## 3 Results and Discussion
### The Microstructure, Flame-Retardant Properties, and Mechanical Properties of EVA Composites  
The microstructures of EVA/MH and EVA/MH/PCS/DVB composites are shown in Figure 8. It can be seen that the MH particle is partially exposed and partially embedded in the EVA matrix (Figure 8a). A noticeable gap exists between MH particles and the EVA matrix, suggesting poor compatibility of MH within the EVA matrix. The MH particle is well covered by EVA and no gap is observed between MH/PCS/DVB and the EVA matrix (Figure 8b), indicating a substantial enhancement of the compatibility between MH and the EVA resin following the PCS and DVB coating modification.  
The flame retardancy of EVA/MH/PCS/DVB is given in Figure 9a. The limiting oxygen index (LOI) of EVA/MH with 50 wt.% MH content was only 24.9, while the LOI values of EVA/MH/PCS and EVA/MH/PCS/DVB were high at 62.0 and 65.1, with increases of 149% and 161% in comparison with EVA/MH, respectively. It is interesting that the vertical burning behavior of EVA/MH/PCS was not improved, while the UL-94 of EVA/MH/PCS/DVB could reach a V-2 rate and the LOI value further increased by 12 percentage points compared with EVA/MH/PCS. Significantly, the flame retardancy of EVA/MH/PCS/DVB was further improved based on the high LOI of EVA/MH/PCS. In contrast, the flame retardancy of EVA/MH/PCS+DVB was decreased, which means the PCS cross-linked by DVB had a more significant synergistic effect with MH than with PCS itself.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Samples** & **MH** & **MH/PCS** & **MH/PCS/DVB** \\ \hline T\({}_{5}\) (\({}^{\circ}\)C) & 369 & 389 & 389 \\ T\({}_{10}\) (\({}^{\circ}\)C) & 383 & 399 & 402 \\ T\({}_{\text{max1}}\) (\({}^{\circ}\)C) & 388 & 404 & 413 \\ R\({}_{\text{max1}}\) (\({}^{\circ}\)C/min) & \(-\)0.66 & \(-\)0.91 & \(-\)0.73 \\ Residue\({}_{600^{\circ}}\)C (\%) & 70.0 & 71.9 & 72.4 \\ \hline \hline \end{tabular}
\end{table}
Table 4: TG and DTG parameters of flame retardants.  
Figure 7: TG **(a)** and DTG **(b)** curves of flame retardants.  
The mechanical properties of the EVA composites were characterized via tensile tests. The tensile strength and elongation at break data of the EVA composites are shown in Figure 9b,c. The tensile strength and elongation at break of EVA/MH/PCS were decreased compared to EVA/MH; on the other hand, the cross-linking of PDC/DVB improved both mechanical properties of the EVA composites. The results show that the introduction of PCS/DVB can improve the mechanical properties of EVA composites, and the resulting EVA composites can meet the wire and cable sheathing requirements.  
The combustion states of EVA composites at different combustion times during the LOI test are shown in Figure 10. The combustion of the EVA/MH composite was slow and showed a mitigatory flame during the test. The char on the top of the composite was in the cycle of accumulation and shedding. While the combustion of the EVA/MH composites with PCS was drastic and showed a roaring flame, the char was firmly fixed on the top of the composites once formed, and the flame was effectively restricted and died out. However, the combustion of EVA/MH/PCS+DVB was intensified compared with EVA/MH/PCS/DVB and EVA/MH/PCS, which indicates an enhanced synergistic effect between MH and PCS cross-linked by DVB.  
Figure 8: SEM micrographs of EVA/MH (**a**) and EVA/MH/PCS/DVB (**b**).  
Figure 9: Flame retardancy of composites (**a**), stress–strain curves of EVA composites (**b**), and histogram of EVA composites (**c**).