Aerospace coatings provide protection in the most demanding conditions
The aerospace sector is one of the most demanding sectors for coatings and materials. Aerospace coatings need to provide protection from corrosion, abrasion, weather, UV radiation, erosion, temperature variations, and more in the most extreme conditions. The aerospace industry can be divided into space, commercial aviation, military and defence, and business aviation. Coatings come in a range of technologies for use in both exterior and interior applications – from wing coatings to heavy-duty cabin coatings – for OEM (Original Equipment Manufacturer) and MRO (Maintenance, Repair, and Overhaul/Operations). With aerospace coatings, the sky is the limit.
In this article we look at the different aerospace segments and the coatings that serve them, as well as the companies and products available in South Africa.
The 4 sectors served by aerospace coatings
Aerospace coatings cover such a range of substrates and purposes it is easy to be confused about how the different sectors use them. Below we look briefly at the aerospace sectors and the coatings they use.
- Commercial aviation – At any given time there are 5000 commercial airplanes in the sky. The commercial sector relies on coatings for protection but also for brand recognition, so colour is a hugely important factor. Long-lasting and durable coatings are needed to withstand the constant usage.
- Business aviation – Private jets and general aviation is a growing industry, and the emphasis for coatings is on appearance – high quality aesthetics with as little downtime as possible.
- Military and defence – The military sector requires specialty applications including for camouflage, chemical agent resistant coatings, infrared reflective coatings, and anti-static coatings.
- Space – Satellites, launch vessels, and other space-bound vehicles need specialty coatings to deal with the very specific nature of their use. These include solar reflection coatings, conductivity coatings, and even Vantablack.
The aerospace coating guide: interior and exterior coatings from nose to tail
It might be a surprise just how many surfaces in aircraft use coatings for protection or decoration (or both). Coatings for interior and exterior are exposed to different stresses and have different requirements in terms of finish and performance. From sealants to ceramic coating, aerospace coatings cover a wide range. Though solvent-based coatings are still the most common choice, air pollution regulations mean that water-based coatings are on the rise. Below we outline the different needs of interior and exterior coatings.
1. Aerospace exterior coatings – Primers to topcoats and selectively strippable systems
The coatings systems used for aircraft exteriors includes primers, intermediate coats, and topcoats, and a 747 will use about 1200L or 300-500kg of paint. An A380 can require up to 3600L of paint. These coatings are spray-applied in very thin layers, ensuring an even and perfectly cured application. Airlines repaint their planes every 5-8 years and the expensive painting period downtime can be days to weeks. In the interest of cutting downtime and increasing efficiency, selectively strippable systems have been introduced. They allow planes to be repainted up to 30-40% faster by removing the top and intermediate coats without damaging the corrosion-resistant primer. These systems also reduce the amount of chemical strippers required.
Example coatings: PPG Desothane HD Basecoat/Clearcoat system which provides a superior colour coverage with a high gloss clearcoat. PPG combine this with the chrome-free Desoprime 7049 primer for aircrafts, which is left intact in future repainting.
2. Wing coatings – from leading edge to stabilisers
Smooth and uninterrupted airflow over a plane’s wing is vitally important to the efficiency of flight. A bird strike may be obvious in its damaging potential, but even bug guts can disrupt and damage an aircraft. Bug guts sticking to the wings of aircraft cause drag during takeoff and landing to such a degree that NASA and Boeing are working on developing a coating to combat the problem. Outside of the bugs, wing coatings need to deal with UV radiation, leading edge erosion, abrasion, and corrosion.
Leading edge erosion is a problem also experienced by wind turbines where the front edge of a wing or blade erodes due to constant impact with rain, dirt, bugs etc. When an aircraft wing arrives at the assembly and paint line it is, like all other commercial airplane skin panels, covered in a green protective coating. Once this is removed, the paint system is applied (see below). Aircraft wings are also coated with an impact resistant paint system. The walkways on aircraft wings are also coated, this time with a non slip walkway coating for escape routes and cargo floor panels like AkzoNobel’s Aviox.
Example coating: Aeroflex HS 113F02 – a three component polyurethane high gloss wing coating with excellent flexibility for exterior use on the upper surface (in-spar) of the wing box and horizontal stabilisers.
3. Cockpit and cabin coating – topcoats for decoration and preservation
Last year, airlines ferried four billion passengers to destinations around the world and at any given time there are some 5000 commercial airplanes in the sky. For a passenger, the cabin is most of the flying experience and ensuring that the cabin provides a comfortable, positive and safe atmosphere is vital for customer satisfaction. The coatings used play a large role in creating this atmosphere. A cabin is high traffic and high use, with people, bags, and trolleys rolling through several times a day. The coatings used for every part of the cabin need to withstand this use without damage. Stains, scratches, and smudges all detract from the flying experience.
Safety is also an important factor for interior cabin coatings. There are strict regulations regarding fire safety and flammability for interior cabin coatings, as well as the aircraft manufacturer’s own specifications. Interior cabin topcoats are often polyurethane based as the resin provides a soft touch, UV resistant, durable, attractive, and scuff resistant finish.
Example coating: Intura 8001 – a one-component, low VOC, cabin interior topcoat that complies with BMS 10-83, Airbus 04-08-002, Embraer’ MEP 10-73, and FAR 25.853
4. Engine coatings – Thermal barrier coating
One of the key concerns in aircraft engineering is safety. Failures can lead to catastrophe, and coatings play an important role in controlling heat exposure, wear, and corrosion of jet engine components. The combination of fuel and heat requires extremely specialised coatings. Safety standards dictate that jet engine turbine blades are replaced when they reach the minimum expected lifetime, regardless of their condition at that point. Coatings are designed to protect and enhance the working of the engine and extend this lifetime as long as possible.
Thermal barrier coating (TBC) is one of the most important coatings used for jet engines. The engine is unique in the combination of damaging elements to which it is exposed – extreme cold on the exterior, extreme heat on the interior, erosion, and corrosion are all threats to its safe and effective functioning. A ceramic thermal barrier coating insulates turbine blades against heat and the stress of high speed rotation for extended periods. TBCs are applied as a system not a single coating, usually comprising of a bond coat, a thermally gown oxide, and a ceramic thermal barrier coating (often yttria-stabilised zirconia).
Example coating: Yttria-Stablised Zirconia Thermal Barrier Coating – a ceramic coating with low thermal conductivity, an expansion/contraction cycle works with the temperature changes, and extreme durability.
5. Fuel tank coatings – Anti corrosion and contamination
A Boeing 747 flying from Pretoria to Dubai will burn approximately 4 litres of fuel per second, adding up to roughly 80,000 litres of fuel across the whole 8 hour trip. The plane itself can carry about 180,000 litres, while the Airbus A380 can carry 320,000 litres of fuel. Given that fuel keeps the plane in the air, it is important to make sure that it is protected from contaminants from the fuel tank, and that the highly corrosive fuel does not eat away at the tanks and lines.
Aerospace fuel tank coatings need to have excellent adhesion to metal substrates, excellent fluid resistance, durability, a wide-ranging service temperature, and comply with a range of safety and performance standards and specifications.
Example coating: PPG DeSoto 823-011 Integral Fuel Tank Coating – protects the interior of an aircraft fuel tank against corrosion from fuel contaminants. 823-011 is a chemically cured coating which provides maximum protection against water, salt water, aircraft fuels, hydraulic fluids, engine oils, and dilute acid solutions.
6. Aerospace specialty coatings – anti-glare, anti-erosion, anti static
Though the temperatures 6 miles in the sky may be cold, overheating is a problem for aircraft due to the heating powder of UV radiation. Traditionally, planes have been painted white for just this reason – it can reduce the temperature of the interior and exterior of the plane by a substantial amount, and save on energy expenditure for air conditioning. There are now solar heat management coatings which allow for a greater colour variety by allowing solar heat to travel through the topcoat pigments and reflect off the white primer coat.
Anti-glare coatings for the cockpit prevent glare from obscuring the vision of instrumentation and dashboards, allowing pilots to perform their jobs more safely. Anti-erosion coatings work to prevent the constant impact of rain, ice , dust, and other particulates causing erosion on exterior surfaces, especially the leading edge of wings and other strike areas. Anti static coatings for aerospace is used for radomes and antennae to obtain surface conductivity on non-conductive substrates.
Example coating: PPG’s Solar Heat Management Coatings
Aircraft paint manufacturers and products in South Africa
AkzoNobel, PPG, and Sherwin-Williams are the top three vendors in the aerospace coatings industry, and all three operate in the South African market. The manufacture, development, and supply of coatings for the OEM and MRO sectors of the aerospace market is a big business, and there are many companies that provide products for it. AkzoNobel products include the Eclipse, Alumigrip, Aerobase, and Aerodur lines, PPG produces Desothane, Andaro, and Desoto, and Sherwin-Williams aerospace coatings include the JetFlex, SKYscapes, and Soft Swade lines.
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