# Effects of Matrix Modification on the Mechanical Properties of Wood-Polypropylene Composites
## 3 Results and Discussion
### Characteristics of the Composites  
#### 3.2.1 Mechanical Properties of the Composites  
According to the aforementioned results, the virgin PP exhibited higher tensile strength in comparison with MPPD0, whereas the composites (WMPD0) with MPPD0 as the matrix exhibited increases of 221%, 178%, and 75% in tensile, flexural, and impact strengths relative to the unmodified  
Figure 4: Effects of DVB concentration on the relative MAH grafting level (**a**) and MFI of PP matrix (**b**). Figure 5: Tensile strength of virgin PP and the modified PP. composites (WP) (Table 3). The increase could be attributed to the effective coupling by the covalent link between the anhydride carbonyl of the modified matrix and hydroxyl groups of wood surfaces, as well as the sufficient wettability of the matrix (MPPD0) with high MFI values, given that the applied force was more efficiently transferred from the matrix to WF. Compared with the modulus of the unmodified composites, the tensile and flexural moduli of MPPD0 were decreased to 8% and 11%, respectively. This result may be attributed to the PP matrix degradation via chain scission reactions. Comparison between the composites (WPMA) containing the MAPP coupling agent and WMPD0 was also studied. The results indicated that much less improvement in mechanical properties by matrix modification with MAH only was observed (Table 3), which could be explained by the following: (i) the lower MAH concentration resulted in a lower MAH grafting level, which failed to provide much more effective fiber-matrix coupling for stress transfer from the modified PP to WF [24]; and (ii) MPPD0 underwent chain scission, which were prone to achieving more effective wetting of WF by MPPD0, thereby promoting the formation of effective coupling between WF and the modified matrix; however, the inferior strength of MPPD0 itself with respect to virgin PP offset the positive effect from improved interfacial adhesion [24]. The mechanical properties of modified composites with different DVB concentrations were also evaluated (Figure 6). The tensile strength and modulus of the modified composites exhibited an increasing trend with respect to WMPD0 when the DVB concentration was increased to 0.4%. Above that value, the tensile strength and modulus decreased (Figure 6a). The increase in tensile strength could be attributed to the improved interfacial adhesion, owing to an increase in the MAH grafting level (Figure 4), as well as the improvement in the tensile strength of the matrix (Figure 5), which benefited the stress transfer from the matrix to WF. A further increase in initial DVB concentrations led to an increase in the grafting level of MAH; however, the modification process was also accompanied by a higher degree of branching and crosslinking, which could lead to insufficient wetting of WF, due to the matrix with lower MFI. The results indicate that beyond the critical concentration of DVB (0.4%) for the modified composites, in the case of constant MAH and DCP concentrations, the wetting of WF by the modified matrix with a lower melt flow rate may decrease, thus forming a weak effective coupling between the fibers and the matrix. The flexural strength and modulus of the composites reflected the same change trend as that of the aforementioned tensile properties by varying the DVB concentration (Figure 6c,d). Interestingly, when the DVB concentration increased from 0 to 0.4%, the composites exhibited a maximum improvement of 32% in impact strength; with a further increase in DVB concentration, the impact strength decreased (Figure 7). The marked improvement in impact strength could be largely associated with the strengthening of interfacial bonding between the modified matrix and WF, allowing the absorption of more energy during impact fracture [9]. At DVB concentrations higher than 0.4%, the melt flow rate of the matrix decreased and was accompanied by a higher degree of branching and crosslinking.