Technology Development - A focus of materials research has always been the prevention of damage and mitigation of failure in materials. In addition to prevention strategies, new approaches to improve the reliability of materials are also being explored. One of the more novel approaches to improve reliability is the use of "smart materials". Smart materials actively respond to the stimulus of mechanical stress imposed on the material, causing the compromised region to self-heal and restore partially or completely, the properties of the material.

Applications of Self-healing Materials

Applications of self-healing materials span several sectors: composites, building and construction materials, foams, films, coatings, plastics, concrete, etc. A brief study of literature indicates that self-healing materials serve four primary purposes: restoration of mechanical integrity of load bearing materials as in composites and concrete; barrier protection of underlying substrates; preservation of aesthetics of surface appearance of coatings, plastics and films; and defect reduction of sensitive components. Fig.1 summarizes a few applications in each category.

Maintaining the aesthetics of surfaces to scratching and marring is relevant to consumer items like cell phones and electronic notebooks, as well as more expensive items from the transportation industry such as automobiles, boats, trains and planes. Consumers care about the look and feel of objects and are willing to pay a premium for surfaces that can be restored to look aesthetically pleasing. The demands on self-healing coatings are application dependent. Flexibility to design coatings to satisfy requirements such as soft touch, multiple healing cycles, and rapid healing are critical. These requirements must also be linked to a mechanism of healing with an accessible source of external stimulus, the simplest being heat.

Defect reduction is another interesting area where self-healing offers a cost performance benefit. Several applications, such as mending of pin defects in fuel cell membranes, repair of tires, defect reduction in the potting of IC chips or the healing of tears in wire and cable components, can benefit from the use of self-healing materials. In this case, the response of the material is expected to be rapid, and specific to the site, but multiple healing may not be required.

Protection of materials from weather is also of critical importance to pipes, conduits, machinery, plants, steel infrastructure, etc. In particular, corrosion protection via self-healing coatings offers benefits of extended lifetime as well as reduction in maintenance costs of such structures. The processes of defect formation are slow and the long term effects of such deterioration are catastrophic. Design of self-healing systems in these cases requires site specific performance through the formation of physical or chemical barriers to the transport of corroding species and also needs to be robust to withstand weathering and aging of the self healing components prior to damage.

Lastly, self-healing materials have also a role to play in critical functions such as load-bearing. Composites of concrete or thermo sets in a variety of applications are subjected to various stresses during their operation. This leads to the formation of defects in the form of micro-cracks that propagate and merge to form larger cracks. Concomitantly, there is a decrease in the strength of the material. Restoring this strength partially is now possible with the use of self-healing technologies. The design requirements in this case are a system that self-heals as cracks propagate, self-heals several times and a system that survives the forces of weathering and aging in the application.

Self-healing technologies have clearly a significant role to play in the future on extending the reliability and durability of materials across a spectrum of applications. It is therefore important to quantitatively value the use of self-healing materials over conventional materials. One perspective of the value is shown in Fig.2. A key feature is the ability of self-healing materials to prolong the durability and hence the lifetime of the article or structure under multiple healing. Self-healing materials must offer positive contributions from a gain in service time in the application and/or a reduction in downtime for maintenance and repair. However, this is highly application dependant and a lifecycle analysis for each application could provide a quantitative estimate of the value of self-healing systems over conventional materials.

Technology Approaches

Self-healing of polymers by recovery and reflow of elastic networks is easily possible and has been successfully adapted in applications such as wire and cable systems, tires, top coats of automobiles, etc.(Fig.3). Encapsulation technologies have been explored extensively for self-healing in polymer composites, in concrete and are being currently validated for corrosion protection applications (Fig.4).

Crosslinked Networks and Self-healing Thermoplastic Elastomers

Most cross-linked coatings made from polymers show viscoelastic behavior. The elastic component of stress exists as stored energy and can be used to recover from deformation, resulting in self healing. The time-dependent viscous component can also aid in self healing via its memory effects, again assisted by a temperature above the glass transition of the polymer.

Other polymers modified with maleimide moieties promote reversible cross-linking, thus creating a self-healing system. An advantage of "reversible systems" is the ability to self-mend repeatedly, even after several fractures have occurred at the same place. This concept has been extended to the design and synthesis of molecules that associate together to form both chains and cross-links via hydrogen bonds. In striking contrast to conventional cross-linked or thermo-reversible rubbers made of macromolecules, these systems, when broken or cut, can be simply repaired by bringing together surfaces to self-heal. The process of breaking and healing can be repeated many times.

Another interesting self healing concept is possible with elastomers. Unlike traditional mechanisms of inter-chain diffusion of elastomers, a class of poly(ethylene-co-methacrylic acid) (EMAA) copolymers and ionomers have shown the unique ability to instantaneously self-heal following ballistic puncture.

Encapsulant Techniques

Self healing in this case is achieved by the curing of a monomer that is released by the rupture of micro or nano-capsules embedded in the polymer matrix when a crack is initiated. The monomer then reacts with a catalyst mixed in the matrix which is activated either by light, heat or moisture. In their report, white and coworkers employed dicyclopentadiene [DCPD] as the monomer. Other oligomers have also been used as self healing agents including hydroxy terminated poly(dimethyl siloxane) [PDMS]. Researchers have also incorporated epoxy monomer and hardener in separate microcapsules to act as reservoirs for healing. In another example, white and coworkers showed solvent based healing of epoxy at room temperature with encapsulated chlorobenzene in urea-formaldehyde microcapsules. Healing of polymeric materials with encapsulated solvents offers an economical, simple, and potentially robust alternative to the recovery of virgin properties of a material after damage has occurred.

Other versions of active materials that have been studied are auto-oxidizable oils. In this case, oils like Linseed and Tung that have unsaturation along their backbone have been encapsulated. Upon capsule rupture and exposure to oxygen in the atmosphere and under UV irradiation, these molecules cross-link to form a hydrophobic thermoset. Another advantage of this approach is the incorporation of additional corrosion inhibitors like di-octyl phosphate, zinc phosphate, spar varnish, camphor, and alkyl ammonium salts in xylene into the healing agent.

In terms of mechanical integrity, self healing capsules have shown an increase in toughness of epoxy coatings and in most cases enhance the service life of coatings. But, the main draw back of microcapsules is that they are designed for single use. Whilst mitigation of small cracks by self-healing capsules has been shown to work efficiently, the same cannot be said of large cracks that require substantially larger volumes of material. In this case, encapsulation technologies may not be able to deliver the requisite amount and that too for multiple insults. Furthermore, the remnant empty capsules can act as defects and in some cases the mechanical properties may not completely recover.

In summary, self-healing abilities imparted to materials through approaches discussed, represent the next generation of technologies coming on stream that seek to extend the lifetime and reliability of products.