Why do powder coatings produce air bubbles?

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

The essence of the Bernard vortex is the change in the surface tension caused by the viscosity change accompanying the melting and curing process of the powder coating, so that the fluid with high viscosity and low surface tension sinks to the middle (concave) of the vortex, while the low viscosity high The surface tension fluid then rises to the perimeter (convex) of the vortex until curing is comple.

Causes of bubbles in powder coatings

Native pinholes in powder coatings: air trapped in the coating

After the powder coating is sprayed, it is deposited on the workpiece in a loose structu.

In order to eliminate the primary pinholes caused by the above factors, 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 pinhol.

Volatile small molecules generated by the curing reaction of powder coatings

The curing reaction of powder coatings can be divided into two categories, one is the direct reaction without release of small molecules, such as the reaction between carboxyl groups and epoxy groups, and the reaction between hydroxyl groups and unblocked isocyanate grou.

Another type of curing reaction releases small molecules, such as polyester/β-hydroxyalkylamide outdoor powder coatings, hydroxyl polyester/tetramethoxymethyl glycoluril powder coatings, and hydroxyl/blocked isocyanates .

Practice has shown that benzoin alone cannot significantly solve this pinhole probl.

Substrate factors

The substrate is another important cause of pinholes in powder coatin.

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, e.

To eliminate such pinholes, first of all, the porous substrate can be preheated to a higher temperature and a longer time before spraying, the substrate can be dried as much as possible, and the powder coating has a lower initial melt viscosity due to the high substrate temperatu.

In order to improve the wetting performance of the powder coating to the substrate, some compounds containing polar groups (such as amide groups, hydroxyl groups, e.

The powder is damp

Unless the conditions are extremely harsh, powder coatings are normally not susceptible to moistu.

For powder coatings that have been seriously damped, a large amount of moisture is introduced into the syst.

Such problems can only be prevent.

Factors of powder raw materials

The high content of small molecules in the raw material is also an important factor causing pinholes in the coating fi.

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

Selection of defoamer

Benzoin is the most effective defoaming and degassing agent for high gloss powder coatings, which can effectively eliminate primary pinholes under normal film thickness (60-90.

Benzoin also has a certain synergistic effect on pinholes caused by other reasons, but the effect is often limit.

The use of a large amount of benzoin will also cause the problem of yellowing of the coating fi.

At the same time, the benzoin itself is also a substance that is decomposed and sublimated by heati.

Excessive use will bring about negative effec.

Defoamers containing amide groups and hydroxyl groups can effectively improve the wetting performance of coatings to substrates, and can also improve the wetting properties of resins to pigments, especially inorganic pigments and fillers, reduce the melt viscosity of coatings, and can effectively eliminate various proble.

bubbles caused by various reaso.

Of course, it is also necessary to select resins with low viscosity and gelation time in the case of serious cell occurren.

Transparent powder defoamer: As a special variety of powder coatings, the principle and defoaming method of the pinholes of transparent powder are the same as those of ordinary powder coatin.

The special point is that the transparent powder is a highly transparent system, the whole system is optically isotropic, and there is no serious phase separati.

Therefore, in the process of defoaming, the added defoaming and degassing agent must be completely compatible with the syst.

In this sense, such defoamers need to be carefully select.

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 carefully add.

Solution

In a word, there are many reasons for the formation of pinholes (cells) in powder coatin.

The fundamental reason is that with the increase of the viscosity of the system during curing, the small molecular gas that was trapped in the coating aggregated and could not be released in ti.

form.

The sources of small molecules entrained in the coating include: loosely accumulated air during powder coating coating (native of powder coatings), water, methanol and other small molecules generated by certain curing reactions, and molecules from inside the porous substra.

Small molecular gas, and small molecular gas introduced by improper storage of powder raw materials or powder produc.

There are different solutions for different reasons: