## 3 Results and Discussion
### Thermal Oxidation Behavior  
Figure 4 shows typical TG and DSC curves of the epoxy composites studied. Figure 3: Raman spectra of MWCNTs (**a**) and EG (**b**). Figure 2: (**a**) Appearance of EG; (**b–d**) SEM images of EG. The thermal degradation process of epoxy resin and epoxy composites when they are heated in an oxidizing environment can be divided into a few steps [44]. The first step is up to \(\sim\)300 \({}^{\circ}\)C. The endothermic peak was observed on DSC curves around 85-120 \({}^{\circ}\)C and this process is accompanied by slight weight loss. The second step (\(\sim\)300-500 \({}^{\circ}\)C) is caused by the thermal degradation of epoxy resin and the formation of gaseous products as a result of interaction of epoxy resin macromolecules with oxygen. Increasing the temperature above 500 \({}^{\circ}\)C leads to the oxidation of the filler and increases its contribution to the degradation process. The TG results of the neat epoxy polymer E0 and all of the epoxy composites are summarized in Table 2.  
The incorporation of MWCNTs into the epoxy matrix led to an improvement in thermal stability. For the epoxy/MWCNTs composites, a sharp rise in temperature of 5% mass loss \(T_{5\%}\) was observed from 300 \({}^{\circ}\)C for the pure epoxy resin E0 to 319 \({}^{\circ}\)C for the epoxy composite filled with 0.1 wt.% of MWCNTs. With a further increase in the content of MWCNTs to 0.5 wt.%, there was a slow increase in \(T_{5\%}\) to 326 \({}^{\circ}\)C. The values of \(T_{5\%}\) were high enough compared to epoxy composites based on expanded graphite and graphene nanoplatelets [45, 46], taking into account relatively low concentrations of EG (E/1.0 EG) and the increased \(T_{5\%}\) (338 \({}^{\circ}\)C). Figure 4: Thermal degradation curves of epoxy composite samples E0, E/0.5 MWCNT, E/0.5 EG: (**a**) TG, (**b**) differential scanning calorimetry (DSC), and (**c**) derivative thermogravimetry DTG curves. The values of temperature \(T_{20\%}\) for the epoxy/MWCNTs composites were slightly higher or close to that for the pure epoxy resin E0. The temperature of 50% mass loss \(T_{50\%}\) slightly increased with increasing the concentration of MWCNTs in the epoxy matrix. The highest temperature \(T_{80\%}\) of 598 \({}^{\circ}\)C was achieved on the sample E/0.5 MWCNT, although for the sample E/0.05 MWCNT, a local maximum with \(T_{80\%}\) of 582 \({}^{\circ}\)C was also observed. This effect can be linked with a high concentration of filler and its ability to absorb the products of oxidative degradation of sample. It is worth noting that the bulk density of the EG and MWCNTs was 0.018 g/cm\({}^{3}\) and 0.065 g/cm\({}^{3}\), respectively. This fact indicates that expanded graphite has a significantly higher volume fraction in the composite at the same weight of filler. Of course, the increase in concentration of carbon nanotubes could neutralize this effect; however, taking into account the wetting of the filler and the results of preliminary experiments, it has been shown that the upper limit of concentration of the MWCNTs was 0.5 wt.%. When adding EG, the value of \(T_{5\%}\) increased compared to that of the pure epoxy resin E0, reaching a maximum at a concentration of 0.5 wt.% EG, and decreased with a further increase in concentration. It is interesting that \(T_{20\%}\) values of the epoxy/MWCNTs and epoxy/EG composites are almost the same and equal to that of the E0 sample, whereas \(T_{50\%}\) is higher in the entire concentration range of MWCNTs and EG compared to the neat epoxy resin. The thermal behavior of epoxy composites within a temperature range of \(T_{80\%}\) is determined mainly by the filler. The temperatures \(T_{80\%}\) are higher for the samples filled with EG that MWCNTs which is caused by the higher graphitization degree of expanded graphite. The higher disorder degree of MWCNTs compared to EG can be confirmed by the I(D)/I(G) ratio of the Raman spectrum (Figure 3). In [47], it was reported that the expanded graphite did not improve the thermal oxidative behavior of the epoxy/EG composite.