## 3 Results
The spectral analysis of unrecycled PP is shown in Figure 4a. From the spectra of WF-PP composites shown in this figure, the characteristic bands assigned to PP and wood structure could be  
Figure 3: ATR-FTIR spectra of polypropylene (PP), recycled polypropylene (R-PP) and wood fiber (WF). seen. The results showed that, with the addition of WF, a fingerprint characteristic band vibration appeared at 1030 cm\({}^{-1}\) (assigned to the stretching vibrations of different groups in carbohydrates), at 1510 cm\({}^{-1}\) (stretching or bending vibrations of groups in lignin), at 1741 cm\({}^{-1}\) (stretching vibration in lignin and hemicellulose) and 3340 cm\({}^{-1}\) (O-H stretching). Moreover, the intensity of vibration bands determined for wood structure increased with the increasing addition of wood fibers. The same characteristic vibration peaks of recycled PP (R-PP) and composites with different WF concentrations (WF-R-PP) are shown in Figure 4b. Spectral analysis showed (similarly as in the case of unrecycled PP and its composites) the presence of wood functional groups with their characteristic peaks at 1741 cm\({}^{-1}\), 1510 cm\({}^{-1}\), 1030 cm\({}^{-1}\) and 3340 cm\({}^{-1}\) assigned to the vibration of lignin, cellulose, and hemicellulose, and clearly suggesting the wood's presence on the surface of the material. Figure 4: ATR-FTIR spectra of (**a**) unrecycled PP, WF, WF-PP composites and (**b**) recycled PP (R-PP), WF, WF-R-PP composites with different concentration of WF. The spectra of PP based composites and R-PP based composites showed that there was no chemical interaction between the fibers and polymeric matrix. Since no treatment of wood fibers or compatibilizers were used, wood fibers act as fillers to the polymeric matrix. To further investigate the influence of wood fibers, the thermal characterization of composites was performed. The effect of WF addition to a PP polymer has also been shown with thermal analysis of WF-PP and WF-R-PP composites. First, it was examined if WF had a role in the crystallization process of WF-PP composites. It is well known from the literature that WF has the role of a nucleating agent in the process of polymer crystallization, resulting in higher polymer crystallinity and increased mechanical properties of such composites [30; 34]. However, treatment of WF or using the compatibilizers also affects the nucleating ability of WF. As the materials used in this study were not chemically treated and were not prepared by adding any additives the analysis of thermal properties enabled a direct comparison of the influence of fibers on the recycled and unrecycled materials. To confirm the influence of the addition of WF to PP and R-PP on the nucleation ability of the composites, measurements of thermal properties were performed with differential scanning calorimetry (DSC) to examine the possible changes of crystalline properties between unrecycled and recycled PP/WF composites. The results of the thermal analysis are given in Table 4, where T\({}_{\text{m}}\) is melting temperature, \(\Delta\)H\({}_{\text{m}}\) is melting enthalpy, Tc is the temperature of crystallinity and \(Xc\) is a degree of crystallinity. The measurements with unrecycled-PP and recycled-PP material showed that the melting temperature did not change significantly with increasing the amount of WF and was in all cases determined at around 165 \({}^{\circ}\)C. The thermal analysis of recycled PP (R-PP) exposed additional melting peak at around 117 \({}^{\circ}\)C. This peak indicated the thermal properties of polyethylene-origin materials and indicated that the recycled PP material contained some amount of PE polymer. However, since the melting enthalpy associated to this thermal transition was considerably smaller compared to the enthalpy related to the melting of PP, we may assume that the amount of PE polymer in the material was small and that it did not significantly affect the overall behavior of the (composite) material.