## 3 Results  
Figure 2 shows the size distribution of wood fibers measured by laser diffraction. As can be seen from optical microscopy, presented in the inserted image in Figure 2, the shape of the fiber was irregular and consisted of fibers, flakes and particles. Based on the measured wood fibers size distribution, the size parameters were evaluated and are presented in Table 3. The mean value of wood fiber was 99.58 \(\upmu\)m. The distribution width of fiber sizes was evaluated through the D10, D50 and D90 values, which represents the values where 10%, 50% or 90% of the population lies below this percentage values. The additional parameter describing the width of size distribution is span, that was calculated as:  
\[\text{Span}=\frac{\text{D90}-\text{D10}}{\text{D50}} \tag{2}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**D10 [\(\upmu\)m]** & **D50 [\(\upmu\)m]** & **D90 [\(\upmu\)m]** & **Mean [\(\upmu\)m]** & **Span** \\ \hline
12.16 & 63.67 & 221.23 & 99.58 & 3.28 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Size parameters of wood fibers. Figure 2: The volume size distribution of wood fibers. Figure 3 shows the ATR-FTIR spectra of unrecycled PP (PP), recycled PP (R-PP) and wood fibers (WF) as reference samples. The characteristic bands of PP (black curve) and R-PP (red curve) at 2917 cm\({}^{-1}\) correspond to CH\({}_{2}\) asymmetric stretching, the band at 2951 cm\({}^{-1}\) to CH\({}_{3}\) stretching, and 2868 cm\({}^{-1}\) to CH\({}_{2}\) asymmetric stretching. The characteristic peaks in the region at 1456 cm\({}^{-1}\) and 1374 cm\({}^{-1}\) were determined as CH\({}_{3}\) CH\({}_{2}\) rocking vibration. In the >>fingerprint<< region of PP and R-PP, the spectra contain several main PP tacticity, which include isotactic, syndiotactic and atactic forms. Typical characteristic peaks for isotactic PP are presented in the region below 1000 cm\({}^{-1}\)[31]. The characteristic peaks for syndiotactic PP (s-PP) at 876 cm\({}^{-1}\) and 808 cm\({}^{-1}\) are shown in the spectra of R-PP and PP. The ATR-FTIR spectra of PP, R-PP indicate the presence of the mixture of isotactic and syndiotactic forms of PP. The characteristic vibration bands of WF (blue curve) are also shown in Figure 3. Wood fibers consist of cellulose, hemicellulose and lignin. Because the wood structure is very complex, the ATR-FTIR spectra were separated into two regions. The first region from, 3800 cm\({}^{-1}\) to 2700 cm\({}^{-1}\), includes the OH and CH stretching vibrations. The second, at 1800 cm\({}^{-1}\) to 800 cm\({}^{-1}\), is known as a >>fingerprint<< region for different functional groups of wood structures. A strong, broad peak can be seen at 3400 cm\({}^{-1}\), which was assigned to different O-H stretching, while, at 2950 cm\({}^{-1}\), a characteristic peak is related to the asymmetric and symmetric methyl and methylene stretching groups represented in wood. The >>fingerprint<< region of the wood consists of several bands which are related to the wood's structure. A characteristic peak assigned to C=O stretching vibration in lignin and hemicellulose could be observed at the 1741 cm\({}^{-1}\). The characteristic bands at 1505 cm\({}^{-1}\) and 1270 cm\({}^{-1}\) were determined as C=C, C-O stretching or bending vibrations of the groups in lignin. The bands at 1452 cm\({}^{-1}\), 1422 cm\({}^{-1}\), 1374 cm\({}^{-1}\) were assigned to C-H, C-O deformation, bending or stretching vibrations of lignin groups and carbohydrates. The bands at 1165 cm\({}^{-1}\), 1050 cm\({}^{-1}\), 1030 cm\({}^{-1}\) were assigned to C=O, C-H, C-O-C, C-O deformations or stretching vibrations of different groups in carbohydrates. The peaks at 1030 cm\({}^{-1}\) were assigned to C=O, C-H, C-O-C, C-O deformations or stretching vibrations of different groups in carbohydrates [10; 33]. Figure 4 represents the ATR-FTIR spectra of unrecycled PP, recycled PP, WF and all wood fiber polypropylene composites with different concentrations of WF (from 5% of WF-PP-5 to 40% of WF-PP-40).