Isothermal Cure with Vitrification

The experiment of Figure 2.2 will now be considered in more detail as a typical example of isothermal cure with vitrification. It shows the nonreversing heat flow (Figure 2.2a), the heat capacity (Figure 2.2b) and the heat flow phase (Figure 2.2c) as a function of reaction time for the quasi-isothermal cure of an epoxy-anhydride resin at 100°C for 200 min. The reaction exotherm obeys an auto-catalytic behaviour the heat flow increases at... 

FIGURE 4.11 Quasi-isothermal cure and vitrification of an epoxy-anhydride resin at 80°C at a frequency of l/60Hz. (From Van Assche, G., Van Mele, B., and Saruyama, Y., Thermochim. Acta, 377, 125, 2001. With permission.)... 

Fig. 9a and b. TBA spectra for a series of isothermal cures showing changes in (a) the relative rigidity and (b) the logarithmic decrement vs. time. Gelation and vitrification are evident in the 80, 125 and 150 °C scans, but only vitrification is observed in the 200 and 250 °C scans. The system studied was a trifunctional epoxy resin, XD7342 [triglycidyl ether of tris(hydroxyphenyl)-methane, Dow Chemical Co.], cured with a tetrafunctional aromatic amine, DDS (diamin iphenyl sulfone, Aldrich Chemical Co.)... 

Thin-films of Resin 5208 with specific isothermal cure histories exhibit three dispersion regions upon heating. The first is attributed to softening, followed by further chemical reaction and finally a peak due to the glass transition of the fully cured resin. Dynamic mechanical testing on thin-films shows that significant reaction takes place between the two DSA loss tangent peaks and that the second DSA peak is associated with vitrification. 

Gel Tg. the temperature at which gelation and vitrification occm simultaneously Tgo. the Tg of the mixed reactants, corresponding to a minimiun cure In isothermal cures, at temperatures between the gel Tg and Tgo, the resin undergoes gelation followed by vitrification. Since Tg is a function of both the degree of cure and the cross-link density, it increases to a point at which aU polymer reaction sites have been consumed. At that point, the Tg reaches a plateau and does not increase further with temperature until the decomposition temperature where a char region occurs (Fig. 2.17). An example of the effect of cure conditions on Tg and CTE values was shown by Konarski " who varied the cure cycles of an anhydride-cured epoxy from 130 to 175 °C as shown in Table 2.10. The value of Tg continued to... 

The potential of MTDSC for the real-time monitoring of reaction-induced phase separation is demonstrated with the cure of an epoxy— aniline-polyethersulphone (PES) mixture [80,104], The epoxy-aniline system allows following the isothermal cure accurately above and below goo (94°C), Choosing an isothermal cure temperature below will provoke a combination of phase separation of a PES-rich phase and vitrification of the epoxy-aniline matrix. Figure 2,17 shows the quasi-isothermal cure at 80°C for both modified and unmodified epoxy-aniline systems. The effect of primary and secondary amine reactions is seen as a positive A Cp,react- In the unmodified system, vitrification is seen after 91% conversion as a stepwise... 

The CHT diagram for the epoxy-amine system is given in Figure 2.32. The experimental points (symbols) in Figures 2.30-2.32 are data obtained with MTDSC and djmamic rheometry. The thick lines are the gelation lines, the vitrification contour (similar to the line of DF% f) and the isodiflfusion contours DF% g and DF. The thin lines display the Tg evolution as a function of time for selected isothermal (TTT) or non-isothermal cure paths (CHT). 

Vitrification during isothermal cure, associated with Z)F o.5, is attained at longer reaction times as the isothermal cure temperature is lowered. 

For the amine system under non-isothermal cure at 0.2°C min goo of 245 °C causes devitrification to occur at a temperature more than 150°C above vitrification. The importance of this extended mobility-restricted cure on the final material s properties should be emphasised. For this tetrafunctional epoxy-diamine system, an increase in Tg of ca. 170°C, corresponding with a residual cure of ca. 44% and a reaction enthalpy of more than 230 J g is caused by diffusion-controlled reactions and drastically influences the flnal network structure (crosslink density). 

To evaluate, in more detail, the effect of the chemical structure of the reactants upon isothermal curing, the rate of conversion at vitrification (dx/dt)DF o.5 can be compared to the average rate before vitrification, (dx/dt), which equals XDF o.5ltDF o.5- It is necessary to work with ratios or relative rates r (Table 2.1) because the amine-epoxy system is much more reactive than the anhydride-epoxy system. For the latter system, the ratio r of (dx/df)Dir o.5 to (dx/df) is lower than 1 5 over the temperature range considered, which is much smaller than the lowest ratio of 1 2.4 for the epoxy-amine system. The ratio r also decreases with increasing cure temperature. 

The variations of this ratio correlate to the differences in final isothermal cure state. Since the rate of conversion at vitrification is non-zero, conversion and Eg further increase in the (partially) glassy state with a rate dependent on the relative rate at vitrification. A relatively lower (dx/d/) )f .o.5 or ratio r results in a smaller increase in conversion and Eg after vitrification. For example, Eg at the end of the isothermal cure at 70°C for the epoxy-anhydride system amoxmts to 85°C, whereas a value of 103°C is determined for the amine system under similar isothermal cure conditions. 

Figure 2.112. Nonreversing (NR) heat flow (a), heat capacity change (ACp) (b), and heat flow phase (( )) (c) for the isothermal cure of stoichiometric diglycidyl ether of bisphenol A (DGEBA) and methylenedianilme (MDA) mixture at 70/80/100 °C the increase in ACp due to reaction and the stepwise decrease due to vitrification are indicated on the graph (1 °C/60s) [data reproduced fromSwier et al. (2004) with permission of John Wiley Sons, Inc.].

---