Development of thermoplastic composite with ESD properties for aeronautic sector

Andrea Tinti, Alessandra Passaro, Giuseppe Buccoliero, Michele Arganese –
Technologies Design and Materials European Research Centre, CETMA
Antonio Greco – Department of Innovation Engineering, University of Salento

This project has been awarded by Compositi’s editorial staff and technical-scientifical Commitee!

The new Europe’s vision for aviation Flightpath 2050, released in March 2011 by ACARE (Advisory Council for Aeronautics Research in Europe), sets out very challenging goals to enable an aviation industry that is clean, competitive, safe, and secure. This should be gained by providing enhanced components.

The focus of this work is the aircraft fuel system. This consists of a number of components with most aircraft having several fuel tanks, with pipes connecting these tanks to each other and the engines. The fuel system must be statically dissipative so that there is no build-up of electrical charge which might create a spark.

Hence, the electrical resistivity needs to be controlled but not so low that the fuel system becomes the preferred pathway for a lightning strike. Typically, rigid self-containing tanks and also connecting pipes are made of aluminum alloys, but some thermoset composite parts are now in use.

There is a body of evidence to support the use of thermosets in aerospace applications whilst the use of thermoplastic composites is still relatively new. The development of a static dissipative thermoplastic composite material suitable for fuel tanks and/or pipes will offer the following benefits:

• flexibility: thermoplastics tend to be less stiff than thermosets;
• formability: thermoplastics soften when heated so they can be repeatedly reheated and deformed;
• fusion bonding: thermoplastics can be welded to themselves. This means that hollow shapes e.g. tanks can be made in two halves and welded together rather than relying on adhesives.

The objective of this work is to develop a thermoplastic composite material which is suitable for use in aircraft fuel systems. The primary target is that the fuel system must be earthed and the source of ignition must be eliminated. Friction between the fuel (which is not an electrical conductor) and the fuel system surface can generate static electricity (triboelectric charging).

If the pipe or fuel tank is made from an anti-static material then triboelectric charging cannot happen. However, should charging occur it cannot be allowed to build up to a level which may create a spark. Therefore, the material needs to be dissipative which means it should have a surface resistivity which is below 108 Ohm/ sq and a volume resistivity below 107 Ω·cm (i.e. electrical conductivity above 10-5 S/m).

Moreover, the material will also need to be suitable for prolonged contact with aviation fuel and have all the necessary mechanical properties for long-term durability in an aircraft, despite in normal use structural loading is low as well as internal fuel pressure.

In the present work, a continuous carbon fiber reinforced polymer (CFRP) has been developed based on a polyphenylene sulfide (PPS) thermoplastic matrix. PPS has thermoplastic matrix to a very wide range of chemicals and is moisture resistant. It has good thermal stability and is intrinsically fire retardant.

On the other hand, it is brittle on its own and requires high processing temperatures. Addition of conductive nano-fillers to the PPS matrix will provide a conductive path for static to run down, when generated in the fuel pipe by the fuel itself, to protect against sparking. The use of multi-wall carbon nanotubes (MWCNT) will allow a lower filler loading. Carbon fibre is also a good conductor so a far lower level of carbon nanotubes will be needed to achieve the resistivity target.