to the cases where high temperatures or external stresses have to be applied. This would be another valid classification for self-healing materials, but as we will find out, very often the limit between them is somewhat blurred. In most cases extrinsic self-healing polymers are totally or nearly autonomous, and intrinsic ones need an external triggering. That is, most intrinsic self-healing coatings should not be regarded as self-healing, but instead just as healable materials. There are as well some notable exceptions to this statement that deserve to be considered.
Extrinsic self-healing polymers were developed several years before that intrinsic ones. However, nowadays intrinsic self-healing polymers are a much faster growing research field, led by polymeric networks based on dynamic covalent bonds (DCBs). We will devote the first part of this chapter to extrinsic self-healing thermosets, and a second part to intrinsic ones, with special emphasis on polymeric networks with DCBs. Finally, we will provide a perspective for future trends regarding self-healing polymeric coatings.
1.2 Extrinsic Self-Healing Polymer Coatings
The most widely explored extrinsic self-healing polymers are systems based on composites containing the healing agent into purposely designed containers that break when the material is damaged. Usually, their self-healing is autonomous, needing no other driving force than the damage itself, or in some cases the action of the surrounding environment. The key for the self-healing to take place autonomously is that the chemical precursors released can readily react at the material’s operation temperature. Quite often this is room temperature, and hence these polymeric composites must be designed in order to carry the appropriate amount of catalyst to complete the curing reaction at this low temperature. Besides this, other relevant aspects of the healing mechanism need to be accounted to achieve high healing efficiencies. The vessels containing the healing agents must break, and the healing agents have to flow to fill the crack before the curing reaction produce its gelation. Hence, a good load transfer from the matrix to the vessels combined with the specific fracture toughness of the vessels is needed, and the viscosity of the healing agents must be low enough to allow them to flow into the crack. These features have also been addressed in several previous works [22].
There are several monomers and catalysts that allow reaction rates high enough to achieve self-healing at room temperature. Grubbs catalysts are one of the most widely used for autonomous self-healing composites based on microencapsulated healing agents. It was first proposed for self-healing composites by White et al., to aid the crosslinking of dicyclopentadiene (DCPD) through a ring opening metathesis polymerization (ROMP) [12]. The DCPD was contained into urea microcapsules embedded within these composites, and the Grubbs catalyst was dispersed in the matrix. After healing during 48 h at room temperature, the researchers observed that the healed sample reached a load of 75% of that of the virgin sample in a fracture test. These results were very promising, and the healing efficiency values were quickly surpassed in the following years by similar systems that incorporated some improvements in their formulations and/or their processing [23, 24]. By protecting the catalyst with wax [25] a considerable raise in the healing efficiency was achieved, thanks to an enhanced stability of the catalyst, but this modification was also associated with a decrease in the damage resistance to impact. These systems were also used to study the effect of the particles’ size and load on the healing efficiency, which allowed to determine that microcapsules smaller than 30 mm are efficient for small cracks (around 3 mm), but larger capsules and higher loads are needed for larger cracks [26].
However, some shortcomings of the DCPD-Grubbs catalyst systems (low long term stability due to possible side reactions with air and the polymer matrix) [27], encouraged the search for other chemistries that could overcome these difficulties. The condensation of hydroxy capped polydimethylsiloxane (HOPDMS) and polydiethoxysiloxane (PDES) with dibutyltindilaurate (DBTDL) in a vinyl ester matrix showed an improved stability, but the healing efficiencies were rather poor [27]. Much better results were observed for a polydimethylsiloxane (PDMS) elastomer filled with poly(urea-formaldehyde) (UF) microcapsules containing the same precursor and Pt initiator used to synthesize the PDMS matrix separately [28]. The authors studied the breaking mechanism of the capsule, and provided an interesting insight from the observation of the process of deformation and failure of a microcapsule within the elastomeric matrix. The sequence is shown in Figure 1.3. For stretch values below 1.5 the micro-capsule is deformed along with the matrix, withstanding the stress, but for higher deformations, the failure of its wall releases the chemical precursors. This study focused on some very important aspects of the use of microcapsules for self-healing composites, such as the microcapsule rupture process, and how its presence affects some properties of the matrix. It clearly showed that the breaking of the capsules is a quite complex matter, and even brittle materials (when evaluated in bulk) such as UF may need relatively high strain values to fail [28, 29]. The authors also observed an improvement on tear strength with the increase in the microcapsules concentration (up to 20 wt% of microcapsules), which is in agreement with previous results for reinforced PDMS [30, 31]. To ease the flow of the healing agent off the microcapsules and into the crack, its viscosity was reduced by using heptane as diluent [28]. Using a similar system, consisting in a silanol-terminated polydimethylsiloxane (STP) as healing agent and dibutyltin dilaurate (DTDL) as catalyst, Kim et al. obtained a self-healing coating that could repair itself at temperatures as low as −20 °C. The viscoelastic material produced by the healing agent was capable of protecting the coated and scratched specimens from saline solution uptake upon immersion for 48 h at −20 °C [32].
Figure 1.3 Images of a single PDMS microcapsule subjected to uniaxial tension at different values of deformation (γ). Reprinted with permission from Ref. [28]. Copyright (2007) John Wiley & Sons, Inc.
Microencapsulated epoxy precursors were also applied as external healing agent in matrices with a dispersed latent initiator [33–37]. Yin et al. utilized UF microcapsules containing an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA), and a latent catalyst consisting in a complex of CuBr2 and 2-methylimidazole (MeIm), dispersed in the matrix [33]. Their results showed very good healing efficiencies, with healed specimens showing fracture toughness values 11% higher than those of the virgin samples. As the authors explained, this is a side effect of the higher fracture toughness of the resin used as healing agent when compared to the matrix, probably due to the higher temperature and/or the different curing mechanism of the healing reaction: the samples were synthesized at temperatures up to 100 °C, and the subsequent healing was performed at 130 °C. They also applied the self-healing matrix to the synthesis of self-healing epoxy laminates, using glass fibers [33, 34]. However, the healing efficiencies were lower in this case, and an important downside of the use of microcapsules was the reduction of the tensile properties for microcapsule loads above 30 wt% [34]. Guadagno et al. assessed the healing efficiency and the mechanical properties of self-healing epoxy systems with different flexibilizers [36, 37]. When the curing temperature of these systems was raised from 170 to 180 °C, the mechanical moduli increased, but the healing efficiency slightly decreased, probably due to an initial thermolytic decomposition of the Hoveyda–Grubbs’ first generation catalyst. Interestingly, the replacement of the Heloxy71 used as flexibilizer by the reactive diluent 1,4-butanedi-oldiglycidylether (BDE) leads to an increase in the compressive modulus, from around 0.7 to 1.4 GPa, with no loss in the healing efficiency (above 80%) [37]. Hia et al. demonstrated multiple self-healing ability in epoxy composites with alginate multicore microcapsules [38]. Three to four healing cycles were produced, though the efficiencies were around 55%, which are lower than for other systems. Moreover, the authors found that self-healing specimens have lower impact strength than the neat epoxy polymers, due to the large size of the capsules, and its high load.
Polyurethanes (PU) were explored for extrinsic self-healing systems [39, 40]. He et al. used isophorondiisocyanate (IPDI) encapsulated into polyurea capsules as self-healing agent for
