Figure 4.22 shows the temperature dependency of E and tan 5 for SPE/QC(1/1.2), SPE/PN(1/0.8), SPE/QC(1/1.2)/WF and SPE/PN(1/0.8)/WF biocomposites mea­sured by DMA. The tan 5 peak amplitude for the biocomposites became weaker with increasing WF content, indicating that amorphous content of the biocompos­ites certainly decreased with WF content. The E curve of at the rubbery plateau region over 120 °C for all the biocomposites was much higher than those of con­trol SPE/QC(1/1.2) and SPE/PN(1/0.8), suggesting a superior reinforcement effect due to the wood fibers. The tan 5 peak temperatures (SPE/QC(1/1.2)/WF20, 30, 40:106.2, 112.7, 107.2 °C) related to Tg for the SPE/QC(1/1.2)/WF biocomposites were significantly higher than that of SPE/QC(1/1.2) (85.5 °C). This trend is marked contrast to the fact that SPE/PN(1/0.8)/WF30 had a lower tan 5 peak temperature (69.9 °C) than that of SPE/PN(1/0.8) did (81.0 °C). Similar lowering of tan 5 peak temperature of WF biocomposite relative to the corresponding cured neat resin had been also observed for GPE/TA/WF biocomposites as was reported by our group.22 Also, the E of the SPE/QC(1/1.2)/WF biocomposites declined at around 90-120 °C due to the glass transition, and then again decreased at around 180-200 °C, probably due to the disappearance of specific interaction between WF and the cured SPE/QC resin. Figure 4.23 shows FT-IR spectra of WF, QC and a mixture of QC/WF 1/1 (w/w) prepared by mixing in THF and drying at 40 °C for 24 h. The band at 1621 cm-1 for QC due to C=C stretching vibration at C-2 and 3 did not shift for QC/WF. In contrast, the band at 1675 cm-1 for QC due to unsaturated carbonyl (C=O) stretch­ing vibration significantly shifted to a lower wavenumber region for QC/WF (1659 cm-1), indicating that there is a hydrogen bonding interaction between unsaturated carbonyl group of quercetin moiety and hydroxy group of lignocellulose component of WF. This interaction is based on the resonance structure of QC generating highly polarized carbonyl group as is shown in Fig. 4.24.

Figure 4.25 shows typical TGA curves of SPE/PN(1/0.8), SPE/QC(1/1.2), SPE/ QC/(1/1.2)WF biocomposites and WF. Since the thermal decomposition tempera­ture of WF was lower than that of SPE/QC(1/1.2), the SPE/QC(1/1.2)/WF com­posite exhibited two-step thermo-degradation. The 5% weight loss temperatures of SPE/QC(1/1.2), SPE/QC(1/1.2)/WF20,30,40 and WF were 342.5, 311.4, 300.8, 299.2 and 295.5 °C, respectively. The SPE/QC showed a comparable 5% weight loss temperature to SPE/PN(1/0.8) (346.3 °C) in agreement with the fact that both QC and PN are aromatic polyphenols. In addition, the cured resin of DGEBA and

QC, both of which are aromatic compounds had a superior 5% weight loss tempera­ture (407.4 °C) as is shown in Table 4.8.

Figure 4.26 shows the relationship between tensile properties and fiber content for SP/QC(1/1.2)/WF composites. The tensile modulus of SPE/QC(1/.12)/WF bio­composites increased with increasing WF content, and SPE/QC(1/1.2)/WF40 had a higher tensile modulus than SPE/PN(1/0.8). However, tensile strength and elonga­tion at break of the biocomposite were lower than those of SPE/QC(1/1.2). Figure 4.27 shows the FE-SEM images of the fractured surface of SPE/QC(1/1.2)/WF bio­composites. The micrograph of WF shows that the fiber length and width of WF are ca. 0.2-0.4 mm and 40-200 mm, respectively. It appeared that the composites are fractured at the interface between WF and the cured resin. As SPE/QC(1/1,2) itself has a high tensile strength (43 MPa), a considerably high interfacial adhesiveness between WF and SPE/QC is necessary to obtain the biocomposite with a higher tensile strength than the cured resin.

FIGURE 4.27 FE-SEM images of WF and the fracture surfaces of SPE/QC(1/1.2) and SPE/ QC(1/1.2)/WF biocomposites.24