Aerospace paints and coatings have to be extremely tough and durable, given the conditions in which they operate and their typical service life. On the exterior, paints and coatings experience extreme temperatures and must protect the fuselage from corrosion and high levels of UV exposure at cruise altitudes.
Coated components are subject to severe stresses. For example, according to body trade the British Coatings Federation (BCF), the coatings protecting aeroengines are subject to temperatures ranging from well below 0C (32F) at the front of the engine to 500C (932F) within the low- and high-pressure compressor and temperatures up to 800C even further in. The fact that access to those complex internal structures may be limited also means some coatings must perform long-term corrosion protection for more than 20 years, the BCF says.
Julie Voisin, aerospace global marketing manager at Sherwin-Williams in Cleveland—which has provided coating systems and technical support to the global commercial airline, military and general and business aviation markets for 80 years—says the company’s paints and coatings are commonly applied to an aircraft’s exterior. In addition to withstanding temperature extremes, it is important that the coatings have the ability to flex with the airframe. “The aircraft is expanding and contracting while in the air, so flexibility is important, along with UV resistance. The Sun can damage components, so being able to cope with that while maintaining gloss and color is crucial,” says Voisin.
Corrosion And Cabin
Coatings must also be resistant to the chemicals found in jet fuel, hydraulic fluid and all the other substances with which aircraft frequently come into contact. Voisin says once a coating has been proven in an aerospace application, it tends to stick. “This is a very conservative industry because of the expense of the asset with which we are dealing. The pace of change is slow. Airlines want to be sure a coating is proven before they use it,” she adds.
“The qualification process for a new coating can take years,” confirms Stefan Jacob, sales director for aviation at Hamburg-based paint manufacturer Mankiewicz.
Voisin explains that a test sample of a new coating may be deployed on an aircraft for up to five years before it goes into standard production. Exterior paints may be expected to last many years in service because of the expense of taking a commercial aircraft out of service, stripping it and repainting it—a process that can take weeks.
In the business jet market, the same rules apply, with owners also expecting their aircraft to be available for service. For commercial aircraft that have been retrofitted as business jets, new owners will expect a perfect finish on the exterior, too. “They want a glass- or mirror-like finish—and they want it to look like that for the lifetime of the aircraft,” Voisin explains.
For aluminum-bodied aircraft, paint provides a crucial protective role in preventing corrosion, which can be very expensive to repair. If corrosion spreads, it could affect safety. “Our corrosion-protection coatings protect the structural parts for the entire lifetime of the aircraft,” Jacob says. Most of those areas are not accessible once the aircraft is assembled, and they need to perform for more than 30 years, he notes. Mankiewicz’s corrosion protection is water-based.
While a commercial aircraft might fly for 5-7 years before its exterior is repainted, aircraft with new operators due to mergers, acquisitions or other changes to the fleet may be repainted earlier. In any case, aesthetics are as important as protection. “There are high expectations from both commercial and business jet owners,” Voisin says. “The aesthetics and appearance of the plane is almost as important as how the coatings perform. It is a balancing act.”
Most surfaces within the cabin are painted, too, including meal trays, storage bins, side walls, lavatories and galleys. Voisin says the finish on internal paints is more subtle. “The gloss is lower—it has more of a calming effect,” she says. Airlines may be trying to present a “home-like” environment to passengers, with neutral colors. In terms of the coatings’ performance, stain and scratch resistance—and for commercial aircraft, flammability requirements—are particularly important in the cabin.
“The paint itself has to be flame-resistant,” says Voisin. This means that there is an extensive testing regime using heat sources and smoke to make sure the paint will not contribute to a fire. In this context, paint is subject to requirements no different than for other important cabin elements, such as seating fabrics or seatbelt loops. The FAA specifies the stringent flammability requirements paint must meet. In terms of aircraft structures, coatings for fuel tanks must also perform to the toughest flammability standards.
The Mankiewicz research and development department is in close contact with engineers at Boeing and Airbus as it develops new materials for these applications, many of which are secret. Mankiewicz R&D resources for new types of aviation paint are high, adds Jacobs. The company has a portfolio ranging from standard primers and fillers to top coats with design effects, such as translucent paints that give plastics a frosted-glass effect. Tactile, soft effects and metallics complete the offerings, and most of these are available as environmentally friendly water-based solutions. Mankiewicz says it was one of the first manufacturers in Europe to introduce water-based paints for aviation interiors some 20 years ago.
The range of paints on offer means there is more than beige and gray in the cabin interior. For example, a new product from Sherwin-Williams features a pearl or mica effect for use in the cabin. This might be employed in business class to emphasize the exclusivity of a certain area. Sherwin-Williams also offers a textured “suede effect” coating with a leathery feel. “Obviously, leather is available, but that can be an expensive option. This provides a more economical option that is also a high-end look,” explains Voisin.
Externally, Mankiewicz emphasizes the importance of repainting the fuselage effectively and achieving the desired finish. Again, durability is crucial. “In high-UV areas such as the Middle East and Australia, there have been examples of finishes not lasting for the required seven years. The colors have faded too fast,” says Jacobs. “Airlines have had to repaint the aircraft much earlier because it does not look nice. But to passengers, the discoloration can be perceived as a security issue and damage the airline’s brand.”
Specific products have been developed for certain parts of the fuselage such as composite engine nacelles. For example, the Mankiewicz FlexPrimer system is designed to prevent cracking in the paint for the nacelle. At best, this can be unsightly for the airline, with damage occurring due to the engine’s movement. “Technologically, we are always looking at ways of painting composites more effectively as their prevalence on commercial aircraft grows,” Jacob says.
Environmental responsibility is of increasing importance to coatings manufacturers, too. Solvents are emitted into the atmosphere when paint is applied, so this must be managed. Jacob says Mankiewicz takes pains to reduce the level of hazardous materials employed in its paint manufacturing to safeguard employees and to source high-quality materials that are kind to the environment in the supply chain. “There is also a large department of engineers in Hamburg working on making painting processes more efficient and environmentally friendly,” Jacob says.
In the MRO or repainting process for an aircraft, paint must be stripped responsibly, and old materials coming off the aircraft must be managed correctly. In the past, some suppliers used chrome in the primer applied to aircraft to help protect the aluminum body from corroding, Voisin notes. “We are now able to offer more chrome-free alternatives,” she says.
Innovation is also centered on increasing the speed at which the coatings dry, whether primer or top coat. “We can speed up the painting process thanks to new technologies,” Jacob says. Products that have shorter drying times, long durability and offer process improvements—such as the “wild-spraying” method in the exterior sector—can help. In that process, the least-used color is applied first and the most-used color last. It also permits the spraying of several color shades at the same time, provided these are not too close to one another, to avoid overspray. Icons seen on aircraft operated by Belgian flag carrier Brussels Airlines were painted in this way.
There is another environmental angle to paint: It is possible to reduce aircraft weight with the introduction of thinner paints and coatings that still exhibit the requisite performance characteristics. “For every kilogram saved, there are benefits in terms of costs for the airline and the environment, and this is an area where coatings and paints also have a role to play,” Jacob says. The aerodynamic profile of the aircraft can be influenced as well. For example, the glossier the surface, the more smoothly air flows around it, he adds.
Paint products are not the only means of achieving weight-savings, says James Frean, ASC business manager for aerospace products at PPG Industries in the UK. “Very often, application is the key coatings contributor to aircraft weight. This is why we work very closely with our customers to optimize their processes for the best combination of product and application techniques,” he says.
Naturally, getting aircraft back into service quickly is another key factor.
“Beyond traditional performance qualities, such as durability and gloss retention, our customers look for coatings that offer process time-savings,” says Frean. “Shorter turnaround times for coatings application benefit both aircraft manufacturers and operators by helping them be more efficient and, therefore, generate more revenue.”