Microcapsule-based Systems
Microcapsule-based systems sequester the healing agent in discrete capsules. When an approaching crack ruptures embedded microcapsules, capillary action draws the healing agent into the crack plane. Subsequently, a chemical reaction occurs and bonds the crack faces closed. A variety of capsule-based schemes have been developed and applied to conventional structural materials including single or dual capsule systems triggered by catalysts or component mixing. The next generation of self-healing capsule systems includes those triggered by a variety of environmental stimuli including temperature, UV irradiation, pH, and specific chemical compounds for use in new applications like corrosion protection, electronic packaging, and abrasion resistant coatings.
In microcapsule-based systems, healing agents are sequestered in discrete microcapsules that are embedded in the host material. A variety of encapsulation schemes are used which depend on the specific healing chemistry and shell wall material desired. Commonly, encapsulation occurs via emulsification of an oil phase (the core material) in an aqueous bath. In situ interfacial polymerization forms a polymer shell wall at the interface of the core material and the aqueous bath. Size and shell wall thickness can be controlled through agitation of the encapsulation medium and temperature and time allowed for the polymerization reaction. Microcapsule-based system leads a singular local healing event as the local healing agent is depleted after release.
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Self-healing concept using a capsule-catalyst system. a, Cracks form in the matrix wherever damage occurs; b, the crack ruptures the microcapsules, releasing the healing agent into the crack plane through capillary action;
c, the healing agent contacts the catalyst, triggering polymerization that bonds the crack faces closed. Ref: White et al,Nature (2001) |
Self-healing epoxy. Microcapsules (in red) contain a healing agent monomer. A solid phase catalyst (black) is dispersed in the epoxy matrix. Crack propagation ruptures the capsules releasing the healing agent into the crack plane (solid red area in lower right). Mixing with the catalyst phase triggers polymerization of the healing agent bonding the crack faces closed. |
Self-healing glass/epoxy composite. Microcapsules (ca. 20 microns) containing a two-part healing chemistry are integrated into woven glass fiber-reinforced epoxy composite in interstitial areas between fiber tows. |