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Analysis of the problem and solution of powder coating defoaming

I. Introduction

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).


Second, the causes of powder coating film pinholes and their solutions:

The formation of pinholes in powder coatings is closely related to its unique melt-solidification process. Therefore, to study the formation mechanism of pinholes in powder coatings, the melt-solidification process of powder coatings must be clarified.

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-mentioned 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.
Even if it is the original degassing problem of powder coatings, 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 in the coating 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.


(2) Small volatile molecules produced by powder coating curing reaction
The curing reaction of powder coatings can be divided into two types, one is the direct reaction type without small molecule release, such as the reaction of carboxyl groups with epoxy groups, and the reaction of hydroxyl groups with unblocked isocyanate groups. At present, polyester / epoxy hybrid indoor powder coatings, polyester / TGIC outdoor powder coatings, GMA acrylic resin / DDDA transparent powder coatings, and pure epoxy powder coatings that are widely used in the market are cured during the curing process. Does not produce additional small molecules.
Another type of curing reaction releases small molecules, such as polyester / β-hydroxyalkylamide outdoor powder coatings, hydroxy polyester / tetramethoxymethyl glycoluril powder coatings, and hydroxy / blocked isocyanates (such as Evonik B1530 ) The powder coating system releases water, methanol and isocyanate blocking agent during the curing process. The small molecules released by the curing reaction will gather into bubbles and be discharged from the coating film. However, due to the increased viscosity of the powder system and the problem of film thickness, some small molecules are too late to be released and are trapped in the coating, resulting in the generation of pinholes. Therefore, such powder coatings not only have original pinholes to be solved, but also small molecule bubbles generated and accumulated additionally need to be eliminated. Of course, it is worth mentioning that because the deblocking temperature of the B1530 sealant is higher, about 160 ℃, before the deblocking system has enough time, low viscosity to level and release the entrapped gas, B1530 curing system The pinhole problem is not particularly obvious.
In addition to the use of benzoin, the elimination of pinholes in unfinished paints needs to be used in conjunction with other defoaming and degassing agents.

(3) Factors of substrate
Substrates are another important cause of pinholes in powder coatings. Porous substrates, such as cast aluminum and cast iron, are high-risk areas for powder coating pinholes.
The reason is that there is a lot of air in the porous substrate itself, or there are a lot of volatile substances (such as undried moisture, etc.) in the voids. After the powder coating is completed, the air or volatile substances in the voids are heated during the heating process Closed by the molten powder coating, the viscosity of the system increases rapidly during the curing process of the coating, which makes the gas in the gap too late to be released from the coating, causing pinholes.
To eliminate such pinholes, first of all, the preheating temperature of the porous substrate can be higher for a longer time before spraying, and the substrate can be dried as much as possible, and the powder coating has a lower initial melt viscosity due to the high temperature of the substrate , Helps to discharge the gas in the substrate. In powder manufacturing, on the one hand, certain substances that can improve the wetting properties of the powder coating substrate can be added to the powder coating formulation, so that the molten coating can quickly penetrate into the porous substrate, as soon as the initial melt viscosity of the system is low Force out the gas in the gap. On the other hand, you should choose a resin with a lower melt viscosity (film-forming substance), or add some substances that can significantly reduce the melt viscosity of the paint to the powder formulation.
In order to improve the wetting properties of powder coatings on substrates, certain compounds containing polar groups (such as amide groups, hydroxyl groups, etc.) can be added to the system. These compounds can not only help to wet the substrate, but also help Reducing the melt viscosity of the system is similar to the solvent or diluent in liquid coatings. Because they show solid characteristics in powder coatings, they can be called "solid solvents" for the time being. "Solid solvent" is a very useful attempt to solve such problems. Of course, in order not to affect the anti-corrosion performance of the system, such "solid solvent" cannot be a non-reactive and water-soluble compound, and the amount of addition should not be too large. .
It is worth mentioning that the discovery of certain reactive "solid solvents" may be very brilliant. This kind of "solid solvent" mainly shows two characteristics: first, a solid substance with a low molecular weight and a low initial melt viscosity; second, it can participate in chemical reactions, but it does not necessarily react with the original powder coating system, It is a self-consistent reaction system. Typical representatives, such as blocked or unblocked isocyanate curing agents (such as B1530 and B1540), their introduction can participate in the reaction of residual hydroxyl groups in the system, which can extend the gel time of the system, reduce the viscosity of the melt, and increase the crosslink density. Such substances are also helpful in eliminating pinholes.

(4) The powder is damp
Unless the conditions are exceptionally bad, powder coatings are not susceptible to moisture under normal circumstances. It is easy for the powder to get wet, often because the powder is stored in a low temperature for a long time, and it suddenly enters a high temperature and humid environment. The powder coating after unpacking is easy to cause moisture in the air to condense due to the low temperature.
For powder coatings after severe moisture, a large amount of moisture is introduced into the system. These moistures are discharged from the coating during the baking process, and some of the gas that is too late to be discharged is in the form of very fine pinholes, usually in the form of haze .
Such problems can only be prevented. The first is to ensure the normal storage conditions of the powder, and the second is to avoid ultra-low temperature storage as much as possible. If it is necessary to store at low temperature, the powder should be allowed to return to normal temperature for a sufficient time before opening the box to prevent moisture.

(5) Factors of powder raw materials
The high content of small molecules in the raw materials is also an important factor causing pinholes in the coating. On the one hand, the volatile content of the powder resin itself is too high, especially the content of small molecules with high boiling point is too high; on the other hand, the raw material of the powder is damp and absorbs a lot of water.
The pinholes caused by the above factors are generally very small, which is mainly manifested as haze on the surface of the coating film.

(6) The choice of defoamer:
Benzoin is the most effective defoaming and degassing agent for high-gloss powder coatings, which can effectively eliminate the original pinholes under normal film thickness (60-90μ). Benzoin also has a certain synergistic effect on pinholes caused by other reasons, but the effect is often limited. The use of a large amount of benzoin will also cause the coating yellow to become a problem. At the same time, benzoin itself is also a substance that decomposes and sublimates by heating. Too much use will bring negative effects.
The defoamer containing amide group and hydroxyl group can effectively improve the wetting performance of the coating on the substrate, and also can improve the wettability of the resin to the pigment, especially the inorganic pigment and filler, reduce the melt viscosity of the coating, can effectively eliminate each Cells produced for various reasons. Of course, it is also necessary to choose a resin with a low viscosity and a gel time to cause serious cell problems.
Transparent powder defoamer: As a special kind of powder coating, the principle and the defoaming method of the pinhole of transparent powder are the same as those of ordinary powder coating. The special point is that the transparent powder is a highly transparent system. The entire system is optically isotropic and there is no serious phase separation. Therefore, during the defoaming process, the added defoaming and degassing agent must be completely compatible with the system. In this sense, such defoamers need to be carefully selected.

It should be noted that the addition of a large amount of non-reactive defoamer is detrimental to the performance of the coating, and the amount of addition needs to be added carefully.

In short, there are many reasons for the formation of pinholes (foams) in powder coatings. The root cause is that as the viscosity of the system increases during curing, the small molecule gas trapped in the coating gathers and is too late While forming. Sources of small molecules entrapped in the coating include: loosely accumulated air during powder coating (primary of powder coating), water and methanol generated by certain curing reactions, and small molecules such as methanol, from inside the porous substrate Small molecule gas, and small molecule gas introduced due to improper storage of powder raw materials or powder products.