It seems so simple: We cut a finger or bruise a knee, and a week or so later we’re all better. You snip a small branch of basil leaves to make pesto, and before you know it there is another branch of basil leaves sprouting from the cut. If plants and animals can heal and thrive, why not other organic material, like polymers used to make coatings?
Let’s face it. Prepainted metal, when bent, may crack a bit, and all prepaint eventually has a cut edge. How cool it would be to have the crack heal or for the coating to creep around the cut edge and form a contiguous film. Cracks and cut edges are pathways for moisture and corrosive chemicals such as salt. Sealing up those cracks could make a big improvement in a prepainted metal product’s corrosion resistance. Let’s investigate where the science is these days.
In its purest form, a self-healing coating heals itself through some kind of molecular interaction. For example, picture a tiny crack. If you zoomed in and examined at this crack, it might look like this:
Image: Apasciuto (CC BY 2.0)
The gap (the crack) is small. To keep things simple, let’s say it’s a hairline crack. So that makes it about 0.1 mm wide. That doesn’t seem so wide … unless you’re a molecule, which can come in all shapes and sizes. So let’s say one of our polymer molecules is about 100 angstroms (Å), which is 10 nanometers (nm). The 0.1 mm crack equals 100,000 nm. To put this in human terms, it’s like a six-foot-tall person looking to leap over a canyon that is 11 miles wide. That’s quite a leap, whether for man or molecule. If you can’t bridge the gap, then there is no possibility of joining up the coating.
The other issue is: What do you do once (if?) you get there — to the other side of the crack? React, of course! But that isn’t so simple. The research to date involves polymers that stretch, break, and form a reactive group (two, actually; one on each end of the torn polymer). With these polymers, the broken ends are physically pushed into each other (i.e., no gap to worry about), and those reactive polymer end groups actually do react to re-form the polymer chain. This might be a covalent bonding reaction or some other non-covalent interaction. Although these polymers might seem interesting, they are quite high in molecular weight (they’d make a very low-solids paint), and their chemistry would not be stable in sunlight. So, interesting stuff if it weren’t for the gap distance to bridge and polymer chemistry of limited interest to us in the coil coating world.
Another approach to self-healing is microcapsule healing. In this approach, there would be tiny capsules in the coating. Each capsule would contain some sort of reactive material. As a crack or scratch forms, the capsule would break, reactants would ooze out, and the injury would be healed. Microcapsule technology is not new. In 1953, two researchers from National Cash Register (NCR) invented non-carbon paper that used microcapsules that broke when crushed by a pen or pencil. They called this NCR (no carbon required) paper. Clever marketers, those researchers. Here’s a schematic how it worked:
Image: Юкатан (CC BY 2.0)
There are two problems with this approach to coatings. The chemistry of the stuff that oozes from the capsule needs to have good weathering properties (e.g., aliphatic isocyanate), and that’s not always easy to achieve. The other problem is making a capsule that is tough enough to make it through the coil coating process (including the bake), but fragile enough to break when the coating is damaged. Very challenging!
As I have suggested before, we should always challenge the cool science that we hear about, but we should never ignore it. The next great invention could be just the solution you need for your problem.
David Cocuzzi, NCCA Technical Director