FEATURE / RAPID CURING EPOXY Figure 6. DMTA measurements representing G'(blue), G " (green), and tan delta (red) as a function of temperature. Figure 7. Effect of cure schedule during HP-RTM on the degree of cure attained by the composite panels. The degree of cure determined via DMTA measurements are shown as the blue symbols while the model calculations are shown as the dashed lines. to be a simple and efficient approach to capture the relative change in the degree of cure for various processing conditions employed in this work. Other approaches for assessment of cure levels such as the use of DSC would be less effective for these composite panels due to the complications involved in separation of heat flow contributions from the fiber fractions and the challenges or uncertainties involved in analysis of residual cure exotherms of very low magnitude. The panel cured for 4 minutes at 115°C, shows a high Tg of 134°C based on the tan delta peak method. The ultimate Tg for the panels was found to be 141 ± 1.8°C based on a second DMTA second for seven samples. Hence, based on Equation 1, the experimental results from the composite panels confirm the potential to achieve 95% cure even for a very short cure duration of 4 minutes, a nominal temperature of 115°C. The values of the experimentally determined degree of cure determined from Equation 1 for panels produced under various processing histories are compared with the model predictions for the same thermal histories in Figure 7. The degree of cure estimated from characterization of the composite panels are found to be in close agreement with the model predictions over the entire range of processing conditions employed in this work, which confirms and validates the fast reactivity of the epoxy system developed in this work. The degree of cure is found to be 95% at cure temperatures of 110-115°C over the short durations employed in this work. The results also show that the degree of cure may vary over a range of 87-95%, depending on the choice of cure temperature and time in this work. The impact of these variations in the degree of cure on the mechanical properties of the composite panels will be explored in detail in the subsequent section. The effect of variation in the degree of cure 28 | SAMPE JOURNAL | JANUARY/FEBRUARY 2022 www. sampe.orghttp://www.sampe.org