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Aerospace coatings - a specialist field
Wednesday, 28 February 2007 13:13

Brian Norton, managing director of Indestructible Paint, provides an insight into the performance requirements for many of the surface coatings used by the aerospace industry. 

The term ‘aerospace coatings’ covers a wide spectrum of paints and other materials.  Examples of the latter include dry film lubricants, attrition coatings and even special wax polishes for aircraft use.Although aircraft and aircraft component manufacturers throughout the world comprise Indestructible Paint’s largest market sector, the company also supplies general industrial coatings and the leisure marine market.

With few exceptions, the performance requirements of the surface coatings used on aircraft are significantly more demanding than those used by other industries.  

An example that illustrates this is that of a short haul airliner.  Often covering around 30,000 miles a year, just on the ground, the aircraft and its coatings can be subject to temperatures of perhaps 40ºC at one extreme or exposed to runway and airframe de-icing chemicals at the other.  

After being subjected to high air temperatures on the ground the aircraft, shortly afterwards, is exposed to temperatures at altitude well below zero.  Aircraft coatings are also frequently exposed to hydraulic fluids, fuel and lubricants.  In short, external airframe finishes have a hard life.  

Not so obvious, but in some cases subject to even more extreme operating conditions, are the heat exchangers, helicopter gearbox housings, jet engine air intakes, undercarriage components, shafts, thrust nozzles, turbine blades and more that are used on modern aircraft.  

The latter all require high performance organic or inorganic protective coatings and finishes for their protection of the type Indestructible Paint Ltd specialises in.

These components may be fabricated from one or more of a long list of materials that includes;  aluminium alloys, carbon fibre, ceramics, honeycomb, magnesium, metal composites, nickel, steel and titanium.

As a result of this diversity, a number of widely contrasting surface coating technologies is needed to meet the various performance specifications.

Possibly the toughest environment for surface coatings is encountered in the harsh environment found inside the gas turbine engines used to power airliners and military aircraft.

Existing in very corrosive conditions, many internal engine coatings currently have to resist temperatures as high as 700˚C and also stand up to chemicals such as ester lubricants and hydraulic fluids.

Organic coatings cannot survive in this environment and the utilisation of inorganic materials is a necessity.

Indestructible Paint’s ‘Ipcote’  products are one of only two ranges of this type of coating available anywhere in the world that can provide the necessary cathodic corrosion protection in these extreme conditions.  

Containing particulate metal, the applied coatings are polished to brightness or treated at high temperature to obtain high surface conductivity and, as a result, maximum corrosion resistance.  

The polishing also improves gas flow through the engine and increases its efficiency.  

A totally different but interesting type of material for engines is a graphite based attrition compound.

Supplied in the form of a two-pack dough or filler, this is applied to the inside of engine casings to provide a seal between the casing and the revolving turbine blades.

After curing it is machined to provide a ‘touch fit’.  The slight contact between the rotating blades and compound then wears away the compound to provide a perfect fit and seal.   

When new engines are developed they are usually more efficient than their predecessors.  As a result they operate at higher temperatures and this increases the demands made on the coatings employed.  

Minimum weight is also a target of designers and the use of magnesium in aircraft construction is becoming more prevalent.  As well as being light, magnesium is also a very reactive and therefore easily corroded metal that requires the best possible protection.  

Introduced specifically for the aerospace industry, ‘Rockhard’  2-pack cold cure and single pack stoving epoxy and epoxy/phenolic coatings have been developed extensively over the years.

Widely used by aerospace companies and other industries, Rockhard coatings are designed to give maximum corrosion protection to metals and versions have been developed specifically for use on magnesium.

The performance of the polymer system alone enables these coatings to be formulated free from heavy anti-corrosive pigments, resulting in a lower film density than that of most protective paints.  

In addition, their high corrosion resistance enables use of fewer coats - a single coat instead of two, or two coats where three would otherwise be required.

These factors help to minimise the weight of certain components.

The Rockhard range and other epoxy coatings from Indestructible provide materials for a variety of diverse aerospace applications.  They may, for example, be single coat protective finishes or primers used in combination with multi-pack polyurethane or acrylic-urethane finishes

A recent addition to its epoxy range is Indestructible Paint’s ‘System-Two’.  The basis of the new system is a specially formulated low VOC 2-pack epoxy primer for application ‘in-mould’ onto fibre-reinforced composite mouldings.

The process involves spray application of the epoxy primer to the inside surface of the mould.  The primer is then either cold-cured or force-cured before the fibres are laid-up and resin injected.

On removal from the mould the primer coat has become an integral part of the composite, and its smooth surface mirrors exactly that of the mould.

The process substantially reduces and often eliminates the need for hand filling any of the porous areas associated with composite panels.  This allows significantly improved production rates and reduced labour costs to be achieved. 

The finishing coats for this system can be either acrylic-urethane or polyurethane depending on the final use of the painted component.

The first application of System-Two is on the composite airframe panels of the ‘Advanced Light Helicopter’ (or ALH helicopter) produced by Hindustan Aeronautics Ltd, of Bangalore, India.

Development of aerospace coatings continues apace at Indestructible Paint often embodying relatively new materials such as nano particles.  

The use of ceramic nano (sub-micron) particles in surface coatings is becoming more prevalent and there are now a number of coatings available from the company that incorporate this technology.  

Some of the inorganic coatings described earlier are examples.  

When used in the organic paint resins common to conventional paints, the tiny nano particles are invisible and do not affect gloss or form any settlement.  

They can reinforce the coated film to produce hardness and wear resistance of a very much higher order than can otherwise be obtained with conventional organic coatings.  

Often described as semi-organic, these materials represent a significant advance in surface coating technology and have been adopted for unspecified purposes on certain military aircraft.

The products mentioned above are only a part of the company’s range of more than 200 aerospace related coatings.

Others include low VOC xylene, toluene and chromate free products, intumescent paints and dry film lubricants.

Tel.  44 (0) 121 702 2485


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