Metal Finishing Guide Book


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increases the powder will melt and start to flow. Its viscosity will have dropped considerably. If this were liquid paint we would expect it to sag. After a few more seconds, depending on the powder, the viscosity increases dramatically and flow essentially stops while the powder starts to cure hard. The time it takes until you reach the high viscosity is called the gel time. It is possible that you are using powder with a long gel time and, hence, there is sufficient time for the powder to form drips. Of course, you can consider looking for a powder of the same color and texture that has a shorter gel time, or you can better control the coating film thickness that your painter applies. The thicker the film, the more prevalent the drips. DETERMINING VOC CONTENT FOR SEMI-VOLATILE COMPOUNDS Q: I just ran some paint solids test on our epoxy/amine catalyzed product according to ASTM D2369 ���Standard Test Method for Volatile Content of Coatings���. The product is a benzyl alcohol containing amine, but essentially no volatile from benzyl alcohol was detected. What do we report to EPA for this compound? A: The ASTM test is conducted at 230��F (110��C) for one hour and at that temperature most volatiles evaporate. However, benzyl alcohol has a boiling point of approximately 337��F (205��C) and very little might evaporate from the coating at the 230��F test temperature. The vapor pressure for this compound is approximately 0.11 mm Hg at 25��C. Many years ago, probably in the late 1970s or early 1980s, the EPA established 0.1 mm Hg as the vapor pressure above which all volatile organic compounds would be considered as ���VOCs���. The EPA was referring to those volatile organics that participated in smog (ozone) formation. Compounds with a vapor pressure < 0.1 mm Hg were considered to have negligible potential to form smog and were not counted in VOC regulations. On the other hand, some volatile organic compounds, such as acetone, methyl acetate and a few others with vapor pressures > 0.1 mm Hg were considered to be exempt from regulations because they do not participate in the photochemical reactions that lead to smog formation. EPA no longer implements the 0.1 mm Hg guideline and ASTM D2369 (which forms part of EPA Method 24A) is now the defining test. However, it is worth noting that benzyl alcohol, with a vapor pressure of approximately 0.11 mm Hg, is on the borderline of VOC status. Since its boiling point is considerably higher than the 230��F temperature at which the ASTM test is conducted, very little will evaporate during the one hour test period. Therefore, it is understandable that the lab that conducted the test on your behalf did not detect any significant amount of benzyl alcohol. Bottom line: even though this compound is volatile at higher temperatures, you need only report the portion that evaporates during the ASTM test. WASH PRIMERS FOR MILITARY SPECS Q: I���m not clear on the difference between MIL-C-8514 and DOD-P-15328 wash primers. What are the benefits/drawbacks of each? A: The two wash primers look very similar to each other, and I cannot discern a difference unless I spend more time comparing the ingredients and their respective percentages. MIL-C-8514C is intended for aircraft metal, predominantly aluminum alloys, whereas DOD-P-15328 is predominantly used on steels. It is possible that MIL-C-8514C contains less acid to ensure that when it is applied to aluminum one does not have excess un-reacted acid remaining on the surface. 616

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