# Effects of Matrix Modification on the Mechanical Properties of Wood-Polypropylene Composites
## 3 Results and Discussion  
### Characterization of Chemically Grafted Polypropylene  
The torque behavior of the peroxide (DCP)-initiated melt grafting system was examined with a HAAKE mixer (Figure 1). With the addition of DCP, the mixing torque values in the PP (dPP) and PP/MAH (dMPP) systems decreased gradually until the end of the reaction with an end-torque value lower than that of virgin PP. This reduction could be attributed to the severe chain scission of the PP backbone (via the tertiary macroalkyl radical) [28]. In the case of the system containing DVB (dDPP), a noticeable second torque peak was observed at the early stage, which also exhibited a lower end-torque value than that of virgin PP, but remained higher than those of dPP and dMPP. The reason was that the highly reactive DVB was preferentially grafted to PP macroradicals, and formed styryl macroradicals, thus restraining the \(\beta\)-chain scission reaction of PP macroradicals [28]. The grafted DVB further reacted with PP macroradicals and resulted in branching or crosslinking, which corresponds to the second torque peak [26; 29]. After the DVB was consumed, the polymer degradation induced by residual peroxide could be dominated by \(\beta\)-chain scission reactions, which corresponds to a slow reduction in torque values at the later stage of the reaction. When both MAH and DVB were added (dMDPP), the peak intensity of the second mixing torque peak and end-torque values were significantly reduced, indicating that parts of the grafted DVB on PP macroradicals were reacted with MAH apart from DVB-assisted branching and crosslinking. The main grafting and competing reactions are shown in Figure 2.  
The FTIR spectra of unmodified PP and modified (dMPP, dMDPP) samples provided structural information of grafting monomers to PP backbone (Figure 3). New absorption bands at 1790 and 711 cm\({}^{-1}\) were observed, which can be assigned to the carbonyl groups (-C=O) of the grafted MAH and the grafted DVB, respectively [25]. In comparison with the characteristic absorption of carbonyl groups in dMPP, the addition of DVB (dMDPP) shifted the characteristic absorption of carbonyl groups from 1790 cm\({}^{-1}\) to 1780 cm\({}^{-1}\). This occurrence may be attributed to the structural change in the grafted MAH, from a single succinin anhydride (grafted on the end chain of the PP backbone, with absorption at 1790 cm\({}^{-1}\)) to other forms of MAH, due to the reaction between DVB and MAH on the PP backbone, which corresponded to the aforementioned analysis of torque behavior [25]. Figure 1: Torque behaviors of virgin PP, dPP (PP + DCP), dMPP, dDPP, and dMDPP systems ([MAH] = 1 wt %, [DCP] = 0.1 wt %, [DVB] = 1 wt %). Figure 3: FTIR spectra of virgin PP, dMPP, and dMDPP ([MAH] = 1 wt %, [DCP] = 0.1 wt %, [DVB] = 1 wt %). Figure 2: The main reactions related chemical grafting of MAH to PP in the presence of DVB. The modified PP matrix at the pilot-plant setting, in the presence of DVB, was also prepared using a twin-screw extruder. As illustrated in Figure 4a, the MAH grafting level increased with increasing DVB concentration, and then decreased. The results indicated that the high-reactivity comonomer DVB could act as an interlinking agent between the PP chain and MAH, resulting in an increase in the MAH grafting efficiency. With a further increase in DVB concentration (above 0.6%), predominance of DVB-assisted branching and crosslinking, rather than MAH grafting would occur, as indicated by a decrease in the MAH grafting level and the reduction in MFI (Figure 4b). The MFI of the modified PP decreased with increasing DVB concentration, but remained higher than that of the virgin PP. This finding suggested that not all tertiary macroradicals can be converted to styryl macroradicals by reacting with DVB to restrain chain scission reactions; the main grafting and branching reaction may occur after chain scission of PP [27; 28].