## 3 Results and Discussion
### Thermal Analysis and Flame Retardancy of EVA and its Composites  
Figure 6 illustrates the TGA and DTG curves of EVA and its composites with 20 wt. % loading of LDHs. The mass loss of EVA shows two degradation steps in the temperature range from 307 to 550 \({}^{\circ}\)C. The first one is attributed to the deacetylation of acetate side groups. The second one is due to the degradation of polyethylene chains at high temperatures [43]. However, all of the composites display three mass loss steps. The first step corresponds to the removal of the interlayer water molecules, dehydroxylation and decomposition of the interlayer anions in the range of 120 to 307 \({}^{\circ}\)C [14]. From the DTG curves, it can be seen that the \(T_{\text{max}}\) of the acetate side groups for all of the composites occurs at 383 \({}^{\circ}\)C, that of the ethylene-based chains is at 496 \({}^{\circ}\)C for S-NiMgAl-Co/EVA, S-NiMgAl-Cu/EVA and S-NiMgAl-Zn/EVA, while that of S-NiMgAl-Mn/EVA is at 503 \({}^{\circ}\)C. These values are higher than that of the EVA (484 \({}^{\circ}\)C) and the earlier reported undoped transition metal composite NiMgAl-LDH/EVA (493 \({}^{\circ}\)C) [22]. Figure 6: **A**: TGA and **B**: DTG curves for ethylene vinyl acetate (EVA) and composites. (**a**) EVA; (**b**) S-NiMgAl–Mn/EVA; (**c**) S-NiMgAl–Co/EVA; (**d**) S-NiMgAl–Cu/EVA and (**e**) S-NiMgAl–Zn/EVA. Cone calorimeter test is used to predict the combustion behavior of materials in real fire scenarios [44]. Figure 7a-d present the heat release rate (HRR), the total heat release (THR), the smoke production rate (SPR) and the production rate of CO (COP) curves of the pure EVA and its composites, respectively. Table 1 lists the corresponding combustion data. From Table 1 and Figure 7a, it can be seen that the peak heat release rate (pk-HRR) of EVA, S-NiMgAl-Mn/EVA, S-NiMgAl-Co/EVA, S-NiMgAl-Cu/EVA and S-NiMgAl-Zn/EVA are 1247, 445, 467, 451 and 485 kW/m\({}^{2}\), respectively. The reductions in pk-HRR for all of the composites are 61% greater than that of EVA. In addition, the pk-HRR time for EVA, S-NiMgAl-Mn/EVA, S-NiMgAl-Co/EVA, S-NiMgAl-Cu/EVA and S-NiMgAl-Zn/EVA are 315, 466, 429, 170 and 149 s, respectively. S-NiMgAl-Mn/EVA shows the lowest pk-HRR and the longest pk-HRR time among all samples. At the same time, compared with the ignition time (\(T_{\text{ign}}\)) of the EVA of 62 s, the \(T_{\text{ign}}\) of S-NiMgAl-Mn/EVA, S-NiMgAl-Cu/EVA and S-NiMgAl-Zn/EVA are postponed to 74, 82 and 66 s, respectively. In contrast, \(T_{\text{ign}}\) of S-NiMgAl-Co/EVA was shortened to 58 s. Therefore, S-NiMgAl-Cu/EVA is the most difficult material to burn for longer \(T_{\text{ign}}\). The fire performance index (FPI) is another important fire resistance parameter. FPI is defined as the ratio of pk-HRR to \(T_{\text{ign}}\). The FPI values of S-NiMgAl-Mn/EVA, S-NiMgAl-Co/EVA, S-NiMgAl-Cu/EVA and S-NiMgAl-Zn/EVA are 6, 8, 6 and 7 kW/m\({}^{2}\)s, respectively. These values are much smaller than that of the EVA (20 kW/m\({}^{2}\)s), confirming that S-NiMgAl-Mn/EVA and S-NiMgAl-Cu/EVA have better flame retardant efficiency. In addition, the THR values for all the composites are slightly decreased compared with that of the EVA. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **pk-HRR** & **Reduction** & **Tign** & **FPI** & **THR** & **pk-SPR** & **pk-COP** \\  & **(kW/m\({}^{2}\))** & **(\%)** & **(s)** & **(kW/m\({}^{2}\)s)** & **(M/m\({}^{2}\))** & **(m\({}^{2}\)/s)** & **(g/s)** \\ \hline EVA & 1247 & - & 62 & 20 & 164 & 0.146 & 0.0147 \\ S-NiMgAl-Mn/EVA & 445 & 64.3 & 74 & 6 & 151 & 0.100 & 0.0044 \\ S-NiMgAl-Co/EVA & 467 & 62.6 & 58 & 8 & 150 & 0.107 & 0.0049 \\ S-NiMgAl-Cu/EVA & 451 & 63.8 & 82 & 6 & 151 & 0.0997 & 0.0045 \\ S-NiMgAl-Zn/EVA & 485 & 61.1 & 66 & 7 & 155 & 0.102 & 0.0044 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cone calorimetry data for composites with 20 wt. % loading of stearate surface-modified LDHs containing transition metal elements (heat flux of 50 kW/m\({}^{2}\)).