## 4 Discussions with Conclusions  
In this paper, the effect of the recycling of PP based wood fiber composites was investigated. As no additives or any kind of chemical fiber treatment was used, the investigation provides the information on the direct polymer--fiber interaction and effects of wood fiber loading. The interactions were evaluated through chemical, thermal and rheological characterization. The ATR-FTIR spectra of all investigated composite materials showed the presence of basic functional wood groups with their characteristic peaks at 1741 cm\({}^{-1}\), 1510 cm\({}^{-1}\), 1030 cm\({}^{-1}\) and 3340 cm\({}^{-1}\), which were determined for the vibration of lignin, cellulose and hemicellulose. No chemical interactions between the fibers and matrix material were observed, which was expected since no compatibilizers were used in the preparation of composite materials. The thermal analysis showed that increasing the WF concentrations increased the degree of crystallinity as wood fibers acted as nucleating agents. While the increase in crystallinity for R-PP based composites was modest (maximal 3.5% increase), the increase in the case of PP based composites was much more decisive. The crystallinity of PP based composite with 40 wt.% of WF increased by 18%. As none of the matrix materials formed chemical bonds with the filler, the increased crystallinity of PP based composites may be related to the molecular structure of the polymer. The values of crystallinity at all loading levels were for R-PP based composites lower compared to PP based composites. This might be a consequence of branched polymeric structure or lower molecular weight of the recycled PP. Additionally, to the increased crystallinity of PP based composites, the XRD measurements showed the formation of all three PP crystalline phases that were not present in pure PP material. In the case of R-PP based composites, PP \(\alpha\)-crystalline phase was unchanged regardless of the WF loading. This leads to an interesting observation. A substantial increase of crystallinity and additional crystalline phases appeared with PP based composites. Both lead to increased mechanical properties, such as elastic modulus and toughness of the material. It was shown that the addition of WF reinforced the PP material and to a lesser extent, also the recycled PP material. However, rheological characterization showed the opposite trend. At concentrations, which were high enough that wood fibers formed a 3D connected network, dynamic rheological properties increased considerably compared to the pure matrix materials. The strength of this wood fiber network was determined by the magnitude of the dynamic moduli. For example, comparing the loss modulus of pure PP with loss modulus of 40 wt.% filled material showed a maximal increase of modulus for about 20%. However, for the R-PP based composites, the reinforcing effect was much larger as the modulus increased by about 110%. In conclusion, it seems that there are two mechanisms controlling the behavior of WF filled composites. In the case of PP based composites, wood fibers promote the crystallinity and have no or limited effect on R-PP based composites. On the other hand, the R-PP based composites form strong  
Figure 10: Crossover point at 190 \({}^{\circ}\)C for unrecycled (WF-PP) and recycled (WF-R-PP) materials. wood particle networks. Which mechanism will have a more significant effect on the mechanical properties is currently still unknown and should be a subject of further research. Nevertheless, the results of this research showed that combining PP or R- PP with natural waste materials, such as wood fibers leads to improved thermal and rheological properties, which could be further improved with the use of compatibilizers or chemical treatment of wood fibers. Conceptualization, K.P.C. and M.B. ; methodology, K.P.C. ; formal analysis, K.P.C., M.B. ; investigation, K.P.C., L.F.Z. ; M.B. ; writing--original draft preparation, K.P.C., M.B. ; writing--review and editing, K.P.C., M.B., L.F.Z.