Self healing coatings now & in the future
No coating is a 100% immune to damage. The damage can be visible scratches, gaps or flaking on the layer or invisible corrosion between metal and the coating. To insure maximum protection and retain the looks, the damage must be repaired which costs time and money. Sometimes the damage is not noticed and corrosion takes over leading to further defect not only on the coating but also on the metal substrate. Now, imagine a self healing coating that fights corrosion while fixing the surface like healing skin.
Self healing coatings are interesting for scientists who are working towards minimum maintenance metal coatings and maximum cost efficiency. The coatings are suitable for example for corrosion prevention and products such as cars to minimise visible damage on the surface. There are already some self healing coating systems on the market, and the technology is also constantly improving promising more self healing systems and applications in the future.
Self healing mechanisms
There are two types of self healing coatings; the differences lay in use and healing mechanisms. These types are;
- Autonomous coating systems which automatically start healing when damage occurs; thus, damage triggers the healing agents.
- Non-autonomous or on-demand healing systems which require an external trigger to activate the healing agents and start the recovery. The trigger can be for example light or heat.
The latter is often a shape recovery systems which makes sure the surface retains its barrier properties. The autonomous systems, however, automatically start healing. These coatings are effective in corrosion protection due to the fact that they contain corrosion inhibitors which are activated by tiny cracks or corrosion species on the metal.
Autonomous healing process
Weaknesses of single action self healing systems
The two types of self healing coatings have naturally numerous advantages; however, they also have pitfalls. The autonomous systems start healing right away when damage occurs but they lack ability to restore the surface weakening the barrier properties. This means that scratches etc. stay on the surface which then remains open for corrosion. The on demand systems on the other hand, leave the surface subject to corrosion until the healing process is triggered meaning that the damage must be noticed and taken care of. In the ideal world, the self healing coating would do both, protect from corrosion between the substrate and coating as well as restore the surface after any damage.
Multi action self healing coating
Self healing smart coatings are constantly a subject to research to enable innovation regarding products and applications. The first developments of the coatings focus on enhancing one healing mechanism; for example making use of multiple corrosion inhibitors or adding shape restoring polymers. However, a combination of the two mechanisms did not exist until 2015 when introduction of a formula combining gap closing and corrosion protection took place. A multiple action self healing coating (MASH) is in progress; it will be the first share-recovery anti corrosion coating on the market.
How self healing coating works
The single action coatings either restore the surface or prevent corrosion under the coating layer. The multi action coating for its part does both. The main problem with a single action coating is that when damage occurs the healing either starts only on the surface allowing corrosion under the layer, or only under the layer preventing corrosion on the substrate but weakening the surface barrier making it vulnerable for corrosion. The non-autonomous systems also do not provide any protection without manually triggering the healing agents.
The combination of the two healing mechanisms tackles these problems. When damage occurs, the corrosion inhibitors start working preventing corrosion on the metal surface. Triggering the surface recovery requires heat or light, which leads to restoring the surface without compromise to the barrier properties.