# Mechanical Properties of Green Synthesized Graphene Nano-Composite Samples
## 2 Materials and Methods
### Flexural Test  
After preparation of the composite mixture, it was transferred to a die. The die consisted of cavities 100 mm in length and 20 mm wide. The mixture was poured to prepare an 8-mm thick composite sample. After post-curing, the surfaces of the hardened samples were polished with soft sandpaper. The sample was placed in a three-point bending fixture with a span length of 80 mm. The bending test was carried out at a 3 mm/min speed, and the force-displacement data was logged. Three samples were prepared and tested for each weight percentage of green synthesized nanocomposites. Figure 4 shows the implementation of the three-point bending test with a universal tensile testing machine. \[\sigma_{f}=\left\{\begin{array}{ll}\frac{3 PL}{bd^{2}}&\text{ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \  
The SEM images of the fractured surface of the graphene nanocomposites are shown in Figure 6. As can be seen in Figure 6, the presence of nanoparticles on the surface of the polymer matrix increased by increasing the concentration of graphene quantum dots in the polyester matrix. The graphene quantum dots were prepared in green synthesis. The morphology of the graphene was spherical dots (20 nm average size). Figure 7 shows the XRD pattern of the carbon graphene quantum dots. The pattern matched the standard peaks of JCPDS card No. 00-08-0415 and confirmed the graphene production. The Fourier-transform infrared (FTIR) spectrum of the as-prepared carbon graphene dots is shown in Figure 8, with a broad and strong absorption band of about 3422 cm\({}^{-1}\) attributed to vibration stretching in the hydroxyl group. Absorptions at 2812, 2850 and 2918 cm\({}^{-1}\) were due to the C-H stretching vibrations. The bands at 1633 cm\({}^{-1}\) were related to the stretching vibration in C=O. The bands at 1383 and 1576 cm\({}^{-1}\) were attributed to the C=C bonds. The absorption band at 1042 cm\({}^{-1}\) was for stretching vibrations in C-O, while the bands obtained at 798 and 760 cm\({}^{-1}\) were attributed to yjr stretching vibration in C-C [29; 30; 31]. Figure 6: Micrograph of the fracture cross-section for (**a,b**) 0.25% wt., (**c,d**) 0.5% wt. and (**e,f**) 1% wt. graphene quantum dots. The influence of graphene quantum dot additives on the fire retardancy of the polymer resin was examined applying the UL-94 test (a sample of 130 \(\times\) 13 \(\times\) 1.6 mm was used). A flame (1.5 cm) was applied to the specimen (time: 10 s) twice. When the sample extinguished in less than 10 s after fire ignition, it was in a V-0 class (drips are allowed as long as they are not inflamed). A V-1 type is for a sample with a fire time of less than 30 s (drips are like those in the V-0 class). The V-2 class has a fire time like the V-1 class while flaming drips are permitted. When the total flaming time is above 50 s, it is not classified (NC). Finally, horizontal burning with a rate of less than 76 mm/min is HB [29; 30; 31]. The UL-94 test for the pure polymer was NC, while the nanocomposites were confirmed for V-1 classification (Figure 9). Fire retardancy of the nanocomposites was due to the high surface-to-volume ratio, which dispersed into the matrix homogeneously, and the formation of a char barrier during combustion. This barrier decreased the evaporation of organic components and reduced the oxygen and flame reaching the sample. Figure 8: FTIR spectrum of the nanocomposites. Figure 7: XRD pattern of carbon graphene quantum dots. Limiting oxygen index (LOI) analysis of the 0.25, 0.5 and 1% wt. graphene quantum dots was implemented and is shown in Table 2. The results showed that by increasing the nanoparticle concentration, the LOI value increased from 17 to 22 (29.4% enhancement), and fire retardancy was also improved. In addition, other investigations on adding nanoparticles (carbon nanostructure, multi-walled carbon nanotubes (MWCNT) to polyester showed that the flame retardancy of the nanocomposites improved, and thermal decomposition of the nanocomposites was shifted toward higher temperatures [32; 33; 34].