European manufacturers. Consider the different markets and OEM
requirements as well as the accommodation of various climate conditions for MROs, and you soon realize that paint manufacturing is complex. One coating formula does not come close to fitting all needs.
How It Works
Why is the new base coat/clear coat process a breakthrough in aircraft coatings? With the conventional, direct-to-gloss
process, a paint shop applies at least two layers of top coat to fully cover the primer. Each layer includes pigments and elements to protect against wear, corrosion and ultraviolet (UV) rays, a mixture that elongates the curing process. The top coat has a glossy element and must fully dry before the aircraft owner’s personal signature—striping, logo, etc.—can be applied. Being cured and glossy, the layer forms a seal that requires sanding so that the paint for the stripes and logo can adhere. Complex paint schemes with multiple colors could take more than a week to complete. With the new BC/CC process, the base coat and clear coat essentially perform separate duties. The high-solid base coat incorporates a lot of pigment in the binder, or resin, giving it optimum opacity. But it includes virtually no protection against ultraviolet rays and harsh chemicals, leaving that duty, as well as the final finish’s glossiness, to the clear coat that covers it. Having abundant pigmentation, the base
coat offers a “high hide” quality, meaning it can fully conceal the primer color usually with just one coat. The base coat has very few solvents, i.e., volatile organic compounds (VOCs) that provide high gloss. Thus there is less evaporation of VOCs during application. Coating manufacturers claim the high-solid base coat renders cleaner, brighter colors than older paints, and for multicolor paint schemes, a high distinctness of image (DOI), or sharpness. “And the process retains the colors longer,” says Voisin of the BC/CC process. Avoiding the elements that render surface protection and glossiness, the base coat can be applied thinner, thus the paint scheme weighs less. The weight savings with BC/CC could be “about 100 pounds for a small commercial airplane and close to 600 pounds for a large airplane,” says Alan Schoeder, commercial coatings global segment manager with PPG Aerospace. He adds, “Each pound taken off a plane represents $1,000 a year in fuel savings.” Thinly applied base coats also provide faster drying times. Succeeding coats can be sprayed on in just two hours instead of the usual six to 10 hours with conventional coatings.
Chart courtesy of Sherwin-Williams.
Paint shops thus can have multiple colors applied in a single shift. They can augment their production schedules and turn around paint jobs faster. The carefully managed curing times also helps improve the paint’s flow and leveling. The BC/CC process allots a window of time between the base coat’s application and that of the clear coat. Usually the timeframe is 40 to 75 hours, though Richardson claims AkzoNobel’s Aerobase basecoat “is recoatable up to seven days.” Within the recoat time, other colors for striping and logo can be applied without sanding because the base coat film doesn’t completely dry. It stays open (doesn’t fully cure), and this allows the over coatings to adhere. Voisin explains the difference in adhesion qualities between BC/CC and older processes in another way: “With the longer base coat/clear coat recoat window, the coating creates a chemical bond between colors, but with direct-to-gloss since the recoat window is much smaller, you typically have to create a mechanical bond, which first requires sanding a dry, painted surface.” The 40-plus-hour recoat time usually is
sufficient to accommodate stripe painting before applying the final, clear coat. However, if the window needs to be expanded—say, because of an interruption in scheduling or shop closure over the weekend—more
32 Aviation Maintenance |
avm-mag.com | April / May 2012
time can be added by putting an “adhesion promoter” on the base coat. Failure to work within the recoat window of time could cause a base coat to be too well cured and thus the risk of delamination.
Durable and Long Lasting The clear coat provides the finish’s durability and high glossiness, usually better than 90 GLUs (gloss level units), according to PPG’s Cancilla. Because of the clear coat’s dedicated protective qualities, a paint scheme’s resin, color and gloss will not degrade as fast with BC/CC. Instead of a typical service life of five to seven years before repainting, a BC/CC coating may last for eight to 10 years, depending on the aircraft’s flight activity and environment and how well its exterior has been maintained. Regarding maintenance and repair, Voisin says, “The clear coat makes it easier to buff out scratches, and if the surface gets a ding, you just cover it with some more clear coat instead of having to match and buy a color.” Will base coat/clear coat become the industry standard in aircraft coatings? It has been developed by at least four coating manufacturers: Sherwin-Williams, AzkoNobel, PPG Aerospace and DuPont, which offers its Imron BC/CC product. “We expect it will be widely used,” says
Cessna’s Tom Heck. “It may well become the standard in the not-too-distant future.” AM
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