Physical aging thermoreversibility

Figure 26 shows the thermal expansion behavior of requenched epoxies. Upon reaging, the densification process was again measurable. Data shown in Figure 26, therefore, supports the notion that physical aging processes are thermoreversible. 

With requenching and reaging, expansivity above Tg again increases with reaging time (96.6 % increase), demonstrating once again the thermoreversibility of physical aging. Table 2 summarizes the results of the thermal mechanical analysis. 

The thermoreversible nature of physical aging has been amply demonstrated. 

To demonstrate the "Thermoreversibility of physical aging ( ), the follov/ing requenching procedure was csr ried out. Some of the specimens aged for 10 rain, were heated to 260°C, v/hich is above the epoxy Tg, for 20 min. and then air-quenched to room temperature inside a desiccator. Five of these requenched specimens were tested immediately, while the rest were subjected to "re-aging" at 1402c in.nitrogen for the time increments of 10, 10 , 10, 10, and up to lO min. At least five specimens were tested for each decade of re-aging time. 

Figure 13 shows the thermal expansion behavior of the epoxy as a function of re-aging time at 140°C. Again, the thermoreversibility of physical aging was demonstrated. 

Thermoreversibility of physical aging has been demonstrated in the time-dependent rate of stress relaxation as v/ell as in the damping behavior of the p-transition. 

---