4.4.1 BIO-BASED EPOXY RESINS CURED WITH TA

The GPE, PGPE, SPE, and ESO were cured with TA in order to compare the proper­ties of the cured bio-based epoxy resins.2123 Because TA is soluble in liquid GPE and SPE, the mixing of GPE/TA and SPE/TA is very easy. In case of PGPE/TA and ESO/ TA, it is necessary to add ethanol to get homogenous solutions. The chemical for­mula for the commercial TA is often given as C76H52O46 (Fig. 4.2). However, in fact it contains a mixture of related compounds. Also, it is supposed that all of the three hydroxy groups of PG moiety of TA are hard to react with epoxy groups. Therefore, the curing temperature, curing time and epoxy/hydroxy ratio were optimized for GPE/TA, SPE/TA and ESO/TA. First, curing temperature and time were changed at the fixed epoxy/hydroxy ratio of 1/1. As ESO with alicyclic epoxy groups has a lower reactivity than GPE and SPE with glycidyl groups, ESO/TA was cured at a higher curing temperature range (150-230 °C) than that of GPE/TA and SPE/TA (120-200 °C). The GPE/TA and SPE/TA had the maximal tan 5 peak temperatures (73 and 95 °C), when cured at 160 °C for 3 h and 2 h, respectively. Also, ESO showed the highest tan 5 peak temperature (57 °C), when cured at 210 °C for 2 h (Table 2). These results indicate that the control of curing temperature and time is very important for the bio-based epoxy curing system containing aliphatic and sug­ar-based moieties with relatively low heat resistance. When PGPE/TA with epoxy/ hydroxy 1/1 was cured at the same curing temperature and time (160 °C and 3 h) as GPE/TA, the tan 5 peak temperature was 77 °C. When the tan 5 peak temperatures of the materials cured at epoxy/hydroxy 1/1 are compared, the higher order was SPE/ TA (95 °C) > PGPE/TA (77 °C) > GPE/TA (73 °C) > ESO/TA (57 °C), as is sup­posed from the relationship between epoxy functionality and the shortest distance between the two cross-linked points.

Next, epoxy/hydroxy ratio was changed at the curing temperature/time, which showed the highest tan 5 peak temperature at epoxy/hydroxy 1/1. The tan 5 peak temperatures of GPE/TA and SPE/TA increased with decreasing epoxy/hydroxy ra­tio (Table 4.3). The increase of Tg with TA content should be attributed to an increase of the content of highly hindered aromatic framework rather than an increase of crosslinking density. However, the cured resins at epoxy/hydroxy 1/1 showed the highest tensile strength and modulus. Judging from the trend of tensile properties, it is thought that the incorporation of TA component in the crosslinked structure is in­sufficient for the TA-rich compositions (GPE/TA 1/1.2—1/1.4, SPE/TA 1/1.2—1/1.4). Consequently, the epoxy/hydroxy ratio 1/1 was selected for both GPE/TA and SPE/ TA, considering the balance of T and tensile properties. The epoxy/hydroxy ratio of PGPE/TA was fixed to 1/1, considering the result of GPE/TA, In case of ESO/ TA, both the tan 5 peak temperature and tensile properties increased with decreasing epoxy/hydroxy ratio. When the epoxy/hydroxy ratio was lower than 1/1.4, the ESO/ TA mixture became so viscous that we could not prepare a void-free cured sample. Consequently, the condition of curing temperature 210 °C, curing time 2 h, and epoxy/hydroxy ratio 1/1.4 was selected for the curing of ESO/TA. When petroleum — based PN is used as a hardener of SPE, the tan 5 peak temperatures for SPE/PN resins cured at epoxy/hydroxy ratios 1/0.8-1/1 were 78-81 °C, which were lower than those of SPE/TA resins cured at epoxy hydroxy ratios 1/1-1/1.4 (95-111 °C).