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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 are characterized by thermal insulation, good lightweight performance, good flexibility and excellent shock absorbent. Zhao et al. [60] developed silicone foam composite with self-healing properties. The concept consists in a material of acquiring different shapes without any fracture. Authors formed hybrid foam with shear stiffening gel (STG) and Methyl vinyl silicone rubber (MVQ) which maintained its shape. Expandable microspheres were used as blowing agent. These microspheres have two structures: a low-boiling volatile hydrocarbon core wrapped in the thermoplastic shell that when heated permits to the liquid inside the core to evaporate. During this process, the thermoplastic shell swells, expanding almost to hundred times. The authors evaluated the self-healing capacity after performing cuts on the samples and then bonding the surfaces together. STG samples were able to stick together, evidencing a very good viscous state. Through tensile tests, a recovery of only 11.1% was determined after healing. In the case of the elongation at break, the recovery was about 34 %. The mechanical properties of the self-healing samples are shown in Figure 3.23 (samples were named considering different MVQ proportions).

Chen et al. [61] synthetized a poly(dimethylsiloxane) (PDMS) elastomer with sacrificial hydrogen bonds and dynamic imine bonds. The self-healing mechanism was characterized through FT-IR, tensile test and dynamic mechanical analysis. The experimental processes and synthesis of the specimens were made in a two-step approach, where segmented PDMS products (named as PDMS-U) was obtained by reacting aminopropyl terminated polydimethylsiloxane (AP-PDMS) and 4,4’-methylenebis-(cyclohexylisocyanate) (HMDI). Then, isophthalaldehyde (IPAL) was added to form imine groups and obtain the self-healing materials with urea and imine groups (PDMS-UI). The samples varies in the molar ratio of urea to imine groups, and the samples are designated as PDMS-UI-x (x = 1–4), being x proportional to the ratio value.

Figure 3.23 Tensile stress–strain curves of self-healed composite foams with different amounts of shear stiffening gel (Adapted with permission from Zhao et al. [60]).

Comparing the self-healing capacity of the samples, there is a decrease in the property with the increment of urea (Figure 3.24). Authors determined that the self-healing capacity depends on the concentration of imine groups.

Sun et al. [62] worked with dielectric elastomers to simulate artificial muscles. Amino terminated polydimethylsiloxane (PDMS-NH₂) and a polymethylvinylsiloxane (PMS-g-COOH) were synthetized, in which hydrogen and ionic bonds would act as physical reversible bonds during the crosslinking process and, then, grant self-healing characteristics. The silicone supramolecular network (SiR-SN) was formulated by incorporating different proportions of PDMS-NH₂ and PMS-g-COOH. Self-healing properties were evaluated through tensile test on thin elastomer films. Previous to the test, samples were cut in half and, then, clipped together to reattach the surfaces. Then, the specimens were heated 1, 2 and 5 h at 80 and 100 °C. Figure 3.25 shows the mechanical characterization of the samples. Authors stated that, after thermal treatment, all samples showed the ability of self-healing. They attributed different self-healing mechanisms according to the temperature used: at 80 °C leads to the reformation of hydrogen bonds, while at 100 °C there is a conversion of the hydrogen bonds into ionic bonds. In this case, there is a big difference between the hydrogen bond proportions of the composites. A better tensile strength is observed for composites with more hydrogen bonds, because it allows to more links to convert into ionic ones. Finally, an electric field is applied in the samples in order to simulate the final application conditions. In those samples self-healed at 80 °C a mechanical recovery of 100% is obtained when an electric field is applied. Even so, they explain that the samples treated at 100 °C show a greater hardness but without reaching a 100% recovery.

Figure 3.24 Evaluation of self-healing efficiency of PDMS-UI-x composites for different healing times (x varies between 1 and 4, being a value proportional to the molar ratio of urea to imine groups) (Adapted with permission from Chen et al. [62]).

Figure 3.25 Stress–strain curves for a silicone supramolecular with different PMS-g-COOH to PDMS-NH2 ratio: (a) SiR-SN 0.1/1; (b) SiR-SN 0.2/1; (c) SiR-SN 0.5/1 before and after healing under different conditions (Reprinted with permission from Sun et al. [62]).