The use of intumescent coatings for fire protection in aircraft

Brian Norton, managing director of Indestructible Paint, Birmingham, describes the role of intumescent coatings on composite aircraft components. Intumescent paints provide effective fire resistance in many situations by producing an expanding ‘char' that is designed to reduce the spread of fire and the transfer of heat. They are used on a variety of components in the aircraft industry including aviation fuel pumps, hydraulic landing gear parts, cabin interior parts and engine nacelles. In recent years the amount of fibre reinforced plastics in aircraft has increased sharply. A large proportion of aircraft components are now constructed wholly or partially from composites; especially carbon fibre/epoxy, because of its strength and lightness. Although composites have many advantages, in the event of a fire they present a greater risk of the fire spreading than would be the case with metal parts. Epoxy based fibre laminates will burn readily and give off toxic fumes but the risk can be substantially reduced by use of the appropriate intumescent coating. Intumescent coatings are generally used as part of a multi-coat system in most aerospace applications. However, it is not always possible to use the same intumescent system for every application and each situation has to be assessed on its own merits. As a result, a period of development work by the coating manufacturer will often be necessary to provide the system that meets the safety requirements specified. In a recent example of this Indestructible Paint's IP9189, an intumescent coating developed for aerospace applications, was adopted by an aircraft manufacturer for the protection of engine nacelles. Constructed from both carbon fibre composite and aluminium, the nacelles in question are used on the latest version of the Saras (Crane). This is an 18-seat light transport designed by the Indian National Aerospace Laboratories and manufactured by Hindustan Aeronautics Limited (HAL) for the Indian Air Force. Relying extensively on composite technology, the latest Saras also utilises carbon fibre laminates in other major components of the aircraft to combine low weight with high strength. To comply with fire safety regulations on the nacelles, a series of independent tests were carried out in India; as a result the fire resistant coating system was approved by the regulatory authority. The key components of this specialised four coat technology-led protective system are Indestructible's IP9189 Intumescent, already mentioned, and a thermal barrier coating MPC50019 R3. In the case of the composite nacelle parts MPC50019 and IP9189 are preceded by a primer specially developed for use on composites. On aluminium nacelle parts this is replaced by a strontium chromate based primer formulated for metal components. In each case, the system is completed with a two-pack gloss white polyurethane finish. All coating materials are supplied to HAL by Indestructible through its Indian distributor, Matcom of Bangalore. The above is an example of an intumescent based system being designed especially to suit a customer's product requirements and specifications. In other applications it would probably be necessary to make substantial variations to the system described. If it is necessary to reduce heat transfer to a greater degree, on electronic ‘black boxes' for example, an insulating coating such as Indestructible's IP50015 or PL95 would be introduced into the system. In other types of application, where aluminium alone is used, the substrate may require a pre-treatment such as anodising. In certain circumstances, simply increasing the intumescent coating's film thickness may increase the level of protection if required. Intumescent coatings have been used by the aerospace industry for many years, but the expanding use of composite parts in aircraft construction has substantially increased demand for these materials.  Intumescent systems are now being ‘tailor-made' by Indestructible for a variety of aerospace applications, especially where composites are employed. www.indestructible.co.uk