# Flame Retardance and Smoke Suppression of CFA/APP/LDHs/EVA Composite
## 3 Results and Discussion
### Flame Retardance and Smoke Suppression Mechanism of Composites  
From Table 1, it can be seen that the LOI value of ELDH-0, from 18% which is of the pure EVA, increases rapidly to 28.5%. Then LOI values of ELDH-Ni, ELDH-Co and ELDH-Cu was increased to 30.3%, 29.7%, and 30.8% due to the addition of LDHs. The LOI test indicates that the CFA/APP system and LDHs have significant synergistic flame retardant effects. The vertical burning (UL-94) test shows that ELDH-Ni, ELDH-Co, and ELDH-Cu reach UL-94 V-2, V-0 and V-2 ratings, respectively. However, ELDH-0 without a UL 94 rating was achieved due to the presence of melts dripping. The UL-94 result also confirms that the synergistic flame retardant effects between CFA/APP and LDHs. To further identify the synergetic effects between LDHs and CFA/APP on the flame retardance and the smoke suppression behavior of composites, Figure 5 shows the heat release rate (HRR) curves of the pure EVA and composites by the cone calorimeter test. The HRR is considered to have a significant influence on fire hazards. Simultaneously, the peak value of HRR (pK-HRR) is used to assess the intensity of a fire. It can be seen that the pure EVA burned rapidly after ignition, and its pK-HRR value rises to 1120 kW/m\({}^{2}\). However, the pK-HRR values of ELDH-0, ELDH-Ni, ELDH-Co, and ELDH-Cu decrease to 258, 214, 231, and 206 kW/m\({}^{2}\), respectively. The reductions in pk-HRR for composites are 77% greater than EVA. In particular, the reduction in pk-HRR of ELDH-Cu reaches 82%. The HRR results also indicate that the flame retardant synergism was observed when LDHs are added into the composites with CFA/APP. Figure 4: TGA-DTG curves of EVA and composites, (**a**) TGA curves; and (**b**) DTG curves. For a long time, the char layer quality is considered to have an important influence on the flame retardance. Figure 6 shows the digital photographs of the residues after cone calorimeter tests. The residue of ELDH-0 has some holes. The residues of ELDH-Ni, ELDH-Co, and ELDH-Cu are not only thin, but also broken into several big pieces. To exhibit the internal morphology of the char residue, the SEM images, after cone calorimeter tests, are shown in Figure 7. The internal residue morphologies of four composites are obviously different. The residue of ELDH-Ni has a smooth structure with some holes of different sizes. The residues of ELDH-Co have many pores. These pores prevented melt dripping of ELDH-Co during combustion. The residues of ELDH-Cu and ELDH-0 have a compact morphology. All of the residues have no intumescent char layer, but metal oxide, char and APP promote charring via the cross-linking of reactive EVA fragments impeding the burning process by reducing the oxygen supply to the bulk phase [35]. Figure 5: HRR (heat release rate) curves of the pure EVA and composites. (**a**) EVA; (**b**) ELDH-0; (**c**) ELDH-Ni; (**d**) ELDH-Co; and (**e**) ELDH-Cu. Figure 6: Digital photographs of the residues after cone calorimeter tests. (**a**) ELDH-0; (**b**) ELDH-Ni; (**c**) ELDH-Co; (**d**) ELDH-Cu. The mechanism of the synergistic effects between APP, CFA and LDHs can be included in the following. The formation of carbonized barrier prevents or slows the transfer of heat, oxygen, and combustible volatiles into the pyrolysis zone, retarding the combustion process [35]. APP decomposes at 200-250 \({}^{\circ}\)C, it releases nonflammable gas such as N\({}_{2}\) and NH\({}_{3}\) to dilute the oxygen [36]. CFA decomposes at 319-450 \({}^{\circ}\)C, it releases H\({}_{2}\)O and NH\({}_{3}\)[16]. LDHs decompose to form H\({}_{2}\)O and CO\({}_{2}\), which could dilute the flammable gases and oxygen, and decrease the temperature [35]. Therefore, the pk-HRR values of composites are obviously lower than pure EVA. Furthermore, the LDH-Cu has a lower maximum mass loss rate temperature of hydroxyls, carbonates and stearate than LDH-Ni and LDH-Co. The consequent LDH-Cu generated H\({}_{2}\)O and CO\({}_{2}\) can cool, dilute and isolate oxygen during combustion early and rapidly.