## 2 Materials and Methods  
### Materials and the Preparation of Composite Materials  
For the investigation, two different polypropylene matrix materials were used: unrecycled PP material AMPPLEO 1020 GA (0.94 g/cm\({}^{3}\), MFI: 20 g/10 min), Braskem, USA and recycled PP material Eco Meplen IC M20 BK (0.90 g/cm\({}^{3}\), MFI: 50 g/10 min) MEPOL S.r.l., Italy. Wood fibers were obtained locally from wood processing company as a side product of plywood grinding. The fibers consisted of spruce and pine wood, approximately in a ratio 80-20. Before mixing wood fibers with a polymeric matrix, the fibers were sieved to remove oversized wood particles.  
All materials were prepared by melt compounding of polymeric matrix and wood fibers, without the use of additives. Before the extrusion, wood fibers, unrecycled PP and recycled PP were dried at 90 \({}^{\circ}\)C for 2 h in a ventilated oven. Wood fibers and PP were mixed and supplied to the feeder of the twin-screw extruder, PolyLab HAAKE Rheomex PTW 16, Thermo Haake, Germany. To assure uniform distribution of WF in the polymeric matrix, all materials were extruded four times. After each extrusion, the material was peltetized by a granulator, and the material was again fed to the feeder.  
Altogether six materials were prepared with unrecycled PP matrix, and six with recycled PP matrix. Unrecycled PP and the recycled PP without the fibers were used as a reference, while other composites were prepared with wood fibers at different loading levels. A complete overview of the prepared materials is presented in Table 1, with designated names for each material mixture.  
A melting temperature of around 190 \({}^{\circ}\)C was used for all materials and all four extrusion cycles. The screw speed was, in all cases, set to 80-90 rpm, and feeding was set to approximately 25 g/min of material. Measured average die temperatures of fourth extrusion cycle for the materials are presented in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Wood Fiber Loading [wt.\%]** & **0** & **5** & **10** & **20** & **30** & **40** \\ \hline Name of the & Unrecycled PP & PP & WF-PP-5 & WF-PP-10 & WF-PP-20 & WF-PP-30 & WF-PP-40 \\ material & Recycled R-PP & R-PP & WF-R-PP-5 & WF-R-PP-10 & WF-R-PP-20 & WF-R-PP-30 & WF-R-PP-40 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Prepared materials and names of individual materials.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Name of the Material** & **PP** & **WF-PP-5** & **WF-PP-10** & **WF-PP-20** & **WF-PP-30** & **WF-PP-40** \\ \hline Average die & 189.9 & 189.9 & 191.4 & 194.0 & 191.9 & 196.5 \\ temperature [\({}^{\circ}\)C] & & & & & & \\ \hline
**Name of the Material** & **R-PP** & **WF-R-PP-5** & **WF-R-PP-10** & **WF-R-PP-20** & **WF-R-PP-30** & **WF-R-PP-40** \\ \hline Average die & 189.9 & 189.9 & 191.4 & 194.0 & 191.9 & 196.5 \\ temperature [\({}^{\circ}\)C] & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Measured average die temperature in the PolyLab HAAKE Rheomex PTW 16 extruder.  
Figure 1 shows the granules of all 12 investigated materials that were used in further investigation: ATR, (DSC), XRD and rheological tests.