How do I...
...know if my coating is cured? The Solvent Rub Test is a well-known and frequently used test throughout the coatings industry. Its primary function is to determine the extent to which a coating is cured and when interpreted properly, it can be a very useful test. The test can be conducted in both the field and in the laboratory to gain a qualitative idea of the chemical resistance of a powder coating.
The test itself is simple; a Q-Tip (or cotton ball or rag) is soaked in solvent and rubbed over the surface of the coating a given number of times. The effect on the surface is then evaluated in terms of gloss loss, softening, or degree of rub off. The results of this test provides a measure of the chemical resistance for a given coating. When the results are compared to those of a control, know to be fully cured, a good qualitative determination of the degree of cure can be made.
To view the Solvent Rub Test in the proper context, it must be understood that the test results are not absolute and are meaningless if a control is not used. Solvent rub data are highly dependent upon the method used, the operator running the test and the type of coating being tested. Various solvents, for example, can affect a coating to different degrees. MEK and acetone are much more aggressive than xylene or toluene and a coating which is unaffected by xylene may be considerably softened by MEK. Similarly an operator who "bears down" on the coating will affect the surface more than one who uses less force. Coating type, of course, strongly influences the results obtained on the test. A standard epoxy powder coating will show little or no effect after 50 double rubs with MEK, whereas, on a standard polyester urethane, softening and slight rub off is normal, even though the coating is fully cured. Accurately judging the degree to cure for either coating without knowing the MEK resistance of the fully cured film is not possible.
Additional information is available by referencing PCI test method #8 Solvent Cure for Chemical Resistance and/or ASTM D1308 Standard Test Method for Effect of Household Chemicals on Clear and Pigmented Organic Finishes.