# New Flame Retardant Systems Based on Expanded Graphite for Rigid Polyurethane Foams
## 3 Results and Discussion
### Fire Behavior of RPUFs
In addition, the THR value was obtained much lower for modified foams in comparison with PU-0 foam. The smallest amount of total heat released was obtained for the PU-EG+SiO\({}_{2}\) sample, which decreased by over 20% compared to the PU-0 sample. Other modified samples had similar THR values. The most common cause of death in a fire is smoke, therefore total smoke release (TSR) is very important for the safety and fire resistance assessment of materials. Based on the data contained in Table 5, it can be seen that the use of fillers effectively reduced smoke during the combustion process. All modified PU foams had significantly lower TSR values. For PU-0, the TSR value was about 37.9 m\({}^{2}\) m\({}^{-2}\) and significantly decreased to 15.6 m\({}^{2}\) m\({}^{-2}\) for the PU-EG+SiO\({}_{2}\) sample, which was probably the result of the formation of a dense carbon layer on the polymer surface, which may limit the total amount of smoke released during the combustion process. The fire-retardant properties of the test samples can also be well assessed on the basis of the observation of the residual samples after combustion in the calorimetric cone test. Figure 6 shows comparative digital photos of PU-0 and PU-EG residues after the cone calorimeter test. Differences \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**System** & **TTI (s)** & **\begin{tabular}{c} **pHRR** \\ **(kW/m\({}^{2}\))** \\ \end{tabular} & **\begin{tabular}{c} **THR** \\ **(MJ/m\({}^{2}\))** \\ \end{tabular} & **\begin{tabular}{c} **MAHRE** \\ **(kW/m\({}^{2}\))** \\ \end{tabular} & **\
\begin{tabular}{c} **TSR** \\ **(m\({}^{2}\)/m\({}^{2}\))** \\ \end{tabular} & **
\begin{tabular}{c} **Char** \\ **Residue (\%)** \\ \end{tabular} \\ \hline PU–0 & 2 & 213 & 21.9 & 180.0 & 37.9 & 13.1 \\ PU–EG & 4 & 71 & 17.9 & 70.8 & 16.1 & 28.7 \\ PU–EG+SiO\({}_{2}\) & 5 & 62 & 17.2 & 61.7 & 15.6 & 31.8 \\ PU–EG+SiO\({}_{2}\)+IL & 5 & 64 & 17.7 & 63.9 & 16.3 & 27.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Parameters determined during the combustion process of RPUFs. in the appearance of the two samples can be seen, indicating two different combustion mechanisms. In Figure 6a it can be seen that the unmodified PU-0 sample was burned almost to the very end. On the other hand, the photo 6b shows a charred boundary layer that does not let fire into the sample, making the intact part larger than the photo on the left. To obtain a more detailed structural analysis and determine the mechanism of action of the modifiers used, SEM analysis was performed after the combustion selected samples; the images are presented in Figure 7. By analyzing the following pictures, it can be seen that the microscopic pictures of the tested composites differ from each other. It can be seen that in the case of PU-0, the carbonized residue has a porous structure with broken cells. The PU-0 structure (Figure 7a) had rough characteristics and presents an indistinct \"molten\" network, without clear cell structure features. In the case of a sample containing expanded graphite, a clear flame retardation mechanism was visible. The pictures (Figure 7b-c) show the characteristic structure in the form of charred ribs between the cells of the porous structure, which prevented the spread of fire into the burning sample.