When things break, you throw them out.
Whether it’s your cheap plastic Tupperware or your pricy prescription glasses, it’s
not going to fix itself, right?
But what if it could?
Scientists are inventing materials that repair themselves, or “self-heal”, so that someday
all kinds of things -- from tires to tools to sidewalks -- will be able to last indefinitely.
One kind of material that’s being developed is a kind of rubber that can stitch itself
back together.
If you cut into it, hold the two pieces together, they’ll reattach!
Eventually.
Scientists at Germany’s Leibniz Institute of Polymer Research created this technology
in 2015, in the hopes of inventing car and truck tires that can fix themselves.
And they did it by making one pretty simple switch in the chemistry of the tires we currently
use.
Tires, it turns out, aren’t just pure rubber.
Instead, sulfur is usually added to the rubber, to make its molecular bonds stronger.
This makes the rubber tough enough to support your car.
The downside is that, once those bonds break -- from something like a cut or a scratch
-- they’re broken forever.
You can’t just bond those molecules back together.
But the researchers were able to make rubber self-heal by replacing the sulfur in the rubber
with a pair of charged molecules, or ions, called butyl imidazolium and bromide.
Butyl imidazolium has a positive charge, and it’s always accompanied by a negatively
charged bromide, just like the ions of sodium and chloride that you find in table salt.
And just like in salt, these pairs of ions tend to bond to other pairs, to form big groups.
That means that an ion pair attached to one rubber molecule can stick to a pair on another
rubber molecule.
And they bind using strong, ionic bonds.
These ionic bonds hold the rubber molecules together and strengthen the material, just
like the sulfur bonds did.
So far, so good.
But the crucial thing about ionic bonds is that they can easily break and reform, just
like the snaps on a jacket.
So when the researchers cut into strips of their new rubber, the ionic bonds broke.
But, pushing the two pieces together allowed the pairs find each other again and form new
ionic bonds.
To heal the cut quickly, the scientists added heat, to jostle the molecules around and help
lonely pairs bump into each other.
And the renewed bonds were strong enough to hold the two pieces of rubber together!
Scientists are now trying to make ionic rubbers that heal without heat, so a tire can fix
itself before it goes flat.
And in the meantime, other researchers are taking a totally different approach to make
self-healing plastics.
The idea here is to mix tiny, tiny packets of a special glue, called a healing agent,
into plastics and paints.
Then, when the plastic cracks, the packets will open up, and spill out glue to seal up
the crack.
The thing is, a lot of glues need either air to dry, or water from the air in order to
set, or cure.
But scientists can’t rely on air or water always being around, to get into every tiny
crack.
So they add a second chemical that helps the healing agent cure.
A team at the University of Illinois tried this, using a healing agent called HOPDMS.
***Its molecules come in long chains, called polymers, and on each end there’s a hydrogen-oxygen
pair, called a hydroxyl group.
Then, the researchers added tiny packets -- each about the size of a grain of sand -- filled
with a second chemical that works as a curing agent, called DMDNT.
When the plastic breaks, the chemicals leak out of their packets, and they mix.
That’s when the curing agent sets into action.
It reacts with HOPDMS in such a way that it breaks off a hydroxyl group from one end of
each chain, so that two chains can join together!
A single molecule of DMDNT can stitch up thousands of polymers without stopping, so the glue
you wind up with is mostly made up of healing agent.
Making these little chains longer might sound like a small change, but the effect is huge:
These longer polymers get tangled into knots that prevent the molecules from moving freely.
And that hardens the healing agent into a solid.
Because the plastic has everything it needs to heal, it can heal anywhere, quickly and
reliably.
Scientists have already used this technique to make a plastic coating that basically acts
like self-healing paint.
In one test, researchers added those tiny packets into a plastic mixture that they painted
on steel, to see if it would prevent rust.
Then they scraped the coating with a razor blade and let the coating heal for a day.
After that, they dunked the metal in corrosive saltwater and let it sit there for a few days...
and it emerged rust-free!
Now, scientists are looking at putting this technology to work in other materials, embedding
these powerful packets in concrete and metal, so that just about anything in your life can
last longer and be safer to use.
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