July 12, 2005
For tube producers who need a colored coating with no volatile organic compounds, powder coatings have been the only choice until now. A new process provides this type of coating in a liquid form. The coating dries nearly instantly under ultraviolet light and performs well when subjected to a variety of tests.
For several decades tube producers who wanted an opaque, colored, temporary coating with no volatile organic compounds (VOCs) have not had a choice as to the type of coating they could use—only powder coatings were available.
However, technologies change over time, and tube producers now have an alternate type of coating to choose from. They now can apply a 100 percent solids liquid coating inline during tube production runs. These coatings are nonflammable and cure instantaneously. They can be formulated to be fully opaque or semitransparent.
The coatings are applied inline after the tube is formed. A multistage cleaning step removes all contaminants from the tube. Air knives then remove excess water from the tube. Next, the coating equipment applies the coating at a thickness from 0.35 to 0.6 mils. An ultraviolet (UV) curing chamber then cures the coating. The UV light reacts with the coating and changes it from its initial liquid form to a dry, cured film within a fraction of a second. The tube then goes to the cutoff area where it is cut to length.
The coatings may be used to differentiate tube types or sizes, identify tubes for different end uses, or to differentiate brands or manufacturers.
And, although the coatings are applied at low film thicknesses, they do impart temporary corrosion protection. In one application, galvanized tubes protected with this type of coating, which was applied during a standard production run, were installed outdoors in Houston. After almost two years of outdoor exposure in the heat and humidity of Texas' Gulf Coast, the tubes exhibited no color change, no chalking, and little corrosion.
In tests, fully opaque, colored, galvanized tubes were produced at line speeds of 60 to 80 feet per minute. The coating was applied with a vacuum coater to control the film thickness and cured with six 600-watt UV lamps placed around the diameter of the tube. The film thickness was measured, and the coating was tested to determine its adhesion, hardness, flexibility, chemical resistance, and corrosion resistance.
It should be noted that these results were obtained using six 600-watt lamps and the line was running 60 to 80 FPM. Because the coating cures when exposed to UV light, additional lights facilitate faster line speeds and achieve similar results. Coating formulations can be tailored for a specific application or requirement.
This technology can benefit tube manufacturers that have an interest in saving money on application costs for opaque coatings on tubes that require only temporary corrosion resistance. In addition, because they contain no VOCs, the coatings comply with regulatory requirements.
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