Traditional coatings serve one or sometimes two functions, which include decoration and substrate protection or both. In addition to these fundamental properties, Multifunctional Coatings can provide an impressive array of additional performance benefits, including:
- Surface resistance to mildew, mold and bacteria
- Anti-fouling without metals
- Self-healing anticorrosion
- Self-healing scratch repair
- Highly conductive coatings
Some Multifunctional Coatings may also fall into the category of Smart Coatings, as in many cases they are responsive to their environment. Their functional properties are essentially independent of each other and include the performance attributes listed above as well as many other possibilities too numerous to detail in this article.
Many of these innovative breakthroughs were adapted from naturally occurring materials and biology.
For example, why does a lotus leaf shed water?
A lotus leaf sheds water due to the unique micro and nanoscale architecture on the surface that minimizes the droplet’s adhesion to the surface. The contact angle of water on the surface of a lotus leaf is greater than 150° and thus water droplets readily roll off the surface.
The cause of self-cleaning properties of the lotus leaf is the hydrophobic water-repellent double structure of the surface. This significantly reduces the contact area and the adhesion force between surface and droplet and results in a self-cleaning process, allowing water to readily roll off the leaf and collect dust deposits on the way. This micron size double structure is formed at the surface of the plant and is comprised of needle-like projections from the surface that are covered by wax.
The wax-covered projections are 10 to 20 µm in height and 10 to 15 µm in width. The waxes are hydrophobic and form the top layer of the double structure. Some plants show contact angles up to 160° and are called superhydrophobic; only 2–3 percent of the surface of a water droplet is in contact with the surface. A surface contact area of less than 0.6 percent leads to the self-cleaning effect.
Why does the Nepenthes pitcher plant provide a slippery surface?
The carnivorous Nepenthes pitcher plant may be found in many tropical regions of the world. Surrounding the entrance or lip of the trap is a structure called the peristome. While colorful to attract prey, it also provides unstable footing due to the presence of a slippery liquid substance. When this surface is wet, it causes prey to slip and fall into the trap.
When this phenomenon is engineered into a substrate surface, it results in a multitude of very unique performance properties which can provide one or more surface properties. These properties include:
- Antifouling without the use of metals
- Bacteria and contaminant repellency
Why is some candle soot conductive?
Candle soot can be conductive as it contains irregular nanotubes. Further research resulted in carbon nanotubes which provide conductive coatings without the use of conductive metal(s).
Carbon nanotubes (CNTs) were first named in the 90s, but their use in coatings has just begun to be harvested in the last five to 10 years. CNTs can be single walled (SWNT) or multiwalled (MWNT). CNTs have exceptional strength due in part to their extremely high length-to-width ratio, extraordinary thermal conductivity, mechanical, and electrical properties. With these properties, carbon nanotubes make effective additives to various structural materials.
The bonds in CNTs are SP2 which, along with CNTs structure, provide their unique properties. CNTs application in coatings is in its infancy, however CNTs are expected to provide differential properties in coatings that will include corrosion resistance, improved mechanical properties and very high conductivity.
Self-healing coatings can be made by incorporating microcapsules and nanocapsules that contain film-forming polymers. When the coating is scratched, the capsules break and fill the scratch with a polymeric fluid that forms a film. Self-healing technology is used by some automakers in clear-coat automotive topcoats.
Other self-healing coatings are film formers, which protect the underlying substrates from corrosion through the release of microcapsules that contain corrosion-inhibiting chemicals.
Thermochromatic paint utilizes special pigments which change color as the temperature changes.
In summary, a wide variety of innovations in material science and coatings technology are enabling an impressive array of new Mulitifunctional Coatings and applications.
A search of Prospector’s database yields a number of materials that can provide a variety of properties when properly formulated into a coating system, such as:
- Smarten Up! A Smart Coatings Primer
- Smart Coatings: Self-Healing, Anti-Fouling & Sensing Material Developments
- Hydrophobic Coatings Explained
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