## 3 Results and Discussion
### Famambability Test  
The evaluation of the flammability of the polymer materials was carried out using indicators such as the ignition temperature and the time-to-ignition. The ignition temperature is the lowest temperature at which a combustible substance, when heated, catches fire in the air and continues to burn. The time-to-ignition is defined as the minimum exposure time required for the sample to ignite and sustain a flaming combustion. The results of the experimental determination of the ignition temperature and the average time-to-ignition for the studied epoxy composites are shown in Table 4.  
According to the obtained results, for the samples of epoxy composites filled with MWCNTs, a sharp growth of ignition temperature was observed from 308 \({}^{\circ}\)C for the pure epoxy resin E0 to 328 \({}^{\circ}\)C for the epoxy composite filled with 0.05 wt.% MWCNTs. With a further increase in the content of MWCNTs to 0.5 wt.%, there was a rapid decrease in the value of ignition temperature to 316 \({}^{\circ}\)C at 0.1 wt.%, followed by a slow decrease in the ignition temperature to 315 \({}^{\circ}\)C and stabilization at the content of MWCNT at 0.5 wt.%. Figure 5: Optical micrographs of E/0.5 MWCNT (**a**) and E/0.5 EG (**b**) samples (the unit scale is 200 \(\upmu\)m). \begin{table}
\begin{tabular}{c c} \hline
**Sample** & **Hardness** \\ \hline E0 & 78 \(\pm\) 2 \\ E/0.01 MWCNT & 79 \(\pm\) 3.2 \\ E/0.05 MWCNT & 79 \(\pm\) 1.7 \\ E/0.1 MWCNT & 81 \(\pm\) 1.1 \\ E/0.5 MWCNT & 75 \(\pm\) 2.3 \\ E/0.5 EG & 80 \(\pm\) 1.1 \\ E/1.0 EG & 76 \(\pm\) 2.5 \\ E/1.5 EG & 73 \(\pm\) 2.2 \\ E/2.0 EG & 77 \(\pm\) 1.4 \\ E/2.5 EG & 78 \(\pm\) 2.1 \\ \hline \end{tabular}
\end{table} 
Table 3: Shore D hardness results of epoxy/MWCNTs and epoxy/EG composites. The data on ignition temperature are in agreement with data on the kinetics of the process. For example, model-free analysis of TG data by ASTM E698 showed the activation energy of the process was 91.4 \(\pm\) 16.76 kJ/mol and pre-exponential factor log(A/s\({}^{-1}\)) = 4.6. The activation energy slightly grows for E/0.5 MWCNT compared to pure epoxy and it was 97.28 \(\pm\) 15.5 kJ/mol (log(A/s\({}^{-1}\)) = 5.11), whereas the activation energy increases significantly for composite based on EG, and for E/0.5 EG, it was 124.44 \(\pm\) 7.37 kJ/mol (log(A/s\({}^{-1}\)) = 7.56). The results on data treatment by model-free analysis are presented in the Supplementary Materials (Figures S3-S5). The incorporation of expanded graphite in epoxy resin also led to an increase in the ignition temperature. In this case, the rise in the ignition temperature from 308 \({}^{\circ}\)C for the unfilled epoxy resin E0 to 326 \({}^{\circ}\)C for the epoxy composite filled with 1.5 wt.% EG was observed. The further increase in the content of the EG in the epoxy resin to 2.5 wt.% caused the decrease in the ignition temperature to 321 \({}^{\circ}\)C.  
Among the studied fillers, the maximum ignition temperature was observed for the epoxy composite filled with 0.05 wt.% MWCNTs. For the epoxy/EG composites, an increase in the ignition temperature was observed at concentrations of EG in the range of 1.0-2.5 wt.%. The char layer formed by carbon-based fillers on the surface of the epoxy matrix acts as a heat barrier [15; 49] and increases the ignition temperature of the epoxy composites. At the same time, for almost all samples, a decrease in the time-to-ignition was observed compared to the pure epoxy resin, except for the sample E/0.5 MWCNT. This effect is caused by the high thermal conductivity properties of MWCNTs and EG [46; 50], and the increase of the epoxy composites thermal conductivity as a result of the MWCNT addition. Therefore, the ignition temperature for the epoxy/MWCNTs and epoxy/EG composites was reached faster than for the pure epoxy resin. The improvement of thermal conductivity and flammability of polypropylene composites with expanded graphite was also reported in [51].