Michel Biron

Exposures to harsh environments easily lead to degradations of polymers, paints and polymer composites. Development of micro-cracks followed by their coalescence can induce catastrophic failures of the plastics, rubbers and composites, and hence can significantly shorten structure lifetimes. For other applications such as paints, scratches are damaging from an aesthetic point of view and can lead to corrosion.

Mimicking the nature, self-healing polymers and polymer composites target the partial or total repair of these injuries allowing a functional usage. The following figure 'Self-Healing-Scheme' displays a general scenario.

According to the used ways, we can distinguish in-situ and extrinsic self-healing polymers.

In-situ self-healing polymers and polymer composites

An external event damages the polymer material and causes the release of healing agents. Then an external parameter promotes or speed up the healing reaction leading to a repaired material recovering partial or total properties of the virgin product.

Extrinsic self-healing polymer and polymer composites

The beginning of the scenario is the same as above: An external event damages the polymer material but an external healing agent must be brought to the polymer product. Then an external parameter can be necessary or not to start the healing reaction leading to a repaired material recovering partial or total properties of the virgin product.

The repair can be permanent or temporary needing, in this last case, an ulterior consolidation by conventional methods.

External aggressions can be:

Resulting damages are diversified:

Healing phenomenon can come from:

The external parameters are often heat and sometimes UV.

The property recovery is often limited and differently affects the various properties. The repair can be permanent or temporary needing in this last case an ulterior consolidation by conventional methods.

Finally there are many occurrences:

In-situ healing items:

Self-healing polymer:

Healing ingredients:

Extrinsic healing agents:

Chemically responsive: heating leading to curing or polymerization

Physically responsive: for example water leading to hydroswelling...

The chemical way through embedded reactive healing ingredients

The chemical way is the most often studied ant theoretically the easiest. A two-part healing system is used. One part can be directly added to the matrix and the other part is encapsulated in such a way that, on the one hand, it cannot react during the usual life, whereas, on the other hand, it reacts when an external aggression breaks the capsules and releases the encapsulated healing agent. It is also possible to separately encapsulate each part of the healing system. The following figure 'Action-Mode-of-Encapsulated-Healing-Agents' shows this last case.

One of the issues is to control the integrity of micro-capsules during the processing and the current life of the polymer.

Instead of micro-capsules, healing agents can be located in hollow glass fibres filled with uncured resin systems that bleed into a damage site upon fibre fracture. Once cured, they provide a method of crack arrest and recovery of mechanical integrity.

Carbon nanotubes also have the potential to act as containers for self-healing liquids but their high cost is a serious issue.

In another way, the healing reaction can be started by a voluntary overheating or by a UV exposition activating a chemical reaction of curing or polymerization.

Further to the healing function, it is also possible to introduce UV fluorescent dye into the healing resin, which will illuminate any damage/healing events that the structure has undergone, thereby simplifying the inspection process for subsequent more permanent repair.

It must be noted that repairing is possible only one time at a defined spot because the micro-capsules or hollow fibres are already broken by the first damage.

The physical way through encapsulated solvent: a promising way for thermoplastics

The Dutch Polymer Institute, Faculty of Aerospace Engineering of Delft University of Technology, Industrial partners such as DSM Innovation Center BV, Teijin Aramid, SABIC Innovative Plastics BV and DOW Benelux BV work on the encapsulation of specific solvents to obtain self-healing thermoplastic materials. After they have succeeded in demonstrating self-healing in thermoplastic polymers, they are studying the industrialisation. In this tender they propose a promising solution to equip industrially processed engineering plastics with a self-healing mechanism.

Wang Xufeng (Materials Science & Engineering, Faculty of Science, UNSW) studies several candidate single-part healing agents including cyanoacrylate, polyurethane, silane and polystyrene cements. Silanes are the most promising from storage stability and healing efficiency points of view but substantial additional efforts are needed to achieve industrial applications. The silane 3-[tris(trimethylsiloxy)silyl]-propylamine leads to a 22% strength recovery and a suitable storage stability. Healing capacity of polystyrene was poor and stabilities of cyanoacrylate and polyurethane were insufficient.

The physical way of intrinsic self-healing polymers

For intrinsic self-healing polymers, the healing process can goes through several ways: - surface rearrangements affecting diffusion and topological features; - wetting, - diffusion and reptation of polymer chains, - randomization, ensuring disappearance of cracking interface. For example, Jud and Kaush ( Polymer Bulletin, 1, 1697-1707, 1979) tested crack-healing behaviour in a series of poly(methyl methacrylate) (PMMA) and poly(methyl methacrylate-comethyl ethylacrylate) (MMA-MEA copolymer) samples of different molecular weights and degrees of copolymerization. They induced crack healing by heating samples above the glass transition temperature under slight pressure and found that full resistance was regained during short term loading experiments. That was attributed to interdiffusion of chains and formation of entanglements for the glassy polymer.

Heating can be the consequence of mechanical treatments. E.g: following Kalista S. J., Ward T. C. (Journal of the Royal Society: Interface, 4, 405-411, 2007) poly(ethylene-co-methacrylic acid) (EMAA) films are self-healed upon ballistic puncture. This occurs through a heat generating frictional process, which heats the polymer to the viscoelastic melt state and provides the ability to rebond and repair damage. In contrast, low speed events cannot generate a sufficient heating to lead to a self-healing behaviour.

Chemical ways for intrinsic self-healing polymers

Reaction of unreacted species, broken molecule rebuilding, labile bonds in lieu of chemical bonds

Self-healing paints penetrate automotive industry and other applications

Paints and clear topcoats are exposed to scratches from car-washing machines, off-road driving and fingernails… Consequently automotive clear topcoats are required to possess a number of performance characteristics, including toughness, durability and excellent resistance to chemicals and scratching. But that is insufficient and automakers and paint suppliers develop new paints and coatings having those properties combined with self-healing characteristics allowing to attenuate or obscure scratches. We have chosen three examples among others.

Nissan's "Scratch Shield" paint or "Scratch Guard Coat" was developed together with the University of Tokyo and Advanced Softmaterials Inc. It contains high elastic resin that helps prevent scratches from affecting the inner layers of a car painted surface. With this self-healing coat, a car scratched surface will return to its original state anywhere from one day to a week, depending on temperature and the depth of the scratch. The water-repellent paint also has a higher resistance to scratches compared with conventional clear paints. A vehicle painted with "Scratch Guard Coat" will have only one-fifth the abrasions caused by a car-washing machine compared with a car covered with conventional clear paint. "Scratch Guard Coat" is effective for about three years. The paint can be applied to car chassis, bumpers, door mirrors, among other parts. The following figure 'Car-Painted-with-Scratch-Guard-Coat' shows, in the red boxes, the evolution of new scratches on the hood of a car (left view) and the same hood after one week (right view).

'Recover' is a clear 2-pack isocyanate cured polyurethane coating, which is spray applied over any HMG basecoat system, available in virtually any colour. It is also 'water clear', which means no slight yellowing effect, highly accurate shade matching and excellent colour retention. Market experience has shown that polyurethane (PUR) coatings based on aliphatic isocyanates are suitable for these coatings but it is necessary to make a compromise between scratch resistance and chemical performance. Scratching subjects the coating to considerable localised mechanical stress and any recovery starts shortly after stress removal, due to the visco-elastic deformation behaviour of polymer networks, known as 'reflow'.

'Recover' is a flexible, yet tough and durable 2K PUR coating, which possesses optimum chemical and scratch resistance characteristics, as well as inherent self-healing or 'reflow' properties. This self-healing nature allows the paint film to recover and obscure any minor marks and scratches caused by mechanical washing, within a time period of anything from 5 minutes to an hour or so, depending on severity. To demonstrate this effect, HMG has even produced sample Recover-coated panels, which technical and sales personnel can abrade with Scotchbrite pads allowing to see the coating recovering before their eyes.

The unique self-healing quality of the Nissan's "Scratch Shield" paint or "Scratch Guard Coat" is also ideal for mobile phone and other various non-automotive applications susceptible to scratches over daily use. NTT DoCoMo, Inc. will license the Scratch Shield for its mobile phones to be introduced in Japan, which will be a value-added feature for customers.

Reverlink™ by Arkema: the breakthrough of the Supramolecular Chemistry resulting from the collaboration of industry and advanced research by CNRS and ESPCI

Traditional or molecular chemistry focuses on the synthesis of molecules by creating covalent bonds between atoms. These « chemical » bonds are strong and generally speaking, are not reversible. So, conventional polymers are barely fluid as well as difficult to process by hot moulding or extrusion.

In supra-molecular chemistry, the bonds holding together small or large molecules are weaker physical bonds, which are reversible. Arkema's R&D focuses in particular on its applications in the field of polymers, and, with regard to hydrogen bonds imparting additional cohesion to solid or liquid bodies. These weak « physical » bonds are reversible and can be broken up by a trigger factor such as temperature. So, supramolecular polymers are easier to process.

With the supra-molecular polymers Arkema is working on two ways, the association of small molecules through reversible hydrogen bonds, or the modification of polymers through grafting to make them associative in a reversible mode thanks to hydrogen bonds.

The supra-molecular approach with polymer molecules made « associative » by hydrogen bonds, allows to envisage medium size polymer molecules that will remain relatively fluid at processing temperatures (opening of hydrogen bonds which do not impair fluidity), while imparting to the materials outstanding mechanical strength and resistance to solvents at service temperatures (closed hydrogen bonds which impart cohesion).

Using patented Arkema's supramolecular technology, the Reverlink™ range covers various products. One can obtain a polymer-type behaviour from small or medium-sized molecules linked through thermo-reversible physical bonds. Arkema proposes new semi-crystalline resins, or elastomeric materials based on renewable and supramolecular chemistries, with very unique properties like a self-healing capacity.

The Reverlink™ range includes bio-based products suitable as additives in many types of formulations with properties including self-healing, adhesion promotion on plastics, metal, glass...; Corrosion protection of metals, creep resistance, solvent resistance etc.

Markets and applications are diversified, such as additives for water-borne paints, epoxy resins and other polymers; oil & gas industries, sports goods, adhesives etc.

Two-parts self-healing composites: For what property recovery?

The question is obvious but the answer is very vague because of the large range of damaging events, the diversity of studied composites, the nature of properties used for the recovery rating, the nature of healing agents, the healing process and so on.

Really, a statistical analysis of actual recovery percentages (see table 1) measured on 'satisfying' experiments shows a contrasted panel with property recoveries ranging from 26 up to more than 100%. For a mean of 77%, minimum is on the order of 26/33% and maximum is about 110/120%. A recovery superior to 100% is obtained with healing agents of a different type that the matrix.

Statistical analysis of actual recovery examples

For a given study using a defined damaging event, recovery percentages can vary from 38 up to 82% according to the measured property, the healing temperature and the rating mode taking into account the mean value or the maximum data.

Self healing silicones

Dow Corning proposes several self-healing gels including 3-6636 Silicone Dielectric Gel and Sylgard 527 Primerless Silicone Dielectric Gel. These two-component, transparent silicone encapsulants cure in place to form a cushioning, self-healing, resilient gel-like mass. The cured gel is claimed to be self-healing while developing much of the dimensional stability and nonflow characteristics of a solid elastomer.

Michael W. Keller, Scott R. White, and Nancy R. Sottos (Adv. Funct. Mater. 2007, 17, 2399-2404) report the development and testing of a low modulus, self-healing PDMS by microencapsulation in poly(urea-formaldehyde) (UF) of the constituent resin and initiator for PDMS in two separate microcapsules. The resin microcapsules contain high-molecular weight vinyl functionalized PDMS and platinum catalyst complexes. Initiator microcapsules contain a PDMS copolymer with active sites that will link to the vinyl functionalized resin via the action of the platinum catalyst. Both types of microcapsules are then embedded in the PDMS matrix to produce a self-healing composite material. After the researchers, tear testing demonstrates the capability of the self-healing elastomer to routinely recover at least 70% or even 100% or greater of the original tear properties.

A new way toward repeated and easy recyclings: Inherently recyclable plastics

Antonius A. Brokehuis, Youchun Zhang and Francesco Piccioni at the University of Groningen in the Netherlands study thermally self-healing thermoset polymers based on furan-functionalised, alternating thermoset polyketones (PK-furan) and bis-maleimide using Paal-Knorr (PK), Diels-Alder (DA) and Retro-Diels-Alder (RDA) reactions to form and subsequently break polymeric crosslinking.

The polymer can be crosslinked to make moulded parts and then moulded again to fabricate other new parts at temperatures of around 150°C.

During the reshaping process, the polymers are in a thermoplastic state and then crosslink again to form again a rigid, structural, crosslinked network by cooling.

Multiple times self-healing composite: Mimic biological pervasive vascular network to supply the necessary healing components

Nancy Sottos's team at the University of Illinois create a bilayer composite: The epoxy base layer includes a network of interconnected horizontal and vertical channels using a technique called direct-write. These channels are filled with the healing agent, a low-viscosity, monomeric dicyclopentadiene (DCPD). The second layer is a solid epoxy top layer covering the vascularised base and incorporating the Grubbs' catalyst.

When a crack of the top layer reaches the underlying channel network, the contained healing agent (dicyclopentadiene) exudes into the crack and comes into contact with the catalyst dispersed into the top layer. In about 10 hours, the polymerized DCPD fills the crack. The DCPD flows out by capillary action and doesn't need any external pressure.

The researchers are able to crack and re-heal the surface as many as seven times before the catalyst wears out and stops working. New developments target many more times crack/re-heal cycles using a two-part system that injects both a healing agent and a catalyst into the crack.

The cost of these composites might keep them limited to certain high-value, high-performance applications concerning air- and spacecraft.

At the cutting edge of technology for next generations of high-performance composites for aerospace and defense

T. Duenas, E. Bolanos, E. Murphy, A. Mal, F. Wudl, C. Schaffner, Y. Wang, H. T. Hahn, T. K. Ooi, A. Jha, and R. Bortolin (MULTIFUNCTIONAL SELF-HEALING AND MORPHING COMPOSITES) combine the use of mendomer materials developed by UCLA and the addition of magnetic microparticles and nanoparticles. The highly cross-linked polymer (Furan-Maleimide crosslinked solid) has thermally reversible linkages, resulting from multiple Diels-Alder (DA) connections. Magnetic microparticles and nanoparticles allow to heat the composite up to its glass transition temperature by application of a magnetic field at a specified frequency depending on the used particles. Actually, micro-graphs proof that cracks can be healed.

Due to the self-healing activation by heat, mendomer self-healing materials may have anti-fatigue properties. During cyclic loading the heat that is generated in the fatigue zone may be adequate to heal cracks after they are formed.

Mendomer materials are also expected to be highly resistant to damage caused by explosive loading since the cracks formed during loading are likely to heal while they are subjected to the temperature field generated by the explosion. Lastly, these materials can have morphing properties.

Conclusion

Exposures to harsh environments easily lead to degradations of polymers, paints and polymer composites and can significantly shorten structure lifetimes. Mimicking the nature, self-healing polymers target the partial or total repair of these injuries allowing to assume a temporary or permanent functional usage.

Multiple chemical or physical ways are investigated: In-situ healing systems, self-healing polymers, embedded healing agents, extrinsic systems.

The chemical ways through embedded reactive healing ingredients are the most often studied using two-part healing systems, micro-encapsulated or contained in hollow fibres or CNTs.

The physical way using encapsulated solvent is demonstrated for thermoplastics but needs further studies for industrialisation.

The physical way of intrinsic self-healing polymers is based on surface rearrangements, diffusion, wetting, reptation, randomization of macromolecules ensuring disappearance of cracking interfaces.

Chemical ways for intrinsic self-healing polymers include reaction of unreacted species, broken molecule rebuilding, creation of labile bonds in lieu of chemical bonds.

Among the industrialised breakthroughs we can quote, for example, self-healing paints penetrating automotive industry, self-healing silicone gels for electronics, the Reverlink™ range of products by Arkema based on the Supramolecular Chemistry.

Bayer, Nissan, HMG Paints and others develop and commercialise self-healing 2-component polyurethane topcoat self-repairing scratches on painted car surfaces, including scratches from car-washing machines, off-road driving and fingernails.

In supra-molecular chemistry, the bonds holding together small or large molecules are reversible physical bonds. Arkema's R&D focuses on its application in the field of polymers, marketing the Reverlink™ range including semi-crystalline resins and elastomeric materials based on renewable sources.

Advanced composites are a promising application field and promote a multitude of studies concerning, for the more sophisticated technologies, multiple times self-healing composites mimicking biological pervasive vascular network to supply the necessary healing components or high-performance composites for aerospace and defense combining the use of mendomer materials and magnetic microparticles and nanoparticles allowing to heat the composite up to its glass transition temperature by application of a magnetic field.

Inherently thermally self-healing thermoset polymers open the way to endless recycling thanks to alternate Diels-Alder (DA) and Retro-Diels-Alder (RDA) reactions forming and subsequently breaking polymeric crosslinking. That is a new step toward the cradle-to-cradle concept.

References