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Over 800 million tons of coil-coated metal are produced and shipped annually in North America alone. Coil coating (see Diagram I) is a very efficient way to produce a uniform, high quality, coated finish over metal in a continuous automated fashion. Coil coating is also referred to as pre-painted metal, because the metal is painted prior to, rather than after, fabrication.
In the coil coating process, the metal coil is first unwound, cleaned and pre-treated, applied on a flat continuous sheet, heat cured, cooled and rewound for shipment. At the fabricator, it is then cut to the desired size and formed into its finished shape. Versus most other application methods, coil coating efficiency is nearly 100%. Application is at very high line speeds as modern coil lines can run at speeds as high as 700 feet per minute and cure the applied paint in 15 – 45 seconds. As opposed to a spray-applied coating, for example, a coil-coated, formed surface offers uniform film thickness rather than the thicker films on edges, corners and bends that is more typical of spray-applied coatings.
Topcoats are applied by reverse roll coat in which the applicator roll travels in the reverse direction of the strip and thus provides a smoother film with fewer defects. Primers and backers are normally applied by direct roll coating. Some lines also apply coil coatings using an extruder or via a solid block of paint with a softening point such that it can be applied smoothly when heated.
The types of paint curing employed in the coil industry include thermal, infrared, induction and UV cure. By and large, the vast majority of coil coatings are cured using gas-fired ovens. Accordingly, the remaining portions of this article will focus on thermal-cured coil coatings. Coil primers and backers are normally applied much thinner than spray-applied liquid or powder coatings, dip or electrocoat paints. Applied primer dry film thicknesses are normally in the range of 4 – 6 microns in thickness, whereas topcoats are normally applied to provide a dry film thickness of 18 – 20 microns.
Special consideration when formulating coil coatings include adjustments in the resin technology, crosslinking, solvent system, catalyst, surface modifiers and lubricants to accommodate proper cure, flow and leveling, and minimization of surface defects. Once cured, coil coatings can offer excellent handling, physical, chemical and environmental properties. From a paint usage standpoint, polyesters make up the bulk of coil coatings. Table II details typical materials utilized in a polyester coil coating.
In summary, coil coating technology has grown more rapidly than more conventional application technologies due to a variety of benefits that include cost savings, versatility, environmental, performance and quality issues.
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Coil coating refers to the process of creating prepainted steel and aluminum. Prepainted steel or aluminum can be found in a high number of household products. From refrigerators to air conditioners and construction material, prepainted steel can be found everywhere. Different coatings can be applied for different demands.
“In some coatings, you are going to need more exterior environment weathering, flexibility, and more impact resistance than you do with other coatings,” says Aaron Small, Corporate Vice President at Kloeckner Metals U.S. and President of the National Coil Coating Association (NCCA). “If we’re going to do roofing and sidewalls, we would often choose either a fluorocarbon, siliconized polyester, or modified polyesters.”
Coil coating is how the coating material is applied to the strip of steel or aluminum. A coil of metal sheet is continuously fed into the process which is controlled via accumulators at each end of the line. They take in a bare coil of metal at one end and let out the finished product on the other end. In the middle, the coil coating process is applied.
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“What we do is actually referred to as high-speed, continuous coil coating,” Small says. “What makes high-speed, continuous coil coating possible is the non-stop application of the cleaning, the treating, the coating, the curing, and potentially the leveling, while the sheet is flat, all in one high-speed path through a coil coating line.”
The key is being able to maintain a constant speed of the strip through the process. The accumulators allow this by letting the middle of the line to continue running at the prescribed rate while new metal is added at the decoil end, or the finished product is removed from the recoil end of the line.
“You don’t have starts and stops,” Small says. “This allows our coaters to consistently clean, treat, seal, apply a paper-thin coating, cure it evenly, and cool it back down without burning the metal or the coating.”
Coil coating has evolved significantly since the process was first developed in the 1930s, but then it was not a high speed or continuous operation like we have today. One of the first major uses of coil coated materials was the coating of emissions lines. In the 1950s and 60s it began to see greater and more widespread use.
“It started with very narrow aluminum coils and paint that would be poured in by hand out of a one or five-gallon bucket,” Small says. “These coils were really tiny. A lot of times a single man could pick up the coil and put it on the coiling line.”
With the development of the accumulator towers, the process was fully industrialized. In the 1970s and 1980s, it was not uncommon for a single system to run continuously for an entire week. Today, some lines run up to 1000 feet per minute.
“That made it a very cost-efficient way to paint metal,” Small says.
The key element of prepainted steel is that the product is cleaned and treated while in a flat state. You don’t have any nooks or crannies, as you do with an already formed metal for surface impurities to hide and hinder adhesion which is ultimately important for film integrity and corrosion resistance. Both the top and bottom of the metal are thoroughly cleaned and treated, which greatly improves the adhesion of the coatings. When one of the prime attributes of high-speed coil coated metal is its ability to be formed after coating. You can’t reach that potential without great adhesion.
You can then form the metal into a variety of uses. Numerous household appliances, including refrigerators, air conditioners, dryers, and hot water heaters, use some form of prepainted steel or aluminum in their construction. The automotive industry also utilizes prepainted steel or aluminum in their parts. A couple of large markets, metal roofs, and garage doors utilize the product, too.
Prepainted metal, even with cut edges, has been found to consistently offer more corrosion resistance than post-painted surfaces, according to a case study conducted jointly by the NCCA, PPG Industries, Inc., and the North American Zinc-Aluminum Coaters Association. In the study, prepainted metal louvers with exposed cut edges were subjected to the same environment as one with a post-painted electrocoat, a second with a powder finish, and a third with a spray finish. After 16, 44, and 68 months of exposure to the elements in Daytona Beach, Florida, the prepainted metal showed consistently better corrosion resistance in terms of the uniformity of the treatment and film thickness.
What sets prepainted steel and aluminum apart is the consistency of the product in both appearance and quality. Coating the metal in a flat state allows for incredible uniformity in the coating, and the industrial process allows for a large amount of control.
“The overall consistency of the whole operation is just fantastic,” Small says.
If you like what you are reading, feel free to check out our previous blog posts on topics such as galvalume metal roofing, enameling steel, metal service centers, PVD, and aluminum. For help with your next coil coated architectural project, call (678) 259-8800.
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