The working principle of defoamer in paint
1. The generation of bubbles in paint and the principle of foam stabilization:
In the production process of paint, air will be involved to form bubbles, and some raw materials in the paint, such as surfactants and dispersants, will stabilize the bubbles.
Air bubbles will also be generated during the construction of the paint, which mainly depends on the way the paint is applied. For example: curtain coating can be continuously drawn into the air in the paint, airless spray is also easy to get in the air, spraying under relatively low humidity conditions or under high temperature conditions can easily generate bubbles. Foam stabilization principle: Marangoni effect (the phenomenon of liquid backflow caused by surfactants, counteracting the effect of gravity).
Static effect: The electrostatic repulsion of the surfactant thickens the liquid film of the bubble, thereby stabilizing the bubble.
2. The principle of defoamer
Anti-foaming agents work during or after the formation of a thin foam layer:
The uniformly dispersed defoamer penetrates into the foam elastic film and distributes in the film, and causes the thin layer to rupture due to the reduction in surface tension.
The uniformly dispersed defoamer penetrates into the thin layer of foam and forms a monomolecular film, which reduces its adhesion and is prone to cracking of the thin layer.
A 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 breaks due to the absence of surfactant.
3. Selection and evaluation method of defoamer:
The defoamer must be able to spread quickly on the foam surface and penetrate quickly, causing the foam to burst quickly. The types of defoamers currently commonly used are silicones and polyacrylates.
Silicone-based defoamers are usually polysiloxane type.
For example: polydimethylsiloxane and polyether modified polydimethylsiloxane containing acrylate functional groups. Silicones have high temperature resistance, low temperature resistance, stable physical properties, chemical inertness, and very low surface tension. They are commonly used defoamers.
Anti-foaming agents such as polyacrylic acid defoam by changing the polarity and molecular weight of the polymer to cause selective incompatibility. The use of such anti-foaming agents requires evaluation of the effect on gloss.
In the future, when selecting a suitable defoamer, the process of foam generation in the system, compatibility and concentration of the system, temperature, and viscosity must be considered. Each of the above factors will have a direct impact on the choice of defoamer.
The evaluation of foam control agent mainly considers the following aspects:
Spreading rate; compatibility with the system; defoaming stability 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 defoaming stability; the poorest with the compatibility, the spreading rate is often the worst fast.
Due to the diversity of coating materials and construction methods, defoamers need to be evaluated based on actual conditions.
1. Add the defoamer to be compared to the varnish according to a certain proportion, put it in a glass bottle, shake it in a shaker for 5 minutes, and take out the observation at the same time to determine the defoaming ability of the defoamer according to the amount of foam. ; After leaving for 10 minutes / 30 minutes, observe the foam height again and compare the defoaming speed;
2. Scrape the paint with a spatula fineness meter to determine the compatibility of the defoamer and the system (with or without shrinkage);
3. After the system foam is eliminated, observe the clarity of the system for turbidity, delamination, oil slick, etc .;
4. Storage stability: After being placed for half a month, repeat the 1, 2, and 3 steps of experiments, and determine the long-term effectiveness of the defoamer;
5. Determine the amount added