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Coatings Ingredients
Article

Smart Coatings Getting Smarter

Dr. Ing. Patricia Geelen – Feb 19, 2021

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.
Water droplet on Superhydrophobic Coating
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


Main Categories of Stimuli - Smart Coatings 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:

  1. UV Light
  2. Temperature
  3. pH
  4. Moisture
  5. Damage


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 »

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:

  1. The dissolution of magnesium (and evolution of hydrogen gas at the AlMn particles)
  2. The formation of magnesium phosphate
  3. The nucleus formation of calcium phosphate and zinc phosphate, and finally
  4. The growth of the crystalline deposit, giving again protection to the magnesium alloy.

Working Principle of Chemical Conversion Coatings
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 Coatings

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.

Encapsulation - Paint Coating Having Microcapsules with Self-healing Protecting Paint on Carbon Steel
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.

Nanotechnology for Functional Coatings - R&D Overview

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 Crosslinking
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/deposition4.

Limescale Formation in Water Pipes
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
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 change6.

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 temperature7, 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
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:

  1. 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.
  2. 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.
  3. Nanomaterials: Due to their extraordinary properties and behavior, nanoparticles are utilized.
  4. Anti-microbial peptides: These can be attached to the surface chemically or by physical attachment.

Antimicrobial Coatings     Zwitter Ion Function in Smart Antibacterial Coatings
Antimicrobial Coatings (L), Zwitter Ion Function in Smart Antibacterial Coatings (R)8


Commercialization 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:

  1. Economic viability
  2. Ease of production
  3. 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.

» Follow Our Exclusive Channel for All the Latest Industry Developments in Smart Coatings

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!

Smart Coatings Update



References

  1. Zeng, R., Zhang, F., Lan, Z., Cui, H. and Han, E., 2014. Corrosion resistance of calcium-modified zinc phosphate conversion coatings on magnesium–aluminum alloys. Corrosion Science, 88, pp.452-459.
  2. Koh, E., Kim, N., Shin, J. and Kim, Y., 2014. Polyurethane microcapsules for self-healing paint coatings. RSC Adv., 4(31), pp.16214-16223.
  3. Zhang, F., Ju, P., Pan, M., Zhang, D., Huang, Y., Li, G. and Li, X., 2018. Self-healing mechanisms in smart protective coatings: A review. Corrosion Science, 144, pp.74-88.
  4. Zhu, Y., Li, H., Zhu, M., Wang, H. and Li, Z., 2021. Dynamic and active antiscaling via scale inhibitor pre-stored superhydrophobic coating. Chemical Engineering Journal, 403, p.126467.
  5. Zhang, D., Peng, F. and Liu, X., 2021. Protection of magnesium alloys: From physical barrier coating to smart self-healing coating. Journal of Alloys and Compounds, 853, p.157010.
  6. Chen, L., Yu, Z., Yin, D. and Cao, K., 2020. Preparation and anticorrosion properties of BTA@HNTs-GO nanocomposite smart coatings. Composite Interfaces, 28(1), pp.1-16.
  7. Yari, H., Mohseni, M. and Messori, M., 2016. A scratch resistant yet healable automotive clearcoat containing hyperbranched polymer and POSS nanostructures. RSC Advances, 6(79), pp.76028-76041.
  8. Li, X., Wu, B., Chen, H., Nan, K., Jin, Y., Sun, L. and Wang, B., 2018. Recent developments in smart antibacterial surfaces to inhibit biofilm formation and bacterial infections. Journal of Materials Chemistry B, 6(26), pp.4274-4292.

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