I n June 2009, Domingo Ureña, with his nomination as CEO of Airbus Military still recent, presented his views on the ‘Present and future of the Spanish and Andalusian aerospace industry’ in the Chamber of Commerce, Industry and navigation in Seville Among other issues, Ureña highlighted the strength of our region in the field of composite materials, although he also warned about the need to continue innovating. Carbon fibre is a technology that has reached its maturity. It can be further developed and optimized, but beyond a given point the improvements become incremental.
The question is: what comes next? Thermoplastics, new alloys? In which direction should go research, development and innovation efforts? Ureña announced then that Airbus Military would prepare a plan of technological development for R&D+i in order to establish strategic lines that could serve as guidance to the auxiliary industry and identify key partners for the next 10 years. The outcome of this plan has not been made public yet, but the growing interest attracted by thermoplastics in the industry stands out among the range of new aerospace technologies and materials.
These composite materials are today the great hope and an opportunity not to be missed to continue at the forefront of composite technology for the aerospace industry.
Thermoplastics, something new under the sun
A composite material is the result of the association between one or more reinforcement materials (fibres, particles, sheets …) and a matrix, without chemical reaction among them. The use of composite materials to manufacture aircraft structures presents well known advantages over more traditional metallic materials. To mention only a few, they have a high load bearing capacity vs. weight ratio, they are immune to corrosion, present good fatigue behaviour, and it is easy to manufacture them in complex shapes, resulting in a dramatic reduction in the number of component assemblies.
Composite materials can be classified by the type of reinforcement material or the type of matrix, among other criteria. If we take a look at the different types of matrices, the best known and most widely used in air-craft structures are thermoset polymers (or simply thermosets), the best example being probably composites made of an epoxy resin matrix reinforced with carbon fibres. However, thermoplastic polymers (or simply thermoplastics) can also be used as matrices. The matrix confers the composite material some properties related to its fragile, elastic or plastic behaviour, and also its porosity.
Usually the matrix also restricts the range of temperatures in which the material can be used. Hence the choice of matrix depends on criteria such as the service temperature and ambient moisture, the toughness and chemical stability required in the final product, and of course the profitability of its manufacturing process. In short, thermosets are polymers that cure irreversibly. Without going into too much detail, one of the properties that condition the use of thermosets is that when they are heated they burn, and therefore they cannot be re-melted or welded.
Some examples of this type of materials are Bakelite, melamine, polyurethane, or epoxy resins. On the other hand, thermoplastics can be softened and melted with the application of heat without suffering an irreversible degradation. These polymers can therefore be welded, reformed and re-melted using heat, and then freeze again to a very glassy state when cooled below a critical temperature Tg (glass transition temperature). Some examples of these polymers are polyethylene, polystyrene, nylon, polypropylene or Teflon.
Weaknesses, Strengths, Threats and … a great Opportunity
Thermoset matrices have been more widely used in industry than thermoplastics mainly due to their economic reasons. Their processing is simpler, and the raw materials and industrial facilities required are less expensive. In addition, the excellent fluidity of these polymers facilitates the penetration of the resin to completely cover the surface of reinforcement materials. Hence the high quality and low cost of elements made with thermoset matrix composites fit better in the business plans of aircraft manufacturers. However, thermoplastic composites improve some of the weaknesses of thermosets.
For instance, they present better resistance to impacts and fire, reduce the issue of moisture absorption, and improve the resist ance to aircraft oil, fuel, hydraulic fluid and other chemicals. In addition, they present certain advantages in terms of processing, such as shorter manufacturing cycles -with the subsequent cost reduction if the volume of production is sufficiently high. The raw material can be stored indefinitely at ambient conditions, can be conformed without the need for the traditional autoclave, and can be recycled. If this was not enough, elements made with thermoplastic composites could potentially be repaired using fusion bonding, i.e. applying pressure and heating with an electrical resistance to effectively weld the structure.
This is a quick process, easy to implement with portable equipment, and very interesting for military aircraft. If we push this concept even further some experts consider that the introduction of this technique in assembly processes could put an end to many of the problems associated to traditional joining techniques used in aircraft such as riveting and bolting. Then why is the use of thermoplastics not more widespread? Basically due to difficulties in the manufacturing process deriving from their physical properties and other factors linked to the embryonic status of this technology. For instance, conforming thermoplastics requires higher temperatures. Also the matrix presents higher viscosity, and it is difficult to guarantee that it will flow to cover completely the reinforcement material without leaving gaps during the manufacturing process. This lack of maturity in the manufacturing process, together with the high cost of prepregs – the format in which the raw material is usually supplied have been the biggest obstacles for the use of thermoplastic composites.Therefore we have a product with exceptional properties, a market where the competition is reduced – although highly specialized and a field of application (we will discuss it next) that has grown continuously over the last thirty years and is ever wider. An excellent opportunity to reinvent the fibre.
A bit of history
The first aircraft to include structural elements made with thermoplastic materials date from the Eighties. Aircraft such as the Fokker 100, the Gulfstream G400, or the Airbus Beluga included floor elements made with thermoplastic sheets.
Soon after, in the nineties, these materials started to be used not only in secondary structures but also in areas of higher structural responsibility such as the ribs in the wings of the Gulfstream G500. Back in 2000, Fokker used a compression moulding technique to manufacture ribs and stiffeners made of thermoplastic matrix reinforced with glass fibres. The raw material was supplied in consolidated sheets
By Rubén Carvajal vázquez, and Manuel Heredia Ortiz.
Authors of http://aergenium.es, first web dedicated since 2008 to the aerospace industry in Andalusia, Spain.
If you want to know more about the topic of this article, visit the web http://sabermas.aergenium.es, where you can find more information, support material, photographs, videos and links of interest. Our gratitude to Amparo Cerisola and Salvador Ortolá for their valuable contributions to this article.