# New Flame Retardant Systems Based on Expanded Graphite for Rigid Polyurethane Foams
## 3 Results and Discussion
### Mechanical Properties of RPUFs  
Cell morphology is a very important factor that translates into the later properties of RPUF. The close correlation with the structure is the apparent density of PU foam, which is one of the most important parameters describing porous materials. It is directly related to such foam properties as dimensional stability, thermal insulation as well as all mechanical properties. By analyzing the results presented in Table 3. It can be stated that for all tested foams the apparent density increased from 41 kg/m\({}^{3}\) for PU-0 foam to a maximum value of over 47 kg/m\({}^{3}\) for the series containing EG and Si.  
Figure 3: Images of foam using 250x magnification. (**a**) PU–0; (**b**) PU–EG; (**c**) PU–EG+SiO\({}_{2}\); (**d**) PU–EG+SiO\({}_{2}\)+IL.  
The increase in density in modified systems was the result of increased system viscosity, impeded bubble expansion, as well as the additional mass of fillers with specific densities. The addition of modifiers also increased the hardness of RPUF, which was the result of increased apparent density, because the higher the apparent density of the composite, the more compact it was and thus the higher the hardness. In addition, the presence of reinforcing fillers in the form of expanded graphite and silica capable of creating secondary structure and for polymer-filler interactions, increasing the hardness of synthesized foams.  
Due to cell anisotropy, the compressive strength (\(\sigma_{10\%}\)) was measured in two directions: parallel and perpendicular to the foam growth direction. As shown in Table 3, the compressive strength (measured in parallel and perpendicular direction) of modified foams was higher than \(\sigma_{10\%}\) PU-0.  
The results distinctly point out that there is a correlation between \(\sigma_{10\%}\) and the density of RPUFs. Samples with higher density had higher values of \(\sigma_{10\%}\). In addition, it was seen that the mechanical strength of rigid polyurethane foams--measured in a direction perpendicular to the growth of the foam--was greater than in the parallel, which was associated with cellular anisotropy manifested in the cell lengthening in the direction of foam growth. The reference foam had a resistance of 162 kPa, which was the minimum compressive strength value declared by PUR manufacturers. In the case of modified foams, a tendency to increase mechanical strength was visible along with the introduction of another modifier. The addition of EG and SiO\({}_{2}\) had a positive effect on increasing the compressive strength up to a maximum value of 261 kPa. This value was over 60% higher than in the case of PU-0 foam. The improvement in \(\sigma_{10\%}\) for modified foams may be due to the increased content of polar urea groups that can form hydrogen bonds and stiffen the structure. Undoubtedly, the tested mechanical properties were also affected by the increasing viscosity of the systems together with the increase in the content of modifiers that could more stabilize the structure of the resulting foam. An analogous improvement in properties could also be seen in the measurement of impact strength and flexural strength of modified PU foams.  
This reinforcing effect could also be explained by the mechanism for introducing fillers into the matrix of porous material. Namely, the filler particles, which due to interfacial adhesion were located in the cell struts of the polyurethane matrix, could disperse and transfer stress, which increased the resistance to mechanical stress. A similar trend was also seen in previous studies [20; 21; 22; 23].