A new bonding mechanism discovered by US researchers could make self-healing composite materials that help fix small cracks in the structures of planes, bridges, and wind turbines more cost-effective.
The research was led by Jeffrey Moore at the University Of Urbana, US. Scientists are highly optimistic about such composite materials that can repair small cracks in their structure. "When you have any damage induced by fatigue, there's usually nothing you can do except wait for catastrophic failure," New Scientist quoted Jeffrey Moore, as saying. This can be changed through self-healing composites. These materials consist of capsules made of a liquid adhesive that leaks out and repairs tiny cracks when they appear. But, to make the adhesives set; generally, some kind of post-processing is needed, such as curing with UV light or heating to high temperatures.
However, engineers would rather choose a material that can automatically heal for itself without any extra intervention. Moore's group developed a similar material in 2001 that relied on the mixing of two different chemicals that set like a two-part epoxy. The material consists of two types of capsule, one containing a ring hydrocarbon called dicyclopentadiene and the other containing a ruthenium solvent that acts as a catalyst, causing the rings to break open and polymerise. If there is a crack the chemicals would mix and set, bonding the crack faces together.
However, ruthenium is rare and that makes it impractical for most applications. "An Airbus fuselage has 60,000 pounds of composites in it. If you used the catalyst approach, a significant fraction of the world supply of ruthenium would be flying around in one plane," said Moore. Thus his team started looking for another alternative.
The group shifted to a nickel-based catalyst, in order to improve the earlier approach. But that lead to changing the solvent as well. The first step was to gauge the new solvent in the absence of a catalyst. Surprisingly, it worked equally well.
Moore said that the solvent was probably dissolving the composite material, which led to its mixing and bonding again. However, he did not reveal the exact mechanism, which remains a mystery. Later they tested another solvent, using a chemical called chlorobenzene.
After fracture and self-healing, the composites containing chlorobenzene recovered up to 100pct of their original strength which was as good as new. And, chlorobenzene, even after being toxic, is a hundred times cheaper than ruthenium and is much more easily accessible. "It's a very interesting way forward! It really improves on the catalyst method." said Ian Bond, a self-healing materials researcher at the University of Bristol.
However, Bond warned that the idea would not attract industries as chlorobenzene is toxic. In the mean time, Moore's team is testing many less toxic, more biodegradable solvents to do the same job.