Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Sam Meure et al.
THE BIOMIMETIC APPROACH TO SELF HEALING
POLYMER COMPOSITE DEVELOPMENT IN THE
AEROSPACE INDUSTRY
Sam Meure and Dong Yang Wu
CSIRO Manufacturing and Materials Technology
Tel: +61 3 9545 2044
Fax: +61 3 9544 1128
sam.meure@csiro.au
The concept of healing polymers was already established in the 1980’s, but the presentation of self healing
polymer composites by Dry[1] in 1993 and then the well cited White[2] publication in 2001 inspired world wide
interest in these materials. Self healing composites were very quickly recognized as having enormous potential
to impact upon the structural materials used in the aerospace industry[3]. Examples of the interest that has been
shown in this relatively new type of material are seen through US Air force and European Space Agency
investments in self healing polymers as well as the extension of continuum damage mechanics modeling to
account for self healing capabilities.
Healing in polymers can be achieved by resealing fractured surfaces or through crack growth retardation, but in
self healing composites it is usually achieved through the physically filling of flaws in the damaged material or
reversing the chemical changes that were caused by the damage. During the development of this new range of
smart materials, the mimicking of biological systems (biomimetics) has been used repeatedly as a source of
inspiration[4, 5], as has also been the case for other recently developed materials[6-8]. One example of
biomimetic healing is seen in the vascular-style bleeding of healing agents in the original self healing composites
proposed by Dry[1].
Despite their inspiration coming from robust and multidimensional responses to damage; the adaptation of
biological healing mechanisms to polymer composites has generally taken a one dimensional approach. In this
presentation we review existing self healing polymer composite technologies and discuss them with respect to
parallel biological healing mechanisms. Avenues for potential improvement in the robustness and efficiency of
self healing composites are also presented with a specific focus on the structural composites designed for
aerospace applications.
Keywords: polymer composite, aerospace structure, active healing ,passive healing,
biomimetic repair , review
1
Introduction
Conceptually, self healing composites are repaired without the need for additional materials.
This healing can be activated either autonomously or after a specific stimuli has been applied.
Healing in polymers can be achieved by resealing fractured surfaces or the crack growth
retardation, but in self healing composites it is usually achieved through the physically filling
of flaws in the damaged material or reversing the chemical changes that were caused by the
damage.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Sam Meure et al.
Like all synthetic materials, polymers degrade and become damaged, with unseen damage in
polymer composites capable of causing significant reductions in strength and working
life[10]. A self healing composite may not only heal unseen damage, but also prevent repairs
that can be costly and time consuming. Throughout the development of this new range of
smart materials, the mimicking of biological systems (biomimetics) has been used as a source
of inspiration[4, 5], as has been the case for a range of newly developed composite
materials[6-8].
Self healing composites were very quickly recognized as having enormous potential to impact
upon the structural materials used in the aerospace industry[3]; with interest shown in this
relatively new type of material exemplified through US Air force[11] and European Space
Agency[12] investments in self healing polymers. As a reflection of the interest in these
materials by the aerospace industry, this review is focused on structural polymers and
polymer composites, such as epoxy and polyester resins.
2
Self healing mechanisms
2.1
Active self healing in polymer composites
2.1.1 Thermally triggered healing
The concept of thermally reversible crosslinking was introduced by Chen[13] in 2002;
reporting self healing composites via Diels-Alder based cycloaddition of polymer chains
containing multi-furan and multi-maleimide functionalities. The authors[13] repeatedly heated
and cooled films of these polymers to show that the Diels-Alder-type crosslinks could be
separated and rejoined a number of times. Healing in these films was based on welding
fractured surfaces back together; with healing efficiencies reaching up to 80%[14]. More
recent developments in this type of thermally healing composite includes, the incorporation of
braided electromagnet elements to heal internal cracks[15] and the mending of superficial
scratches[16].
The incorporation of thermoplastic additives into thermoset composites was first reported by
Zako[17] in 1999. On heating the composites, thermoplastic beads melted, flowed into
internal cracks or flaws and then re-set. These composites were shown to have a healing
efficiency of 100% in three point bending and cyclic fatigue tests, when damaged composites
were healed at 120°C. A second embodiment of this healing mechanism, using “solid
solutions” instead of discrete additives, has subsequently been patented by Jones in 2005[18];
however the reported healing efficiency were lower than those of Zako[17].
2.1.2 Light triggered healing
The first example of a light-induced self healing composite was reported by Chung[20] in
2004. Although Sriram[19] had earlier suggested the use of photoinitiated catalysts in self
healing composites; he did not report the production of any self healing composites using this
technique. Chung[20] used a 280nm light source to crosslink branched cinnamoyl groups and
form solid transparent films. The ability of branched cinnamoyl groups to self heal was tested
by blending this crosslinking agent with methacrylate-based monomers and then polymerizing
films using a visible-light photoinitiator.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Sam Meure et al.
Healing of fractures in these films was shown to only occur under exposure to light of the
correct wavelength, proving that healing was photoinitiated. Healing efficiencies in flexural
strength reached up to 26% when both light and heating (100°C) sources were used during
healing.
2.2
Passive self healing in polymer composites
2.2.1 Hollow fiber-based technologies
Dry[1, 21] reported the production of self healing composite consisting of hollow fibers that
had been filled with healing agents. One example of these composites involved setting epoxy
resin- and hardener-filled 100μl glass pipettes in blocks of glass bead reinforced epoxy[22]. In
this example, healing takes place when adhesive agents fill internal flaws and then set, acting
as a patch or an in situ cured wedge of new resin. Dry[22] presented a second example of
these self healing composites by replacing the two part epoxy healing agent with a one part
cyanoacrylate adhesive and the replacing glass beads with a reinforcing wire. Healing in both
these systems occurred in most samples after repeated exposure to impact and bending tests
followed by an 8-12 month healing; however healing efficiencies were not reported. Since the
presentation of Dry’s work, a range of hollow fiber-based self healing composites have been
produced[12, 23-25]. In more recent reports[5], heating was used to initiate curing of the
resins.
2.2.2 Hollow microsphere-based technologies
In 1997, Jung, Hegeman and associates[26-28] reported the development of self healing
composites based on urea-formaldehyde microspheres rather than hollow fibers. Jung[28]
reportedly tested healing agents designed to react with polyesters containing an “added
functionality,” however these materials described as unworkable. Jung[28] then focused on
the “natural functionality” of polyester resins to produce composites that were healed using
styrene mixtures. In 2001, collaborators on the Jung‘s work[28] started releasing a series of
publications on self healing composites which used dispersed Grubbs catalysts to active a ring
opening polymerization in dicyclopentadiene healing agents[2]. During the development of
these composites, a range of healing agents[19] and microsphere production conditions[29]
were considered, including the more recent development of diene monomer blends[30, 31] as
healing agents. Recent advances in hollow microsphere-based self healing composites have
included the attachment of the Grubbs catalyst directly to the exterior of the hollow
microspheres[32], the use of “added functionalities” enabling the bonding of fractured
surfaces to healing agents[33] and the encapsulation of Grubbs catalysts instead of healing
agents[34]. Cho[34], chose to improve the stability of the agent-catalyst system, shifting
toward more robust reactions between siloxane monomers and a dilaurate catalyst. Using
phase separation in the selected polymer blend to create discrete pockets of healing agent, and
then encapsulating the catalyst, Cho[34] produced a more robust self healing composite.
When tested under ambient conditions, fracture toughness healing efficiencies of up to 24%
were achieved; when tested on samples that were healed under water, efficiencies of above
15% could still be achieved.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Sam Meure et al.
2.2.3 Molecular rearrangement-based technologies
Self healing via molecular rearrangement has been found to occur in some ionomers[35];
including puncture reversal after ballistic impacts in ethylene ionomers[36]. Re-closure of
bullet wounds was subsequently observed in Surlyn®, Surlyn®-carbon fiber composites and
Nucrel® ionomers, but not in comparable non-ionic materials (such as low density
polyethylene)[35]. Healing in these materials[37] only occurs after damage that results in high
energy transfer (such as ballistic impact and sawed cuts [35]), during which localized melting
of the ionomer can occur (healing efficiencies were not reported).
Another method of self healing via molecular rearrangement involves the catalytic
reconnection of broken chains, and has been reviewed by Takeda [4, 7]. Careful selection of
functionality in polycarbonates[38] and polyphenylene ethers[39] facilitates the joining of
terminal hydroxide and phenyl groups via carbonate and copper catalyst activation. This type
of reaction can be used to repair polymer chains degraded by oxidative and thermal
treatments[4]. Studies on the polycarbonate systems containing sodium carbonate catalysts
revealed that after being damaged via hydrolysis, healing efficiencies of up to 94% and 98%
could be achieved in molecular weight and tensile strength respectively[38]. However, it was
revealed that chain end mobility was critical to healing in these systems[4], and it is possible
that nitrogen atmospheres and temperatures of 130°C were used to assist healing in cases
where the highest healing efficiencies were achieved[7].
3
Potential routes for improvement
Although the development of self healing composites has been largely based on the
mimicking of biological healing, there is still a long way to go before even the simplest of
biological healing mechanisms has been replicated within these synthetic materials.
One immediate difference between biological and existing synthetic healing mechanisms is
that biological systems invariably use multi-step healing solutions. For example, healing in
vertebrates and invertebrates is based on a “patch then repair” mechanism, even though the
actual healing processes are significantly different. Invertebrates such as mollusks patch
breaks in their shell with fast forming plastic; then after a new seal has formed the crossedlamella microstructure typical to shell material is slowly regenerated[40]. Human healing
processes also rely on fast forming patches to seal and protect damaged skin before the slow
regeneration of the final repair tissue[41]. In contrast to these mechanisms, all the self healing
composites discussed above attempt to complete healing in a single step; either through the in
situ curing of a new phase or the permanent re-sealing of newly exposed surfaces. The closest
that synthetic healing has come to a multi-step healing process has been through the use of
monomer mixtures[31]; where the in situ polymerization of a reinforcing wedge includes a
secondary and slower forming rigid polymeric component. There is no doubt that the
introduction of more multi-step healing processes will improve the performance of new self
healing composites.
A second difference in the dimensionality of biological and synthetic healing systems can be
seen in the multi-mechanistic approach used by biological systems. Even the simplest
biological systems use multiple healing mechanisms simultaneously. The healing process
used by cells suffering from membrane damage involves both a chaotic coalescing of lipids to
block the hole[42] and then a purse-string-like to pull the edges of the hole closer
together[43]. The repair mechanisms for bone[44], tendons[45] and skin[41] in humans are
also based on a multi-mechanistic approach, involving an initial inflammatory response in
conjunction with the regeneration of the damaged material.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Sam Meure et al.
However, in synthetic healing either wedging or bridging is used as the sole repair mechanism
despite the establishment of numerous other crack growth retardation mechanisms such as
crack surface sliding and zone shielding[46]. It could be argued that the addition of healing
agent filled microspheres to epoxies increased fracture toughness via crack growth retardation
mechanisms such as crack pinning[27, 47]; however, these improvements contribute to the
intrinsic toughness of the composites rather than acting as a repair mechanism. Through the
development of self healing composites that deliberately use multiple repair mechanisms,
improved efficiencies and system robustness are likely to be achieved.
In addition to the development of broader healing mechanisms, changes in the nature of
healing agents are can be used to improve the existing self healing composites. Limitations in
existing self healing composites such as working temperatures and healing agent lifespan
have already been identified[48] and are being addressed to produce self healing composites
that work in more extreme environments[5]. Further developments in healing agents may also
include; biomimetic fillers that enable improved bending and buckling resistance through the
use of sandwich-type cellular agents[49], improved surface adhesion through the use of
branched fibrous agents that possess higher pullout energies[50], or improved healing
consistency through the use of self-assembling agents[51, 52]. Whether achieved though the
use of possible multistage / multi-mechanistic healing methodologies or via evolutionary
improvement of the materials used in existing methodologies, it is certain that the continued
development of self healing composites will produce a new generation of structural materials.
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