Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Other scratch testers are based on the impact of some flying particle on the surface. The falling sand abrasion device is one, but the precision of the test is poor. One reports the number of kilograms of sand required to wear through the coating film, but the expected error between labs is about 150%. The auto industry has a "Gravel-o-meter" that throws small "standard" stones at a paint- ed surface, as well.9 A more precise abrasion technique uses an air-blasted silicon carbide particle stream to measure resistance of a coating to sand blasting. ASTM reports that their collaborative study found that differences in precision between labs were small. The Taber Abrader, an abrasive rubber wheel device, is more commonly used, but ASTM reports poor precision. Scrubbing resistance may also (at least in part) be considered an abrasion-re- sistance test, although it is more often used on interior household paints than for metal coatings. The big difference in these tests is that the substrate is con- tinuously wetted, as well as being abraded. ASTM D 2486 uses a brush with an abrasive cleanser in a cyclic scrubbing machine, and the abrasive certainly scratch- es through the coating. ASTM D 4213 uses a sponge with a specified surfac- tant\N silica aqueous scrub medium. ASTM reports fair precision, although a re- cent critique of paint testing for the General Services Administration cited poor results for the former method. CONCLUSION The test of mar or scratch resistance is mainly on coated substrates. Independent laboratory testers may indicate some measure of the coating hardness, but only agreement between coating supplier and customer can find the test and the measurement level that is satisfactory to both. REFERENCES 1. Konstandt, F., Organic Coatings: Properties and Evaluation, pp. 19–20 ff; Chemical Publishing, New York; 1985 2. Bull, S.J. and D.S. Rickerby, Advanced Surface Coatings, D.S. Rickerby and A. Matthews (Eds.), pp. 317–24, Chapman and Hall, New York; 1991 3. Brown, R.P. (Ed.), Handbook of Plastics Test Methods, p. 112; Longman Sci- entific and Technical, Harlow, U.K.;1988 4. Sato, K., Journal of Coatings Technology, 56(708):47; 1984 5. Toronto Society for Coating Technology, "Correlation of Hardness in Coat- ing Films Using Koenig and Sward Pendulum Hardness Testers," Society pa- per presented at the 68th Annual Meeting of the Societies for Coating Tech- nology, Washington; Oct. 29, 1990 6. Wilkinson, W.H., Scientific Section Circular, No.184; National Paint and Coatings Association (formerly the National Paint and Varnish Association), Washington; 1923 7. Smith, W.T., Official Digest, 28:232; 1956 8. Gardner, H.A., and H.C. Parks, Scientific Section Circular, No. 228; Nation- al Paint and Coatings Association (formerly the National Paint and Varnish Association); 1925 9. ASTM D 3170 290

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