Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 282 of 331

ml) sample taken from the well-mixed fluid. The Class 1 tests are the following: 1.Total solids content (nonvolatiles) 2. Density 3. Viscosity 4. Grit content (filter through 100 to 325 mesh screen) For waterborne coatings, pH is also essential. Powder or other extremely high solids coatings will not need all of these tests. TEST FILMS FOR SOLID PROPERTIES The performance on the test of a coating film is likely to depend on the physi- cal form of the film. For instance, film thickness is an important factor in phys- ical and appearance measurements (until the coating gets too thick), so there must be some control of the film thickness. Appearance is also related to how smooth the film surface is, and care taken to make the film appropriately will ensure that appearance measurements are germane to the end use. The primary concern in making films for tests is that the film be homogeneous and consistent with previous or future films for the same test. The jargon of the trade calls the art of making films "casting," but many film formation process types are used to form the film, and each process has its own set of advantages and drawbacks. Test Requirements of Films Think carefully about the three types of stresses (tensile, compression, shear) and recognize that all are blended in a hardness or adhesion evaluation by indenter, pencil, mandrel bend, or dart impact test on a coating. Recognizing these stress combinations can make you aware of your needs in making the film. In cases where adhesion is stronger than cohesion of the film, you will want to be able to detect the adhesive or the cohesive failure. The preparation of the film should not con- flict with these objectives. Another portion of the film influence on the test is the thickness of the film. Combinations of rheology and shear rate of application, as well as oth- er details, govern the amount of coating left after the process. Knowing the process variables in your film casting process is essential to getting what is needed for the ultimate test and result therefrom. Certainly, other properties of the film (opacity, erosion rate by some attacking mode, etc.) will depend on the thickness of the film, so understanding thickness control in the casting process is crucial to getting meaningful results in terms of meeting specifica- tions, reproducibility and repeatability. In instances where permeation resistance or corrosion barrier properties are to be tested, a "pinhole," "holiday," or "mud crack" in the film would be a fatal flaw. The film preparation technique must eliminate (as much as possi- ble) any such fatal flaw, or the test method should prescribe what is to be done in cases where an unseen fatal flaw is detected by the test. 281

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