Professor’s question sparked research into self-healing composites

When researcher Patrick Smith joined the mechanical engineering department at Sheffield University, he gave a talk about his research interests, which were influenced by his background in chemistry and materials science. Professor Alma Hodzic was intrigued – and had one question: “Can you print polymers?” “This question has led to three years of very fascinating research”, recalls Smith, adding that the research is continuing.

So far, the research resulted in a process to produce self-healing carbon-fiber composites. The carbon-fiber composites consist of sheets of woven cloth of carbon fibers infused with epoxy resin. The sheets are known as ‘pre-preg’. In the process, a polymer ink pattern is printed onto each sheet of pre-preg. The polymer-layers that form the self-healing agent are sandwiched between the carbon-fiber sheets. To activate the self-healing properties, external heat needs to be applied.

“What makes inkjet such a vital part of the process is the ability to print a wide variety of materials and patterns,” explains Smith.  These characteristics are guiding the way to future projects: “To date, we’ve only published work on printing one material at a time and one pattern, but we are working with material combinations and varying print patterns,” says Smith. Farther off into the future, he envisions being able to print materials that lead to composites with embedded functionalities.

Scanning electron micrograph of printed droplets of polymer on the surface of carbon fibre composite in accurate hexagonal patterns

Same quality, more benefits

The new technology has many benefits but one quality stands out: “One of the greatest advantages is that our technology barely increases the overall weight of the composite”, Smith emphasizes. Thus, it is possible to produce carbon fiber composites with improved mechanical properties while retaining the lightweight quality that makes carbon fiber so well suited for many products.

One example where stiffness, strength and low weight provide a clear advantage is in aircraft construction. Any reduction in weight translates into a decrease in the amount of fuel necessary to operate it, which, in turn, can result in lower greenhouse gas emissions.

Both Boeing’s 787 Dreamliner and Airbus’s A350 have a high proportion of carbon fiber composite in their wings and fuselage, which makes them more fuel-efficient. The development of carbon fiber composites is also of interest to the US Air Force Office of Scientific Research that supports the research at Sheffield University. “We’ve also found that composite prepared using our approach exhibit different machining behaviors,” says Smith and gives the examples of smoother edges in drilled holes in the inkjet printed composites.

For Smith, the general advantages of the new technology are similar to those offered by additive manufacturing. “We no longer have to accept the ‘one size fits all’ mode of thinking”, he says. “We can design composites that have localized variations in properties and embedded functionalities in specific regions of the same piece of composite.”

The research may also find its way into the production of sporting equipment such as lightweight racing kayaks or stand-up paddleboards that are made of carbon and are often in need of repair. The technology would add a step to the production process and requires extra equipment but research conducted with Smith’s colleagues at Freiburg University has shown that the current technology can be used with an affordable Dimatix Materials Printer, which may lower the financial barriers to using this technology.

Smith hopes to see the technology commercially available in the near future. The US Air Force Office of Scientific Research encouraged the team of scientists lead by Professor Hodzic and Smith to explore ways to make the technology accessible for larger scale production.