Analysis on the problem of powder coating defoaming and its solutions

Abstract: This article introduces the causes of pinholes (bubbles) in the powder coating film and the corresponding solutions
.
Studies have shown that the root cause of pinholes (cells) in the powder coating film lies in the small molecules that are too late to be released during the curing process of the coating film
.
The reasons for this phenomenon include the inherent characteristics of the powder coating system, the small molecules produced by the curing reaction, the reasons for the substrate (porous, high activity, pretreatment, etc.
), and the moisture during the powder storage process.
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.
For different reasons, this article proposes corresponding solutions
.

1 Introduction

Powder coatings, water-based coatings, light-curing coatings and high-solid coatings are collectively referred to as the four new environmentally friendly coatings

Pinholes (pinholes), that is, bristle pores, a kind of needle-like pores, are small film defects that are prone to occur during the curing process of the coating
.
Its appearance not only affects the beauty of the coating film, but also may seriously affect the corrosion resistance of the coating
.

　　Due to its unique application conditions, thermosetting powder coatings usually exhibit a higher curing temperature (above 120°C), a thicker film thickness (above 50u), a shorter curing time (within 20min), and a higher initial melting Viscosity (no solvent dilution) and other characteristics
.
Practice has shown that it is precisely 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, because the viscosity of the thermoplastic powder coating does not increase, the probability of pinholes is relatively small
.

　　As a kind of coating film defect, pinholes are especially obvious in high-gloss powder coatings, while low-gloss powder coatings, especially matte sand grain powder coatings, are usually not obvious
.
How to prevent and eliminate pinholes in medium and high-gloss powder coatings has become a topic that powder coating technicians must face (the following research is only for thermosetting high-gloss powder coating systems)
.

2.

　　The formation of powder coating film pinholes is closely related to its unique melting and solidification process.

　　Powder coating, as the name suggests, is a kind of powder coating.

　　During the melting and solidification of the powder coating, the small volatile molecules entrapped in the powder coating can be divided into the following situations:

　　(1) Native pinholes in powder coatings: air trapped in the coating

　　After spraying, the powder coating is deposited on the workpiece in a loose structure.

　　In order to eliminate the above-mentioned factors that cause primary pinholes, degassing agents are necessary raw materials in the formulation of high-gloss powder coatings, and benzoin (benzoin) is an efficient degassing agent for eliminating the above-mentioned pinholes

　　It should be noted that although benzoin is a very effective powder coating defoamer and deaerator, it cannot solve all the pinhole problems of powder coatings

　　(A) Benzoin is easy to decompose under heating and cause the coating film to turn yellow.

　　(B) As the thickness of the coating film increases, especially when it exceeds 120μ, even if a larger amount of benzoin is added, there will usually be obvious pinholes (thick film pinholes) on the surface of the coating film

　　(C) Benzoin cannot completely eliminate pinholes in some low-temperature curing powder coatings:

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　　It is worth mentioning that the discovery of certain reactive "solid solvents" may be very colorful
.
This type of "solid solvent" mainly exhibits 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 kind of self-consistent reaction system
.
Typical representatives, such as blocked or unblocked isocyanate curing agents (such as B1530, B1540), their introduction can participate in the reaction of the residual hydroxyl groups in the system, which can extend the gel time of the system, reduce the melt viscosity, and increase the crosslink density
.
Such substances are also helpful in eliminating pinholes
.

　　(4) The powder is damp

　　Unless the conditions are abnormally bad, powder coatings are not easy to get damp under normal circumstances
.
Moisture of the powder is prone to occur, often because the powder has been stored in a low-temperature storage condition for a long time and suddenly entered a high-temperature and humid environment
.
Due to the low temperature of the powder coating after opening the box, it is easy to condense the moisture in the air and become damp
.

　　After severely dampened powder coating, a large amount of water is introduced into the system, and this water is discharged from the coating during the baking process.
Part of the gas that is too late to discharge is in the form of very small 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 low-temperature storage is really needed, the powder should have enough time to return to normal temperature before opening the box to avoid 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 film
.
On the one hand, the volatile content of the powder resin itself is too high, especially the content of small molecules with high boiling points is too high; on the other hand, the powder raw materials are damp and absorb a lot of water
.

　　The pinholes caused by the above factors are generally very small, which are mainly manifested as haze on the surface of the coating film
.

　　(6) Selection of defoamer:

　　Benzoin is the most effective defoaming and degassing agent for high-gloss powder coatings, which can effectively eliminate native 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 problem of yellowing of the coating film.
At the same time, benzoin itself is also a substance that is decomposed and sublimated by heating.
Excessive use will bring negative effects
.

　　The defoamer containing amide groups and hydroxyl groups can effectively improve the wetting performance of the coating to the substrate, and can also improve the wettability of the resin to the pigment, especially the inorganic pigments and fillers, reduce the melt viscosity of the coating, and can effectively eliminate the Cells caused by various reasons
.
Of course, it is also necessary to choose a resin with a low viscosity and gel time to cause serious problems to the cells
.

　　Transparent powder defoamer: As a special type of powder coating, the principle and defoaming method of the pinhole of transparent powder are the same as those of ordinary powder coatings
.
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, in 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 harmful to the performance of the coating, and the amount of use needs to be added carefully
.

　　In short, there are many reasons for the formation of pinholes (cells) in powder coatings.
The fundamental reason is that as the viscosity of the system increases during curing, the small molecule gases that are entrapped in the coating gather and are too late to release And formed
.
The sources of small molecules entrapped in the coating include: the loose accumulation of air (native to the powder coating) during the coating of powder coatings, water, methanol and other small molecules produced by certain curing reactions, and small molecules from the inside of the porous substrate.
small gas molecules, and small molecules and other powdery raw material gas or improper storage of finished powder introduced
.
You can take corresponding solutions for different reasons: