## 3 Results and Discussion
### Cure in a Convection Oven  
Once the co-monomer composition was set for the resin system under study, a DEA experiment was conducted in order to determine the optimal cure time for the resin. Figure 2 shows the curve of log ion viscosity _vs._ time for the cure carried out in a convection oven at 140 \({}^{\circ}\)C. As mentioned previously in the text, this temperature was chosen based on previously published data of related resins [11, 30], and preliminary tests. According to the sample preparation method employed in this work, it is expected that polymer chain growth starts immediately after mixing the resin components with BF\({}_{3}\cdot\)OEt\({}_{2}\), following a cationic polymerization mechanism that predominates during the initial 30 min resting time that precedes heating. Because no effort has been made to suppress oxygen from the capped vials used, an auto-oxidation process triggered by the presence of oxygen at high temperatures cannot be excluded under the conditions employed [39], especially during longer reaction times. As the temperature is increased to 140 \({}^{\circ}\)C, it is possible that both cationic and free radical reactions coexist in the system, contributing to the final cure of the thermoset. Indeed, the thermal polymerization of similar tung oil-based resins in the absence of cationic or free radical initiators has been demonstrated and addressed previously in the literature [11].  
After initial heating (first 45 min. not shown), the growth of polymer chains predominates in the system, and a steady increase in the ion viscosity, due to the lower mobility of the resin components, is noticed during the following 30 min (Figure 2). After 1 h and 15 min, the increase in ion viscosity slows down gradually until the resin is fully cured and the ion viscosity plateaus. The complete polymerization of the resin is attained after 8 h. However, the change in ion viscosity between 6 and 8 h is negligible for practical purposes, and, as will be shown next with the DSC results, further cure of the system can't be detected after 6 h.  
In order to confirm the ideal cure time, sample mixtures were prepared and heated at 140 \({}^{\circ}\)C in a convection oven, for different times. Each one of these samples was analyzed by DSC in order to check for residual cure of the polymer. Indeed, the eventual residual cure of a sample leads to an exotherm at approximately 230 \({}^{\circ}\)C. Figure 3 presents the DSC curves of samples cured for 1.5 h, 3.5 h, 5.5 h, and 6 h. From the curve of the sample cured for 1.5 h, an obvious exothermic peak is seen at approximately 230 \({}^{\circ}\)C (Figure 3). The area under that peak clearly decreases with increases in cure time. Indeed, a clearly lower area is observed, at the same temperature, for the sample cured for 3.5 h. The trend is maintained with the sample cured for 5.5 h, but when the resin is heated for 6 h, no exothermic peaks could be detected by the DSC. Therefore, the DSC data confirms the results obtained from the DEA, and 6 h was selected as the ideal heating time for the preparation of the tung oil thermosetting resins reported here. The more discrete transitions occurring at other temperatures will be discussed later in the text, when analyzing the DMA data. A quick analysis of Figure 3 suggests that the cure behavior matches a self-catalyzed process. Further kinetic experiments are necessary in order to fully elucidate the cure mechanism.  
Figure 2: Dielectric analysis (DEA) of the cure of a tung oil-based thermosetting resin at 140 \({}^{\circ}\)C. The resin is composed of 70 wt % of tung oil, 15 wt % of divinylbenzene (DVB), 10 wt % of styrene (ST), 3 wt % of soybean oil, and 2 wt % of BF\({}_{3}\cdot\)OEt\({}_{2}\).