Ironically, the most striking aspect of any airframe is also its most overlooked: Everyone notices an aircraft’s paint-job, but almost no one considers the intricate chemistry and technological advances that underpin its application and durability.
There are few environments that are more challenging for paint than the exterior of a fuselage at 30,000 ft. Near the engines, extreme heat threatens the integrity of epoxy and polyurethane polymers, while in other areas temperatures can fall as low as -60F, causing expansion and contraction of the airframe that demands flexibility. Meanwhile, UV radiation at altitude is intense enough to break the chemical bonds of traditional paints, which would also suffer from the range of cleaning and de-icing fluids used on commercial airliners.
To protect against these and other factors such as weathering, coating manufacturers must carefully select resins and additives to cope with a multiplicity of stresses. In addition, new airframe materials—notably carbon fiber—have weaknesses that need to be accounted for.
And although the basic requirements of aerospace coatings remain flexibility, fluid resistance (from cleaning products and moist, 500-mph winds) and durability, the evolving priorities of aircraft manufacturers and airlines necessitate constant research and development. This might be to satisfy new environmental regulations, achieve faster curing times or to extend repainting intervals.
“Just as the design goals of new aircraft incorporate improvements in efficiencies in production and operation, the requirements for coatings to aid in the achievement of these efficiencies have increased,” says Mark Cancilla, global platform director for aerospace coatings at manufacturer PPG.
During the recent era of high oil prices, significant research was devoted to marginal fuel-efficiency gains. And although fuel is much cheaper in 2016, surface technologies that hasten the flow of air around the fuselage are now in advanced development.
“We are working with institutes and OEMs to investigate how coatings can improve aircraft operating performance. This includes structured coatings to reduce drag and low-density coatings,” confirms John Griffin, business director, North America, for AkzoNobel Specialty Coatings.
Perhaps the most famous development has been a sharkskin-inspired coating from Airbus that uses a micro-engineered, denticular surface to better channel air flow. This is applied via a transparent lacquer that is transferred onto an aircraft from a silicon mold.
Although the technology is still being tested, it could obviously pose major challenges for paint shops, which might need to incorporate new tooling to strip and reapply such advanced coatings. Keeping the roughened surfaces clean also will be vital, as their aerodynamic qualities are impaired by dirt.
Thin on Top
In some ways, aircraft used to be painted like houses, with multiple layers applied to ensure a strong finish. This has changed, however, with the advent of basecoat-clearcoat systems that require only a single colored base layer, followed by a clear coating over the top.
Basecoat-clearcoat systems benefit from improved gloss and color retention, longer service life, lower weight and quicker drying because of their fast-curing topcoats. AkzoNobel says its Aerodur/Aerobase products double the life of traditional aerospace coatings, while Cancilla says PPG’s Aerocron is up to 30% lighter than mono-coat applications. These use gloss and matte layers and can weigh as much as 1,000 lb. per aircraft—about the same as a grand piano.
Such advantages mean that basecoat-clearcoat is rapidly becoming the industry standard, as airlines seek to better project and support their brand through glossy fuselage liveries, and manufacturers push for faster curing times to accommodate rising production.
“OEMs see the benefit of faster cycle times and are encouraging customers to convert to this new technology,” says Griffin.
As manufacturers and others in the industry have become more environmentally aware, there has been a strong push to reduce the chrome content of paints. Chrome—particularly hexavalent chromium—has long been used to counter corrosion, but its carcinogenic properties pose risks to paint workers and the environment.
Cancilla recognizes that the incorporation of chrome-free products is accelerating, but he also notes that adoption has proved easier on the aircraft structures least prone to rusting or those that require frequent repainting for other reasons.
“Chrome is a very robust corrosion inhibitor, and as non-chrome products are being evaluated for corrosion resistance, unique issues arise that are not evident in coatings that contain chrome,” he says.
Adhesion, flexibility and protection are key attributes, and while AkzoNobel is investing heavily in chrome-free coatings that deliver each, Griffin says they only “show great promise” in delivering better performance than chrome-based products. “The critical issue is to identify test methods that accurately predict in-service performance, and this has slowed implementation and adoption of chrome-free primers,” he says.
Lengthy and complex testing that seeks to simulate a paint’s service life of up to 10 years is a challenge also highlighted by Cancilla. Each small hiccup in the trial of a new, corrosion-resistant coating requires another tweak to its formula, whereupon testing must begin afresh.
While improvements to coatings may be welcomed by airlines, they may not always be so popular with the shops that provide painting, stripping and repainting services. U.S.-based Dean Baldwin Painting reports that the average interval between paint services for an aircraft was 4-6 years, but that has now stretched to 6-8 years.
“The newer paints are advertising 8-10 years between paint jobs, so more durable paint will affect throughput, but only time will tell for sure,” says Rick Smith, vice president of business development for Dean Baldwin.
On the flip side, the spate of airline mergers over the past decade has ensured huge volumes of repainting work. In the U.S., carriers such as American Airlines and US Airways have been harmonizing their fleets, “pushing hundreds of aircraft into paint facilities in a rush to rebrand as quickly as possible,” says Smith.
Once aircraft are in the hangar, inspections and other maintenance tasks obviously occur before any painting. Surfaces also need to be prepared. This usually means applying of stripping agents at least twice over until the old layers are removed. Sometimes airlines will choose to simply sand and paint to hasten an aircraft’s return to service.
But for stripping, new systems like basecoat-clearcoat can complicate matters because of their hardiness.
“The implementation of basecoat-clearcoat and new chrome-free primers with much greater durability than their predecessors have necessitated the development of better paint strippers, which must remove these systems in at least the same amount of time as current strippers remove the older coatings,” observes Cancilla.
New environmental regulations also have made paint removal more difficult, but AkzoNobel’s Griffin says that “unfortunately” stripping technology has not kept pace with advances such as basecoat-clearcoat, and he calls for more research and collaboration in this area.
One elusive technology has been laser stripping, which was tested and then abandoned by Lufthansa Technik several years ago and considered too expensive for maintenance providers to implement. Instead, cheaper chemical stripping still dominates, though using selectively strippable layers during the initial painting can ease later removal. Selective stripping might be done to avoid penetrating conductive coatings or OEM-applied bonded primers or to coat components that were replaced after a full paint job.
Watching Paint Dry
In addition to regular service interval maintenance, rebranding and adding specialty liveries, aircraft enter the paint shop because of aircraft trading. Smith says trading constitutes a small volume of the specialist business, as sold or released aircraft are usually painted during bridging maintenance.
Smith also says the retirement of older-generation aircraft has not yet had any noticeable effect on volumes. “We don’t see the steady flow of new aircraft into the industry as a threat, for now,” he says. “Rather, we look forward to booking them through our facilities within the next 10 years.”
Once those aircraft have undergone the stripping processes outlined above, they are typically masked before the sanding of their wings, engine and composite areas. A power-abrade and wash follows, then surfaces are remasked for primer. A conversion coating and primer are applied, followed by base colors. The final stages involve layout of livery stripes and logos, then the application of maintenance and emergency placards, registration numbers and door bands.
Turnaround time for the whole process plus price are the key distinguishing factors for paint shops, says Smith, who adds that “it’s not uncommon to have 5-7 bidders competing for a contract.”
Since turn periods are mostly influenced by paint drying times, manufacturers have devoted considerable research to accelerated curing. Basecoat-clearcoats, for instance, incorporate faster-drying topcoats. The results benefit both aircraft OEMs and MRO shops as they seek to deliver and return aircraft to their respective customers. Maintenance providers can also speed up the process with heated hangars.
As Cancilla notes: “Watching paint dry is a non-value-added activity.”