# New Flame Retardant Systems Based on Expanded Graphite for Rigid Polyurethane Foams
## 3 Results and Discussion
### Fire Behavior of RPUFs  
The flame retardant behavior of RPUFs was determined using a cone calorimeter test, which is recognized as the best method to simulate small-scale fire conditions. The results of ignition time (TTI), peak heat release rate (pHRR), total heat release (THR), the maximum average rate of heat emission (MAHRE) as well as the total smoke release (TSR) and char residue are presented in Table 5.  
The cell structure had a great impact on the fire behavior of PU foam; however, the addition of a filler could change it significantly ([PERSON] et al., 2016). As can be seen in Table 5, the ignition time (TTI) was increased by the addition of EG and EG with Si and the peak heat release rate (pHRR), total heat release (THR), total smoke release (TSR) decreased with addition each modifier. This phenomenon probably resulted from the effective heat assimilation by expanded graphite and silica, which delay the burning of foams. Figure 4: Dependence of the thermal conductivity coefficient at 20 \({}^{\circ}\)C on the apparent density of PU foams. Figure 5: Dependence of the thermal conductivity coefficient at 20 \({}^{\circ}\)C on the total porosity of PU foams. The parameter that should be taken into account is the heat release rate--defined by the peak heat release rate, which provides information on the spark ignition of the exposed burned material. Based on the results presented in Table 5, it can be noticed that the addition of the modifiers to the RPUFs decreased the pHRR value compared to the unmodified foam. Namely, for PU-EG, PU-EG+SiO2 and PU-EG+SiO2+IL samples, the pHRR value was 71, 62 and 64 kW/m\({}^{-2}\), respectively, which means a reduction of approximately 67, 71 and 70% compared to reference foam. The decrease in pHRR was due to the fact that the expanded graphite exposed to the heat source expands into the carbonaceous layer on the RPUF surface. The formed char layer effectively hinders the diffusion of oxygen and the transfer of mass and heat between the flames and the polyurethane matrix, thus interrupting the self-sustaining combustion of the foam [29]. The addition of an additional filler, i.e., pyrogenic silica, resulted in the creation of an additional thermal barrier, because in addition to the high thermal stability of silica, it also favored the ceramization process, which further enhanced the barrier effect. In contrast, no positive effect of IL on pHRR was observed. Probably, despite the content of anion [BF\({}_{4}\)]-, the ionic liquid decomposed during the combustion process, as evidenced by a slightly lower content of carbonized residues than in the case of PU-EG+SiO2. However, as could be predicted at the end of combustion, RPUFs with addition only EG and EG in the presence of SiO\({}_{2}\) or SiO\({}_{2}\)+IL retained more carbonized residues than PU-0 foam. This trend was comparable to the residual weight obtained from TGA, although the TGA residue was slightly lower compared to the results of the cone calorimeter. This was probably the result of oxidative decomposition which enhances pyrolytic decomposition under forced-flame conditions. As shown in Table 5 the total char residue of the PU-0 sample was about 13.1%, and the addition of flame retardants increased the char yield by up to 31.8% for the PU-EG+SiO\({}_{2}\) sample. The increased char yield of PU foams containing expanded graphite may be due to the presence of a protective char layer acting as a swelling, thermal barrier, which contributes to a reduction in the diffusion rate of volatile compounds as well as a significant reduction of oxygen supply to the interior of the sample. As a result of this action, the further distribution of the sample was reduced, and the weight loss of modified foams was less than for a sample without the addition of a graphite flame retardant. It should be noted that for RPUF used as insulation materials the available pHRR value was 300 kW m\({}^{-2}\)[30], while for the analyzed PU foams the pHRR values did not exceed 71 kW m\({}^{-2}\).