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Self-healing material

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A self-healing material is a material that has the built-in ability to partially repair damage occurring during its service lifetime. Usually, certain properties of any engineering material degrade over time due to environmental conditions or fatigue, or due to damage incurred during operation. This damage is often on a microscopic scale, requiring periodic inspection and repair to avoid them growing and causing failure. Self-healing materials address this degradation through the inclusion of an "active" phase that responds to the micro-damage by initiating a repair mechanism. Several mechanisms have been proposed for a range of engineering materials (e.g. metals, polymers, ceramics, cemetitious, elastomeric and fibre-reinforced composite materials;[1]) most are currently under investigation at a laboratory or research scale.

Self-healing in polymers and fibre-reinforced polymer composites

Liquid-based healing agents

The first report of a completely autonomous man-made self healing material was by the group of prof Scott White of the University of Illinois at Urbana-Champaign. [2] They reported an epoxy system containing microcapsules. These microcapsules were filled with a (liquid) monomer. If a microcrack occurs in this system, the microcapsule will rupture and the monomer will fill the crack. Subsequently it will polymerise, initiated by catalyst particles (Grubbs catalyst) that are also dispersed through the system. This model system of a self healing particle proved to work very well in pure polymers and polymer coatings.

A hollow glass fibre approach may be more appropriate for self-healing impact damage in fibre-reinforced polymer composite materials. Impact damage can cause a significant reduction in compressive strength with little damage obvious to the naked eye. Hollow glass fibres containing liquid healing agents (some fibres carrying a liquid epoxy monomer and some the corresponding liquid hardener) are embedded within a composite laminate. Studies have shown significant potential [3].

Solid-state healing agents

In addition to the sequestered healing agent strategies described above, research into "intrinsically" self-healing materials is also being performed. For example, supramolecular polymers are materials formed by reversibly connected non-covalent bonds (i.e. hydrogen bond), which will disassociate at elevated temperatures. Healing of these supramolecullary based materials is accomplished by heating them and allowing the non-covalent bonds to break. Upon cooling new bonds will be formed and the material will potentially heal any damage. An advantage of this method is that no reactive chemicals or (toxic) catalysts are needed. However, these materials are not "autonomic" as they require the intervention of an outside agent to initiate a healing response.

Biomimetic design approaches

Self-healing materials are widely encountered in natural systems, and inspiration can be drawn from these systems for design. There is evidence in the academic literature [4] of these biomimetic design approaches being used in the development of self-healing systems for polymer composites.

Commercialization

At least one company attempting to bring these new materials to the market, Autonomic Materials Inc.,[5] with a product expected in 2009.[6]

References

  1. ^ S. van der Zwaag (Editor). "Self-healing Materials: an Alternative Approach to 20 Centuries of Materials Science" 2007. (Dordrecht, The Netherlands: Springer).
  2. ^ S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N. Brown, S. Viswanathan: "Autonomic healing of polymer composites", Nature. 2001 409, 794-797.
  3. ^ R.S. Trask, G.J. Williams, I.P. Bond: "Bioinspired self-healing of advanced composite structures using hollow glass fibres" J R Soc Interface. 2007. 4(13). 363–371.
  4. ^ R.S. Trask, H.R. Williams, I.P. Bond: "Self-healing polymer composites: mimicking nature to enhance performance", Bioinspiration and Biomimetics. 2007. 2. 1-9.
  5. ^ autonomicmaterials.com
  6. ^ technologyreview.com– First Self-Healing Coatings
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