All the bio-based materials used in this study (PGPE, GPE, and TA) are water-solu­ble and hydrophilic substances. The average number of epoxy groups per molecule of PGPE and GPE is 4.1 and 2.0, respectively. Because the viscosity of GPE (150 cps at 25 °C) was much lower than that of PGPE (1300 cps at 25 °C), a mixture of GPE, TA, and WF can be compounded without solvent. However, it was necessary to add a solvent in case of a mixture of PGPE, TA, and WF. Since some precipitate was liberated when aqueous solutions of PGPE and TA were mixed, ethanol was used as a mixing solvent. The mixture of PGPE, TA, and WF or GPE, TA, and WF was cured at the condition of 160 °C for 3 h with epoxy/hydroxy ratio of 1/1, at which most balanced thermal and mechanical properties were attained for the cured products of GPE and TA in the previous section.22 Although the biocomposites with WF content higher than 70 wt.% can be prepared, the obtained composites became brittle and the surface was rough. Figure 4.5 shows FE-SEM photographs of the fractured surfaces of PGPE/TA(1/1)/WF and GPE/TA(1/1)/WF composites with WF contents of 60 and 70 wt.%. It appeared that WF is tightly incorporated into the crosslinked epoxy resins and their interfacial adhesion is good. This result may be attributed to the fact that the polyphenol moiety of TA and lignocellulose moiety of WF resemble each other. There are some voids on the fractured surface of PGPE/ TA(1/1)/WF, probably generated during the evaporation of ethanol when compared with GPE/TA/WF.

Figures 4.6 and 4.7 show the temperature dependency of storage modulus (E’) and tan 8 for PGPE/TA(1/1)/WF and GPE/TA(1/1)/WF measured by DMA, respec­tively. The E’ at the rubbery plateau region over 80 °C for all the composites was much higher than that of control cured resins, suggesting a superior reinforcement effect due to the wood fibers. The tan 8 peak temperature related to Eg for the com­posites was a little lower than that of the corresponding neat resins. The reason is not clear, but it is thought that some components of WF react with the epoxy resins, and/or that WF disturbs the crosslinking reaction.

FIGURE 4.7 DMA curves of GPE/TA(1/1) and GPE/TA(1/1)/WF biocomposites.22

Figure 4.8 shows typical TGA curves of GPE/TA(1/1), GPE/TA(1/1)/WF60 and WF. Since the thermal decomposition temperature of WF was lower than that of GPE/TA(1/1), the GPE/TA(1/1)/WF composite exhibited two-step thermo-degra­dation. The 5% weight loss temperatures of all the composites are summarized in Table 4.4. Consequently, the 5% weight loss temperatures of all the composites were lower than those of the corresponding cured neat resins.

Figures 4.9 and 4.10 show the relationship between tensile properties and fiber content for PGPE/TA(1/1)/WF and GPE/TA(1/1)/WF, respectively. Although ten­sile modulus (4.3 GPa) of PGPE/TA(1/1)/WF was much higher than that of PGPE/ TA(1/1) (2.7 GPa), the tensile strength of the composite was lower than that of PGPE/TA(1/1). On the other hand, both the tensile modulus and strength of GPE/ TA(1/1)/WF were much higher than those of GPE/TA(1/1) (2.4 GPa and 37 MPa). Those values increased with WF content, became maximal values (5.1 GPa and 51 MPa) at WF content 60 wt.%, and were lowered at 70 wt.%. In general, although the tensile modulus of polymer/plant fiber biocomposites is higher than the control polymer, the strength is rather lower because of a poor interfacial adhesion. It is noteworthy that the tensile strength of GPE/TA is improved by the addition of WF without any interfacial modification. This result should be attributed to the superior interfacial adhesion between GPE/TA and WF. As the reason that the tensile strength of PGPE/TA(1/1)/WF did not increase, the following factors are considered. As ten­sile strength of PGPE/TA(1/1) is much higher than that of GPE/TA(1/1), the in­terfacial adhesion strength between PGPE/TA(1/1) and WF is not higher than the strength of PGPE/TA(1/1). The structural defects due to some voids are observed as was shown in Fig. 4.5. Also, the fact that the tensile modulus and strength of both PGPE/TA(1/1)/WF and GPE/TA(1/1)/WF composites with WF content 70 wt.% are lower than those of the composites with 60 wt.% suggests that the packing of ma­trix resin between the WF particles is relatively insufficient for composites with 70 wt.%.

□ PGPE/TA(1/1) О PGPE/TA(1/1 )/WF50 I I PGPE/TA(1/1)/WF60 PGPE/TA(1/1)/WF70

As the tan 5 peak temperature of GPE/TA(1/1)/WF was lower than that of GPE/ TA(1/1) (Fig. 4.7), the epoxy/hydroxy ratio appropriate for GPE/TA/WF compos­ites was investigated. Table 4.4 summarizes the tan 5 peak temperature of GPE/ TA and GPE/TA/WF60 prepared at epoxy/hydroxy ratios from 1/0.6 to 1/1.2. In case of the control GPE/TA, the tan 5 peak temperature related to Tg increased with decreasing epoxy/hydroxy ratio. Considering that all of the three hydroxy groups of PG moiety of TA are hard to react with epoxy groups of GPE, it is supposed that an actual stoichiometric epoxy/hydroxy ratio should be lower than 1/1. Although GPE/TA(1/1.2)/WF60 exhibited the highest tan 5 peak temperature among the GPE/ TA/WF60 composites in a similar manner to the control resins, GPE/TA(1/0.8)/ WF60 had a little higher tan 5 peak temperature than GPE/TA(1/1)/WF60. In case of GPE/TA(1/0.8)/WF60, there is a possibility that the excess epoxy groups of GPE reacted with the hydroxy groups in WF at the curing temperature of 160 °C. As a
result, GPE/TA(1/0.8)/WF60 had almost the same tan 5 peak temperature as GPE/ TA(1/0.8).

Table 4.4 also summarizes 5% weight loss temperature of GPE/TA and GPE/ TA/WF60 prepared at epoxy/hydroxy ratios from 1.0/0.6 to 1.0/1.2. Regarding the control GPE/TA, the 5% weight loss temperature a little decreased with decreas­ing epoxy/hydroxy ratio. As TA itself has the lowest 5% weight loss temperature (285 °C), the presence of unreacted TA moiety in the GPE/TA with a higher TA con­tent caused a decrease of the 5% weight loss temperature. In case of the composites with WF content 60 wt.%, GPE/TA(1/0.8)/WF60 exhibited the highest 5% weight loss temperature among the GPE/TA/WF composites.

Figure 4.11 shows the tensile properties of GPE/TA/WF60 composites prepared at various epoxy/hydroxy ratios. The GPE/TA(1/0.8)/WF60 showed the highest tensile modulus (5.22 GPa), strength (54.9 MPa) and elongation at break (1.35%), indicating that the best ratio of epoxy/hydroxy is ca. 1/0.8. In case of polypropylene (PP)/WF composites, it is known that the preparation of the PP/WF composite with WF content higher than 50 wt.% is not easy, and that the addition of maleic anhy­dride-grafted polypropylene (MAH-PP) improves the tensile properties. The tensile modulus and strength of PP/MAH-PP/WF (45/5/50) composite are reported to be 4.55 GPa and 40.4 MPa, respectively.64 It is also known that that tensile modulus and strength of high-density polyethylene (HDPE)/WF (35/65) composite are 2.6 GPa and 15.6 MPa, and those of HDPE/poly(ethylene-cobutyl acrylate-co-maleic anhy — dride)/WF (32.5/2.5/65) are 2.5 GPa and 18.6 MPa, respectively.65 The GPE/TA/ WF composites in which neither modifier nor compatibilizer is added have higher tensile modulus and strength than these petroleum-based plastics/WF composites.