Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 290 of 331

compression energy as time passes. The more elastomeric a coating, the fewer rocks can be counted. The Sward rocker is two parallel wheels with two levels between. The Koenig (and Persoz) tester is a pendulum on a bar that has balls on the bar that contact the coating surface. Sato4 analyzed the physical significance of a similar device, the Walker Steele pendulum tester, concluding that it is meaningless to compare a hard coating to a visco elastic (rubbery) coating with such a device. The Toronto Society for Coating Technology Technical Committee showed that the results for Koenig and Sward hardness are dependent on the film thickness.5 Statistical analysis showed the measurements to be varied based on substrate, ex- perimenter, and film thickness for Sward tests and for the latter two with the Per- soz tester. The scratch hardness testers vary from fingernails, coins, and penknives to engineering drawing pencils. People introduced to the pencil hardness test are charmed by its simplicity and the low cost of a test kit. Simply use a quart jar labeled "Pencil Hardness Test Kit" with a complete set of Eberhard-Faber pen- cils of varied hardness acquired at a college bookstore or art store for less than $10. One caution is that the user must push the pencil hard. A technique for do- ing the test is outlined below. 1. Sharpen pencil to expose graphite slightly. 2.Peel wood away from graphite about ½ in. 3.Hold end of graphite on 600-mesh grit paper, and smooth until point is a com- plete circle. 4. Hold pencil as if writing. 5.Set point on coating at 45 degree angle pointing away from body. 6. Push straight forward and down to make a mark ¼ to ½ inch in length. Ro- tate pencil 90 degrees along long axis and repeat. 7. Pick up pencil and examine mark for indentation or chipping. One may need to rub off the graphite residue or hold the panel up to light to examine for po- tential damage. 8.If damage is done, repeat with softer pencils until no damage is detected. 9.If no damage is done, repeat with harder pencil until damage does occur. 10.Record pencil at which first failure occurs, and record last pencil tested with no failure. A good history has been established on the pencil hardness test. Wilkinson6 first described it in 1923, and Smith7 making the "standard" brand an important agreement detail for the test. Gard- ner8 examined the point shape impact on the test results, and recommended that the wood be peeled back to expose the whole graphite cylinder for a length of ½ in. or so, and that the point be flattened with fine sandpaper (e.g., 600 grit). The pencil hardness test can give two results. One is to mar or indent the film, and the other is to penetrate the coating all the way to the substrate. Crit- ics of the latter claim that the test is now an adhesion test instead of a hardness test. One may not agree, as the softer pencils do not break the film. The pencils are certainly more reliable than a coin, fingernail, or pocket knife, as they are of known gradations of hardness. Precision among many testers may be within one pencil, but occasional deviations from this do occur. Every lab should have a pencil hardness test kit, although many paint companies do not. 289 found wide variation in brands of pencils,

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