## 3 Results and Discussion
### Thermal Properties of the Composites  
The thermal stability of pure epoxy and epoxy/70 wt.% G150/10 wt.% (G44 or G75) composites was investigated, and the TGA and DTG curves are shown in Figure 7. Maximum decomposition temperatures and weight losses of the epoxy and epoxy/G composites at different temperatures are tabulated in Table 2. The samples are almost thermally stable,  
Figure 6: The tensile strength values of epoxy/G composites: (**a**) epoxy/G150 prepared at 110 \({}^{\circ}\)C, 60 min; (**b**) epoxy/70% G150/G75 and epoxy/70% G150/G44 prepared at 110 \({}^{\circ}\)C, 60 min; (**c**) epoxy/70 wt.% G150/10 wt.% (G44 or G75) prepared at 60 min and different curing temperatures; (**d**) epoxy/70 wt.% G150/10 wt.% (G44 or G75) prepared at 150 \({}^{\circ}\)C and different curing times.  
with minor weight loss percentages occurring up to 200 \({}^{\circ}\)C, reaching approximately 0.24% and 0.33% for epoxy/G150/G75 and epoxy/G150/G44 composites, respectively, at an overall filling load of 80 wt.%. This minor weight loss might be attributed to the evolution of moisture and volatiles [54]. The weight loss percentages at the maximum decomposition temperatures were 11.84% for epoxy/G150/G75 and 14.01% for epoxy/G150/G44 composites. This might be attributed to the degradation of epoxy itself [54]. [PERSON] et al. [16] also reported a 0.6% weight loss of the novalac epoxy/NG composite material at a temperature of 100 \({}^{\circ}\)C. As expected, the degraded weight decreased by filling the epoxy with G particles [55]. [PERSON] [56] attributed the improvement of thermal stability of epoxy/G composites at 6 wt.% G content to the uniform distribution of G particles and the formation of tortuous path, which hinders the diffusion of the volatile decomposition products compared to that in pure epoxy. The addition of G filler of different sizes (G150/G75 or G150/G44) had a slight difference in the values of the degraded weight and the maximum decomposition temperatures and a better thermal stability of epoxy/G150/G75 composite compared to epoxy/G150/G44 composite. According to the DSC results shown in Figure 8, attention was given to the glass transition temperatures (T\({}_{\mathrm{g}}\)) of the epoxy and its composites. It was observed from the DSC curves that there was a slight deviation of T\({}_{\mathrm{g}}\) from 66 \({}^{\circ}\)C for epoxy/G150/G44 to 69 \({}^{\circ}\)C for epoxy/G150/G75 composites compared to that of pure epoxy (68 \({}^{\circ}\)C). [PERSON] et al. [57] showed that the nano-sized fillers in epoxy composites had a clear effect on the reduction of T\({}_{\mathrm{g}}\) compared to composites having micro-sized fillers. They also indicated that the surface conditions of the fillers and their dispersion besides the interaction at the filler-resin interface play a crucial role for determining the T\({}_{\mathrm{g}}\) of the composites. [PERSON] et al. [58] found that the lower aspect ratio filler with the higher specific surface area has better interfacial interactions between the polymer matrix and the filler, which restricted the molecular mobility of the polymer and resulted in a higher T\({}_{\mathrm{g}}\) value. In this study, using a narrow range of particle sizes of the secondary filler (i.e., 75 \(\upmu\)m and 44 \(\upmu\)m) might not display much difference on T\({}_{\mathrm{g}}\).  
Figure 7: TGA and DTG curves of pure epoxy, epoxy/G150/G44 and epoxy/G150/G75 composites at 70 wt.% G150 and 10 wt.% G44 or 10 wt.% G75 prepared at 150 \({}^{\circ}\)C for 120 min.