Non-Contacting

The requirements for a self-healing coating is different than a general structural self-healing polymer. Namely, the coating should be relatively inexpensive, otherwise it will be limited to very small components. In addition, evaluation of a coating is different than evaluation of a structural polymer. For a structural polymer we are mostly interested in fracture and fatigue performance, as well as the recovery of fracture resistance after a crack occurs. For a coating we are more interested in protection from environment and extended life of the coating. To evaluate the performance of a self-healing coating we use Electrochemical Impedance Spectroscopy (EIS). The equipment pictured here is used during the EIS test. This is fundamentally an electrical test, where the voltage is controlled and the resulting current is measured. With that information we can determine the impedance of the system. The impedance will change as corrosion of the steel substrate progresses. By tracking how the impedance changes over time it is possible to compare the performance of a self-healing coating to a regular coating.

Developing a commercial self-healing coating requires that the extended life and improved protection outweighs the added cost of incorporating self-healing components. The self-healing components developed for structural self-healing polymers is too expensive for a self-healing coating application. Alternative polymer chemistries, monomers and catalysts, need to be evaluated for many different properties, for example: stability in storage, stability once incorporated in the coating, encapsulation quality, flow of the healing agent into the damage site, catalysis of the healing-agent after release in the coating, protective properties after release, resistance to washing out (think rain, waves, splashes), compatibility with the coating host material, compatibility with the substrate, among many others. Here you can see the data from an EIS test of some candidate healing agents. Higher data points indicate better performance.

After choosing a healing agent, successfully encapsulating it with stable microcapsules and successfully distributing it in the coating material, we still need to evaluate how the coating mechanically. Sure the self-healing coating may release healing-agent and heal damage, but that does not help if the coating will not adhere to the substrate, or if the coating is very fragile due to the addition of the healing system. After creating a candidate self-healing coating we evaluate the strength of the coating and compare it to a similar coating without healing components. Here you can see the bending of a steel substrate with a self-healing coating applied. In all cases the self-healing coating had improved mechanical performance.

A typical test of a possible self-healing coating consisted of a number of steps.

  1. Encapsulate the healing-agent to make microcapsules
  2. Distributing the microcapsules in the coating material
  3. Applying a uniform, thin coat on a steel substrate
  4. Allow the coating to cure (paint to dry)
  5. Damage the coating (usually an X shaped scratch)
  6. Expose the specimen to the desired environment, for example, periodic salt sprays or full submersion in salt water
  7. Repeat the exposure protocol over the time period desired (usually 7 - 14 days)
  8. Over the exposure protocol periodically take pictures and EIS measurements

This image shows a comparison of a normal (control) coating to a self-healing coating. Both the control and the self-healing coating specimens had the same exposure protocol. Picture (a) is the control coating after the exposure protocol and picture (c) is the corresponding SEM image of the scratched area in the coating. Picture (b) is the self-healing coating and picture (d) is the corresponding SEM image of the scratched area.