**3. Mechanical characteristics**

### **3.1 Effects of a covalent interphase**

Many graphene-based polymer composites with significantly improved properties have been prepared and tested. Graphene with its possession of remarkable physical properties have been proposed as an efficient reinforcement agent for elastomers [94–100]. To make the best out of graphene in reinforcement, improved dispersion of graphene and higher interfacial interactions are important [101–104].

Graphene oxide is efficiently reduced by tannin derivatives. At the same time, *ortho*-quinone derivatives produced during the reduction of graphene oxide are adsorbed on the graphene surface [105, 106]. The quinones are reactive toward thiols via Michael addition [107, 108]. Polythiols are usually produced during the sulfur-cure of an elastomer [108, 109]. The *ortho*-quinone derivatives are quite suitable for constructing covalent interphase cross-linking between graphene and the rubber matrix.

In a study by Yang et al. [106], graphene/styrene-butadiene rubber (SBR) nanocomposites were prepared by latex co-coagulation of SBR latex with a suspension of graphene. They pursued a strategy to construct a strong interface in graphene/SBR nanocomposites. The *ortho*-quinone covalent interphase was realized in the graphene/ rubber system. The main feature was superior dispersion and controlled reinforcement properties with the quinone-modified graphene in the rubber matrix. In this nanocomposites system, the energy loss by a tire was lower compared with those for carbon black-filled elastomers. **Figure 2** represents the manufacture of graphene/ SBR nanocomposite and the mechanism of formation of covalent interphase between graphene and the SBR Matrix. The developed nanocomposites were applied to a dynamic elastomeric product, the auto tires. The very low rolling resistance coefficient is considered a great advancement in the production of tires.

#### **3.2 Mechanical behavior of a graphene/SBR nanocomposite**

With the introduction of covalent interphase, the dispersion status of graphene in the rubber matrix and the interfacial interaction between graphene and SBR were remarkably enhanced. The stress-strain isotherms of SBR and SBR nanocomposites are illustrated in **Figure 3**. The effect of graphene on the mechanical properties of

#### **Figure 2.**

*Fabrication of graphene/SBR nanocomposites and a schematic illustration of proposed formation mechanism of covalent interface between graphene and rubber matrix [109].*

**Figure 3.** *Stress-strain curves of SBR/graphene nanocomposites [107].*

graphene/SBR nanocomposites is apparent. For purposes of comparison, a stressstrain isotherm of 10% graphite-filled SBR composite was also made. The presented results indicate much higher efficiency in the reinforcement of SBR. In **Figure 4**, the effect of graphene loading on the mechanical properties of the nanocomposites is demonstrated. Compared with the neat SBR, the tensile strength of graphene/SBR nanocomposites with only 1.1 phr of graphene has increased by 223%. With the incorporation of 5.6 phr of graphene, the tensile strength of the nanocomposite can reach 21.5 MPa, which is over ninefold that of the neat SBR. This is a marked enhancement in reinforcement compared with other 2D reinforcing fillers such as clay [110–112].

As an additional confirmation of reinforcement, a percolation phenomenon is observed in **Figure 5** in the tensile strength with graphene loading, as has been reported earlier [106, 113–118]. Although for CNT-filled composites, 0.001 wt% percolation concentrations have been reported. In other studies, 2 wt% concentrations are reported that depend on the processing, alignment, chemistry of the CNTs, and matrix compatibility [116–118].

*Processing of Graphene/Elastomer Nanocomposites: A Minireview DOI: http://dx.doi.org/10.5772/intechopen.104849*

**Figure 4.** *Tensile modulus and strength of graphene/SBR nanocomposites [107].*

**Figure 5.**

*Percolation phenomena of SBR nanocomposites, the intersections of the dash lines represent the percolation point [107].*

In what would be considered the first stage, below the percolation threshold volume fraction of graphene, the phenomenon could be explained by the rubber strengthening mechanism that graphite nanofillers induce the formation of straightened polymer chains during the stretching process [119]. In the second stage, when the distance between the fillers decreases to a specific threshold, chains attached to the adjacent fillers will form straightened rubber chains that will enhance and strengthen the rubber [119]. In **Figure 5**, the tensile stress of the composites could be observed to initially increase slightly and then shows an abrupt increase. The percolation point of graphene/SBR

nanocomposite appears surprisingly at as low as 0.42 phr, which is far below that of CB/SBR composite (5.1 phr), as seen in **Figure 5**. The ultralow percolation threshold of graphene/SBR composites might be ascribed to the excellent dispersion of ultrathin graphene layers and the strong covalent interface. The different result obtained for graphene and carbon black (CB) in the reinforcement of SBR composites is evidence that graphene possesses significantly higher reinforcing efficiency toward SBR [106].

The effect of the presence of the nanostructures on the thermal stability of natural rubber was evaluated using thermogravimetric analysis (TGA). The results indicated that the presence of GO does not affect the thermal stability of the rubber [106, 119].
