ones [41]. They show that complete healing (efficiencies around 100%) of both epoxy and PU matrices is achieved for capsules with diameters of 96 mm or higher. Smaller capsules produced poorer healing performances. Gil et al. used microencapsulated diisocyanate to improve the tensile strength of collagen [42]. They used two different diisocyanates: IPDI and 4,4’-diphenylmethane diisocyanate (MDI); the isocyanate groups react with the collagen, creating new crosslinks and mending the damages. Other healing agents employed for extrinsic self-healing coatings, including thiol-ene and azide-alkyne precursors, as well as vinyl ester and unsaturated polyesters have also been tested, as pointed out in a review by Hillewaere and Du Prez [43].
A smart approach proposes to develop self-healing systems especially designed so that the healing can be activated by the environment surrounding the material when a crack propagates. Different research groups used encapsulated isocyanate that reacts with water when released into aqueous environments, producing coatings with potential applications in offshore and other marine devices. Di Credico et al. also used encapsulated IPDI to provide self-healing capacity to a DGEBA-based epoxy matrix, thanks to the crosslinking reaction of isocyanate with the surrounding water [44]. Figure 1.4-I shows the damaged coating, and the healed coating after immersion for 48 h in salty water. The authors emphasized that the rough outer surface of the microcapsules played a key role improving the adhesion to the matrix, allowing the capsule to fail and release the IPDI. Wang et al. also used IPDI as healing agent to repair cracks in an alkyd varnish coating (AVC) [45]. Figure 1.4-II shows the aspect of the mended scratch on different substrates after different times of exposure to marine salty water. Though some seawater could penetrate into the coating and reach the substrate, the self-healing prevented a much larger damage.
Light is another possible external stimulus that can be harnessed to trigger the healing response. Sunlight was proposed by some authors to induce the self-healing of polymeric coatings. Song et al. designed the first photoinduced microcapsule-based self-healing coating from tetraethyl ortosilicate and a polysiloxane, with encapsulated methacryloxypropylterminated polydimethylsiloxane (MAT-PDMS) and benzoinisobutyl ether (BIE) photoinitiator [46]. The conversion of the healing agent upon exposure to real sunlight was somewhat lower than when an artificial UV light was used instead. However, several tests (optical and SEM examination, and measurements of water uptake and chloride ion penetration) showed that relatively short times (4 h) of sunlight exposition are enough to produce an acceptable healing. Figure 1.5 shows the results obtained, comparing the performance of the self-healing coating with the control coating (without the encapsulated healing agent). Khalaj Asadi et al. successfully encapsulated a sunlight curable silicon based resin, and studied several parameters affecting some final properties of the capsules [47]. They used polyvinylpyrrolidone as emulsifier, and established the optimum amount to obtain higher yields. These capsules showed good resistance to water and xilene, but ethanol produced a rapid deterioration of the microcapsules. The authors remarked that these microcapsules can be readily used for self-healing polymers formulations.
Figure 1.4 (I)—SEM micrographs of crack in a coating with PU/PUF microcapsules (a) before and (b) after immersion in salt water for 48 h. Reprinted from Ref. [44]; Copyright (2013) with permission from Elsevier. (II)—Alkyd varnish coatings on a titanium surface after 200 and 1,200 h of seawater immersion. Reprinted from Ref. [45] with permission from The Royal Society of Chemistry.
The use of UV light to trigger the mending reactions was explored as well by other researchers. Gao et al. encapsulated a photosensitive resin obtained from a mixture of Bisphenol A epoxy resin diacrylate ester (BAEA) and trimethylolpropane-triacrylate (TMPTA) with 1-hydroxy-cyclohexyl-phenyl-ketone into UF capsules, and embedded them into an epoxy-amine matrix [48]. The microcapsules were synthesized containing TiO2 in its shell in order to absorb the UV light and protect the photosensitive resin into the undamaged capsules from curing before being released. Anticorrosion tests were performed after scratching the samples and healing during 30 s with UV irradiation. The neat epoxy-amine matrix and a composite with capsules without TiO2 were used as control (CC1 and CC2 respectively). Corrosion was observed in CC1, but CC2 and the self-healing coating could protect the steel substrate. When the experiment was repeated with a new scratch on the same samples, CC2 could no longer protect the steel, and corrosion was observed. Figure 1.6 shows the images of the samples after each test. For the self-healing coatings, the experiment was repeated 5 times, and the self-healing ability was eventually lost after 5 irradiation events. SEM images of the scratch and the electric current measured for each case confirmed the previous observations.
Zhu et al. also used a UV-curable healing agent into microcapsules with a rapidly degradable inner polymeric shell and an outer TiO2 shell that can absorb UV radiation [49]. The action of the TiO2 shell helps to degrade the inner shell, releasing the healing agent. Hence, the self-healing composite displays a dual release mechanism that enhances its efficiency. The micro-encapsulated healing agent consisted in an epoxy silicone with a photosensitive initiator (triarylsulfonium hexafluorphosphate salt) and the matrix was based in silicone resins. Figure 1.7 shows a scratch on the coatings after 12 h of UV irradiation. The comparison was made using composites with microcapsules without the healing agent (labeled as “BS-xx”), and composites prepared with capsules filled with the healing agent but unable to fail and release it by UV irradiation, due to a low concentration of TiO2 NPs in its outer shell (labeled as “CS-xx”). The self-healing coatings were labeled as “SH-xx”. The numbers xx represent the wt% of microcapsules. The effect of the healing agent released within the crack is very clear, and for a microcapsules load of 60 wt% the healing seems to be excellent.
Figure 1.5 (I)—Scheme of the sunlight induced healing mechanism: the crack breaks the microcapsules and release the healing agent, which undergoes the crosslinking reaction upon exposure to sunlight. (II)—Water uptake measurements for the plain mortar, and mortars coated with the control and the self-healing coating. (III)—Chloride penetration tests. Current vs. elapsed time, and accumulated charge during 6 h for the undamaged control coating (a), scribed control coating (b) and scribed and healed self-healing coating (c). Reprinted with permission from Ref. [46]. Copyright (2013) American Chemical Society.
Figure 1.6 Steel substrates coated with (a) CC1, (b) CC2 and (c) self-healing coating, after successive scribing and healing sequences. Reprinted with permission from Ref. [48]; Copyright (2015) American Chemical Society.
Some drawbacks of the use of microcapsules/hollow microfibers are worth to mention. Samadzadeh et al. [50] have mentioned some of them, including the negative side effects on the mechanical properties of the material, such as Young’s modulus and ultimate stress [50, 51]. Adhesive properties can also suffer a decrease due to the presence of microcapsules [50]. In most cases a compromise between an acceptable healing with a minor deterioration of the resistance has to be reached. Additionally, there are some aspects that should not
