Turning up the heat

An aeroplane wing that can de-ice itself in flight is one application of a new type of composite technology. Lou Reade reports



A spray-on, electrically conductive coating has been used to make self-heating composite parts.
The technology – developed by US company Boyce Components – is at an early stage, but has already been used to build a prototype aircraft wing that could be de-iced in flight.
Because moulds to make composite parts are often made from composites themselves, the method could also be used to make moulds that can be heat-controlled for optimum part curing.
Porous aluminium strips are built into the composite part, and embedded in the conductive material, which owner Darren Boyce calls 'K-Factor'. The part can then be heated by applying a controlled current to it. Parts can be heated to around 60ºC. The company also sees potential applications in anti-static flooring. heating pads – for industries such as food or agriculture – and industrial heaters. Temperature can be controlled via a special box, supplied by Boyce, or using thermoswitches.
The conductive layer relies on carbon nano-fibres (CNFs) for its performance, though Boyce has been reluctant to sell it on this basis.
“Originally, my focus was simply to sell the heating system,” he says. “If it was to be successful, I wanted it to stand on its own merit and not use the hype surrounding the term 'nano'. I won't hide what is in the resin, but to attempt a marketing push with emphasis on the nano ingredient -- and not the utility of the product -- is to essentially market a red herring.”
Despite his reluctance to jump on the 'nano bandwagon', he says that the dispersion of the nano additive is crucial to the performance of the product due to the special nature of the additive. When he developed the heating system, he says the hardest segment was to disperse the CNF into the resin – which on its own took two years to accomplish.
“One of the most valuable aspects of the system is the effective dispersion of the carbon nanofibres into a resin for use as the conductive resin matrix for heating,” he says. “There are few companies existing today which can perform the function of dispersion.”
He says there are many potential uses of a conductive resin that can still be processed as a resin, such as EMI shielding, electrostatic discharge (ESD) for discharge and painting purposes, mechanical strengths and smart materials.
“I did not know at that time that the dispersions would be the focal point of the process by the users,” he says.
A cheaper alternative to CNF might have been to use standard carbon black as the conductive element, but CNF has a crucial advantage. Carbon black is typically seen as a 'ball' under the microscope, he explains. These would all need to be touching each other if current is to flow.
“Visualise it as throwing basketballs into a swimming pool,” he says. “You would need a lot of basketballs before they are all touching each other from one side of the pool to the other.”
CNFs, however, have a high aspect ratio, meaning they are much longer -- approximately 1000 times longer -- than they are wide.
“Visualise small pieces of hair. Fewer will be needed to stretch across the pool,” he says. “This is why the loadings of CNF can be 0.3% by weight, while carbon black and other fillers require more than 20%.”
A 'heat treated' part is made in a similar way to a typical composite part, with the K-Factor layer being applied in the same way as a resin or gelcoat layer.
As a demonstration, Boyce built a mould to function as a candle warmer or modelling clay heater. It was made with a layer of black gelcoat as the surface with 2oz mat placed behind to reinforce the gelcoat. After this a textured gritty surface was placed down under the K-factor to show the K-factor’s ability to overcome variations in the surface. In addition to the gritty surface, the K-factor is used on top of the rough backside layer of 2oz glass.
Porous aluminium conductors are laid onto the surface in parallel fashion, and more K-Factor is sprayed on top. Copper connectors are then laid on the top surface, and connection made between the copper and aluminium layers, to close the circuit. Wiring is then attached and the system is ready. (See diagram for full description.)
According to Boyce, the sample was built in a single day using very basic tools. It is important to note that the technique can be used to make a composite part or a compsite mould.
“The difference between the mould and part are mostly in the use of the product,” says Boyce. “Many fibreglass products are made from moulds made with fibreglass. The moulds will typically use the same construction techniques as the parts that are made from them.”
For instance, he says a mould will typically have a surface layer of gelcoat, which is reinforced with fibreglass cloth or mat. When a fibreglass product is built it is usually built in the same fashion by a coloured gelcoat and reinforcement with fibreglass.
“Since the heating technology can be incorporated within the mould using the standard materials, the parts may also have the heating built into them as well.”
Boyce has sold the heating system to a number of aerospace companies, which are developing their own applications, as well as to international companies in the UK, Europe and the Middle East.
“I never intended to enter into the high-end aerospace fields or working as a sub-contractor in governments contracts in just a year – but that is were I have ended up,” he concludes.


Author
Tom Shelley

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