Q: I’m selling powder coating in South America, so now I have many questions with regard to powder coatings. But the most important is this: How long is a powder coating acceptable after the due date, when it still is at storage?
A: Thanks for the question. I have traveled and worked in Colombia and have very fond memories of my time spent there.
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 glass transition temperature (Tg) 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.
In addition, 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°C and 50-70% relative humidity.
Now to answer your question (are you asleep yet?). Standard curing powder coatings (175-200°C) that have been stored in a reasonable environment can remain usable long past their “due date.” Low-temp-cure powders and those that have been stored in environments exceeding 27°C for long periods of time can suffer from clumping and chemical advancement.
Before use, inspect (for clumps, etc.) and then spray and bake a sample of any powder that is past its due date. Then 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.