## 1 Introduction
[27], and they found that the highest electrical conductivity was obtained in EG/phenolic composites with the largest EG size of 300 \(\upmu\)m. The effect of particle size and graphite morphology on the electrical conductivity of graphite/polypropylene (G/PP) composite bipolar plates was studied by [PERSON] et al. [28]. The load of conductive filler material was maintained at 78 wt.%, and compounds with three different sizes of flake-shaped conductive fillers with sizes of 5, 10, and 20 \(\upmu\)m were produced. The results showed that the electrical conductivity of G/PP composites increased with the decreasing particle size of flake-shaped graphite and that the flake-shaped particles were able to produce G/PP composites with higher electrical conductivity than those containing sphere-shaped graphite. [PERSON] et al. [29] investigated the effect of hybrid conductive filler loading up to 50 wt.% on the electrical conductivity and tensile strength of conductive polymer composite materials. As the main filler, 74 \(\upmu\)m graphite particles were used, and 13 and 5 \(\upmu\)m G particles were used as the secondary conductive fillers. They found that the incorporation of a smaller conductive filler resulted in a significant increase in electrical conductivity and tensile strength compared to CPCs using only the main conductive filler with a size of only 74 \(\upmu\)m.  
On the other hand, many studies have highlighted the optimization of the processing parameters of fabrication techniques to improve the electrical and mechanical performance of CPCs [24; 30]. The improvement of the properties is associated with the dispersion of the filler in the polymers, which depends on the adopted processing method [31; 32]. A study on the composites polyethersulfone (PES) filled with G and CB prepared by solution blending technique reported an electrical conductivity four times higher than that prepared by powder mixing method [33]. [PERSON] et al. [34] conducted a study on the effect of curing and mixing processes on the electrical conductivity of epoxy multiwall carbon nanotube (MWCNT) composites. They found that the mixing conditions and curing temperature were the controlling factors for increasing the electrical conductivity of the resulting composite material. [PERSON] et al. (2017) studied the mechanical-thermal-electrical performance of epoxy/GNPs processed by two dispersion methods: three-roll milling and sonication combined with high speed shear mixing technique. Composites prepared by three-roll milling showed higher electrical conductivity of almost three orders compared to that prepared by the sonication process, which was related to the dispersion degree and the spatial distribution of filler. [PERSON] et al. (2017) analyzed the influence of post-curing temperature and preparation method on the mechanical, electrical, and physical properties of G/CF/copper/phenolic resin composites. It was found that the properties of composites improved with increasing post-curing temperature. Finally, most of the previous studies in the literature focused on the improvement of the electrical and mechanical properties of polymer/G composites either by using a specified size of G particles incorporated into the composite ([PERSON] et al., 2016; [PERSON] et al., 2017; [PERSON] et al., 2018; [PERSON] et al., 2019) or by hybridizing large G particles with other types of smaller conductive fillers such as CB, CNTs, and CF ([PERSON] et al., 2016; [PERSON] et al., 2017; [PERSON] et al., 2018; [PERSON] et al., 2019). Few studies investigated the synergistic effect of using different sizes of the same kind of conductive filler in the composite ([PERSON] et al., 2018). [PERSON] et al. (2018) fabricated a PP/G composite with a 75 wt.% G content and found that the combination of two G sizes of 40 and 200 \(\upmu\)m exhibited better electrical conductivity and maintained good flexural strength.