The most common approach to preventing corrosion is to paint the surface with a protective coating. Typically, paints composed of an inorganic powder embedded within a polymer matrix have only limited ability to resist abrasion. Attempts to improve durability are ultimately constrained by the requirements that the coating be relatively thin (e.g., <100 µm) and easy to apply. While repainting and touch-ups can be performed as part of regular maintenance, many defects go unnoticed before significant damage occurs. Accordingly, self-healing coatings have been developed that autonomously repair scratches below some maximum width, thereby delaying the onset of corrosion and increasing the time between maintenance cycles.
The most common strategies utilized in developing self-healing polymer coatings are to supply energy to the system to form new bonds, or supply additional material to the damage zone. However, heating is logistically impractical for large objects, and UV activation may not provide complete healing if pigments in the coating interfere with light absorption. Another approach achieves self-healing by supplying additional material to the damage zone, for example, the use of embedded polymer microcapsules incorporated into paints and primers. However, appropriate materials should be used to fabricate the microcapsule and its contents, or else it may "deploy" before the coating is applied or, upon application, spontaneously deploy improperly, i.e., without a physical compromise of the coating such as abrasion or nicking. Further, unless the microcapsule is compatible with both its contents (the encapsulated repair compound) and its surrounds (the solvent), the "application" life of the resultant mixed product may be less than desirable.
Accordingly, there is a continued need for improved self-healing coatings that can be made in a simple, cost efficient manner.
Researchers at The Johns Hopkins University Applied Physics Laboratory have successfully synthesized a self-healing coating that meets the needs outlined above. Through a combination of emulsification, interfacial polymerization, and electroless metal deposition, they have demonstrated the successful encapsulation of a water-sensitive chemical–isophorone diisocyanate (IPDI)–within a nickel shell. Measuring less than 50 µm, each microcapsule hermetically seals its contents, and provides the ability to release the contents on demand using either a mechanical, electrical, or magnetic stimulus.
The synthesis of these liquid-filled, metal microcapsules have demonstrated: (1) the ability to hermetically seal a chemical that is water or air sensitive within a discrete, microscopic package; (2) the ability to prevent outgassing of volatile encapsulants; (3) the ability to deposit metal coatings directly upon polyelectrolyte multilayer films; (4) the ability to encapsulate chemicals that may be sensitive to a static discharge; (5) the ability to resistively heat microcapsules to trigger chemical release using an electrical stimulus; and (6) the ability to hysteretically heat microcapsules to trigger chemical release, using a magnetic stimulus. The ease with which a metal may be applied to a negatively charged polymer from aqueous solution has implications in the fabrication of novel metal-polymer composites and metamaterials with unique mechanical, optical, magnetic, and electrical properties.
As the synthesis steps are based entirely on industrial processes, the JHU/APL method of synthesis should scale easily to large batch sizes.CONTACT:
Mr. M. T. Hickman