Inspired by natural biological systems, continuous efforts are being made to mimic natural materials and integrate self-healing capabilities into coatings, polymers and polymer composites.
Research on self-healing materials and coatings has grown significantly in the last decade, as they are an attractive option for extending asset life and improving safety in response to environmental changes. The need for sustainability use of oil-based materials is of increasing importance and the use of self-healing materials and coatings can greatly increase their lifetime.
Currently available self-healing materials with strong mechanical properties require energy input to trigger the healing process while the materials with autonomous self-healing ability are not robust enough for practical applications.
Main application markets for self-healing materials and coatings are:
- Automotive coatings that heal post damage and restore functionality).
- Aerospace composites.
- Consumer mobile devices (mainly scratch repair at present)
- Wind turbine blades.
- Concrete capable of repairing cracks.
- Self-healing batteries for wearable electronics.
- Self-healing elastomers (seals, tires).
What are self-healing materials?
Self-healing materials and coatings can heal or repair themselves automatically and autonomously from damage (e.g. mechanical or corrosion) without any external intervention. This process leads to the (partial) restoration of the original properties of these materials, in particular the mechanical properties.
Their investigation has largely been inspired by fracture healing that occurs as a self-repair process in nature. The encapsulated-monomer/catalyst, supramolecular self-assembly, and reversible or covalent bond formation are the main strategies.
Extrinsic and intrinsic and self-healing strategies can be employed to mitigate the effects of local damage to (partially) restore a lost property or functionality and to avoid premature catastrophic failure of the entire system.
Extrinsic self-healing materials rely on the release of healing agents which are polymerized to heal the damage. These materials generally comprise capsule based and vascular systems. Compared to the intrinsic system, the extrinsic system has a major advantage in healing larger damage volume.
In capsule-based self-healing materials, small capsules containing a liquid able to fill and close cracks are embedded under the material surface. When the material is damaged, cracks cause some capsules to rupture, releasing the liquid and closing the gap.
For vascular self-healing materials, the capsules are replaced by an embedded microvascular network similar to a tunnel network, containing a fluid healing agent and dispersed catalyst particles.
The healing agents fill the gap when a crack occurs and breaks the vascular network, allowing the healing agent to fill the crack and react with the catalyst. The mechanism and behaviour of healing agents are fundamental to the recovery process and restoration of mechanical properties. Healing is repeatable due to the vascular nature of the healing agent supply.
Another self-repairing strategy relies on utilizing reversible chemical bonds or physical interactions. Intrinsic self-healing materials contain a latent functionality that triggers self-healing of damage via:
- thermally reversible reactions
- dynamic covalent bonds (e.g. the Diels–Alder reaction)
- radical-based systems.
- supramolecular interactions, such as hydrogen or halogen bonds
- ionomeric arrangements
- ionic interactions.
- π–π interactions.
- host–guest interactions.
- metal–ligand interactions.
Figure 1: Schematic of self-healing polymers. Capsule based (a), vascular (b), and intrinsic (c) schemes for self-healing materials. Red and blue colours indicate chemical species which react (purple) to heal damage. Image credit : Blaiszik et al, 2010.
The matrix is self-healing inherently and thus no release of self-healing agents is required. Examples include:
- swelling of shape memory polymers.
- melting and solidification of thermoplastic materials.
- increasing viscosities of pH-sensitive micro-gels.
This method avoids problems of integration and healing-agent compatibility that are found in capsule and vascular based healing systems.
Such self-healing system can withstand repetitive damage due to their intrinsic stimuli-responsive self-healing properties. By employing the reversible but relatively strong dynamic covalent bonds, synthetic self-healing materials based on the equilibrium of bond breaking and reforming possess both structural and mechanical stability, which is well-suited for self-healing purpose
In terms of healing volume:
- Microvascular: can heal small to large volume due to its interconnected network;
- Capsule: can heal small to moderate volume
- Intrinsic: the proximity of the damaged surfaces is important hence limit the healing volume for efficient healing.
Another crucial element of self-healing is multiple healing cycles:
- Microvascular: can heal multiple times as long as the healing agent supply is available.
- Capsule: healing ability exhausts when the healing agent is depleted by healing events.
- Intrinsic systems: can heal multiple times -however the efficiency decreases for each cycle at the same crack path
The healing of the three systems can be triggered by damages within which the efficiency depends on the environmental factors, damage types and the nature of the materials. For example, temperature and damage surfaces are critical factors for intrinsic systems.
Table 1: Comparison of self-healing systems.
|Microcapsule System||Microvascular System||Intrinsic System|
|Healing agent||Required||Required||Doesn’t use healing agent|
|Multiple healing||x||o (require supply of agent)||o|
|Healing volume||Limited by amount of agent||Broad if have enough supply||the opening need to contact|
|Shape limitation||Limited by capsule size||Limited by the embedded network||Without limit|
Source: Future Markets.
It is required of new-generation protective coatings to intelligently respond to mechanical or chemical damage caused by the external environment and to reproduce their original properties, including their adhesion to the substrate and integrity. Self-healing protective coatings are produced using macromolecular compounds, ceramics, metals and composites and allow for:
- Increased asset service life.
- Ease of repair.
- Lower environmental impact.
- Improved part functionality and reliability.
- Improve public safety.
The self-healing action of such coatings is activated by appropriate stimuli: temperature changes, radiation, pH changes, pressure changes and mechanical action.
The surfaces of metal products are protected against the detrimental effect of the corrosion environment by organic or inorganic metallic coatings. However, these coatings degrade over their lifetime and the most effective corrosion systems are based on chromium(VI) compounds. Their use is now restricted by regulation, leading to opportunities for corrosion protection self-healing coatings that automatically repair and prevent corrosion on underlying substrates. These coatings feature both passive characteristics (from matrix material) and active behaviour towards the local environment (through incorporated or surface-mounted compounds acting as inhibitors).
An approach for solving the problem of scratches and/or abrasion of organic glasses is to protect the lenses with self-repairing coatings that can revert completely or partially to the initial non-scratched condition. Several companies have developed scratch repair self-healing coatings, with focus on automotive and consumer electronics sectors.
Self-healing polymer composites
Brittle polymers (i.e. epoxies) used in the aerospace and other sectors are particularly susceptible to damage in the form of cracks which form deep within the structure where detection and repair is nearly impossible. By autonomously healing without human intervention when damage is introduced, self-healing polymers reduce maintenance costs, increase safety, and extend the life-cycle of polymer composite parts.
Self-healing composite materials include polymer matrix composites, ceramic matrix composites (CMCs), metal matrix composites (MMCs) and cementitious composites. Polyurethanes and polyureas have been utilized for intrinsically self-healing polymers as they feature hydrogen bonds. These weak bonds are highly dynamic, and the shape-memory effect of polyurethanes can be utilized to enhance self-healing.
Figure 2: Self-healing mechanism of polymers.
The microscopic defects that are distributed randomly in metals are not only hard to detect, but also may cause catastrophic failure, leading to safety issues and reduced service life for structures. Therefore, development of self-healing metals is desirable, but is more challenging than in polymers. Metals do not have the ability to probe and identify damage, and their atomic mobility or diffusivity is so low and melting points are so high that a healing process is difficult to activate. Consequently, introducing energy and matter into the metallic materials become a prerequisite for triggering necessary interactions as well as healing.
Metal matrix composites
Metal matrix composites (MMCs) are metals or alloys are reinforced through the addition of particles, whiskers or fibres to specifically tailor properties. These materials can be tailored to be lightweight and with various other properties including:
- High specific strength and specific stiffness
- High hardness and wear resistance
- Low coefficients of friction and thermal expansion
- High thermal conductivity
- High energy absorption and a damping capacity.
Shape memory self-healing MMCs are under development for application in aerospace and automotive components.
The ability to oxidatively heal surface cracks in elevated temperature ceramics and metallo-ceramics, thus restoring mechanical strength is desirable. Self-healing in ceramics is mainly induced by the passive oxidation of a non-oxide ceramic-the healing agent (such as SiC)-pre-incorporated into the ceramic matrix.
When micro-cracks are initiated in such composites, unreacted SiC on the fracture surface oxidizes at elevated temperatures. Then, the newly formed oxidation products fill the crack-gap volume and re-bond to the fracture surfaces, resulting in complete recovery of the deteriorated strength to their initial or even more robust states. In this system, oxidizability of the healing agent under the operating conditions is a key determinant of the strength recovery rate
Studies on self-healing elevated temperature ceramics have mainly focused on extrinsic self-healing, in which the crack filling reaction is due to the presences of discrete reactive particles homogeneously distributed in an inert ceramic matrix. Recent research has focused on intrinsic self-healing.
Carbon nanotubes (CNTs), graphene and other 2D materials can be used as nanofillers to obtain nanocomposites of exceptional mechanical, electrical, thermal, and self-healing properties with the added advantage of lower weight. The advantages of nanomaterials are based on their high specific surface area, high stiffness and This allows for multi-stimuli-responsive materials with tailored functionality for high-performance applications in:
- smart textiles and apparel.
- smart medical devices.
- flexible electronic devices.
- sensor and actuators.
- self-deployable and self-folding structures in aerospace.
- self-healing structural health monitoring.
- self-healing coatings.
Biocompatible self-healing materials demonstrate potential for applications in:
- drug delivery hydrogels for injectable controlled release.
- soft actuators able to resist mechanical impacts.
- self-healing biosensors for electronic skin
- tissue and organ repair.
- shape memory functions.
3d printing of self-healing materials
The use of 3D printing opens opportunities for the development of self-healing polymers. 3D printers can to produce complex structures with high resolution for the creation of:
- Microvascular networks.
- self-healing hydrogels.
Self-healing single-walled carbon nanotubes
SWNTs exhibit important electric properties that are not shared by MWNTs. They are also more pliable than MWNTs, yet more difficult to produce cost-effectively, limiting their use to niche/high-priced applications. Properties include:
- Electrical (extremely high in current density)
- Thermal (comparable in specific thermal conductivity to diamonds)
- Optical (emit light in an optical communication band of wavelengths)
- Hydrogen storage capability
- Metal catalyst supporting capability.
SWNTs can be either semiconducting or metallic, depending on their diameters and chiral angles, and therefore have drawn attention as materials for nanoelectronics devices, field-emission displays, energy storage and nanosensor devices.
CNTs are used reinforcing and electrically conducting agents in self-healing materials and polymers. MWNTs and SWNTs have both been utilized and they have been integrated into a self-healing sportscar from Lamborghini in 2017.
Graphene self-healing materials
Graphene possesses a range of unique properties – an exciting electronic character, described as a zero-gap semiconductor, unparalleled strength (breaking strength ~40 N/m, Young’s modulus~1.0 TPa), and record thermal conductivity. Charge carriers, described as massless Dirac fermions, exhibit ballistic movement across submicron distances approaching relativistic speeds, with intrinsic carrier mobilities up to 200,000 cm2 V–1 s–1.
Graphene can maintain current densities six orders of magnitude greater than that of copper. All of this can be achieved with little electronic noise (exhibiting little extraneous noise from outside sources), which is increasingly important as microelectronic devices continue to shrink in size. With thickness on the order of atoms, graphene has a high surface area-to-volume ratio while maintaining incredible flexibility.
As additional layers are introduced, the structure becomes increasingly complex, resulting in more distinct and/or unique behaviour. Depending on the number of layers, the magnitude of the electric field applied, and the edge orientation, the band gap of the material can be engineered to achieve a wide range of values.
Due to these outstanding properties graphene has been explored for application in self-healing polymers:
- Used as triggers in click polymerization.
- Sensing for self-healing.
- Hyperbranched polyurethane (HPU) nanocomposites as reinforcing fillers.
- Rapid, self-healing reversible polymers.
- Self-healing hydrogels.
- Lignin-modified graphene self-healable, UV-resistant, and conductive polyurethane nanocomposite coatings.
Shape memory composites
Shape memory composites including shape-memory polymers (SMPs), alloys, hybrids, ceramics and gels. Properties include:
- low density
- potentially recyclable at relatively low cost
- high recoverable strain within a wide range of stimuli
- chemical stability and modification
- biocompatibility and biodegradability with an opportunity to adjust the degradation rate.
SMPs are also programmable (multi)stimuli-responsive shape-changing polymers and possess the can recover their initial shape upon direct or Joule heating, radiation and laser heating, microwaves, pressure, moisture, solvent or solvent vapours and change in the pH values.
SMPs suffer from several drawbacks including:
- comparatively low tensile strength and stiffness
- actuation restricted mostly to heat-related treatment and lack of proper function
- low thermal conductivity
- inertness to electrical, light and electromagnetic stimuli accompanied
- slow response ability and low recovery time during actuation.
This limits application in high-performance applications necessitating the use of nanomaterials such as carbon nanotubes in the polymer matrix to enhance thermal, mechanical and electrical properties.
Markets for self-healing materials and coatings
Self-healing coatings are desirable for numerous industrial applications as they can increase materials lifetime, reduce replacement and labour costs, improve product safety and disruption associated with the recoating of surfaces. The automotive, aviation and aerospace and military industries have benefited from their development. Self-repairing car paints have been developed in which temperature triggers the repair action.
Figure 3: Microspheres incorporating self-healing materials.Figure 3: Microspheres incorporating self-healing materials.Image credit: Nature.
Among commercialized self-healing coatings are polymer nanocomposites coatings that heal both surface scratches and mesoscopic damage (e.g., micro-cracks and cavitation). These have been applied in automotive coatings, marine surfaces and consumer electronics. Microcapsulation is a common approach for self-healing coatings, whereby an active ingredient is only released when triggered by external stimuli. For example, a self-healing coating may release a healing agent when the coating is mechanically damaged, automatically fixing microflaws that can allow corrosive agents to penetrate the coating. Main markets for self-healing materials and coatings include:
- Structural health monitoring.
- Concrete, cementitious products, architectural blocks.
- Utility pipes (e.g. sewage).
- Metallic framework.
- Chemical plants.
- Pulp and paper mills.
- Oil and Gas refineries including down hole.
- Off-shore oil drilling platforms,
- Hydroelectric & Power.
- Mining (ore processing, surface and underground mining, drilling).
- Wind power at sea.
- Solar cells.
- Flexible energy harvesting.
- Implants and devices for bone tissue engineering.
- Drug delivery.
- Artificial skin.
- Shipping containers.
- Brakes, valve trains, bearings, and gears.
- Automotive coatings and composites.
- Interior/exterior of ships.
- Underwater structures.
- Military equipment.
- Navy ships.
- Military vehicles.
- Anti-corrosion coatings.
- Aerospace equipment.
- Aerospace coatings and composites.
- Anti-corrosion, electric conductivity, and durability of metallic components used in PEM fuel cells.
- Flexible batteries and supercapacitors.
- Anti-corrosion pipes.
- Flexible electronics.
SELF-HEALING MATERIALS COMPANY PROFILES
Advanced Softmaterials Inc.
The company’s SeRM Super Polymer SH Series can be utilized in self-healing polymers. The materials are polyrotaxane derivatives having radical crosslinkable functional groups. http://www.asmi.jp/en/product
Autoclear LV Exclusive is a self-healing clear coat used in automotive coatings. https://www.akzonobel.com/
Reverlink® supramolecular technology enables polymer-type behaviour from small or medium-sized molecules linked through thermo-reversible physical bonds and/or polymers with properties such as self-healing and extreme dampening. http://www.arkema.com/
The company offers self-healing agents for elastomeric, thermosetting, and powder coatings and has developed an epoxy primer developed in partnership with a leading U.S.-based paint manufacturer that features AMI’s proprietary self-healing technology. http://www.autonomicmaterials.com/
Self-healing oligomer filled microbeads are manufactured by Battelle to mitigate corrosion and impact damage. https://www.battelle.org/
The Global Market for Self-Healing Materials and Coatings https://futuremarketsinc.com/global-market-self-healing-coatings-materials/