Technical Guide for Selection of Defoaming Agents in Water-Based Dip Coating Systems

Technical Guide for Selection of Defoaming Agents in Water-Based Dip Coating Systems

The core technical advantages of water-based dip coating process: The bath solution is circulated for long-term use, accompanied by continuous pumping shearing and air mixing throughout the process. It must not only quickly eliminate existing bubbles but also achieve long-term stable suppression of new foam formation. Under the premise of strictly controlling shrinkage holes and pinholes, it must not significantly impair the paint film's water resistance, salt spray resistance, or intercoat recoatability. Most short-term defoamers commonly found on the market lose their defoaming effect after a few hours of circulation.

I、 Requirements for the hardness performance of defoamers in immersion coating process (priority needs to be given to ensuring)

1. Good ability to remove microbubbles; The most common problem with immersion coating of paint film is pinholes and prickly heat caused by the precipitation of small internal bubbles.

2. The importance of resistance to cyclic shear and long-term foam suppression is far greater than instantaneous foam rupture; The tank solution is pumped and overflowed throughout the entire process, and the short acting defoamer that can only quickly burst bubbles will soon become ineffective.

3. Do not significantly increase the hydrophilicity of the system to avoid a decrease in water and salt spray resistance indicators; Mineral oil products should be used with caution as they can easily cause paint film loss of gloss and deterioration of water resistance.

4. Compatibility needs to be maintained in a moderate state: excessive compatibility will significantly weaken the defoaming ability; Poor compatibility can easily lead to shrinkage and oil spot problems, and after long-term cycling, defoamers will also precipitate and float on the surface of the tank solution.

5. pH tolerance needs to be stable and cover 7.0-9.5; Most water-based immersion coating solutions are within this pH range, and defoamers cannot experience demulsification failure when the pH fluctuates slightly.

6. Excellent recoating adaptability; Defoamers with high silicon content are prone to poor adhesion during subsequent recoating processes.

II、 Performance comparison and selection priority ranking of three types of defoamers

1. Silicon free polymer and polyether defoamer (preferred for high shrinkage risk dip coating systems)

-Core features: completely silicon free, extremely low risk of shrinkage, good interlayer recoating performance; The instantaneous defoaming ability is relatively weak, and the long-term defoaming performance is moderate; Especially suitable for silicon sensitive systems, such as two-component waterborne epoxy and waterborne polyurethane immersion coating scenarios.

-Suitable for: workpieces with strict requirements for shrinkage control and recoating performance, as well as transparent immersion coating oil scenes.

-Reference grades: HR-3062G, HR-3062H (polymer silicon free).

-Addition amount: 0.2-0.4%, generally slightly higher than the amount of silicone defoamer used.

2. Polyether modified organosilicon (mainstream preferred category for dip coating process)

-Features: Fast defoaming response speed, long-lasting and stable foam suppression, excellent microbubble release performance; Relying on adjusting the polyether EO/PO ratio to balance the compatibility of the system is the most commonly used type in water-based epoxy, water-based PU, and water-based acrylic immersion coating systems.

-Suitable for water-based immersion coating process of most metal components such as chassis, hardware, castings, etc.

-Representative reference grades: HR-3062E, HR-3062

-According to the total paint quality calculation, the addition amount should be controlled within the range of 0.1% -0.3%, with a gradient test starting from 0.1%. It is strictly prohibited to exceed the specified amount.

3. Mineral oil-based defoamer (it is recommended to use as little as possible in immersion coating scenarios and can only be used for low-end working conditions)

Disadvantages: Short duration of foam suppression, and precipitation problems are prone to occur during long-term circulation of the tank solution; It is easy to cause paint film loss of gloss, deterioration of water resistance, and oil spot defects; It is not recommended to use it in immersion coating systems that require salt spray resistance.

-Only suitable for black thick film immersion coating conditions with low performance requirements, as well as application scenarios with cost control as a priority goal.

❌The dip coating scene should be avoided as far as possible: pure dimethyl silicone oil lotion; This type of product has poor compatibility and is prone to oil drifting and shrinkage problems, resulting in poor stability of the tank fluid circulation.

III、 Quick selection based on resin system

1. Waterborne acrylic lotion dip coating

The range of optional and adaptable product categories is relatively wide; Mainly using long-acting silicone polyether defoamers, a small amount of silicone free defoamers can be used in combination if necessary.

2. Immersion coating process of single component and two-component waterborne epoxy system

Priority selection: Silicon free polymer defoamers or low silicon content polyether modified organosilicon; Because the system is highly sensitive to silicon components, it is prone to inducing shrinkage problems.

3. Water dispersible polyurethane immersion coating application system

Priority selection: polyether modified organosilicon with moderate compatibility; It can simultaneously balance defoaming efficiency, long-term foam suppression performance, and comprehensive paint film performance.

IV、 Add operation methods and control points for exclusive immersion coating processes

1. The defoamer is added in two parts: one part is added during the grinding stage to eliminate the bubbles generated during the pigment dispersion process; The other part is added during low-speed mixing of paint to enhance the anti foaming performance of the tank solution; It is not recommended to leave everything for later processes before adding it.

2. It is not recommended to stir the defoamer at high speed for a long time, as it may damage its uniform dispersion and cause premature failure and oil drifting of the defoamer.

3. Exclusive compounding strategy for long-lasting tank solution: Polyether modified organosilicon is compounded with a small amount of silicone free polymer defoamer to meet the requirements of rapid defoaming and long-term cyclic foam suppression. This is a mature solution that has been implemented for many mass-produced immersion coating tank solutions.

4. The core verification items that must be done for dip coating process: tank liquid circulation simulation test, continuous circulation for 4 to 8 hours, tracking and observing the speed change of foam recovery.

-Store the tank solution for 24-72 hours and check for any floating oil or defoamer precipitation on the liquid level.

-Appearance of paint film: Focus on checking pinholes and shrinkage defects, and simultaneously carry out three performance tests: water resistance, salt spray resistance, and interlayer recoating adhesion.

V、Quick selection checklist