The onset and peak temperatures of the exothermic curve on the first heating DSC thermogram for the SPE/TPG compound with a standard epoxy/hydroxy ratio of 1/1 were 142.3 and 192.9 °C, respectively. Based on the DSC data, the curing tem­perature of the SPE/TPG(1/1) was changed between 150 and 190 °C. The tan 5 peak temperature (43.0, 43.5 and 53.5 °C) measured by DMA increased with an increase of curing temperature (150, 170 and 190 °C). Also, the 5% weight loss temperature (344.3, 344.8 and 361.1 °C) increased with an increase of curing temperature. When the mixture was cured at a temperature higher than 190 °C, the cured material con­siderably colorized. The curing temperature was fixed to 190 °C, considering the stability of SPE/TPG and wood flour which is subsequently added.

Figure 4.31 shows DMA curves of the SPE/TPG(1/1)/WF biocomposites cured at 190 °C. The E’ at the rubbery plateau region over 50 °C for the composites was much higher than that of SPE/TPG, suggesting a superior reinforcement effect due to the wood fibers. The tan 5 peak temperature related to T for the composites (WF40:45.6 °C; WF50:45.7 °C; WF60:44.5 °C) was a little lower than that of the corresponding neat resins (53.5 °C). The reason is not clear, but it is thought that hydroxy groups of WF reacted with epoxy groups of SPE and the stoichiometry of epoxy and hydroxy is deviated. A similar decline of Tg by the addition of WF was also observed for the GPE/TA/WF biocomposites.22 Figure 4.32 shows TGA curves of WF, SPE/TPG(1/1) and SPE/TPG(1/1)/WF composites. Since the thermal de­composition temperature of WF was lower than that of SPE/PGT, the SPE/PGT/WF composite exhibited two-step thermo-degradation, and the 5% weight loss tempera­ture decreased with increasing WF content (0 wt.%: 361.1 °C, 40 wt.%: 294.7 °C, 50 wt.%: 286.3 °C, 60%: 279.6 °C).

Figure 4.33 shows the tensile properties for SPE/TPG(1/1)/WF composites. The tensile modulus of SPE/TPG(1/1)/WF increased with increasing WF content in the range of 0-50 wt.%. However, the tensile modulus of SPE/TPG(1/1)/WF60 was lower than that of SPE/TPG(1/1)/WF50 in agreement with the influence of WF content on the E’ measured by DMA. Also, the tensile strength of the composites with WF content 40-50 wt.% was a little higher than the corresponding neat resin (SPE/TPG(1/1)). The fact that the improvement of tensile strength is not so high as that of tensile modulus is related to the decease of elongation at break for the WF biocomposites. In the previous our study on GPE/TA/WF and SPE/QC(1/1.2)/ WF biocomposites, the tensile strength considerably decreased by the addition of WF.22,25 When TPG was used as an epoxy-hardener, the tensile strength of the WF composite did not decrease.

Figure 4.34 shows SEM images of WF and the fractured surfaces of SPE/ TPG(1/1) and SPE/TPG(1/1)/WF composites. The micrograph of SPE/TPG(1/1) showed no phase separation, indicating that SPE is homogeneously cured with TPG. 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. All the micrographs of SPE/TPG(1/1)/ WF biocomposites show that WF is tightly incorporated into the crosslinked epoxy resin and their interfacial adhesion is good. The fact that tensile strength did not decrease by the addition of WF is related to the good affinity of SPE/TPG(1/1) and WF. The good affinity is inferred from what TO is widely used as a coating material for woody surface and the structure of pyrogallol moieties of TPG resembles that of lignin of WF.