Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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testing TESTING COATINGS: AN OVERVIEW BY ROBERT D. ATHEY ATHEY TECHNOLOGIES, EL CERRITO, CALIF. A coating is applied for one of two reasons — decoration or protection. In ei- ther case, there are specifications to be met and tests to show how well the coating performs in meeting those specifications. Most coating tests are phys- ical assessments that imply a performance in use, but some are appearance tests as well. The ideal test is one that exactly replicates some aspect of the decorative or protective function. Unfortunately, most tests are hurry-up approximations and may not directly measure actual performance. The ideal test would measure ex- actly some parameter that directly relates to end use; however, because a phys- ical test applies a complex stress to the coating, the coating may fail owing to only one or another of the actual pure stresses. For instance, a coating may mar in an impact test (one type of failure); it may scratch (another type of failure); or it may chip off (loss of adhesion, yet another type of failure). Each type of failure needs to be reported differently, and each says something about the coating formulation, processing or substrate. STANDARDIZATION The standardized lab test is the common point of communication between customer and manufacturer/seller. Both should be aware of the differences between lab test and use and be aware of (and report) the expected error (pre- cision) in the lab test. For instance, an expert witness was discredited for re- porting only the lowest test value for a traffic paint coefficient of friction, when his test notes showed an average acceptable to safety standards and an error of +25% in the measurements. The standardized test is aided considerably by having standards organiza- tions establishing guidelines for what the standard test means,how it is done, what it is done to, and how it is reported. There are national standards agen- cies [American Society for Testing and Materials (ASTM)] and international agencies [German Industrial Standards (DIN) and the International Stan- dardization Organization (ISO)]. Trade associations sometimes develop stan- dards. These agencies publish the test methods for members\' reference and oc- casionally publish discussions of the methods. The Powder Coating Institute has a collection of "recommended" test procedures for their products that re- flect the uniqueness of the materials, although they are published "for infor- mational purposes only." The ASTM methods are moving to include discussions of expected error and bias in their methods. Most of the standards are developed for very specific applications by collab- orative groups within an industry; however, one can find instances where a stan- dard metal industry coating test is used in the hardboard industry simply because the seller and buyer agree to it, how to do it and how to interpret the results. Ideally, a test can be done in a short time in the lab, and the result relates to the actual end use. In addition, the ideal test can be done with the coating in free- film form or on a small panel of some sort for easy storage. These samples can 279

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