## 3 Results and Discussion
### Microstructure and Thermal Conductivity
[59], who investigated PUR foam composites with silica aerogels and silica nanoparticles. Even though the closed cell content (Table 3) of all PUR foam composites is  
Figure 4: Comparison of thermal conductivity of PUR foam composites with different fillers at (**a**) 2.5 wt.%; (**b**) 5 wt.%; (**c**) 7.5 wt.%, and (**d**) 10 wt.%. higher than that of a control PUR foam, [PERSON] et al. [60] determined that the cell size causes the highest impact on thermal conductivity. They explained that in larger cells, the contribution of radiative heat transfer to the thermal conductivity value increases. Figure 4d shows that the further addition of all fillers, i.e., 10 wt.%, yields a thermal conductivity value almost identical to that of the control PUR foam. The excessive addition of fillers led to even worse results, with the cell size becoming larger and non-uniform. The microstructure of all PUR foam composites at 10 wt.% filler concentrations (Figure 5c,e,g,i) was also destroyed and exhibited interconnected cells, unlike the control PUR foam, which displayed the typical cellular structure. This could be attributed to the worsened thermal conductivity values of the PUR foam composites. Therefore, the maximum addition of all fillers based on thermal conductivity is 7.5 wt.%, which yields the highest thermal insulation performance in PUR foam composites. However, the statistical analysis of the thermal conductivity of PUR foam composites with 2.5 wt.% different fillers showed (Figure 4a) that there are no differences between the PUR/Per-2.5, PUR/Verm-2.5, PUR/Per-Dex-2.5, and PUR/VermDex-2.5 composites. The F-criterion is 0.48, \(p\) = 0.70, and R\({}^{2}\) = 0.154. Further, the statistical analysis of the thermal conductivity of PUR foam composites with 5 wt.% different fillers showed (Figure 4b) that there are also no differences between the PUR/Per-5, PUR/Verm-5, PUR/PerDex-5, and PUR/VermDex-5 foam composites. The F-criterion is 1.14, \(p\) = 0.39, and R\({}^{2}\) = 0.299. Moreover, the statistical analysis of the thermal conductivity of PUR foam composites with 7.5 wt.% different fillers showed (Figure 4c) that there are no differences between the  
Figure 5: Optical imaging of PUR foam composites’ microstructure at x80 magnification: (**a**) control PUR foam; (**b**) PUR/Per-5; (**c**) PUR/Per-10; (**d**) PUR/Verm-5; (**e**) PUR/Verm-10; (**f**) PUR/PerDex-5; (**g**) PUR/PerDex-10; (**h**) PUR/VermDex-5; and (**i**) PUR/VermDex-10. PUR/Per-7.5, PUR/Verm-7.5, PUR/PerDex-7.5, and PUR/VermDex-7.5 foam composites. The F-criterion is 0.98, \(p\) = 0.45, and R\({}^{2}\) = 0.269. In addition, the statistical analysis of the thermal conductivity of PUR foam composites with 10 wt.% different fillers showed (Figure 4d) that there are no differences between the PUR/Per-10, PUR/Verm-10, PUR/PerDex-10, and PUR/VermDex-10 foam composites. The F-criterion is 1.94, \(p\) = 0.20, and R\({}^{2}\) = 0.421. After repeated comparisons of the PUR/Per, PUR/Verm, PUR/PerDex, and PUR/VermDex foam composites with the control PUR, F-criterion is 0.39, \(p\) = 0.81, and R\({}^{2}\) = 0.135, indicating a statistically insignificant difference between the mean values.