Analysis of the problem and solution of powder coating defoaming
Powder coatings and water-based coatings, light-curing coatings and high-solids coatings are also called four new environmentally friendly coatings. As an important branch of coatings, powder coatings are widely used in household appliances, general hardware, aluminum profiles and automobiles due to their high efficiency, environmental protection and economic characteristics, as well as excellent decorative and corrosion resistance properties.
Pinholes (pinholes), that is, mane holes, a kind of pinholes, are small defects in the coating film that are likely to occur during the curing process of the paint. Its appearance not only affects the appearance of the coating film, but also seriously affects the corrosion resistance of the coating.
Due to their unique application conditions, thermosetting powder coatings usually exhibit higher curing temperature (above 120 ℃), thicker coating thickness (above 50u), shorter curing time (within 20min), and higher initial melting Features such as viscosity (without solvent dilution). Practice shows that it is because of the above characteristics that powder coatings are more prone to pinholes during the curing process than solvent-based coatings with lower initial viscosity. It is worth mentioning that the thermoplastic powder coating does not increase the viscosity of the system, so the probability of pinholes is relatively small.
Pinholes, as a kind of coating film defects, are particularly obvious in high-gloss powder coatings, and low-gloss powder coatings, especially matte powder coatings, are usually not obvious. How to prevent and eliminate the pinholes of medium and high gloss powder coatings has become a problem that powder coating technicians must face (the following research is only for thermosetting high gloss powder coating systems).
Causes and solutions of pinholes in powder coating film ①
The formation of pinholes in powder coatings is closely related to its unique melt-solidification process. Therefore, in order to study the formation mechanism of pinholes in powder coatings, the melt-solidification process of powder coatings must be understood.
Powder coating, as the name implies, is a powdery coating. In the coating process, it is firstly electrostatically sprayed to absorb or accumulate on the surface of the substrate in a loose structure. After the spraying is completed, the workpiece enters the hot drying tunnel, the substrate and the coating are melted and flowed by heat, and the original loose structure or accumulation mode is destroyed as the powder particles melt and flow. It should be mentioned in particular that a local vortex effect generated by the liquid flow during the film formation process is called Bernard vortex.
The essence of the Benard vortex is that the change of surface tension accompanying the change of viscosity during the melting and solidification of the powder coating causes the fluid with high viscosity and low surface tension to sink into the middle of the vortex (recess), and the low viscosity is high The fluid of surface tension rises to the periphery (convex part) of the vortex until solidification is completed. In this process, the gas (air) in the original loosely packed voids will form bubbles after the powder melts and collapses, and small molecules from the coating or the substrate will also form bubbles And was discharged. As the viscosity of the system continues to increase as the curing progresses, the bubbles entrapped in the Bernard vortex will eventually form pinholes during the discharge process. Therefore, to prevent and eliminate pinholes in powder coatings, it is necessary to analyze the root cause of small molecules (bubbles) in the coating, and then apply the right medicine to prevent and solve pinhole defects in the coating film.
In the process of powder coating melting and solidification, the volatile small molecules entrapped in the powder coating can be divided into the following situations:
(1) Native pinholes in powder coating: air trapped in the coating
After being sprayed, the powder coating is deposited on the workpiece in a loose structure. This loose structure causes a large number of gaps between the powder particles and the powder particles to be filled with air. As the ambient temperature increases, the powder coating particles melt causing this This kind of loose structure collapses, because the thickness of the powder coating film is generally greater than 50μ (the loose structure after spraying is much larger than this thickness), the powder particles in the middle position and the temperature rises slowly melts slowly, so that the air between the particles is melted Wrapped by the paint, as the curing progresses, the viscosity of the system gradually increases, and the air trapped in the coating causes the formation of pinholes in the coating film. The pinholes of this kind of coating film are necessarily possessed by thermosetting powder coatings due to their own characteristics. Therefore, strictly speaking, pinholes are the original defects of powder coatings.
In order to eliminate the above-mentioned factors and lead to the original pinholes, the degassing agent is a raw material that must be used in the formulation of high-gloss powder coatings, and benzoin (benzoin) is an efficient degassing agent to eliminate the above pinholes. The defoaming mechanism of benzoin is very complicated. In addition to eliminating the above-mentioned pinholes, benzoin also has a certain effect on the elimination of pinholes caused by other factors.
It should be noted that although benzoin is a very effective defoamer and deaerator for powder coatings, it cannot solve all the problems of pinholes in powder coatings.
1.1. Even if it is the original degassing problem of powder coating, it still needs to pay attention to:
(A) Benzoin easily decomposes under heating and causes yellowing of the coating film. The addition of too much benzoin will cause discoloration of light-colored powder coatings.
(B) As the thickness of the coating film increases, especially when it exceeds 120 μ or more, even if a large amount of benzoin is added, there are usually obvious pinholes (thick film pinholes) on the surface of the coating film. Such thick film pinholes need to be combined with other types of defoamers and benzoin to eliminate.
(C) Benzoin cannot completely eliminate pinholes in some low-temperature curing powder coatings:
In order to reduce the curing temperature, a curing accelerator is usually added to the polyester / TGIC system or the polyester / epoxy mixed powder coating system, which causes the melt viscosity of the system to increase rapidly during heating and curing, causing a large amount of gas to be entrapped The layer cannot be completely released, resulting in pinholes. Practice shows that benzoin cannot completely solve this problem, and it needs to be solved with other defoamers.