Blistering Defects and How to Avoid Them

Verney Denerville is an application field specialist with TIGER Drylac.

Q. Do you have recommendations for avoiding or mitigating blistering defects in powder coatings?

Dealing with surface defects can be a nuisance and costly endeavor for any coating operation. Especially if the cause of the defect is coming from the process. A common surface defect that happens during the powder coating process is blistering. Blistering is described as the formation of bubbles that range in various sizes and elevations within the coating. These bubbles can be broken or unbroken. Blisters are a unique surface defect because they form between the coating and substrate and will guarantee no adhesion due to enclosures at the part surface. Ultimately, this will lead to a complete failure of the finish and possibly damage the metal surface. 

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There are several factors that can lead to any form of blistering, but the most common form of blistering is due to a contaminant of some kind left on the surface after cleaning. These contaminants can lead to the accumulation of moisture within the coating film and can be composed of solvents, gasses, water, oils and more. During the curing cycle, temperatures rise and the trapped impurities expand between the coating and the substrate, causing the blistering effect. Bubbles will form because the film is tight but they will eventually collapse back into the surface. Substrates such as steel and other porous metals can retain and trap moisture easier, so it is important to pay extra close attention when handling these metals.

To help minimize and/or eliminate the potential of having blisters on the finished goods, take these recommend steps and incorporate them into the maintenance schedule. 

• Check dryer and hanging configurations and take note of part geometry. Drying time, temperature and hanging configuration may need to be altered. Air blast workpieces to ensure there is NO water on the workpiece.
• Optimize pretreatment system to have solvents that properly remove corrosion, oil and grease residues.
• Inspect pretreatment, final rinse stage and drying of media/chemicals because salt residues or chemical remnants can be left on the workpiece surface due to insufficient wetting from the final rinse stage. Demineralized or RO water is strongly recommended for use in rinse stages.

In conclusion, it should be understood how important the pretreatment process is to all coating operations and that it can also be the source of blemished products. A majority of surface defects occur during the pretreatment stage of coatings. Take the time to evaluate the pretreatment process in place and make sure it is up to industry standards. Many coating and chemical companies offer services that can audit and help enhance the process. We hope that this article will help provide solutions and insight for anyone that may be experiencing this issue.