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  • Writer's pictureNed Patton

Self Healing Composites

So, what is a picture of the Parthenon doing in a blog post about composites? That’s a very legitimate question. Aside from the fact that I love the Ancient Greek and Roman architecture, this doesn’t seem like it’s a good picture to start a post about self-healing composites – does it.

Well, in fact, this post was inspired by an article I saw in ASME Tech Briefs (a regular email I get that helps me keep up with the new stuff). The article headline is something like “Roman Buildings used Stronger Concrete”. What scientists have discovered recently is that since the Romans used a hot mixing technique rather than the ambient temperature mixing that is in use today, lime clasts got incorporated into the concrete. It was originally thought that this meant their concrete was inferior to what is in use today. But, Roman concrete still survives today, so the researchers looked to find out why. What they found was that as the Roman concrete cracks, water seeps into the cracks. In modern concrete, those cracks just keep growing and eventually the water freezes and spalls the concrete or the steel rebar rusts. But in Roman concrete, the lime clasts mix with the water, filling in the cracks and sealing up the concrete. Pretty remarkable if you ask me.

Romans used self-healing concrete!! That’s why the Parthenon – and so many of the other Ancient Roman structures – still stand and are in as good a shape as they are. Thus, the inspiration to write about self-healing composites. And that is because, by and large, self-healing composites use a very similar concept to self-heal. There is an intermediate stage – a ready glue if you will – incorporated in to the self-healing resin formula that releases when a microcrack develops or fatigue damage happens in the composite. Then the incorporated glue heals the crack(s).

The approaches to self-healing in composites run the gamut, from allowing chemical bonds to detach and reattach in a different place – covalent dissociation-reassociation – to entrained capsules that have a resin and hardener in little packets that when a crack develops they break open, mix together as they fill the crack, and cure to a hard resin.

One concept uses a dissolved thermoplastic in something like a glass/epoxy composite. When the glass/epoxy cracks slightly, the thermoplastic can be pushed into the crack by heating the material and applying pressure to it to make the thermoplastic flow. And the energy to initiate the cure can come from infrared (heat) or even ultraviolet sources. Lots of ways to do this.

The University of Illinois’ Department of Chemistry1 has been looking into self-healing polymers for some time. And, they have a very unique way of doing it. They incorporate a vascularized structure – a structure with many tiny tunnels – that they can circulate fluids in. When these fluids encounter a crack (microcracks are the most prevalent) the fluid is capable of wicking into the microcracks and creating a glue of sorts that seals up the crack, and structurally bridges it. And, they have even incorporated a two-part fluid – one in one set of channels and one in another - in some of their newer creations that will handle larger damage like a ballistic impact (like someone hitting the composite with a hammer). The original fluids would just leak out and make the surface wet, whereas these new two-part resins, when they mix in the hole that is created from the larger damage will gel very quickly and heal up the surface so that the composite can still function.

While none of these concepts is quite ready for commercialization, I thought that it would be a good idea to introduce everyone to at least the ideas out there and what people in the research labs are cooking up. Once these self-healing composites reach the stage where they are commercially available, I am absolutely certain that they will begin to take over in structurally critical applications like aerospace. There are already composites used in most of the modern aircraft with embedded damage sensors so that the maintenance folks know where to look for problems. Pretty soon there will be self-reporting, self-healing composites where the structure will keep the user informed about its structural integrity as well as how much repair it has had to do to itself. And if this sounds like the Jetsons, it really is not that far off – maybe a decade or so.


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