A Brief Analysis of Defoaming Problems and Solutions in Powder Coatings

As one of the four major new environmentally friendly coatings, powder coatings have become a mainstream choice in home appliances, hardware, aluminum profiles, automobiles, and other fields due to their advantages of high efficiency, environmental friendliness, and economy, coupled with excellent decorative properties and corrosion resistance. However, many people encounter a troublesome problem during use—pinholes (commonly known as "pigmentation" or "fuzzy pores") appear in the coating after curing. These tiny defects not only ruin the appearance of the coating but also severely weaken its anti-corrosion performance, especially noticeable in medium-to-high gloss powder coatings, while relatively hidden in low-gloss or textured coatings. Why are pinholes prone to appearing in powder coatings? And how can they be effectively solved? This article will break down the core causes of pinholes in detail and share practical solutions to help you easily deal with the defoaming problem in powder coatings.

1. The Core Mechanism of Pinhole Formation

Powder coatings (especially thermosetting high-gloss types) are prone to pinholes due to their unique application conditions: high curing temperature (above 120℃), large film thickness (above 50μm), short curing time (within 20 minutes), and high initial melt viscosity caused by the absence of solvent dilution. The key to pinhole formation lies in the inability of gas to escape in time: After spraying, the powder accumulates on the substrate surface in a loose structure, with air filling the spaces between particles. Upon entering the drying tunnel, the powder melts and flows under heat, causing the loose structure to collapse. During this process, a "Bénard vortex" is generated—high-viscosity, low-surface-tension fluid sinks, while low-viscosity, high-surface-tension fluid rises. Air in the gaps, small-molecule gases inside the coating, or in the substrate accumulate and form bubbles during the melting process. These bubbles should be expelled with the flow. However, as curing progresses, the system viscosity increases rapidly, and the bubbles become trapped in the vortex and cannot be completely released, ultimately forming pinholes in the coating. Thermoplastic powder coatings, because their system viscosity does not increase, have a much lower probability of pinholes.

2. Main Types and Causes of Pinholes

Pinholes are not caused by a single factor. They can be broadly categorized into primary pinholes and derivative problems, each requiring different solutions:

2.1 Primary Pinholes

This is an inherent problem with thermosetting powder coatings—after spraying, a large amount of air exists between the loosely packed powder particles. When the powder melts and collapses, the intermediate layer of powder heats up slowly and melts sluggishly, causing air to be trapped within the coating, ultimately forming pinholes.

2.2 Derivative Pinholes

Thick Film Pinholes: When the coating thickness exceeds 120μm, even with the addition of conventional defoamers, air is difficult to completely escape, and obvious pinholes easily appear on the surface; Low-Temperature Curing Pinholes: Accelerators added to lower the curing temperature cause the system's melt viscosity to rise rapidly, preventing gas from being released in time, which is difficult to solve with conventional methods; Other Potential Issues: Improper use of defoamers (such as excessive dosage or incompatible types) may cause light-colored coatings to yellow, thus affecting the coating quality.

3. Practical Solutions

The core of solving pinhole problems is "helping gas escape smoothly," which can be addressed through formula optimization and raw material selection:

3.1 Core Solution

Preferred Degassing Agent: Benzoin (benzoin) is an essential degassing agent in high-gloss powder coating formulations. It effectively eliminates native pinholes by helping trapped air gather and escape in an orderly manner, resulting in significant defoaming effects.

Dosage Control: Benzoin decomposes easily upon heating; excessive addition can cause light-colored coatings to yellow. It must be used strictly according to the formula ratio.

Combined Use: For thick films (above 120μm) or low-temperature curing scenarios, the effect of benzoin alone is limited. It needs to be combined with other types of defoamers to completely eliminate pinholes.

3.2 Auxiliary Optimization

Controlling Coating Thickness: Avoid blindly pursuing thick films. If a thick coating is required, the defoamer formula should be adjusted simultaneously to increase gas venting channels.

Optimizing Curing Parameters: Adjust the curing temperature and time according to the coating characteristics to avoid rapid temperature increases leading to a sudden increase in viscosity, allowing sufficient time for gas venting.

Ensuring Raw Material Purity: Select low-impurity, low-volatility powder raw materials to reduce the generation of small-molecule gases within the coating.

Substrate Pretreatment: Ensure the substrate is clean and dry to reduce additional gas hazards caused by surface oil and moisture.

4. Summary

The core of solving the pinhole problem in powder coatings is to address the key issue of "gas venting": Primary pinholes can be mitigated with specialized degassing agents such as benzoin. For special scenarios such as thick films and low-temperature curing, defoamers need to be compounded. Simultaneously optimizing process parameters can effectively reduce pinhole formation. For manufacturers, it's crucial to precisely adjust defoamer formulations based on coating type (high-gloss/low-gloss, thermosetting/thermoplastic), film thickness, and curing conditions. For users, selecting powder coatings suitable for their application and strictly adhering to application techniques can significantly reduce the risk of pinholes. In fact, pinhole problems in powder coatings are not insurmountable. Understanding their formation mechanism and selecting targeted defoaming solutions allows for a balance between aesthetic appeal and protective performance. Hopefully, this guide will help you clarify your thinking, easily tackle powder coating defoaming challenges, and achieve more stable and superior spraying results.