Self-Healing / Self-Repairing Paints and Coatings

  Summary: This article provides general information (history, terminology, etc.) on Self-Healing / Self-Repairing Paints and Coatings, a groundbreaking new class in the fields of industrial, marine, aerospace, automotive, conformal and textile paints/coatings. It examines which main types comprise these technologies and what advantages/disadvantages they possess.

1. Introduction

Breaks and scratches occur, and we accept them as part of life. For example, if you drop your mobile phone, you might scratch or even crack the screen. An even more serious situation is when your car's paint gets scratched or cracked. While the cost and time required to repair the phone screen are relatively modest, the same cannot be said for repairing your vehicle's paint. More troubling still is when microscopic cracks form in critical equipment and structures due to mechanical damage—cracks that grow progressively and can lead to catastrophic consequences ending in accidents. A case in point is the China Airlines Flight 611 aircraft, where 22-year-old micro-cracks in the fuselage caused the hull to rupture, resulting in the aircraft crashing into the sea off Taiwan in 2002. However, just as when you slightly cut your hand, the bleeding area clots and heals itself after a certain time (your body repairs itself), the same phenomenon occurs on metal, wood, concrete, composite and even textile surfaces through the use of Self-Healing / Self-Repairing Paints and Coatings. This technology can impart self-healing/self-repair functionality to products.

Figure 2: Blood clotting and skin self-healing

To explain this simply, when a scratch or crack appears in paints and coatings, a microscopic break or tear occurs in the polymer chains that form the building blocks. Fundamentally, what Self-Healing / Self-Repairing Paints and Coatings do is repair these breaks/tears in an Autonomous or Semi-Autonomous manner using various technologies and mechanisms.

Industry 4.0 and the Paint / Coating Industry: Autonomous and Semi-Autonomous Self-Repairing Paints and Coatings

As we enter the Industry 4.0 era, just as with new generation vehicles, paints and coatings can be expected to self-heal / self-repair either autonomously (without any intervention) or semi-autonomously (through external stimulation such as temperature changes, heat, UV or IR radiation application, and pressure changes). These types of paints/coatings can heal and restore surface scratches and mesoscopic damage (e.g., micro-cracks and cavities). They follow a two-stage process in doing so: 1. Sealing the void, 2. Healing/repairing the damaged area.

Figure 3: Two-stage self-healing and repair process of paints/coatings.

Different types of Self-Healing / Self-Repairing Paints and Coatings possess different technologies. These differences cause some variations in the two-stage self-healing process and will be examined briefly in the Main Types section of this article.

Brief History

Historical records indicate that the oldest known self-healing/self-repairing material was a type of lime mortar used during Roman times. In the modern sense, numerous articles and presentations on Self-Healing / Self-Repairing Paints and Coatings have been conducted since the mid-1980s. The first successful examples of this technology began to be produced in laboratories at technical universities in America, Europe and Australia in the early 21st century. From 2010 onwards, this technology found commercial application in various sectors. The most active research and commercial developments in this field are observed in the following areas/sectors.

Which Sectors Use These Products?

Self-repairing paints/coatings are most frequently seen in the automotive and OEM sectors (particularly for removing paint scratches on metallic or plastic vehicle surfaces). Through ongoing research in different countries worldwide, these technologies have been proven applicable in smartphones, aerospace, defense and military equipment, industrial structural steel, and even textile products (See References section).

2. Main Types

Self-healing chemical production methods can be examined under 5 main categories: I. Microencapsulation II. Heat-activated Thermoplastic Polymers III. Micro-vascular network IV. Reversible chemical reactions V. Hollow-fiber approach However, Microencapsulation and heat-activated thermoplastic polymers are highlighted as leading methods in the production of Self-Healing Paints/Coatings. Let us now examine these two technologies more closely.

Microencapsulation

Healing/repair agents and chemicals are embedded within this type of paint and coating as capsules at micro and even nano scales. These capsules function as pockets within the paint or coating; when subjected to external impact (impact, scratching, tearing, etc.), these capsules rupture and release adhesive-like healing chemicals and agents into the external environment. Subsequently, they react with other catalyst chemicals present externally to form a larger and more durable copolymer by volume. Thus, it repairs the scratch/tear/hole in that area and heals the paint and coating. Of course, all these reactions occur in an instant, at a scale invisible to the human eye, at microscopic and even nano scales. (The relationship of this technology with nanotechnology will be explained in the next section.)

Figure 4: Simple operating mechanism of microencapsulation-based self-healing / self-repairing paints and coatings

Heat-activated Thermoplastic Polymers Microencapsulation

(Heat-activated Thermoplastic Polymers) This type of Self-Healing/Self-Repairing Paints and Coatings can be divided into 2 sub-categories: 1. Shape Memory Polymers - SMPs 2. Reversible Polymers

1. Shape Memory Polymers

Many of us frequently use shape memory materials in our daily lives but do not notice it. For example, we have witnessed how our eyeglasses made of Nitinol (Nickel-Titanium) return to their original form/shape no matter how bent they become. Generally, Shape Memory Polymers function in a much more complex (and extremely interesting) manner. However, we will not delve into these details. (Interested readers can examine the link at the end of the article.1) What matters here is that, due to their structure, these types of Shape Memory Polymers are flexible and require a heat or energy source to return to their pre-damage condition. For example, many automotive manufacturers and coating suppliers producing paints for these manufacturers incorporate paints with this technology in the Clear-coat layer, which is the final stage of the vehicle's paint system. Thus, after the paint is scratched, it becomes possible to remove these scratches using a simple heating device.

Figure 5: Applying heat to a scratched paint on a vehicle causes the paint to self-heal / self-repair

2. Reversible Polymers

In this type of self-healing paint/coating, polymers such as those with Thermoreversible chemistry containing Furan and Maleimide functional groups are used. Following damage, through the application of heat or heat-generating energy to the surface, special reversible chemical reactions such as Diels-Alder (DA) and retro-Diels-Alder (RDA) enable these reversible polymers to return to their original undamaged state. The greatest characteristic of these polymers is that the ends of their polymeric structure are highly active; even if the polymers are separated into fragments due to external mechanical damage, they have the potential to attract each other like magnets to return to their original state.

Figure 6: Shape Memory Polymers (SMPs) and Diels-Alder (DA) and retro-Diels-Alder reactions

3. Relationship with Nanotechnology

Nanotechnology—science, engineering and technologies conducted at the nanometer scale, which is one billionth of a meter (1/1,000,000,000 meter)—currently affects nearly all sectors from pharmaceuticals to construction. The use of nanotechnology is particularly essential in the class of Self-Healing/Self-Repairing Paints and Coatings produced through microencapsulation. This is because specially formulated chemicals placed in nano-scale capsules can transform an ordinary paint into a Self-Healing / Self-Repairing paint that will release its chemicals in a controlled manner when subjected to external impact. Furthermore, nanotechnology is heavily used in the production of the aforementioned Shape Memory Polymers.

4. Problems and Disadvantages

The most frequently encountered problems and disadvantages related to these technological paints and coatings, whose main types we have examined, can be summarized in 4 main points: I. Most microcapsules can only repair defects resulting from mechanical damage and are vulnerable to natural corrosion processes such as metal rusting. To date, these paints/coatings cannot detect electrochemical processes such as corrosion in advance and take precautionary measures accordingly. II. Because the capsules used during the microencapsulation process are extremely small (nanometer-sized; if larger, they can cause weaknesses in the paint's chemical structure), the healing agents/chemicals they can carry are limited. For this reason, most often these capsules can only heal and repair one or a few times when damage occurs in the same location. (It can be said that their healing power is extremely limited.) III. During the microencapsulation process, most of the catalyst and healing agent chemicals required for healing must be stored separately and then thoroughly mixed into the paint. Since the location where damage will occur is not known in advance, if these two different materials are not mixed sufficiently homogeneously, they may not be close enough to each other and healing may not occur. IV. The healing chemical agents are extremely unstable; they react immediately with water and oxygen. For this reason, their shelf life is quite short and storage conditions must be carefully observed.

5. Recent Developments and Trends

To address the problems and disadvantages mentioned above, numerous universities and research centers worldwide are conducting research. For example, scientists at the BATTELLE Memorial Institute research center are working on a single-component smart particle that can detect and heal corrosion formation in advance. (Battelle Smart Corrosion Detector® bead) These spherical particles are 30 to 50 micron diameter spheres that appear as a whitish powder in bulk form. Thus, it is envisioned that these particles would provide early warning signals for corrosion formation on metal surfaces, triggering the self-healing/repair process. The development of paints and coatings using microencapsulation techniques that respond to external stimuli targeting bacteria and microorganisms is also under consideration. The goal is to have capsules transmit toxic chemical agents from within themselves onto bacteria and microorganisms detected by the microcapsules. As a triggering mechanism, a specific protein in bacterial cell walls can be used as a stimulus, a technique documented in the literature. Another recently popular approach is the research into producing hybrid self-repairing paints/coatings by using the technological methods described above in combination. For example, the production of dual-action (or even triple-action if other technologies are also employed) Self-Healing Paints/Coatings possessing both microencapsulation and reversible chemical reaction technologies is one of the subjects being researched. Although this technology can be more easily applied to the polymer structures that form the framework of paints and coatings, innovative solutions are being developed in different countries worldwide for use in metal, ceramic, cement and even asphalt materials. You can read more extensive information on this topic in the article at the following link2:

6. Conclusion

With the industrial and technological developments of the 21st century, innovations continually enter our lives. Self-Healing/Self-Repairing Paints/Coatings represent one of the innovative breakthroughs entering our lives as a result of the industrial and technological developments of the 21st century. Scientists in academia, research academics and industrial R&D center experts are essentially developing innovative paints and coatings with longer service lives by imitating the Self-Repair Mechanisms of living organisms, just like humans, thereby progressively extending the service life of the substrates these paints and coatings protect. In this innovative technology, which is just taking its first steps, more advanced types with higher performance and fewer disadvantages can be produced by taking living organisms' vital functions, also known as Biomimetics, as examples. In our 21st and 22nd century world increasingly surrounded by smart technologies, the concept of Self-Healing/Self-Repairing Paints/Coatings will be a continuously evolving technology. It is an indisputable fact that we will hear about this technology more frequently in the future and use it more often in our daily lives. PCS. Tolga Dıraz - Chemical Engineer - Protective Coatings Specialist - Chair of TUCSA TK-4 Surface Protection Committee

1.http://www.bilimgenc.tubitak.gov.tr/makale/sekil-hafizali-yeni-polimerler ) 2.https://www.rvo.nl/sites/default/files/bijlagen/282975_ANL_V2011-10%20AgentschapNL%20IOP%20Selfhealing%20materials%20A5%20C.pdf

References 1. Self Healing materials – concept and applications, Ministry of Economic Affairs, Agriculture and Innovation, NL Agency Second edition – 2011 2. Self-Healing Coatings For Textile I. De Vilder 1 and M. Vanneste 1 3. https://www.slideshare.net/LakshiNandanBorah/self-healing-of-composite-material 4. https://www.forbes.com/sites/fionamcmillan/2017/12/21/the-rise-of-self-healingmaterials/#66cd943f64c3 5. https://www.explainthatstuff.com/self-healing-materials.html 6.https://www.researchgate.net/publication/283550665_Self-healing_materials_A_review_of_advances_in_materials_evaluation_characterization_and_monitoring_techniqueshttps://www.oceanit.com/products/healable-nano-coating 7. https://www.pcimag.com/articles/101732-the-science-of-corrosion-busting-smart-coatings 8. https://www.corrosionpedia.com/definition/1480/self-healing-coating 9. https://www.compositesaustralia.com.au/the-reality-of-self-healing-polymers/ 10. https://pubs.acs.org/doi/pdf/10.1021/acsami.8b06985 11. https://pubs.rsc.org/en/content/articlelanding/2014/TA/C3TA13389C#!divAbstract 12. https://www.graphene-info.com/northwestern-team-develops-graphene-based-self-healing-coating 13. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5510056/ 14. https://www.wikizeroo.org/index.php?q=aHR0cHM6Ly9lbi53aWtpcGVkaWEub3JnL3dpa2kvU2VsZi1oZWFsaW5nX21hdGVyaWFs