## 3 Experimental
### Characterization and Testing  
The purified maleic anhydride-grafted polypropylene was prepared by dissolving the modified polypropylene in xylene at a concentration of 1 g/100 mL. The solution was heated and refluxed at 140 \({}^{\circ}\)C for 1.5 h, and an excess of acetone was added to precipitate the polymer, followed by rinsing. The maleic anhydride grafting rate (G) was determined via rapid titration of the purified sample in hot xylene, using phenolphthalein as an indicator and a potassium hydroxide-ethanol standard solution [41]. The purified polymer sample was hot-pressed into a film at 190 \({}^{\circ}\)C and analyzed using attenuated total reflectance (ATR) on a Nicolet 6700 Transform Infrared Spectrometer (Nicolet Company, Madison, WI, USA).  
The melt mass flow index (MFI) was measured using an XRZ-400 tester (Jilin University Teaching Instrument Factory, Changchun, China) under the conditions specified in GB3682-2000 [42] (at 230 \({}^{\circ}\)C and 2.16 kg). The shear rheological properties of the polymer samples before and after modification were tested using a Discovery HR-2 rotational rheometer (TA Instruments Inc., New Castle, DE, USA), with the temperature set to 185 \({}^{\circ}\)C. The final gap was 2 mm, strain amplitude was 1%, and the angular frequency scanning range was 0.1 rad/s to 500 rad/s. The melting and crystallization behaviors of the polymer matrix (5 mg) were analyzed by differential scanning calorimetry (DSC, Diamond DSC, Perkin Elmer, Shelton, CT, USA) using a heating/cooling rate of 10 \({}^{\circ}\)C/min in the temperature range of 60-200 \({}^{\circ}\)C under nitrogen flow. The crystallinity of rPP samples was calculated using the following equations [33]:  
\[X_{\mathrm{c}}=\frac{\Delta H_{\mathrm{f}}}{\Delta H_{\mathrm{f}}^{0}}\times 1 00\%\]  
where \(\Delta H_{\mathrm{f}}\) is the melting enthalpy value of the samples and \(\Delta H_{\mathrm{f}}^{0}\) is the melting enthalpy of the hypothetically 100% crystalline rPP [37].  
Tensile and flexural tests were conducted using a universal mechanical testing machine (RGT-20A, ARI Medical Technology Co., Ltd., Hong Kong, China) to assess the mechanical properties. In accordance with ASTM D 638-10 [43], the plastic tensile specimens were dumbbell-shaped, with dimensions of 165 \(\times\) 13 \(\times\) 4 mm\({}^{3}\) and a tensile rate of 50 mm/min. The wood-plastic sheet was prepared into dumbbell-shaped specimens (165 \(\times\) 13 \(\times\) 4 mm\({}^{3}\)) for testing, with a tensile rate of 5 mm/min and a gauge length of 50 mm. The flexural specimens were rectangular (80 \(\times\) 13 \(\times\) 4 mm\({}^{3}\)) according to ASTM D790-10 [44], with a crosshead speed of 2 mm/min and a span of 64 mm. Notched impact specimens of the plastic matrix (80 \(\times\) 13 \(\times\) 4 mm\({}^{3}\), notch depth 2.7 mm) were tested using a Charpy impact tester (XJ-50G, Chengde Precision Testing Machine Co., Ltd., Chengde, China) following ASTM D256-10 [45]. The pendulum energy used was 5 J. Impact specimens of the composites (80 \(\times\) 10 \(\times\) 4 mm\({}^{3}\)) were tested with a span of 60 mm and a pendulum energy of 2 J using a Charpy impact tester (XJ-50G, Chengde, China) according to ASTM D4812-06 [46]. For each of the tests mentioned, eight specimens were tested. A dynamic mechanical analyzer (Q800, TA Instruments Inc., New Castle, DE, USA) was used to investigate the dynamic viscoelastic behavior of the composite. Specimens (35 \(\times\) 13 \(\times\) 4 mm\({}^{3}\)) were tested in the single-cantilever mode with a 50 \(\upmu\)m amplitude, 1 Hz frequency, and a temperature range from \(-\)10 \({}^{\circ}\)C to 120 \({}^{\circ}\)C at a heating rate of 3 \({}^{\circ}\)C/min.  
A scanning electron microscope (Quanta 200, FEI Company, Hillsboro, OR, USA) was used to observe the morphology of modified polypropylene. The samples were frozen in liquid nitrogen for 5 min and immediately broken by the fixture, and further sputtering with a thin gold layer at an acceleration voltage of 15 kV.