## 2 Materials and Methods
### Experimental Methods  
The volume size and volume size distribution of wood fibers were characterized using laser diffraction by a Particle Size Analyzer (PSA 1190), [PERSON], Austria. To measure wood fiber sizes a particle-water suspension was first prepared. Before the measurement, the suspension was exposed to ultrasound for 30 s to break up potential wood fiber agglomerates. After this procedure, the measurements were conducted by exposing the flowing suspension to a laser beam for 10 s. Based on the diffracted light, particles sizes of wood fibers were determined, where the particle size is reported as a volume equivalent sphere diameter. This procedure was repeated seven times to determine the average size distribution of the particles. For the investigation of the chemical structure, the ATR-FTIR spectra were performed using the Perkin Elmer Spectrum GX NIR FT-Raman. The sample was in contact with a diamond crystal. Each spectrum was determined as an average of 32 scans at a resolution of 4 cm\({}^{-1}\) for the measuring background. Spectra of materials were measured in the range from 400 to 4000 cm\({}^{-1}\) at room temperature. All spectra were baseline corrected and smoothed after the measurements. Thermal properties of materials were investigated using differential scanning calorimetry, with a Mettler Toledo DSC 2 calorimeter. For the characterization, Al crucibles with a sample size between 10 and 30 mg were used. Tests were performed in an N\({}_{2}\) atmosphere, following the heat-cool-heat thermal cycle. Samples were first tempered at \(-65\,^{\circ}\)C for 5 min, followed by heating to 180 \({}^{\circ}\)C with the rate of 10 \({}^{\circ}\)C/min. This step was followed by the isothermal segment at 180 \({}^{\circ}\)C for 5 min, and cooling from 180 \({}^{\circ}\)C to \(-65\,^{\circ}\)C, again with the rate of 10 \({}^{\circ}\)C/min. The second heating cycle was repeated using the same conditions after isothermal segment at \(-65\,^{\circ}\)C.  
Degree of crystallinity was evaluated by taking into account the actual content of the polymer in the composite following the Equation (1),  
\[Xc=\frac{\Delta\mathrm{Hm}}{\Delta\mathrm{H}_{0}f_{PP}}\times 100 \tag{1}\]  
where \(X_{C}\) is the degree of crystallinity, Hm is the melting enthalpy, \(\Delta\mathrm{H}_{0}\) is the melting temperature of the 100% crystalline PP and \(f_{PP}\) the weight fraction of PP in the composites. \(\Delta\mathrm{H}_{0}\) for PP was found to be 209 (J/g). The crystalline structure of composites was determined by X-ray diffraction (XRD) analysis, by a PANalytical PRO MPD diffractometer using a Cu K\(\alpha\) radiation source at 40 kV. The X-ray diffraction  
Figure 1: Granulated materials with different wt.% of wood fiber (WF) for unrecycled (**a**) and recycled (**b**) composite materials. patterns were recorded for the angles in the range of 20, from 10 to 40\({}^{\circ}\), with a step of 10\({}^{\circ}\)/min (\(\lambda\) = 0.154 nm). Rheological tests were performed by using a rotational rheometer, MCR 302 Anton Paar GmbH, Austria. Tests were performed with parallel plate sensor geometry, with sensor diameter 25 mm and gap 1 mm at constant temperature T = 190 \({}^{\circ}\)C. For the rheological characterization, the strain amplitude sweep and frequency sweep tests were performed. Two repetitions were made, and average values were calculated for each measurement. Strain amplitude sweep tests were performed at a constant frequency of 6.28 rad/s and increasing shear strain. The shear strain between \(\gamma\) = 0.01-100% was used for samples up to 20 wt.% of wood particles' loading, while for the samples with 30 wt.% and 40% wt.% of wood particles loading, the shear strain range was lower, i.e., between \(\gamma\) = 0.001-1%. Frequency sweep tests were performed in the frequency range from 0.062 to 628 rad/s at a constant shear strain. The value of strain was for each material selected in the linear viscoelastic (LVE) region of material behavior determined by the strain amplitude sweep tests.