Scientists are reporting the development of a coating for steel that heals itself after being scratched, without any catalyst and at relatively low temperatures.
The technology involves the use of polyurethane microcapsules that contain the highly reactive healing agent hexamethylene diisocyanate (HDI), to create a one-part, self-healing, anticorrosion coating, Singaporean researchers report in “Facile microencapsulation of HDI for self-healing anticorrosion coatings,” just published in the Journal of Materials Chemistry.
Self-healing coatings have become a very hot area of research, much of it involving microencapsulation of various reactive agents, note the researchers, led by Professor Jinglei Yang, of Nanyang Technological University. UV exposure has been reported as a promising catalyst in much of the recent self-healing research.
Images: Journal of Materials Chemistry
|Steel panels show corrosion test results after 48 hours of immersion in a salt solution. Panel (a) was coated with an epoxy coating mixed with HDI-filled microcapsules; panel (b) received a control epoxy coating.|
In this case, the researchers used liquid isocyanates, which are highly reactive with moisture. The material was microencapsulated partway through the reaction process, then the microcapsules were dispersed into epoxy resin at ambient temperature.
Puncturing the microcapsules—by scratching, for example—releases the HDI, which reacts with water to form polyurea, theoretically sealing the damaged region and preventing corrosion.
A hardener was added, and the coating was degassed by vacuum for 20 minutes.
Testing and Results
A pure epoxy coating was prepared as a control.
Both coatings were applied with a final thickness of 300-350µ to steel panels that had been sanded, degreased, washed and scratched with a razor blade. Both coated panels were then immersed in 10% salt solution for 48 hours to simulate accelerated corrosion.
|SEM images show the scratched steel substrates before and after immersion. The control coating (a) still shows an open crack (b) after immersion. The self-healing coating (c) has filled with new material (d).|
After 48 hours, the researchers found the panel with the self-healing coating to be “nearly fully free of corrosion,” while the other panel showed severe corrosion. Not only that, the researchers said, but scanning electron microscopy (SEM) images found that “newly formed materials filled the crack” in the self-healing coating.
“The crack was in this way sealed and healed autonomously to retard the diffusion of salt ions and thus protect the substrate from the corrosion process,” the team writes.
No sealing was seen in the scratches of the control panel.
Tweaking the Recipe
Experimenting with multiple variables in the process, the team found that increasing the HDI content in the microcapsules produced more fill content, but also produced more microcapsules of bad quality. Experiments with temperature found the optimal reaction temperature to be 40 degrees C (104 degrees F).
Other factors under continuing study include the influence of agitation rate, the initial mass of core material, the surfactant concentration, and the material’s shelf life.
Nevertheless, the team concluded: “Preliminary results indicated significant corrosion retardancy happened in the self-healing coatings under an accelerated corrosion process, showing the great potential of this facile microencapsulation technique in the development of catalyst-free, one-part, self-healing coatings for corrosion control.”
Moreover, the healing behavior “was completely autonomous without any external intervention such as heating or UV exposure, and it did not require catalyst or other assisting materials either, making it easier for the development of self-healing materials, which is of considerable technical and commercial importance.”