## 3 Results
Similar behaviour was also observed for composites with 5 and 10 wt.% WF concentrations. This indicates that the presence of fibers did not significantly affect the disentanglement process of polymeric molecules. Figure 7: X-ray Diffraction patterns of R-PP and WF-R-PP composites. A more gradual decrease of storage modulus was observed for the composites with filler loading higher than 20 wt.%. As the wood fiber concentration increases, a percolation point is reached, where wood fibers start to interact with each other directly and form a 3D network. With increasing shear strain, this fiber network is impaired, which results in a decrease in storage modulus. This process occurs at lower shear strains compared to strain values where polymer molecules disentangle. Consequently, the change from linear to non-linear behavior for the composites with 30 and 40 wt.% of wood fiber, occurred at very low strain \(\gamma\sim 0.001\). In this respect, no significant differences were observed between the recycled and unrecycled composite materials. The addition of relatively rigid (compared to molten polymer) WF to the polymer matrix, the elasticity of composite materials increased. This can be seen as an increase in storage modulus with the increasing concentration of wood fiber. This behavior has already been well-reported and observed for other types of polymer-fiber composites as well [44; 45]. From Figure 8 it can be seen that the increase of storage modulus with the increasing concentration of wood fiber was more pronounced for R-PP polymeric matrix (Figure 8b). These results suggest that, for the recycled material (R-PP), a stronger WF network was formed at concentrations higher than 20%, resulting in higher values of storage modulus. Following the amplitude sweep test, the frequency tests were performed in the linear viscoelastic region (Figure 9). For both cases, the R-PP and PP based composites, it is evident that the addition of WF up to 10 wt.% had no or limited influence on the rheological behavior of the melts in linear viscoelastic range. The values of storage and loss modulus over the entire frequency range were for pure PP and R-PP materials very comparable. Above this WF concentration, it seems that fibers start to interact. This is evident by an increase of values of the dynamic material function and especially by the material behavior at low frequencies where fiber-fiber interactions are dominant [45]. For composites up to 20 wt.% the hydrodynamic effects of the polymer matrix becomes dominant at higher frequencies. Figure 8: Storage modulus, G\({}^{\prime}\) as a function of shear strain, \(\gamma\) at 6.28 rad/s and 190 \({}^{\circ}\)C for (**a**) unrecycled material and (**b**) recycled material. In this region, the values of dynamic material functions regardless of the filler concentration were very similar. A more substantial change of dynamic properties compared to the pure PP and R-PP can be observed at composites with 30 and 40 wt.% of fillers for PP based composites and for 20, 30 and 40 wt.% for R-PP based composites. The increased values of dynamic properties were contributed to the wood fiber network formation that resulted in higher storage and loss modulus and showed that the strong interactions between the polymer and the fiber occurred at higher concentrations. Comparison between the matrix materials also showed that the addition of WF had a significant effect on the rheological behavior of the recycled PP (R-PP), as it resulted in higher dynamic moduli compared to PP based composites. Moreover, the results in Figure 9 show that wood concentration affected the viscoelastic character of the materials. This is evident by the crossover between G' and G\" and was analyzed separately in Figure 9 for all composites. From Figure 10 it can be seen that with the increasing WF concentration the crossover point shifted to lower frequencies, meaning that frequency range with solid-like (G' > G\") material behavior broadens.