A group of German researchers say they made “two enormous strides” towards developing nanocapsules with self-healing anti-corrosive capabilities.
One of the nanocapsules releases anticorrosive payloads until the corrosion stops and the other re-seals itself to restore its anti-corrosive effect, scientists from the Max Planck Institute for Polymer Research in Mainz and the Max-Planck-Institut für Eisenforschung GmbH in Düsseldorf announced.
Ideally, the researchers say material scientists would like to develop anticorrosion coatings that can self-heal after a corrosive attack in a similar way to how human and animal skin heals a cut. If coatings could heal in a similar fashion to skin, fine cracks and small holes would not pose a short- or long-term risk to the underlying metal.
Photos: ©Advanced Materials 2013 via www.mpg.de
A team of German scientists say they developed two anti-corrosive nanocapsules. One is a re-closable anticorrosion container and the other re-seals its protective barrier.
"We have made two breakthroughs in the quest for intelligent corrosion protection," says Michael Rohwerder, leader of the research group at the Düsseldorf location.
Receiving Corrosive Signals
The two groups tested nanocapsules made from polyaniline, a conductive polymer, to contain anticorrosive substances. Metal nanoparticles were then added to the capsules to generate electical contact between the containers and a metal that had a coating with the capsules as components.
They then exposed the metal to corrosion by trickling salt water into a defect in the protective coating. However, the corrosive attack had no effect because the polymer capsules became porous, allowing the anticorrosive contents to escape, blocking the oxygen reduction process.
"What is crucial here is to select the correct signal for opening the capsule walls," Rohwerder explains.
The right signal, such as a scratch in the protective coating or an increase in pH value, allows the coatings to open mechanically.
Instead, the team decided to exploit the electrochemical potential as a way to open the capsule by puncturing the polyaniline cover via a chemical conversion process.
According to Rohwerder, "This potential always falls when corrosion starts ... So it provides the most reliable signal for the capsules to open."
The researchers now want to figure out how to chemically combine the two nanocapsules.
Also, an electrical contact is necessary for the capsules to realize the electrochemical alarm. The metal nanoparticles between the capsule wall and the metal provide this contact. Using this same channel, the capsules detect when corrosion ceases, and then its wall restructures itself and re-seals its pores.
The anticorrosive payloads inside the containers can form a polymer skin in a defect, effectively sealing the crack or hole.
However, the scientists didn't apply the capsules to a metal using a coating in order to test them for corrosion. Instead, they replicated the chemicals conditions that exist at beginning and end of the corrosion process and opened or closed the capsules with reducing and oxidizing substances.
"We were able to repeat this redox process with the polyaniline capsules over 80 times," Daniel Crespy, a research group leader at the Mainz location who supervised the study, says.
End Goal: Attack and Heal
Researchers at the Mainz location say they are working to develop a technique that would produce an emulsion from an aqueous solution, in which drops of oil float. The small drops of oil are similar in size, but remain almost completely stable. Before the emulsion process, the components for the polymer capsules are added in, creating a chemical reaction that triggers the polymerization right on the surface of the oil drops, the researchers say.
"This is how we can encapsulate oily fluids in an aqueous environment," Crespy explains.
Proving that the capsules would only release the healing remedy when necessary was not an easy task. Capsules had to be isolated after each step, replaced with solvents, and examined them to determine the volume of the substances in the capsules.
"We now want to enclose the healing substances and the anticorrosion substances together in the same capsules," Crespy says. Combining both substances would be the only way to provide comprehensive protection against rust.
So far, encapsulating both substances under the same chemical conditions has not been possible, but Crespy says it remains a goal for the scientists.
The research was published in two separate papers:
"Redox-Responsive Self-Healing for Corrosion Protection," Advanced Materials, Sept. 24, 2013; and
"Redox Responsive Release of Hydrophobic Self-Healing Agents from Polyaniline Capsules," Journal of the American Chemical Society, Aug. 20, 2013.