Nothing lasts forever, any synthetic material, used in anything from fiber to bridges will eventually fails, if not maintained and fixed. Traditionally, this problem has been addressed through extensive inspection and expensive replacement of damaged parts. Biological systems however, have evolved to include alternative ways to repair internal and external damage through healing mechanisms1-2. Self-healing materials are no more an illusion, and we are not far away from the time when synthetic materials can re-establish their structural integrity in case of a failure.
Such Self-healing materials, when triggered by a crack or tear, can repair themselves and recover their original functionality using only the materials that are inherently available to them3-8. e.g., the cracks in buildings can close on their own or the scratches on car bodies can recover their original shiny appearance by itself. Indeed, this is what everyone can see in case of the natural healing of wounds and cuts in living species.
Virtually, all materials are susceptible to natural or artificial degradation and deteriorate with time. In the case of structural materials the long-time degradation process leads to micro cracks that cause a failure. Thus, repairing is indispensable to enhance reliability and lifetime of materials. Scientists are inspired by the natural process of blood clotting or repairing of fractured bones, incorporating the same concept into engineering materials is far from reality due to the complex nature of the healing processes in human bodies or other animals. However, the recent announcement from Nissan ltd. on the commercial release of scratch healing paints for use on car bodies has gained public interest on such a wonderful property of materials9.
Definition of Self-healing
Self-healing can be defined as the ability of a material to heal (repair/re-build) damages automatically and autonomously, that is, without any external intervention. Many common terms such as self-repairing, autonomic-healing, and autonomic-repairing are used to define such a property in materials. Incorporation of self-healing properties in synthetic materials very often cannot perform the self-healing action without an external trigger.
Self-healing can be of the following two types:
- Autonomic (without any intervention)
- Non-autonomic (needs intervention/external triggering, UV-radiation is required for the healing to occur)
In addition to kinetic modeling, rheological information is essential to take into account the thermo mechanical behavior during healing. The self-healing mechanism under study is based on a temporary increase in local mobility due to a decrease in viscosity upon heating. The change in viscosity and viscoelastic behavior should be limited in order to maintain the mechanical integrity of the coating on the substrate. The flow behavior of the model network is studied by dynamic rheometry and allows determination of a gel-point temperature. Different types of materials, such as plastics/polymers, paints/coatings, metals/alloys, and ceramics/concrete have their own self-healing mechanisms, depending on the material characteristics.
The different strategies of designing self-healing materials are as under:
- Release of healing agent
- Reversible cross-links
- Miscellaneous technologies
- electro hydrodynamics
- shape memory effect
- nanoparticle migration
Release of healing agent
Liquid active agents such as monomers, dyes, catalysts and hardeners containing micro-capsules, hollow fibers, or channels are embedded into polymeric systems during the stage of manufacturing. In the case of crack, these reservoirs are ruptured and the reactive agents are poured into the cracks by capillary force where it solidifies in the presence of pre-dispersed catalysts, and heals the crack. The propagation of cracks is the major driving force in whole process. This process is an autonomic, as this does not need external intervention.
There are different possibilities to explore this concept of designing self-healing materials are as follow:
- Microcapsule Embedment
- Hollow Fiber Embedment
- Microvascular System
Cross-linking, which is an irreversible process, of polymeric materials is performed to achieve superior mechanical properties, such as high modulus, solvent resistance, and high fracture strength. However, it adversely affects the refabricating ability of polymers. Moreover, highly cross-linked materials have the disadvantage of brittleness and have the tendency to crack. One approach to bring processability to cross-linked polymers is the introduction of reversible cross-links in polymeric systems13. In addition to refabricating and recyclability, reversible cross-links also exhibit self-healing properties. However, reversible cross-linked system does not show self-repairing ability by its own. An external trigger such as thermal, photo, or chemical activation is needed to achieve reversibility, and thereby the self-healing ability. Thus, these systems show non-autonomic healing phenomenon. Following are different approaches that are considered to bring reversibility in cross-linked polymeric.
- Diels–Alder (DA) and Retro-DA Reactions
- Supramolecular Polymers
Polymeric properties in traditional polymers are achieved due to the length and entanglement of long chains of monomers, which are held together by covalent bonds. Recently, low molar mass monomers are assembled together by reversible non-covalent interactions to obtain polymer-like rheological or mechanical properties15. As non-covalent interactions can be reversibly broken and can be under thermodynamic equilibrium, this special class of macromolecular materials, that is, the so-called supramolecular polymers show additional features compared to usual polymers. These features include switchable environment-dependent properties, improved processing, and self-healing behavior.
Self-healing microcapsules and slow release microspheres in paints
All over the world the corrosion is a serious problem that could be controlled by different methods. Classical methods that can reduce the corrosion rate are cathodic protection16, passivation17, and barrier protection caused by coatings18. Coatings on metal surfaces provide a barrier against the corrosive species. The effectiveness of the coating depends on its chemical composition, on the parameters they are developed and on the permeability towards corrosive species.
Anticorrosion protection could be improved by combination of different anticorrosion techniques. Smart coatings that could be alternatives to traditional ones can improve the protection of metals, alloys and provide superior resistance to corrosion especially then, when the coating is mechanically or chemically stressed. When the paint is damaged either by mechanical forces or by changes in the environment (pH, heat, light) the anticorrosion additives involved into the dry paint cannot further control the corrosive processes. One resolution is when the active anticorrosion chemicals are involved into the coating19. Another possibility is when the inhibitors and sealants are encapsulated. The idea of microencapsulation of active ingredients was taken from the nature.
In case of paints microcapsules are added to the coating material. When a mechanical damage weakens the coatings, the microcapsules will release their protective agents and heal the coating by preserving the barrier properties. The different copolymers (often urea-formaldehyde ones) can form the wall of containers.
The active components will be free on pressure (mechanical destroy, abrasion, stress), on shell degradation (change in pH/temperature/ ionic strength) and on rupture after swelling (solvent effect). There are a number of advantages in using microencapsulated chemicals. A primary advantage is the ability to extend the life expectance over a period of time to avoid the need for frequent treatments. The other benefit of application of microcapsules in coating systems is that the core material is protected from the environment and their release is controlled, sustained or timed.
The microencapsulation is a combination of materials. The two main components of a capsule are the shell and the core. The shell must be compatible with the components of paints; it could be permeable, semi permeable or non-permeable. A permeable shell allows slow release of active additives from the core. This is good in those cases when elongated active period is expected (e.g. in case of antifouling paints). When the shell is non-permeable, the active ingredient will get freed on special environmental effects like mechanical injury, change in temperature, pH, etc. The chemicals of core could be pigments, catalysts, polymeric substances, etc. The variety of chemicals in microcapsule shells is very wide which allows the application of the most suitable ones in all cases. The size of the microcapsules influences the permeability, diffusion and the controlled release of active components of the core.
There are a lot of additives used as antifoulants20. The most effectives are tin-derivatives but because of their toxicity they are banned. Nowadays several commercial products (e.g. Zn pirithyon) and paints with high Cu2O concentration (which limits its use because of the copper accumulates in the natural waters) are used.
Healing performance — microcapsules in paints
After mechanical crack the microcapsules incorporated into paint should break immediately and release the healing core material. This self-healing process when a mechanical injury is followed by the escape of the core material from the capsule is shown in Fig.9. Fluorescence microscopic images of injured capsules show how the core material invades its environment, fill the gap, and heal the destroyed surface. The release of the oil from microcapsules can be seen in mentioned Fig. Capsules have a green halo because of the fluorescence. There are two perpendicular scratches (cut with lancet) as deep as the thickness of the paint. As the figure shows, when the paint is scratched the self-healing agent leaks and additionally fills the gap. When the coating with microcapsules will be mechanically damaged, along the scratch line the gap will be covered by the core material and it can prevent the metal surface from a direct contact with aggressive environment and so from a further corrosive deterioration.
Effectiveness of self-healing coatings
The corrosion of a metal is caused by aggressive materials and oxygen in presence. When the coating cracks, the accessibility for corrosion accelerating components to the metal-coating interface is easier. But when microcapsules are involved into the paint, due to the healing process, the oil and the inhibitor from the capsules will be freed, activated and the consequence is that they prevent the metal from the corrosion.
Antifouling microspheres and coating
The ban of tin-based antifouling paints increased the application of formulas with Cu2O more and more. But it also has a disadvantage, i.e. in these paints the concentration of Cu2O is high. The consequence is that the copper load of the natural waters increases.
Applications of self-healing materials are expected almost entirely in all industries in future. The very few applications being developed to date are mainly in the automotive, aerospace, and building industries. For example, Nissan Motor Co. Ltd has commercialized world’s first self-healing clear coat for car surfaces. The trade name of this product is ‘‘Scratch Guard Coat’’9. According to the company, this hydrophobic paint repairs scratches (arising from car washings, off-road driving, or fingernails) on coated car surfaces and is effective for a period of three years. This newly developed paint contains high elastic resins that prevent scratches reaching the inner layers of a painted car surface. Depending on the depth of the scratch and the temperature in the surrounding environment, the entire recovery occurs between 1 and 7 days.
Another example in this category is the two component polyurethane clear coats from Bayer Material Science22. The trade names of the raw materials used to formulate this coating are Desmodur and Desmophen. According to company sources (Fig.10), this coating heals small scratches under the influence of heat (sunlight) and the trick employed to design such coatings is based on the use of dense polymer networks with flexible linkages. For both the above examples the scratch discussed is in the range of few micrometers, which is obviously visible to the naked eye, and therefore the products are suitable for keeping the aesthetics of the coating. Energy required to overcome the resistance of materials to create a scratch is higher in the case of thermosetting polymers compared to thermoplastic polymers. In case of thermoplastic polymers, the energy is lost in the process of viscous flow in the absence of residual stress.
Thus the most important driving force that helps the reflow of materials from the side to the groove is surface tension. However, for thermosetting polymers, the energy (below it yield’s strength) incorporated to create a scratch is stored in the neighborhood of the conduit. When the mechanical stress is removed, the stored energy is relieved and the distorted polymer chains returns leveling the groove. This recovery process is highly dependent on the mobility of the polymer chains that is on their glass transition temperature (Tg).
However, while scratching, if the mechanical stress also leads to cracking besides scratch formation, the stored energy will be released at the inappropriate time and a partial recovery may be expected as surface tension-driven viscous process will not take place here due to the presence of opposing elastic force in the system. Thus, scratch with fractures is a permanent damage for thermosetting polymers, and therefore a compromise has to be considered between the above two processes for designing self-healing polymer coatings. External trigger can be useful in this case. Thus, polymers with high (Tg) in combination with high elastic response could be an option for the recovery of small scratches. In case of small fractures, triggering by temperature will enhance the mobility of the polymer chains and surface tension will play an important role for self-healing.