Catch it if you can

Inline colored coatings cure at the speed of (UV) light

By Brian Maddox

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.

Uses for Temporary Coating

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.

Putting the Coating to the Test(s)

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.

• Film Thickness. The film thickness of the coating applied to the tubes was on average 0.45 mils. This average thickness was obtained by taking readings at intervals around the tube. The lowest coverage was 0.35 mils and the highest was 0.55 mils.
  
  
• Adhesion. The adhesion of the coating was tested in four areas around the diameter of the tubes. The tests used the ASTM crosshatch adhesion method. All of these locations showed 100 percent adhesion after immersion in copper sulfate.
  
  
• Hardness. Hardness, which was measured with standard pencil leads in four locations around the diameter of the tubes, was 2H.
  
  
• Flexibility. The bend was tested using a tube bender to produce a 90-degree bend over a 3-inch radius. The bent area then was dipped into a copper sulfate solution and then examined. The bend did not exhibit coating failure.
  
  
• Chemical resistance. Methyl ethyl ketone (MEK), an aggressive solvent, was applied to a cloth until it was completely saturated. The cloth then was rubbed over the coating with moderate pressure. After 150 passes, or rubs, the coating did not show any signs of failure.
  
  
• Corrosion resistance. The coated tubes were tested for corrosion resistance by the salt spray method. This testing was conducted according to ASTM B-117. Coated tubes were removed from the test after 300 hours. Approximately 5 percent of the total area of the coated tubes was covered with white rust.

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.

Brian Maddox is vice president of Polymeric Processes Inc., 1507 Ricefield, Suite 218, Houston, TX 77084, 281-579-7433, fax 281-579-7112, bmaddox@polymericprocesses.com, www.polymericprocesses.com.