Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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the paint applied between two sheets of the substrate (tensile machine pulls on both substrate pieces). Both of these tests apply forces (shear, peel, etc.) that the paint does not often undergo in use, so one may question their validity. Some of these tests are described, along with scrape/scratch adhesion tests, in Kon- standt's book.1 SCRATCH TESTS Scratch tests are complex and may infer something of adhesion, although not as quantitatively as the tensile or "pull-off" tests. One of the more interesting is the simple penknife scratch test required by a U.S. government standard (FTMS 141, method 6304), the so-called ribbon test. One takes the penknife in hand, with the blade at an angle to the surface and the point in contact with the paint. The knife point is then moved with pressure through the coating to force the coat- ing to come off in ribbon form. The hard coatings tend to come off as chips, and the hardest are simply scratched, if harmed at all. Operator variability may be a real problem with this type of test. A British standard (BS 3900 or ISO R 1518) uses a mechanized moving pan- el under a stationary needle under a specified load. The "pass/fail" given to the panel is simply based on visual observation of whether or not there is a scratch. Note that there will be substantial force concentration at the needle point, but is this an adhesion test or a hardness test? OTHER TESTS Remember also that some hardness tests indicate adhesion as well. The chip- off of coating during any test is indicative of adhesion failure whether the test is by pencil hardness, gravelometer, impact (falling dart, etc.) or mandrel bend. Reports on these tests require noting the chip-off of the coating. Strictly speak- ing, these impact tests may indeed be tensile normal to the interface; however, one does not find a good characterization of the actual force concentrated at the point of impact or calculations of the interfacial stress between substrate and film. As the substrate flexes, there is a tensile force parallel to the interface as well. Zehn- der and coworkers2 note an increase in auto coating impact, so these character- izations maybe forthcoming. Some of the chemical resistance tests will cause the coating to blister, bubble up or even fall off or dissolve in the worst cases. Again, this loss of adhesion should be noted. 286

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