Thermoplastics to revolutionaise the composites industry?

Nothing about composites is average or conventional. While offering some of the best strength/stiffness-to-weight ratios of any material, it is difficult to produce and inherently expensive.

The good news is there are improvements in almost every aspect of the materials properties from being cheaper and easier to produce, to being lighter and stronger. The bad news, however, is that you still can't have everything.

The ability to tailor the properties of a composite has always been a major advantage. And this goes well beyond mechanical properties. Production, cost and even disposal are all factors that do not come as standard and need to be traded-off against one another. It is, however, these compromises that continue to spur research and development in to resin systems.

"There is a lot of work going in to resin development as they are generally the weakest link," says composites expert Mike Richardson, a project chief engineer at design and engineering consultancy Atkins. "Thermoplastic resins seem to be getting much higher on the agenda. They are being looked at by quite a few industries and do offer significant advantages."

Thermoset vs. Thermoplastic
Thermoset resins, like epoxy, are liquid state at room temperature making it fairly straightforward to impregnate fibres. However, to get the resin to solidify requires the impregnated material to be run through a continuous heat and pressure cycle inside an autoclave, which typically lasts between 90mins to 12 hours, though it can be longer.

Once cured the material is 'set' and cannot be reformed, reshaped, or easily recovered. If the part doesn't turn out as required, or cannot be machined to the required dimensions, it is normally destined for the bin and cannot be reworked.

The process is unforgiving and notoriously difficult to get right, leaving many to consider it a black art. Warping, voids and shrinkage are difficult to predict and can frequently occur, particularly if you are inexperienced. It's a material system that everyone learns the hard way. Boeing and Airbus have thrown away tonnes of cured composite parts as they developed the processes and structures of the 787 Dreamliner and X350 respectively.

Hence the interest, and development, around thermoplastic resins. Using thermoplastics with continuous carbon fibres can yield similar mechanical properties yet doesn't need to be cured, can be reformed, reshaped, and reused. However, it is not quite as simple as that. The obvious problem is that thermoplastics are generally solid state at room temperature making them difficult to use as a resin to impregnate fibres.

One potential solution has come from German industrial research Institute Cetex. It has been developing machines to produce unidirectional fibre-reinforced thermoplastic prepregs called CE-Preg.

The prepregs are created by melting a thermoplastic film on both sides of unidirectional spread fibres. The 'Fibrefilm' technology is set to produce 'cost efficient' continuous fibre-reinforced thermoplastic materials with excellent mechanical properties.

The machinery it has developed creates pre-consolidated thermoplastic sheets up to 600mm wide with defined fibre volume content and weight per unit area. The process can use carbon, glass or basalt fibres and, at the moment, Polypropylene, Nylon 6 and Nylon 6.6 films are used at a thickness between 40 and 100┬Ám. However plans are already in place to develop the process to work in conjunction with PPS, PEI and PEEK as a matrix material.

Thermoplastic resins present an interesting and tantalising opportunity for the automotive industry as it lends itself toward high volume mass production. Standard thermoplastic reinforced carbon fibre composites could be made in to, and supplied to OEMs, in stock sheets and rods. These could then be heated and reshaped, and even stamped, on the mass scale. And that is the attraction, the ability to use it and process it much like stock metallic materials.

"Thermoplastics have the potential to offer the automotive industry similar technology, similar tooling and similar thinking which you can't do with thermosets," says Richardson. "Think of an application like stamping out a bonnet. The OEMs have already invested in hot presses and they are familiar with that technology and manufacturing process. They know that enables them to make 1000s of consistent parts. There are manufacturing issues with it, of course, but it is being investigated and could be quite interesting."

Application of thermoplastic composites
Like thermoset resins, the initial applications for thermoplastics are coming from the aerospace industry. AgustaWestland has used a thermoplastic composite for the tailplane of its AW169 helicopter. The main load-carrying member is 3m long and consists of four unidirectional preformed panels orientated at different angles that have been melted together under pressure.

Replaceable thermoplastic leading and trailing edges are attached to the main structure made of consolidated thermoplastic laminates, supported by a number of thin press formed ribs.

Thermoplastics were used for the leading edges because of their good impact properties. The trailing edges are made of thin thermofolded thermoplastic laminates supported by press formed ribs. All the components are made of TenCate Cetex carbon/PPS fabric-based materials.

The strongly curved winglets are conventionally laminated using carbon and epoxy prepreg parts, however, the new design results in a 15% weight reduction compared to AgustaWestland's previous composite tailplane design.

The low-weight solution is made possible by the toughness of the thermoplastic material and by the strong, stiff multi-spar torsion box design. Applying co-consolidation of simple preforms to create the main structural element of the single-piece torsion box has also reduced costs significantly.

AgustaWestland expects the new AW169 to be highly successful in the civil market, and that the company will easily exceed sales of 500 aircraft. The new concept could also be applied to more tailplanes and its co-consolidated multi-spar concept is also suitable for other products such as aircraft floor panels.

At present, however, the processes are still being established and thermoplastic composites are not yet reaching the mass markets. However, thermoplastic do seem to be increasingly competing with thermoset composite materials.

Chemical giant Arkema earlier this year launched Altuglas claiming it to be the first thermoplastic resin that could make composite parts using the same equipments and processes as thermoset composites, yield the same mechanical properties, yet with the added advantages of being able to thermoform and recycle it.

The Methacrylic based thermoplastic composites can be reinforced with continuous carbon, glass or even flax fibres and used in the same RTM or Infusion type processes commonly used today. It recently succeeded in developing the first large demonstration part using room temperature RTM-light process with its resin.

The company hopes that Altuglas composite will be suitable for high performance structural composite parts in wind turbines, automotive and sports equipment, directly competing with today's epoxy resin systems.

Thermoplastic resins do show a great deal of potential at providing the missing link between exotic lightweight and strong composite materials and the mass markets. The automotive industry is desperate to reduce the mass of vehicles and is no doubt heavily researching and developing the possibilities that are beginning to be shown. However, the material still has many challenges that need to be solved if it is to truly be exploited.

And by no means are thermoset resin composites going to disappear. The market for thermoset composites is going to continue to grow, and it is a material that should not be ignored by engineers hoping that thermoplastic resins are going to soon solve all their production issues. While thermoplastic composites are going to increasingly find niches, the smart money is that its mass market appeal is still at least five years away.

Justin Cunningham

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