**2. Liquid phase exfoliation**

Liquid phase exfoliation is an efficient and productive way for synthesizing of single and few layered graphene. It has been considered as one of the most feasible approach for industrial production of graphene due to its scalability and low cost. Solvent – carbon source suspension was first sonicated for preparation of exfoliation. Due to not having defects and oxide groups in the graphene products synthesized by LPE, they are more suitable for use in the electronics industry than that are produced by other techniques.

The LPE can form a stable dispersion of monolayer or few-layer defect-free graphene, which only involves the exfoliation of natural graphite via high-shear mixing or sonication [19]. Prepared graphene dispersion was stabilized by used solvent. Solvent type has also importance in productivity of the graphene dispersion [20]. Solvent ensures both the stability of synthesized graphene mixture and its productiveness. Tetrahydrofuran (THF) and N,N-dimethyl-formamide (DMF) are advantageous solvents to get high quality of graphene merely they are poisonous and show low efficiency. Dibasic ester (DBE) is an a nontoxic and environmentalfriendly solvent and it was used for cleavage of graphite by Jiang et al. Its surface tension is 35.6 mJ/m−2 and solubility parameter is 9.7 [20].

Graphite can be exfoliated in liquid medium exploiting sound waves to form single layer, **Figure 3** [21]. Basically, exfoliation of carbon materials is a relatively economical and easy way to produce graphene [22].

The exfoliation step of the LPE can be conducted by the sonication of graphite in different solvents. There are two types of sonication: tip and bath sonication. In this study, tip sonication treatment was applied to the graphite-solvent dispersions. Epoxy/graphene composite shows better mechanical properties due to direct ultrasonication of tip sonication, that generates higher sound pressures and intensity compared to bath sonication which is indirect ultrasonication [23, 24]. The direct sonication of graphite in a solvent having similar surface energy to graphite enables a

stable graphite dispersion [25]. Several studies have been performed in order to find the most appropriate solvent as well as the optimum operation conditions for the sonication process [26–29].
