Figure 4.39 also shows the temperature dependency of E’ and tan 5 for SPE/ PGVNC(1/2.65)/WF composites measured by DMA. The E’ at the rubbery pla­teau region over 150 °C for the composites was much higher than that of SPE/ PGVNC(1/2.65), suggesting a superior reinforcement effect due to the wood fibers. The tan 5 peak temperature related to Tg for the composites was a little lower than that of the corresponding neat resins. 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.

Figure 4.40 also shows the tensile properties for SPE/PGVNC(1/2.65)/WF com­posites. The SPE/PGVNC(1/2.65) composites showed higher tensile modulus than SPE/PGVNC(1/2.65) in agreement with the result of storage modulus by DMA. Tensile strength also improved by the addition of WF. Figure 4.41 shows SEM im­ages of WF and fracture surfaces of SPE/PGVNC(1/2.65) and SPE/PGVNC(1/2.65)/ WF20. The photograph of WF shows that the fiber length and width of WF are ca. 0.2-0.4 mm and 40-200 mm, respectively. Although some voids due to vaporiza­tion of THF were observed in the microphotograph of SPE/PGVNC(1/2.65), the cured resin itself is homogeneous, suggesting that SPE was homogeneously cured with PGVNC. In case of SPE/PGVNC(1/2.65), it appeared that WF was tightly in­corporated into the crosslinked epoxy resins and their interfacial adhesion is good. This result may be attributed to the fact that the structures of guaiacyl and pyrogallol moieties of PGVNC resemble that of lignin of WF. The fact that tensile strength and elongation at break did not decrease by the addition of WF should be related to the good affinity of SPE/PGVNC and WF.

Figure 4.42 shows TGA curves of WF, SPE/PGVNC(1/2.65), SPE/PN(1/1), SPE/PGVNC(1/2.65)/WF composites. Since the thermal decomposition tempera­ture of WF was lower than that of SPE/PGVNC(1/2.65), the SPE/PGVNC(1/2.65)/

WF composite exhibited two-step thermo-degradation, and the 5% weight loss temperatures decreased with increasing WF content (0 wt.%: 319.2 °C, 10 wt.%: 312.1 °C, 20 wt.%: 301.8 °C, 100%: 293.2 °C). The 5% weight loss temperature of SPE/PGVNC(1/2.65) (319.2 °C) was lower than that of SPE/PN(1/1) (346.3 °C). The feed ratio of 1/2.65 for SPE/PGVNC was selected based on the highest tan 5 peak temperature. In order to get the cured material with higher thermal stability, the epoxy/hydroxy ratio should be approached to 1/1, as is obvious from Table 4.9. Regarding the biodegradability, it is supposed that SPE/PGVNC and SPE/PGVNC/ WF are fairly resistant to both aerobic and anaerobic biodegradation because their materials contain highly crosslinked aromatic structure, which is similar to that of lignin.80,81

4.6 CONCLUSION

The biocomposites composed of bio-based epoxy resins (GPE, PGPE, SPE, and ESO), bio-based polyphenol hardeners (TA, QC, TPG, and PGVNC), lignocellu — losic fibers (WF and MFC) were prepared and their thermal and mechanical prop­erties were investigated. Tan 5 peak temperatures and tensile properties of all the biocomposites are summarized in Table 4.10. Among the bio-based epoxy resins cured with TA, SPE/TA(1/1) showed the highest tan 5 peak temperature (95 °C), and

PGPE/TA(1/1) showed the highest tensile strength and modulus (63.5 MPa and 2.71 GPa). Among the SPE cured with various bio-based polyphenol hardeners, SPE/ PGVNC(1/2.65) showed the highest tan 5 peak temperature (148 °C), and PGPE/ TA(1/1) showed the highest tensile strength and modulus. Regarding the biocom­posites with WF, although the tensile modulus increased with increasing WF, tensile strength rather decreased by the addition of WF for all the WF biocomposites ex­cept for GPE/TA(1/1)/WF. In case of GPE/TA(1/1)/WF biocomposites, the tensile modulus and strength were improved by the addition of WF. The GPE/TA(1/0.8)/ WF60 had the highest tensile modulus (5.22 GPa) among the all biocomposites, and had a superior tensile strength (54.9 MPa). The tan 5 peak temperature of all the WF biocomposites except SPE/QC(1/1.2)/WF biocomposites were lower than those of the corresponding cured neat resins. When QC was used as a hardener, the tan 5 peak temperature considerably increased, probably due to a hydrogen bond­ing interaction between WF and QC. Although the maximal fiber content of MFC biocomposite (ca. 10 wt.%) was much lower than that of WF biocomposite (ca. 60 wt.%), the tensile strength and modulus increased with increasing MFC content. Tan 5 peak temperature also increased a little with increasing MFC content. Among all the biocomposites, SPE/TA(1/1)/MFC10 showed the highest tensile strength (78.6 MPa) and a superior tan 5 peak temperature (108 °C). As a whole, it can be said that WF biocomposites are suitable to get the materials with a higher tensile modulus, and that MFC biocomposites are suitable to get the materials with a higher tensile strength and Tg than the corresponding cured neat resins.

KEYWORDS

Bio-based Epoxy Resins Bio-based Polyphenols Biocomposites Microfibrillated Cellulose Wood Flour