## 3 Results and Discussion
### Combustion Behavior of Composites  
The cone calorimetry method is used to predict the burning behavior of materials in real fire scenarios. All cone calorimeter tests were carried out in a cone calorimeter at an incident heat flux of 50 kW/m\({}^{2}\) according to the ISO 5660-1 standard. Better understanding of the underlying phenomena governing fire initiation and growth has led to the development of HRR test methods. HRR is considered to be a key indicator of fire performance and is the rate at which a material produces energy while burning [50,51]. Simultaneously, the peak value of HRR (pk-HRR) is used to express the intensity of a fire. Figure 7a,b illustrates the heat release rate (HRR) and the production rate of CO\({}_{2}\) (CO\({}_{2}\)P) curves of the pure PP and the composites. The pk-HRR of the pure PP is 1057 kW/m\({}^{2}\), and the pk-HRR values of PP-5%LDH, PP-10%LDH and PP-20%LDH were obviously decreased to 878, 705, and 500 kW/m\({}^{2}\), respectively. [PERSON] et al. reported that when 50 mass% ATH is added to the PP matrix, the pk-HRR of PP/ATH composite is 539.8 kW/m\({}^{2}\)[52]. Compared with the reported PP/ATH with 50 mass% ATH, the PP-20%LDH shows better flame retardancy. Furthermore, the pk-HRR time of PP-5%LDH, PP-10%LDH and PP-20%LDH are 295, 340 and 315 s, respectively, which are longer than 245 s of PP. The pk-HRR time of PP/LDH composites were extended, which is more beneficial to people's evacuation and rescue of firefighters. At the same time, a crucial phenomenon was observed. The peak production rate of CO\({}_{2}\) (pk-CO\({}_{2}\)P) time are as accurate same as the pk-HRR time for PP and its composites. This important finding confirmed that the production rates of CO\({}_{2}\) essentially mirror the HRR data. It is consistent with the report of [PERSON] et al. [53]. Figure 8a-c shows the photographs of residues for composites PP-5%LDH, PP-10%LDH, and PP-20%LDH after cone calorimeter testing, respectively. For PP-5%LDH and PP-10%LDH, the cracked residues can be observed, and the gray substance is mainly the magnesium, aluminum, and nickel mixed oxide, and compounds (NiAl\({}_{2}\)O\({}_{4}\)) produced from the thermal decomposition of Ni\({}_{0.2}\)Mg\({}_{2.8}\)Al-LDH [54]. In comparison with PP-5%LDH and PP-10%LDH, the residues of PP-20%LDH is more intact. It is well known that the continuous and compact residues layers can slow the heat and mass transfer between gas and condensed phases and also protect the underlying materials from further burning. Therefore, the pk-HRR of PP-20%LDH is lower than those of PP-5%LDH and PP-10%LDH. The effective heat of combustion (EHC) is an important parameter to understand how the flame retardant acts. The EHC values of the pure PP, PP-5%LDH, PP-10%LDH, and PP-20%LDH are 34.28, 35.98, 35.36, and 34.34 MJ/kg, respectively. At the same time, the average mass loss rate (AMLR) values of the pure PP, PP-5%LDH, PP-10%LDH and PP-20%LDH are 0.116, 0.100, 0.0928, and 0.0848 g/s, respectively. It can be seen that pk-HRR and AMLR show the gradual decrease with increasing loading of Ni\({}_{0.2}\)Mg\({}_{2.8}\)Al-LDH in the PP matrix. Furthermore, the EHC values of different composites show small changes in comparison with the pure PP. The above pk-HRR, AMLR, and EHC analyses confirmed that the condensed phase fire retardant action of Ni\({}_{0.2}\)Mg\({}_{2.8}\)Al-LDH in the composites, there is nothing significant in the gas phase [55]. Figure 6: TEM images of composites (150,000\(\times\)); (**a**) PP-5%LDH; (**b**) PP–20%LDH. In addition, it is well known that most deaths result from the toxic gases CO in the case of fire for PP products [9; 10; 11; 12]. The production rate of CO (COP) curves are shown in Figure 9. The peak values of COP (pk-COP) for the pure PP, PP-5%LDH, PP-10%LDH, and PP-20%LDH are 0.011, 0.0084, 0.0076, and 0.0043 g/s, respectively. Furthermore, MCOY is the mean release yield of CO of burning materials. The MCOY values of PP, PP-5%LDH, PP-10%LDH, and PP-20%LDH are 0.035, 0.035, 0.034, and 0.029 kg/kg, respectively.