Hi Joe:I'm selling powder coating in South America. So now I have many questions in regards to powder coatings. But the most important is this: Can powder coatings be used past the expiration date given by the supplier?
Thanks for the question. I have traveled and worked in Colombia and have very fond memories of my time spent there. I will be very happy to answer your question.
The stability of a powder coating is influenced by many factors. First I must differentiate two distinct types of stability. Physical stability involves the requirement for the individual particles to resist clumping or sticking together. Chemical stability refers to a powder coating's resistance to premature polymerization during storage and handling.
Physical stability is influenced by the melt point or more specifically the Tg (glass transition temperature) of the powder coating and the storage conditions. A powder coating with a low melt point will sinter and clump more readily than one with a higher melt point. Accordingly powders stored in high-temperature environments will clump more readily than those stored in cooler areas. Additionally, powders exposed to high-humidity environments tend to absorb moisture, which will also cause clumping. (Please note that most powders are supplied in moisture impervious containers and the exposure to moisture occurs after the package has been opened.) Clumping obviously makes a powder difficult to fluidize, transport and spray in an application system.
On the other hand, chemical instability causes a powder coating to chemically advance or polymerize during storage. Fast-reacting and low-temperature-curing powders can start to polymerize in high-temperature storage conditions. This premature chemical advancement reduces the melt flow of a powder coating, causing a more textured finish.
Both physical and chemical instability can be present in a powder coating. Typically fast-cure (or low-temperature-cure) powders also have low melt points to help facilitate better melt flow at low cure temperatures. Consequently these types of powders are most susceptible to clumping and chemical advancement, and should be stored and applied in a climate-controlled environment. I recommend less than 27 ⁰C and 50 to 70% relative humidity.
Now to answer your question (are you asleep yet?). Standard curing powder coatings (175 -200 ⁰C) that have been stored in a reasonable environment can remain usable long past their "due date". Low-cure powders and those that have been stored in environments exceeding 27 ⁰C for long periods of time can suffer from clumping and chemical advancement. I would inspect (for clumps, etc.) then spray and bake a sample of any powder that is past its due date and make the determination yourself. If the finish is still within your requirement for smoothness and appearance (no blistering, dirt, etc) then I would continue to use it.
Claudia, I hope that this helps you. Someday I hope to return to Colombia to enjoy the beautiful weather, excellent food and wonderful people.
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