
Silicones are usually of the polysiloxane type. For example, polydimethylsiloxane containing acrylate functional groups and polyether modified polydimethylsiloxane. Silicone has high temperature resistance, low temperature resistance, stable physical properties, chemical inertia, very low surface tension, is a commonly used defoamer.
The use of antifoamers such as polyacrylic acid, which defoamates by changing the polarity and molecular weight of the polymer leading to selective incompatibility, should be evaluated for the effect on gloss.
In the future, the selection of suitable defoamer must take into account the process of foam generation in the system, the compatibility and concentration of the system, temperature and viscosity. Each of these factors will have a direct impact on the choice of defoamer.
The evaluation of foam control agent mainly considers several aspects: spreading rate; Compatibility with the system; Stability of defoaming and cost performance. However, the above factors are often contradictory in a formula. For example, the defoamer with the best compatibility with the system has the worst defoamer stability. The ones with the worst compatibility tend to have the fastest roll-out rate.
Due to the diversity of coating raw materials and construction methods, defoamer needs to be evaluated according to the actual situation.
1, according to a certain proportion of the need to compare the antifoam agent added to the varnish, put in the glass bottle, shake in the oscillator for 5 minutes, while taking out observation, according to the number of foam to determine the antifoam ability of the antifoam agent; After 10 minutes /30 minutes, observe the height of the foam again and compare the defoaming speed;
2, use the scraper fineness meter to scrape the paint solution to determine the compatibility of the defoamer and the system (whether there are shrinkage holes);
3, after the system foam is eliminated, observe the clarity of the system, whether there is turbidity, stratification, oil slick and other phenomena;
4, storage stability: after half a month of placement, repeat 1, 2, 3 steps of the experiment, and determine the long-term effect of the antifoam agent
5. Determine how much to add.
