Powder Coatings Clinic
Marek Urban, Ph.D.
University of Southern Mississippi-Hattiesburg
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Thermal conductivity of powder coatings
Q: I would like to know what the thermal conductivity of powder coatings is. I want to determine if a powder coating applied to an aluminum heat sink will degrade the effect to dissipate the heat. B.J., Virginia Beach, Va.
A: Thermal conductivity of powder coatings is more or less the same as any other thermosetting polymer network, which compared with other nonpolymeric materials is low. The term that is often used is thermal diffusivity,which is a function of thermal conductivity, heat capacity, and density. Thermal diffusivity is also a function of temperature, which may vary from system to system; that is, it may decrease, increase, or not change with temperature because thermal conductivity and heat capacity may exhibity opposite trends. The main difficulty is that thermal conductivity and heat capacity cannot be separated experimentally.
While the above concepts apply to polymeric networks, one may alter thermal diffusivity by using highly conductive additives, like metallic pigments and other nano-particles that may effectively alter thermal diffusivity and, subsequently, thermal conductivity and heat capacity. In all cases, not only the pigment type but also the pigment volume content (PVC) will create another opportunity for altering thermal conductivity and heat resistance.
Paint discoloration in cold temps
Q: A company I do some work for now has a problem from another vendor, and they've asked me if I can help solve it. If I do, I could get a significant amount of business. The problem is that gray painted steel discolors with red when exposed to weak sulfuric acid, but only when the temperatures change from warm to cold. I've examined it, and it permeates the surface through the thickness of the paint. any ideas about how to prevent it or repair it without stripping the entire part? Thanks. C.W., Berea Ky.
A: It's quite complicated to answer this question because I don't know the coatings chemistry and pigments.
It's not surprising that discoloration occurs because iron may form complexes in the presence of sulfuric acid as a result of permeation of the acid into the substrate. If iron sulfate was formed, it's pure form is greenish, but if it was oxidized from Fe2+ to Fe3+, it will appear as reddish.
Another possiblity is that the sulfuric acid reacts with pigments and changes oxidation state (red color) to for reddish discoloration. Because this process begins at the interface between the paint and the steel substrate, it's likely that Fe3+ is formed, which permeates and results in the formation of reddish discoloration. the easiest way to check it is to determine the formulation of the paint and identify if there are any ingredients that would react with sulfuric acid. You could also deposit the paint on other substrates and expose them to sulfuric acid to see if the same processes are observed. If not, it's likely that these are reactions within the coating that lead to red discoloration, and this would require adjustment of the formulation. If the sulfuric acid does react with the substrate and cause discoloration, one way to avoid it is to passivate the substrate surface. Another approach is to increase the crosslink density of the coating so that sulfuric acid won't penetrate the coating.
Not knowing much about the coating formulation and the chemical reactions leading to the crosslinked networks, it's hard to determine the cause. Unfortunately, once the process starts it's hard to stop, and the most effective way to prevent it is not to allow it to happen in the first place. Finally, the temperature dependence may result from solubility differences of Fe3+ and the resulting phase separation, which may occur at lower temperatures, and it's not surprising. PC
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Marek W. Urban is director and professor of polymer science in the School of Polymers and High Performance Materials at the University of Southern Mississippi-Hattiesburg (USM), Box 10076, Hattiesburg, MS 39406-0076; 601/266-6454. He is also director of the National Science Foundation Materials Research Science Engineering Center (MRSEC) established at USM in 2002 and co-director of the National Science Foundation Cooperative Research Center in Coatings.He received an MS in chemistry from Marquette University, Milwaukee, Wis., and a doctorate in in chemistry and chemical engineering from Michigan Technological University, Houghton, Mich. He has also done post doctoral work in macromolecular science at Case Western Reserve University, Cleveland. Urban has written more than 300 research papers and three books and has edited eight books. He has won numerous awards, including the Technical Focus Speaker and Roon Foundation Awards (1999) from the Federation of Societies for Coatings Technologies (FSCT), and an outstanding research faculty award from USM. His e-mail address is [Marek.Urban@usm.edu].