## 3 Results
On the other hand, the addition of WF significantly changed the crystallization process of WF composites. The pure materials showed different degree of crystallinity. The unrecycled PP with 41.4% exhibited higher crystallinity (for about 19%) compared to recycled PP (R-PP) with 34.9%. The difference indicates a difference in the molecular structure of both materials. The degree of crystallinity increased with the addition of WF for both matrix materials (PP and R-PP). For PP based  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Tm\({}_{\text{1}}\) (\({}^{\circ}\)C)** & **\(\Delta\)H\({}_{\text{m}}\) (I/g)** & **Tc (\({}^{\circ}\)C)** & **Tm\({}_{\text{2}}\) (\(  
composites, the degree of crystallinity increased from 41.4% to 48.9%, which represented an 18% increase. On the other hand, the degree of crystallinity of R-PP based composites increased from 34.9% to 36.1%, which represented only 3.5% increase. These results showed that the addition of WF promoted crystallization process and confirmed that the fibers act as nucleating agents in the case of unrecycled PP matrix. The effect was less pronounced and marginal in the case of recycled based (R-PP) composites. The temperature of crystallization of unrecycled PP was 3.5 \({}^{\circ}\)C higher compared to the unrecycled PP (R-PP). In Figure 5, the exothermic peaks represent the crystallization process of PP (Figure 5a) and R-PP (Figure 5b). The temperature of crystallinity was also affected by the addition of wood fibers. Compared to pure PP, the Tc of WF-PP-40 composite was 8 \({}^{\circ}\)C lower, i.e., at 122 \({}^{\circ}\)C, which represented about 6 % decrease of Tc when the highest amount of WF was added to pure PP. A similar decrease in crystallization temperature was also obtained for recycled based (R-PP) composites. The temperature of crystallization decreased from 126.82 \({}^{\circ}\)C for R-PP material to 122.54 \({}^{\circ}\)C for WF-R-PP-40 composite material. In this case, the difference was about 3%. These results are in line with the results of the degree of crystallinity, as they show that the addition of wood fiber significantly affects the crystallization process of PP based composites. The addition of  
Figure 5: Differential scanning calorimetry (DSC) thermograms of (**a**) WF-PP composites and (**b**) WF-R-PP composites. WF has a lesser effect on the temperature of crystallization of R-PP based composites. In the literature, we can find information on the nucleation ability of lignocellulosic materials such as WF [25; 27; 30]. The researchers reported that untreated WF in thermoplastic composites act as good nucleating agents, while different chemical treatment of WF can lead to either increased or decreased nucleating ability. For example, the chemical modification of the fillers that causes smoothing of the filler surface leads to lower nucleation ability. In the case of composite investigated within this study, we were dealing with untreated wood fibers; however, they exhibited different nucleating ability depending on the used matrix material. To further investigate the effects of fillers and their role as nucleating agents, XRD measurements coupled with X-ray diffraction were performed to obtain a detailed picture of the nature of the crystallographic structures. It is known that the presence of some fillers with nucleating ability leads to the formation of the \(\beta\)-form in PP [35; 36; 37]. WF as natural fillers are responsible for the changes in the crystallinity process and morphology. PP crystalize predominately in the \(\alpha\)-monoclinic form by isothermal crystallization. The \(\beta\)-hexagonal, \(\gamma\)-triclinic phases and sematic modifications, all with a \(3_{1}\)-helix conformation, may be crystallized from the melt under high undercooling or high-pressure conditions, or by the addition of nucleating agents. The \(\alpha\)-monoclinic form is one of the thermodynamically stable crystalline modifications.