Figure 3.10 Healing efficiency of tensile strength with different carbon black content (Reprinted with permission from Khimi et al. [9]).
Hernandez et al. [35] analyzed the influence of the disulfide and poly-sulfide bonds on the healing ability in NR with a conventional accelerator/ sulfur ratio fixed in 0.2. The samples were vulcanized at two conditions: t90 (optimum time of cure) and t50 (partially vulcanized), obtaining a recovery of the mechanical properties in the last condition after a treatment at 70 °C for 7 h. The result highlight the compromise between healing capability and mechanical performance, in which the crosslinks density, the amount of sulfur, and the amount of labile disulfide and polysulfide crosslinks are key parameters. Through electron spin resonance it was stated that the healing mechanism is based on free sulfur radicals that are thermally induced.
Recently, Utrera-Barrios et al. [46] reported the development of an ENR with thermally reduced graphene oxide (TRGO) nanocomposite vulcanized with dicumyl peroxide (DCP). They found that the mechanical performance improved with the incorporation of TRGO compared with pristine ENR in more than 100% and promotes the hydrogen bonding interactions. In addition, the presence of TRGO gives self-healing capability to the system.
During the vulcanization of with organic peroxides there are two possible reactions: (1) the crosslinks can be generated by the abstraction of the adjacent hydrogen to the double bond to form the C−C bond and (2) the rupture of the double bond results in the chain growing. When ENR is involved, another reaction can take place, which is shown in Figure 3.11: the ring opening of epoxy groups and consequently form of hydroxyl groups (−OH), creating in the matrix a thermo-reversible supramolecular network [47–49]. Therefore, a high concentration of epoxy groups promotes the formation of hydroxyl groups and promotes the formation of hydrogen bonds, and consequently the self-healing capability.
Figure 3.11 Proposed scheme of self-healing mechanism in epoxidized natural rubber and thermally reduced grapheme oxide composites (ENR/TRGO) (Reprinted with permission from Utrera-Barrios et al. [46]. Copyright 2020 American Chemical Society).
3.3.2 Styrene Butadiene Rubber (SBR)
Styrene butadiene rubber is the most used system among the synthetic rubbers. Due to the huge amount of scrap from tires, some researchers focused its interest on the self-healing behavior, which will allow to increases the time of use and the development of high performance smart tires.
Hernandez Santana et al. analyzed the self-healing behavior in SBR compounds containing ground tire rubber (GTR) particles and the coupling agent bis[3-(trietoxysilyl)propyl] tetrasulfide (TESP) [50]. Due to healing process requires chain mobility, authors prepared samples varying the accelerant/sulfur ratio (A/S) in order to evaluate the influence of the density and type of crosslinks: mono, di and polysulfides [51]. The sulfur amount was fixed in 0.7 phr.
The healing efficiency was evaluated through tensile tests: once the test was performed, the two pieces were repositioned together at 70 °C in a press at 10 bar for 7 h. Afterwards, the sample was tested again and the obtained results are shown in Figure 3.12. It was observed that systems with the lowest tensile strength exhibit the higher healing efficiency, corresponding to the ratios A/S = 0.2 and 1. The healing efficiency is attributed to the chain entanglement between the dangling chains on each piece, followed by thermal scission of the di and poly-sulfide bonds, which increases the chain mobility in the broken area. Also was confirmed that the healing efficiency is strongly reduced if the chain mobilization decreases due to a denser crosslinking network or by the use of reinforcement particles.
Araujo-Morera et al. [52] incorporated different amounts of GTR (10, 20 and 30 phr) into an SBR compound. The samples were submitted to 20 stretch cycles up 70 % strain in each cycle. After that, each sample was stored 12 h at room temperature and 12 h at 70 ºC. Through dynamic mechanical analysis (DMA) it was stated that the elastic modulus (E’) diminishes in all the samples once the damage process was applied. That behavior is attributed to the Mullins effect, which is a result of different contributions like: chain scission, breaking of crosslinks, chains disentanglement, filler deagglomeration and softening of the chains that were near the filler surface (bound rubber) [53]. Once the healing protocol was applied, an efficiency of 110 % was obtained in the unfilled sample, which is attributed principally to the entanglement of the dangling chains and to the re-crosslinking of the damaged poly and disulfide crosslinks. Same authors used dielectric spectroscopy to analyze the type of interaction within the rubber composites. Figure 3.13 shows dielectric spectrum of different samples at −25 °C. In the case of the unfilled sample, either in the virgin and damage states, a similar spectrum was obtained, which indicate that the relaxation processes of the sample were exactly the same (Figure 3.13a). After the healing protocol, the spectrum presents a slight widening at high frequencies, indicating a higher number of entanglements promoted by the thermal healing treatment. In the case of the compound with 10 phr of GTR, a particular behavior was observed in Figures 3.13b and c, since the virgin sample indicates the presence of a percolated network that is broken with the cycled deformation, which cannot be recovered after the healing treatment.
Figure 3.12 SBR vulcanized with different values of the ratio accelerant/sulfur (A/S): (a) Tensile strength of pristine and healed samples and (b) Healing efficiency and crosslink density as function of A/S (Reprinted with permission from Hernandez Santana et al. [50]).
Kuang et al. [38] crosslinked furfuryl grafted SBR (SBR-FS) with bismaleimide (M2) via DA reaction at different molar ratios (1/1, 2/1 and 3/1). After the tensile test, the broken samples were pressed and healed at different thermal conditions (70 °C for 30 min, 100 °C for 30 min and 100 °C for 5 h) to induce the healing behavior giving by the reversibility of the DA/ rDA reaction. In addition, author included carbon nanotubes functionalized with furfuryl groups (MWCNT-FA) were included in the compound.
Figure 3.13 Dielectric parameters (e” and s”) as function of frequency at −25 °C: (a) unfilled SBR and (b, c) SBR/10GTR compound (Reprinted from Araujo-Morera et al. [52], open access).
Figure 3.14 (a) Stress–strain curves of SBR-FS/M2 = 3/1 with 5% of MWCNT-FA and (b) Healing efficiency as a function of MWCNT-FA; both for the original sample and healed samples at the specified healing conditions (Adapted with permission from Kuang et al. [38]).
Figure 3.14(a) exhibits the stress–strain curves of samples whose molar ratio was 3/1 of furan/maleimide and 5% of MWCNT-FA. It can be observed that the stress–strain curves with the 3 different conditions after the healing process are overlapped. Moreover, the healing efficiency increases with temperature and time during which the temperature is applied. In Figure