Fully Cured Powder
Is there a quick way to determine if a powder coating is fully cured? What Mil-Spec covers powder coating?
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Q. Is there a quick way to determine if a powder coating is fully cured? What Mil-Spec covers powder coating? J. B.
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A. The simplest way to check for cure on a powder coating is to use the Powder Coating Institute Test Procedure #8 “Solvent Cure Test.” This test method is available directly from the PCI and is included in their Powder Coating Handbook. This test procedure details how you can use two different “reagents” to check cure of a powder coating. Reagent A is pure MEK and Reagent B is a mixture of 10% MEK and 90% Xylene. You can purchase these solvents at most paint stores or home centers.
This test method works well for providing a simple yes or no answer to the question if a powder coating is cured. It works on the premise that solvent resistance is the last property that is achieved during the cure of a powder coating. Therefore, only a fully-cured powder coating is cross-linked enough to provide the anticipated solvent resistance of that particular powder coating formula. Having said that, you must understand that not all powder coatings have the same solvent resistance. For instance, epoxy is more solvent resistant than a polyester powder coating. Therefore, only epoxies are tested using Reagent A, while most all other powder coatings are tested using Reagent B.
Closely follow the directions in this test method to ensure accurate results. Be sure to obtain the solvent resistance information from the powder supplier before you test to ensure that you accurately interpret the test results. The stated value of solvent resistance using this test method is “double rubs” and is related to a single back and forth motion of the test instrument. For instance, a solvent resistance for your particular powder formula may state “100 double rubs with some slight color transfer using Reagent A.”
If you are looking for a quantitative test result for cure, your only choice is sending your part to a laboratory for Differential Scanning Calorimeter (DSC) testing. The laboratory will need a sample of the uncured powder, as well as the part. Their reported results will be very accurate and will state cure in percent (i.e. 85% of full cure). Be aware that this test is destructive to the coating on your part and can be very expensive. This would not fit your request for “a quick and simple” test for cure.
As for your last question, the only Mil Spec (Military Specification) that mentions powder coatings that I am aware of is MIL-C-24712. It may or may not apply to your particular application. Review it carefully before using it.
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