illustration of hybrid rubber network in cured ENR/CABt composite CABt."/>
Figure 3.7 Schematic diagram of hybrid rubber network in cured ENR/CABt composite CABt (Reprinted with permission from Xu et al. [42]).
Figure 3.8. Schematic representation of the chemical reaction between: (a) NR and MA, (b) NR-g-MA and FFA, (c) NR-g-furan and bismaleimide (Adapted with permission from Tanasi et al. [27]).
More recently, Khimi et al. studied the influence of carbon black on the self-healing efficiency of NR and investigated an intrinsic self-healing NR by ionomeric interactions [9]. Compounds using NR were mixed in a conventional laboratory two roll mill, combining with Zinc Oxide, Stearic acid, Zinc thiolate, dicumyl peroxide (DCP) and Carbon Black grade N330 in different amounts. The presence of carbon black generate a network, which contains rubber with different degrees of movement restriction: trapped rubber, occluded rubber and bound rubber (Figure 3.9). These constrained rubber region improves the stiffness and elasticity of the matrix increasing tensile strength. However, in Figure 3.10 it can be seen that the healing efficiency decreased with the addition of carbon black from 98% to approximately 40%. This could be explained because of the lower mobility of the chains rubber for the healing process.
Zhan et al. [44] studied graphene (GE)/NR composites with a conductive segregated network prepared by latex compounding, with focus in electronic devices due to rubber-like conductors are considered as one of the most important components in flexible electronics [45]. Usually, the electrical conductivity of a typical conductive rubber decrease due to the tensile cycles in service can destroy the conductive network. Therefore, the objective of the work was to obtain rubber-like conductors that can recover its original properties by a post-treatment. Samples with 10 phr of GE were made and an electrical conductivity of 2.7 S/m was obtained, with a relatively good flexibility. Then, after 4 tensile cycles and subsequent thermal treatment, the electrical conductivity of the sample increased nearly 2 times than that without treatment, indicating that the network destroyed during the tensile cycles can be healed during the post thermal treatment.
Figure 3.9 Schematic presentation of carbon black filler network (Reprinted with permission from Khimi et al. [9]).
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
