In the printing and dyeing factory workshop, because there is too much foam in the dye vat, the craftsmen often ask the operators to add some defoaming agent, but the defoaming principle of the defoaming agent is probably only partially understood by everyone. This article explains to you in detail the principle of defoaming! Lets come look!
1. The generation of bubbles in paint and the principle of foam stabilization:
During the production process of paint, air will be involved to form bubbles. Certain raw materials such as surfactants, dispersants, etc. can stabilize bubbles.
Bubbles will also be produced during the application of paint, which mainly depends on the way the paint is applied. For example: Curtain coating can continuously entrain air into the paint, and airless spraying can also easily entrain air. Spraying under relatively low humidity conditions or under high temperature conditions can easily generate bubbles.
Foam stabilization principle:
Marangoni effect (liquid backflow caused by surfactant, counteracting the effect of gravity)
Electrostatic effect: The electrostatic repulsion of surfactant thickens the liquid layer of the bubble, thereby stabilizing the bubble.
2. The working principle of defoaming agent
The defoaming agent is in the thin layer of foam Acts during or after the formation stage:
The evenly dispersed defoamer penetrates into the foam elastic film and is distributed in the film, causing the thin layer to rupture through a reduction in surface tension.
The evenly dispersed defoaming agent penetrates into the foam thin layer and forms a monomolecular film, which reduces its adhesion and makes the thin layer prone to rupture.
Defoamers containing hydrophobic particles have a third mechanism. These hydrophobic particles reach the surface of the thin layer and adsorb surfactant on the top of the thin layer. The thin layer breaks due to lack of surfactant.
3. Selection and evaluation method of defoaming agent:
The defoaming agent must be able to It spreads quickly on the foam surface and can penetrate quickly, causing the foam to burst quickly. The commonly used types of defoaming agents currently include silicones and polyacrylates.
Organic silicone defoamers are usually polysiloxane types. For example: polydimethylsiloxane containing acrylate functional groups and polyether-modified polydimethylsiloxane. Organosilicon has high temperature resistance, low temperature resistance, stable physical properties, chemical inertness, and very low surface tension, and is a commonly used defoaming agent.
Defoaming agents such as polyacrylic acid defoam by changing the polarity and molecular weight of the polymer to cause selective incompatibility. The impact on gloss needs to be evaluated when using this type of defoaming agent.
When choosing a suitable defoaming agent in the future, we must consider the process of foam generation in the system, the compatibility and concentration of the system, temperature and viscosity, etc. Each of the above factors will have a direct impact on defoamer selection.
The evaluation of foam control agents mainly considers several 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 defoaming agent with the best compatibility with the system often has the worst defoaming stability; the one with the worst compatibility often has the worst spreading rate. quick.
Due to the diversity of coating raw materials and construction methods, defoaming agents need to be evaluated based on actual conditions.
1. Add the defoaming agent to be compared into the varnish according to a certain proportion, put it in a glass bottle, shake it in a vibrator for 5 minutes, and take it out for observation at the same time, and make a preliminary determination based on the amount of foam. The defoaming ability of the defoaming agent; after leaving it for 10 minutes/30 minutes, observe the height of the foam again and compare the defoaming speed;
2. Use a scraper fineness meter to scrape the paint liquid to determine the defoaming The compatibility between the agent and the system (whether there are shrinkage holes);
3. After the system foam is eliminated, observe the clarity of the system to see whether there are turbidity, stratification, oil slicks, etc.;
4. Storage stability: After half a month, repeat steps 1, 2, and 3 to determine the long-term effectiveness of the defoaming agent
5. Determine the amount to add. </p