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Indestructible Paint’s technical sales manager, Graham Armstrong describes the progress of developments in coating materials for aerospace composites.
The rapid increase in the use of composites in aerospace applications requires a totally different approach to the coatings used for metals and presents a number of challenges to the surface coatings industry.
Coatings for composites fulfil many technical and decorative functions. Indestructible Paint first became involved with coating composite components over 10 years ago, working initially on systems designed for helicopters. Since then, the company has cooperated with designers and engineers from aerospace companies to provide coatings for the variety of the composites now used in many aircraft.
This has included further work on helicopters for Hindustan Aeronautics (HAL) during the development of its Advanced Light Helicopter (ALH). Designed for both commercial and military use, this aircraft was conceived 8-10 years ago to become the first fully carbon fibre reinforced epoxy composite airframe helicopter. Its rotor blades were also constructed from carbon fibre/epoxy prepreg.
Its airframe components were laid-up by hand in moulds before autoclaving. However, the condition of components from the mould may be varied, with both ‘resin rich’ and ‘resin weak’ areas at the surface. Therefore, it was the thought initially that much hand filling and sanding would be necessary - a time consuming and expensive process. But, by working closely with HAL engineers on site in Bangalore, India, Indestructible technical personnel, assisted by its Indian distributor, developed a spray coating system requiring minimal sanding and localised hand filling.
The coatings used included a low VOC, high solids epoxy primer-filler and primer-surfacer that would provide a smooth surface for further finishing coats. Of special interest was the primer-filler. As well as having excellent deep hole properties, it provided the added benefit of low weight. It was also capable of high film build in a single coat with no sagging.
In addition, the lightweight filler (‘pigmented’ with glass micro balloons) provided a degree of performance as a thermal barrier, reducing the heat throughput to the composite panel surface. This made the coating ideal for use in areas where heat transfer to the composite was to be avoided, especially near engine exhausts and on the firewall between engine and passenger cabin.
While the above is typical of the approach used by Indestructible to engineer coatings that meet the specific requirements of its customers, there are many other diverse examples.
Critical exterior applications
The first is the use of elastomeric polyurethanes on exterior aerospace components. The main benefit of these materials is their resistance to erosion caused by rain and other particles. As such, they are now used widely in applications where this is of importance.
A recent series of coating failures on a specific engine spinner brought about a research project into the properties of their coatings. The major difficulties encountered with elastomeric polyurethanes are the application of the products as sprayable coatings and their normally poor adhesion to carbon fibre reinforced bisphenol A epoxy composites.
Indestructible’s development teams studied the areas of concern and the use of a suitable primer-basecoat system, plus the manufacture of an ‘application friendly’ elastomeric coating. They also researched the use of UV absorbers to prevent substrate and basecoat degradation where coatings are used in areas of high UV radiation such as aircraft engine nose spinners.
Following discussions with the engineers of the engine manufacturer, a series of 14 coating combinations were examined. Tests included exposure to QUV UV light, direct adhesion pull off and bell peel adhesion.
The best results were obtained using a correctly formulated elastomeric coating, including UV absorbers applied over a suitably prepared composite, base coated with a clear or pigmented two-component epoxy. This system successfully provided the required adhesion, exterior durability and life expectancy.
Intumescent and thermal barrier coatings
The expanding use of composites increases potential fire risks. Indestructible’s first major project involving composite coatings - an intumescent system formulated for hatches and doors - had a requirement for five minutes minimum protection in an aviation gasoline (Avgas) fire. In this situation temperatures of up to 1,100ºC can be expected.
Epoxy resin-based composites burn readily, giving off toxic smoke and fumes. This is of particular importance where composites are used in aerospace applications for aircraft interior components, e.g. control boxes and recorders, as well as airframes.
There is a specification for non-burn and non-smoke emission coatings for interior components that includes bulkheads, overhead bins and washroom units among others. In this context, the company’s latest project involving intumescent and thermal barrier coatings was the development of a low VOC lightweight thermal barrier two-component epoxy primer.
This material, over-coated with a specially formulated non-burn, non-smoke emission polyurethane topcoat, has been proven to meet all the requirements of the FAR 25853 specification. It’s now in use on commercial aircraft components and within military and commercial helicopters.
In-mould primer applications provide another example of the technologies currently being used to solve surface coatings issues on composites. While it’s an acceptable route to achieving a surface coating on the composite, decorative aspects may be important.
‘Fibre telegraphing’, for example, can take place to such an extent that, particularly with dark body colours, the carbon fibre blanket pattern becomes visible in sunlight through the final finish. To achieve the high quality surface finish required, an excessive amount of hand filling and sanding would be needed.
In conjunction with the moulding division of a car manufacturer with similar problems, Indestructible investigated the use of in-mould priming systems. This involved first coating the inside surface of the panel mould with an epoxy compatible, silicone free, release agent followed by the application of a coat of low VOC two component epoxy primer. The resin system used to formulate the primer was matched to that used in panel production. After curing the primer, the composite panel was laid-up and injected in the normal way.
Following the curing process, when removed from the mould, the ‘already primed’ panel exhibited a very smooth even surface, which mirrored the smooth internal surface of the mould itself. By utilising this method, the number of man hours for further filling operations was drastically reduced.
There were concerns regarding ‘resin weak’ areas under the in-mould primer, but NDT methods were employed, and a system devised to fully qualify control panels manufactured this way. This process has been discussed in several other composite manufacturing areas, and ongoing testing in both aerospace and defence industries is now in progress.