LOI of different PURFs were tested and given in Table 4.
The addition of PEPA could increase the LOI of PURF, while CaCO3 was less effective on the increase of LOI. LOI of PURFs with 8 wt% PEPA and 6 wt% CaCO3 reached 23.5% so that it would not be sustained combustion in air environment.
3.3.2. Performance of PURFs in direct fire impact effect
Penetration times of PURFs modified with PEPA and CaCO3 were tested to study the char stability, which could indicate the protection effect of the char layer to the inner part of origin polymers. The results were shown in Table 5.
Penetration times were mainly dependent on char residue and the char stability. As listed in Table 5, the penetration times of PEPA or CaCO3 modified PURFs were slight longer than that of pure PURF, which could be attributed to the rise of char residue. However, in the absence of PEPA, the influence of CaCO3 on penetration time was insignificant. There are two reasons. One is that CaCO3 is not stable at high temperature. The other is that inorganic/char nanocomposites could not form when char residue is very low.
When PEPA and CaCO3 were added together to PURFs, the penetration times were dramatically increased. Char stability was enhanced by formation of inorganic particles/char nanocomposites which could be proved by SEM imagine of char of PURF, and the enhanced char had better anti-oxidation property, which was in accordance with TGA of PURFs in different environments.
The result was in accordance with SEM and FTIR shown in Fig. 8, Fig. 9 and Fig. 10, it can be seen that surface of the char of PURFs modified with PEPA and CaCO3 was enhanced with nanoparticles after combustion, either covering model or buried model. According to FTIR (shown in Fig. 10), nanoparticles were mixture of calcium phosphates and calcium oxide.
Enhanced char layer by nanoparticles could reduce the oxidation of carbon element in the char by lowering the permeation of oxygen which resulted in good anti-oxidation property of char layer and longer penetration time.