## 2 Results and Discussion
### Properties of Composites
#### 2.2.2 Analysis of Mechanical Properties and Fracture Mechanism  
The mechanical properties of the modified waste polypropylene/wood flour composites are presented in Figure 7. The tensile strength of WrPM increased by 59% compared with that of WrP, while the tensile modulus decreased by 13%. This increase was attributed to improved interfacial bond strength between the matrix and wood flour [39]. MAH graft modification improved the fluidity and polarity of rPM compared with unmodified PP, facilitating the dispersion of wood flour in the matrix and resulting in more extensive interfacial bonding within the composite. Under tensile stress, the interfacial layer facilitated stress transfer from the degraded PP short chains to the rigid wood flour [33]. Improved interfacial compatibility restricted short-chain molecular slippage at the interface, leading to more wood flour fracture and enhanced tensile strength of the composites during deformation. However, the lower modulus of rPM negatively impacted the composites'  
Figure 6: Fracture morphology of the composites: (**a,b**) WrP; (**c,d**) WrPM; (**e,f**) WrPOM; (**g,h**) WrPOMD. resistance to deformation, leading to a tensile modulus lower than that of WrP. With the addition of DVB at the same MAH level, co-grafting further increased the tensile strength of WrPMD by 20% compared with that of WrPM, while the tensile modulus increased to match that of WrP. This improvement could be attributed to the increased grafted MAH groups on the matrix molecular chain, the introduction of branched structures, an expanded interfacial bonding area, and enhanced matrix strength, ultimately resulting in improved composite mechanical strength. In the elastomer toughening system, the tensile strength and modulus of WrPOM and WrPM were nearly identical. The tensile modulus was shown to depend mainly on the content of the embedded structure formed by MAH-grafted elastomer-coated wood flours [26]. Therefore, the rigid embedded structure formed by MAH-grafted OBC-coated wood flour can partially mitigate the negative impact of the elastomer on the modulus [31]. In the DVB and MAH co-grafted blend matrix, WrPOMD showed a 10% increase in tensile strength and an 84% increase in tensile modulus compared with WrPM, and a 60% increase in tensile modulus compared with unmodified WrP. Co-grafting modification enhanced the interfacial compatibility among PP, OBC, and wood flour, improving stress transfer efficiency and thereby increasing the contribution of rigid wood flour to the composite's mechanical properties, while offsetting some negative impacts of the low-modulus OBC [26; 31]. Additionally, the extensive embedded structure increased the reinforcement efficiency of wood flour and enhanced the tensile modulus of the final composite material [25]. The flexural strengths of WrPM and WrPMD increased by 49% and 60%, respectively, compared with that of WrP (Figure 7b). This increase could be attributed to enhanced interfacial compatibility between wood flour and modified rPP, as well as improved matrix strength. For WrPM, despite the improved interfacial compatibility, the flexural modulus decreased by 11% compared with that of WrP. However, the addition of DVB restored the flexural modulus of WrPMD to match that of WrP. After adding OBC, the flexural strength and modulus of WrPOM were similar to those of WrPM. Further addition of DVB resulted in a smaller increase in the flexural strength of WrPOMD. Figure 7: Mechanical properties of the composites: (**a**) tensile strength and modulus; (**b**) flexural strength and modulus; (**c**) impact strength. After MAH graft modification, the impact strength of composite WrPM increased by 69% compared with that of unmodified WrP. The impact strength of WrPMD was further improved by 20% with the addition of DVB co-graft modification (Figure 7c). This increase could be attributed to the enhanced grafting of MAH on the PP molecular chain, which reacted with the hydroxyl groups on the wood flour's surface to generate more chemical bonds.