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November 4 - November 8, 2019

Is the MEK rub test a conclusive test to check the cure of IOZ coatings?

Selected Answers

From Larry Muzia of Exceletech Coating & Applications, LLC on December 5, 2019:
First, I disagree with one of the replies which says that the MEK rub test is not suitable.  Here is my position on the subject: Ethyl silicate zincs cure by hydrolysis, thereby requiring moisture in the air, i.e., it is very difficult to get a proper cure in the desert or in environments below 50% humidity.. It has been an allowable procedure, after a short time after application, to apply a water mist directly onto the zinc-coated steel. In reference to testing for cure, it is important to have sufficient cure, especially before overcoating, to avoid splitting the zinc layer. Inorganic zincs do not have high tensile strength, and overcoating can stress the zinc film to the point of causing splitting. The MEK rub test is one method to test for cure, as well as the pencil hardness test and the coin rub. My preference is the pencil hardness test, using the correct tool designed for this test. The coin rub is subjective but acceptable if being done by a well experienced inspector. It should be noted that  the data sheet may give a specific time/temperature curve a variable which must be seriously considered. The film thickness on the data sheet  is usually at a 2-3 mil dft; however, if the inorganic zinc is applied at 4-5 mils, the published cure time is invalid, and this has been the reason zinc splitting failure often occur when overcoating. The film appears hard but that means little as it relates to proper and full cure without proper testing.

From Aajjay Sunkay of ASSETREIFURB ENGINEERRS on December 2, 2019:
IOZ coatings contain 100% inorganic zinc and once sprayed on a metal surface, forms an inorganic zinc layer. There is no organic liquid or resin  left on surface after spray, as resin is not part of the formulation of an IOZ coating. MEK cure test is used on coatings formulated with organic resins as binder. When organic resin-based coatings (i.e., epoxy, polyester, vinylester resin-based coatings) are sprayed onto the metal surface, they cure by polymerization. Full cure of these coatings depends on the mixing ratio of base and hardener. quantity . To test the full cure, MEK  solvent is used. If there is mismatch in the mixing ratio or water inhibition, this results in uncured or partially cured coating on the surface, though it appears tough when we press a finger on it. MEK is a solvent and when it is rubbed on a sprayed coating surface, the uncured part of the resin gets softened, which then confirms that the coating is fully cured or not. The main action of MEK on a coating is to soften the coating by its dissolving action as a solvent. Due to inorganic nature of IOZ coating , there is no.such organic part  to get softened when it is in uncured condition. Hence, the MEK rub test is not recommended to test extent of full cure of inorganic zinc coatings.

From Rob Francis of R A Francis Consulting on November 18, 2019:
Thomas Starr in JPCL, March, 1986, investigated various field methods for cure of ethyl silicate IOZ, comparing against actual cure using infrared spectroscopy. He concluded "the only test that correlated well with the spectroscopic results was the MEK rub test". A similar test has been developed by Ted Riding of Jotun Australia for the waterborne products, rubbing with a water-soaked rag. The zinc needs to be soaked for 5 minutes with the wet rag before the test is carried out. As far as I know, this test has not been standardized.

From Tom Schwerdt of TxDOT on November 14, 2019:
Jon, be aware that the pencil hardness test is quite variable, and correlation between manufacturers (and even batch to batch) can be poor. ASTM does not have an accepted method for verifying the calibration of pencils for hardness. So, we're left with three common field methods, each with issues. Of the three, I'm most confident in the MEK rub for reproduceability, and many hardware stores carry MEK.

From Geir Christianslund of Aker Solutions MMO on November 13, 2019:
There are many roads that lead to Rome... What's conclusive for any test that an inspector shall perform the test that is acceptable in the procedure and standard that this job shall follow..

From rON Lewis of Corrosion Management Ltd on November 12, 2019:
First, forgive me if it appears that I am boasting, but as an American working for the DIMET COATINGS CO. in Australia, I found I was working for the inventor's for DIMETCOAT Inorganic Zinc Coating. There are several varieties, with air-cured being most preferred. However, during a familiarization tour of the Whyalla Pipeline in South Australia, I saw the first major application for Dimetcote - a moisture-cured zinc coating. Soon thereafter, AMERON became the DIMET USA licensee, with whom I had several Los Angeles visits. AMERON developed the first chemically cured DIMETCOTE 6 product. To my knowledge, all Dimetcoat products were successfully inspected by the coin rub test, using  the edge of the flat side of the coin. Only the chemically cured Dimetcote 6 was more aggressively inspected. 

From Jon Cavallo of Jon R. Cavallo PE LLC on November 8, 2019:
Given my druthers, I would druther use pencil hardness for several reasons, including 1) limiting personnel exposure to MEK for health reasons, 2) limiting use of MEK for safety reasons (flammability, disposal, etc.) and 3) TSA won't let me take MEK through airport security! All of the coating manufacturers that I have dealt with to date will provide me with pencil hardness = cure data for their coatings.

From Bryant Chandler of Greenman-Pedersen, Inc. on November 7, 2019:
IOZ coatings are both solvent- and water based. Problems developed with the water-based coating after both the MEK rub test and coin rub test determined the coating was cured. CCC &L (now GPI Labs) did extensive testing and was able to determine that the water-based coating could resolublize under certain conditions.

From Kalpesh Patel of Endura Manufacturing Co. Ltd. on November 7, 2019:
I agree with Jeff. Curing of IOZ Coating is dependent on moisture, temperature and time. So, uncured IOZ coatings can give you problems, like cohesive break, delamination of topcoat, and zinc splitting. So it is important to make sure IOZ coating is cured properly before applying an intermediate coat or topcoat. Even though MEK rub or coin rub are not conclusive tests,  they are  good tools to determine IOZ coatings' cure and give you at least some confidence.

From Jeff Kim of Sherwin-Williams on November 5, 2019:
I would not call it "conclusive" or indicative of the entire surface coated. When performed properly, the MEK rub test can give you a good indication of cure but only for the specific area tested. The same holds true for the coin test.

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Tagged categories: Curing; Performance testing; Quality Control

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