TAGS: Smart Coatings
What are Functional Coatings?
In the field of coating technology, many coatings have been developed over decades of research, leading to high-quality coatings which have:
- Good adhesion
- Mechanical properties, and
- Other important functions which are defined by the final application and use.
Example of Water Droplet on Superhydrophobic Coating
However, next to good mechanical and chemical properties additional functionalities have been introduced to further increase the added value of these coatings. Well-known examples of such functions, just to name a few are - Anti-reflection, Anti-fingerprint, Corrosion protection, Superhydrophobicity, etc.
Let's understand when a functional coating is smart and the key considerations while designing smart coating applications.
Smart Coatings Versus Functional Coatings
Next to functional coatings, the term smart coating is often misused.
The question remains: when is a functional coating also smart?
Even though there is no fixed definition of when a coating is smart, the consensus is that the smart coating has the added functionality that it actively reacts to an external or internal trigger, whereas the functional coating is a passive layer which is insensitive to environmental conditions.
Stimuli - Response to External or Internal Factors
As it was mentioned before, smart coatings react to an
external or internal stimulus. The main categories that may induce the response of the smart coating include:
- UV Light
- Temperature
- pH
- Moisture
- Damage
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Chemical Conversion Coating
In the specific case of
corrosion protection,
chemical conversion can be used as the smart function within the paint or coating. These chemical conversion coatings contain inorganic compounds which rely on the principle of the dissolve – reprecipitation effect in local defects.
In the figure below, we can see the principle of a chemical conversion coating. The active protection of the magnesium alloy takes place in 4 stages:
- The dissolution of magnesium (and evolution of hydrogen gas at the AlMn particles)
- The formation of magnesium phosphate
- The nucleus formation of calcium phosphate and zinc phosphate, and finally
- The growth of the crystalline deposit, giving again protection to the magnesium alloy.
Working Principle of Chemical Conversion Coatings1
Chemical conversion coatings have the important advantages of having excellent mechanical-stability and the simple preparation method of the coatings. Their limitation lies in the low-efficiency self-healing performance, which limits their application in certain ways.
Self-healing Paint and Coatings
The
self-healing paint and coatings can be designed using various approaches, such as:
- Encapsulation of active ingredients
- Reversible crosslinks
- Controlled/dynamic release
- Active leaching or shape memory
Amongst these approaches, encapsulation and reversible crosslinks are explained below.
Encapsulation
Self-healing paint and coatings contain
capsules which are loaded with active repair agents, which can transform the coating from a barrier role to an active role. The stimuli can occur either internal or external, such as:
- Ion release
- pH increase
- Dynamic bonds, and
- Shape memory effect
Besides microcapsules, hollow tubes are being used as the carrier for the active materials, such as fibers or capillaries.
Schematic of the Self-protection Process: Paint Coating Having Microcapsules with Self-healing Protecting Paint on Carbon Steel2
Compared with chemical conversion coatings,
encapsulation coatings rely more on the functions of the coating materials and intelligent designs; this makes for the encapsulation coatings having a more complex preparation process but poses a higher efficiency and controllability. Currently, research focuses on the encapsulation of ions, inhibitors, nanoparticles, and nano/microcapsules.
Reversible Crosslinks
Another way to create self-healing properties is by the incorporation of reversible crosslinks in the coating matrix, also called
covalent adaptable networks (CANs). Suitable mechanisms for this include:
- Diels-Alder/ Retro Diels-Alder
- Ring-opening metathesis polymerization using DCPD
- Schiff base (imine) bonds
- Reversible hydrazone bonds
- Oxime bonds
- Disulfide bonds
These covalently crosslinked networks are formed such that triggerable, reversible chemical structures persist throughout the polymer matrix. These reversible covalent bonds can be triggered through:
- Molecular triggers
- Light or other incident radiation, or
- Temperature changes
Upon application of this stimulus, rather than causing a temporary shape change, the CAN structure responds by
permanently adjusting its structure.
SEM Image of Self-healing Coating with DA Reversible Crosslinking3
All of the reversible crosslinking reactions described are within the chemical linkage category. The other category which can be used for reversible crosslinking is the
physical linkage category. Examples of reversible physical linkages are:
- Hydrogen bonding
- Host-guest interactions (such as Cyclodextrins), or
- Electrostatic/ionic interactions
However, the coatings typically have inferior mechanical properties due to the lack of covalent crosslinks. The choice of self-healing mechanism depends on the matrix composition, trigger and overall product requirements.
Smart Coatings – Fields of Application
Anti-scaling
In industrial environments, scale formation is a key problem. The most effective method for the protection of pipes and metal is the use of (smart) coatings. Next to functionalities such as (super) hydrophobicity and corrosion protection, gradual and dynamical leaching of scale inhibitors causing chelating and prevent further scale formation/deposition
4.
Limescale Formation in Water Pipes
Related Read: Sol-Gel Based Nanotechnology Solutions for Advanced Smart and Functional Coatings
Corrosion Protection
In the case of corrosion protection, the conventional coatings are of very high quality but in certain complex service environments, the damage will inevitably occur. In these cases, smart coatings or self-healing properties5 enhance the lifetime and provide long-term protection of the protected substrate.
Example of Corroded Surface
An important class of smart coatings, in the field of corrosion protection, are the chemical conversion coatings as described above. Next to this, many coatings are described in the literature which makes use of encapsulated
corrosion inhibitors which leach out and repair upon inflicted damage or other suitable triggers, such as pH change
6.
Automotive Coatings
In order to enhance the lifetime of
automotive coatings and improve the aesthetic appearance,
enhanced scratch resistance and scratch repair are major drivers towards the development of self-healing or self-repairing topcoats.
To improve scratch resistance,
fillers and nanoparticles play a major role in increasing this next to the hardness. However, there is a maximum amount of
fillers which can be used which when exceeded the mechanical properties will deteriorate and appearance will be affected.
To further improve the scratch resistance, several pathways towards
self-healing and self-repair are described. Partial replacement of covalent bonds by physical linkages, to provide dynamic chain movement for thermal healing treatments at high temperature
7, is one of them. And the second one is the use of micro/nanocapsules which will actively repair the damage.
Self-repairing Paint by Nissan
Anti-microbial Coatings
For many years,
antimicrobial coatings have been a field of attention. To provide antimicrobial properties to the coating, 4 different categories can be used:
- Inorganic and metal-based: Silver and copper are the most used metals. Despite their effectiveness, there are concerns regarding their toxicity over a period of time due to leaching. Also, the long-term dosage effects are not well reported.
- Organo-compounds: A very popular solution is the use of organo-silanes. They provide (super) hydrophobic properties to the surface incapable of hosting any colonization properties. Zwitterions are also very efficient due to their response behavior, enabling both antibacterial properties and antifouling properties to the surface.
- Nanomaterials: Due to their extraordinary properties and behavior, nanoparticles are utilized.
- Anti-microbial peptides: These can be attached to the surface chemically or by physical attachment.
Antimicrobial Coatings (L), Zwitter Ion Function in Smart Antibacterial Coatings (R)8Commercialization of Smart Coatings
The class of smart coatings has become embedded in the research towards the functionality of coatings in many fields and even more applications. Due to the intensive research, following the exponential growth in academic publications, companies such as PPG, Croda and AkzoNobel have commercialized smart coatings, such as:
- Self-healing vehicle refinish coatings
- Smart anti-fingerprint coatings, and
- Antimicrobial coatings, amongst others
Challenges that remain to be the focus in the development of these smart coatings are:
-
Economic viability
- Ease of production
- Lifetime or durability of the coating
For self-healing coatings, it is no doubt that
active materials and coating designs are the core parts of the coatings. However, currently, the utilization efficiency of active materials is relatively low. Therefore, novel and efficient active materials and intelligent coating designs need to be explored in further studies.
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Main Considerations While Designing Smart Coatings
In addition, it is also
vital to evaluate how the basic properties of polymer coatings are affected by the introduction of these smart functionalities. Naturally, the smart function may not have a negative effect on the main (mechanical) properties of the coating.
Other considerations which need to be made when designing a smart coating are:
- Costs of raw materials
- Availability of raw material in commercial volumes
- Added benefit of the smart functionality with respect to the
- Coating lifetime
- Substrate protection
- Added value
- Health and safety issues
- Efficiency of the smart functionality
- Durability of the smart functionality
The main balance will still be finding the optimum between the price of the raw materials used and the added benefit over the “standard” functional coating.
Capture fresh ideas and gain insights into next concepts for your functional coatings by reviewing Smart Coatings trends in our exclusive innovation round-up. The new materials, latest commercial launches, promising concepts, & innovations can be a game-changer for you as well, if you act quick. Join today!
