Heat-based disbonding promises to transform composite use

Technology demonstrated by researchers at the National Composites Centre in Bristol promises to make it easier to dismantle, repair and recycle composite parts.  

Developed in collaboration with a team from Oxford Brookes University, the technology – which involves the use of carefully modified structural adhesives – enables composite structures to be separated (or disbonded) quickly and cheaply using a simple heat source.

It is claimed that the research could have a transformational impact on the design, use and end-of-life recycling of products including cars, aircraft and wind turbines.

Project leader, NCC research engineer Lucy Eggleston, said that the group has explored the use of two specific off-the-shelf additives which can be added to any structural adhesive.

The first of these additives, expandable microspheres, have been explored before for disbonding applications, but never for industrial scale applications. These are effectively thermoplastic bubbles containing hydrocarbons with relatively low boiling points, explained Eggleston.

“When they’re exposed to heat the thermoplastic bubble becomes malleable and stretchable and the hydrocarbons boil and expand,” she said. “That causes the whole bubble to expand which, when used as part of a joint, causes two surfaces to push apart from one another and breaks apart an adhesive bond.”

The second additive, expandable graphite, which has never before been explored for this application, works in a slightly different way, she added.

“We use these stacks of graphite with really thin carbon layers between these graphite plates. When they’re exposed to heat those oxidise and that causes expansion. That expansion forces the joined parts apart from each other.”

During trials of the technology the team demonstrated potential applications on a battery box supplied by Williams Advanced Engineering.

“We designed three different features that would be industrially relevant, bonded them together and demonstrated that application of this technology by releasing all of those different parts,” said Eggleston.

A number of different heat sources were trialled, including a bespoke induction coil that could and also be used on a disassembly line, an infrared lamp, a heat gun and an oven.  The induction coil method was able to disbond a joint in just six seconds, whilst the IR lamp and heat gun took slightly longer.

By enabling composite components to be easily repositioned and reused during manufacturing – and simplifying repair and recycling processes – the technology should, said Eggleston, lead to far more efficient use of composite materials.

“We’re seeing a greater emphasis on sustainability and end of life and being able to reuse what you’ve made, or repair what you’ve got, as opposed to just creating a new one and this is one of those enabling sustainable manufacturing technologies.”

She added that it could also help drive the increasing use of adhesives, which are attractive for a number of reasons but which still have drawbacks.

“We’re seeing a move away from bolted joints. They take a long time to put together, increase the weight of a structure, and if you want to take them apart at end of life they take a long time as well. Using a quick cure adhesive is a lot faster and more economically feasible than using bolts. But the problem with those permanent adhesives is that you can’t just take them apart if you get something wrong. With a technology such as this you get the best of both worlds.”

This, she added, could in turn have an impact on the design of composite components. “We’ve been talking to a number of customers and members about design for disassembly – and I think this is one of those key technologies that’s going to enable a lot of that. It’s going to expand the design envelope and give the designers a little bit more freedom when it comes to how they make sure something is structurally sound and reliable but also has this added bonus at end of life or during use.”

Run as part of NCC’s technology pull through programme – which is aimed at accelerating the commercialisation of promising university research projects – the composite disbonding initiative has already generated a high level of interest.

Eggleston said that NCC customers and members had already identified a range of potential applications including helping the auto industry meet the end of life vehicle directive and enabling the wind energy industry to segment turbine blades more easily.

“There’s a huge amount of applications, which is quite interesting for a single, quite simple technology,” she said.

Despite the range of potential applications, she added that use of the technology would clearly be limited to environments where operating temperatures are relatively low.

“If you’re sticking together bits of an engine you probably don’t want to be using this technology. But for external bits of a car, internal bits of aircraft, and bits of wind turbine…there are a number of applications that don’t see those sort of temperatures at which the use of these is perfectly OK.”

Eggleston and the team are now looking at taking the technology to the next step and investigating a number of specific applications.

“We’ve got a couple of customers who are looking to progress this further with us,” she said. “We’re looking to optimise the technology further for their specific applications and work with them to ensure that the technology can deliver everything they need it to.”