HISTORICALLY, primers and finishes for aircraft have evolved slowly
with only minor changes to formulations taking place over periods of many
years. However, the increasing complexity of modern aircraft, plus the
increased performance and environmental requirements of recent times,
has resulted in more rapid development of the surface coatings required.
Aerospace companies now use widely contrasting technologies for a variety
of purposes on fibre reinforced composites, steel, aluminium, magnesium
and nickel alloys.
Magnesium helicopter gearboxes, undercarriage assemblies, heat exchangers, jet engine air intakes and thrust nozzles, not to mention fuselages and wings, are just a few of the aircraft components requiring modern high performance coatings. Nitro-cellulose based paints and dopes provide a good example of a coating type commonly used on aircraft for many years. Cellulose was first used on aircraft just after WW1 and is still sometimes used today for re-finishing older light aircraft.
Harrier's epoxy protection
 Indestructible supplies the RAF Harrier with at least four types of coating. |
Epoxy primers, often with chromate pigments, have been supplied by Indestructible to protect aircraft components since the 1950's. Continuous development has ensured that these coatings still provide the highest level of corrosion protection currently available on many of the metal airframe components listed previously. Now known as the Rockhard range they were first used to provide an effective primer for the magnesium skin of Westland helicopters.
Rockhard two-pack epoxies also protect items such as fuel pump diaphragms on the Harrier and also serve as primers on some of the composite plastic structures used on the same aircraft. These epoxy and epoxy phenolic coatings can be supplied as single pack stoving materials or two-pack or cold cure coatings. They come in a range of colours and gloss levels.
Polyurethane systems
Often used in conjunction with epoxy primers are Indestructible's modern high performance acrylic or polyester based polyurethane finishes. The generic term polyurethane describes an enormous range of coatings and plastics with widely varying properties and a multitude of uses. There are also a number of curing reactions and methods used with these products.
A polyurethane can be simply defined as a polymer containing repeating urethane linkages. These linkages are produced by the reaction of a polyisocyanate with another another compound containing at least two hydroxyl groups per molecule. The polyisocyanate component will often be contained in one component of a two-pack system. The hydroxyl containing polymer, such as a suitable acrylic or polyester resin plus pigments, will be contained in the other system component. Indestructible Paint are manufacturers of both polyester and acrylic based polyurethane coatings.
 Gliders re-finished in two-pack acrylic. |
These materials are well suited to modern aircraft construction materials and performance requirements. They can be used on metal or non the glass, carbon and other fibre laminates used in the construction of military aircraft and light civil aircraft including gliders.
Two-pack acrylics are the most widely used of these products. Two-pack polyester-urethanes are less likely than acrylics to absorb certain chemicals and their use on military aircraft makes decontamination easier.
Inorganic innovation
Based on a less conventional, and more recent technology, and in complete contrast to the coatings mentioned above, are the inorganic materials now used internally on jet engines. These coatings are able to withstand very high operating temperatures, and therefore protect turbine blades from corrosion and erosion in an environment harsh enough to make conventional organic paints simply disappear.
Originally introduced in 1994, specifically to resist the increasingly high operating temperatures required for greater engine efficiency, Ipcote 9183 is used on turbine blades operating at temperatures between 500°C and 600°C. This material also has exceptionally high corrosion resistance and has been salt spray tested to 3000 hours (ASTM B117) on mild steel. All test panels were completely rust free at the end of the test, including the cross scribed areas.
To obtain maximum corrosion protection it is necessary for the surface of the Ipcote to be electrically conductive - to achieve this it contains particulate metal. Maximum conductivity is obtained in one of two ways. The paint film can either be processed after application up to 540°C or processed to 350°C and then vibro-polished.
 Ipcote 9813 coats Rolls Royce and Pratt & Whitney turbine blades for civil and military aircraft. |
Ipcote 9183 is also completely impervious to organic solvents, even at elevated operating temperatures. Apart from its use on turbine blades Ipcote may shortly be used on engine rotors, shafts and even on undercarriage parts. Simplicity of application for this type of material, plus high performance make it an economical alternative to IVD aluminium and even chrome plate on certain products.
Used globally on Rolls Royce and Pratt & Whitney aero engines by air forces, including the RAF, and several civil airlines, Ipcote products are one of only two ranges of this type of coating available.
Graphite innovations
Another product produced by Indestructible, and used on various aero engines including the Harrier AV8B is a two-pack graphite attrition compound. This material, supplied in the form of a two-pack dough or filler, is applied to the inside of engine casings to provide a seal between the casing and turbine blades. After curing it is machined to provide and exact fit. Slight contact between the rotating blades and the compound wears the compound away to provide a perfect fit.
The trend towards higher engine operating temperatures will no doubt continue and, in turn, place further demands on internal engine coatings in the future.
NOTE: the above article was published in the Polymers Paint Colour Journal, July 1999.
