100 °C for the first 10 min of the total time. It was found that the healing effect increases with time and the obtained stress–strain curves are similar to that one from the uncut sample, as can be seen in Figure 3.18. A good self-healing is obtained due to the reversible characteristic of the ionic bonds and to the good mobility of the isobutylene chains (which is improved at higher temperatures). They did not obtain self-healing in the compound crosslinked with sulfur.
Figure 3.17 Reaction scheme for the conversion of BIIR with butylimidazole for posterior self-healing by ionic cross-linking (Adapted with permission from Das et al. [55]).
Figure 3.18 Stress–strain measurements to characterize self-healing behavior in BIIR samples modified with imidazolium (BIIR-i) when the healing was made at room temperature at different length times (blue lines) and when the first 10 minutes of heal was made at 100 °C (red lines), compared with an uncut sample (black line) (Adapted with permission from Das et al. [55]).
Lee and coworkers evaluated the self-healing property in pristine BIIR when is used as a coating to prevent corrosion, demonstrating that the healing process is highly stimulated by temperature by using a copper nanofiber as heater [56]. Kim et al. proposed a bilayer of BIIR over another one of BIIR with carbon nanotubes (in a concentration between 7 and 10 wt.%). The latter was used as heater to improve the self-healing efficiency [57]. The bi-layer was prepared through different methods: casting, painting and spraying. The angle between the applied current and the performed linear crack was varied (0, 45 and 90°) in order to study the influence of the current direction in the self-healing process. In the case of a current parallel to the crack orientation, the healing process took 2.5 h, while in the case that the current and the crack direction are parallel just 1 h is required. Then a repeating healing was performed, founding that up to a third cycle the material self-healed successfully when the sample is heated during 1 h in each cycle, as can be seen in Figure 3.19. Finally, they performed corrosion tests by applying the bi-layer in a steel substrate. To this aim, a deep scratch was made in 2 samples which were submerged in saline water and only one of the samples was heated during 1 h. After 72 h under the saline bath, the bi-layer was removed from the samples and the steel surface was observed, founding rust in the sample that was not heated (Figure 3.20).
Figure 3.19 Bi-layer of BIIR and BIIR-CNT. Repeated healing test under water, the electrical current is perpendicular to the crack direction and is applied during 1 h in each cycle (Reprinted with permission from Kim et al. [57]).
Figure 3.20 Bi-layer of BIIR and BIIR-CNT in a steel substrate to evaluate corrosion test in saline water during 72 h. (a) without heating, (b) with heating (Reprinted with permission from Kim et al. [57]).
3.3.5 Silicones
The silicone is characterized for its insulating properties, chemical and thermal stability, outstanding weatherability and transparency [58–60]. Some of the most common applications are automobiles, electronics, medical implants, sportswear and shoes, among others.
In some cases, it is necessary that the insulating material needs to transmit the heat generated, for example, in electronic circuits. Several authors [61] are currently working on the development of insulating materials with thermally conductive loads with desirable self-healing properties.
Zhao et al. [58] used thermally conductive composites based on silicone for electronic packaging materials. The challenge is to fabricate functional composites with two main characteristics: self-healing ability and high thermal conductivity. Self-healing silicone was formulated with boron nitride (BN) in order to induce the DA reaction. After the tensile test, samples were submitted to pressure and temperature to join the broken surfaces. The final material exhibited a high self-healing efficiency (almost 90%) and an increase in thermal conductivity about 500% with just 50 wt.% of BN. From Figure 3.21, it is possible to compare the stress–strain curves before and after healing process of the silicone elastomer and composites, observing a high structural recovery with 50 wt.% of BN. Authors explain that, due to the hindrance of the BN nanosheets, the mobility of low molecular oligomer is reduced, so it is necessary to apply certain external force (for example pressure) to facilitates the chain diffusion in the damaged interface of the composite to produce an optimal self-healing process.
Figure 3.21 Stress–strain curves of the silicone elastomer and composites before (solid line) and after healing treatment (named with “R” and in dotted line) (Adapted with permission from Zhao et al. [60]).
Xiang et al. [14] studied a reversibly cured silicone elastomer which is obtained by a condensation reaction between α,ω-dihydroxyl polydimethylsiloxane and disulfide bond that contains silane coupling agent as cross-linker. This work explores the self-healing composite under sunlight by using microcontainers (for example capsules and glass capillaries), with liquid healing agents that are released when the material is damaged. The self-healing capacity was determined with a tensile test, in which samples were broken and, then, recombined under pressure to be healed by xenon exposure or natural sunlight exposure for a certain period of time. Authors observed that the healing efficiency depends on the exposure time, reaching a maximum after 48 h. Also, a misalignment of the fractured surfaces is an important variable to obtain a better self-healing. The crosslinks density was also evaluated due to a high crosslinks density reduces the wettability and diffusion of dangling chains on the fracture surface, affecting the self-healing capacity.
As it was explained before, the vulcanization agents play an important role during the dynamic reversible of disulfide bonds since they promote a higher healing efficiency. Figure 3.22 shows 80% recovery in mechanical test for healed and recycle samples, being the last one obtained by cutting the silicone sheets in small chips that were pulverized into powder and then compressed and molded.
Figure 3.22 Tensile stress–strain curves of virgin, healed and recycled silicone compounds (Adapted with permission from Xiang et al. [14]).
Another interesting application is silicone foams, which
