Vitrification time

Figure 5.7 shows the superposition of Tg vs lnt data for the diepoxide (DGEBA)-aromatic diamine (TMAB) system, to form a master curve at 140°C (Wisanrakkit and Gillham, 1990). Vitrification times, defined as the time at which Tg equals the cure temperature, are marked by arrows (Tg was defined as the midpoint of the baseline change during a DSC scan). 

The polymerization is kinetically controlled up to the vitrification time, for every cure temperature. Moreover, as in this range of temperatures, gelation arrives before vitrification, the passage through the gel point does not have any influence on the reaction rate this is a general experimental observation for stepwise polymerizations. After vitrification, a significant decrease in the reaction rate occurs, leading to the observed departure of experimental curves from the master curve. 

Comparison of these vitrification times will show the differences in curing kinetics. 

Fig. 8 The DMA cure profile of a two-part epoxy showing the tyrpical analysis for minimum viscosity, gel time, vitrification time, and estimation of the action energy. (From Ref l) (View this art in color at www.dekker.com.)...

By using the point at which the first derivation of raw conductivity data became zero at the end of the reaction, vitrification time of radiation crosslinked UP resins was determined. Vitrification times in the range 293-345 K were approximately constant below upper liquid-liquid transition temperature but exhibited significant temperature dependence above the transition, which can be seen in the Arrhenius plot in Figure 13.3b. In the same manner, electrical field dependence of vitrification was also observed—the stronger the field, the earlier the system reached vitrification. 

Hydroxy terminated polysulfones have been found to increase epoxy fracture toughness (46). Amine terminated polysulfones have been found preferable to hydroxy terminated polysulfones as modifiers for DGEBA because they react directly with the epoxy and do not require a catalyst (32). Fracture toughness of TGAP has been tripled by addition of polysulfones and the system has been studied with frequency dependent electromagnetic sensing technique which can measure vitrification time and provide information about size and shape of occluded particles in the cured resin. 

In an experimental exploration [120], all approaches show the expected decrease in vitrification time with increasing frequency (LMDSC results shown in Figure 2.25). However, even with the extended frequency range (ca. 2 decades for LMDSC) the simultaneously measured heat flow is not accurate enough to correlate a specific frequency with the reaction kinetics of the different epoxy thermosetting systems. Indeed, there is a considerable experimental error (see scatter in Figure 2.25) and the variation of the vitrification time and the conversion at vitrification associated with 2 decades in frequency is only about 15 min and 6%, respectively. The latter is below the accuracy of the (partial) reaction enthalpy determination (LMDSC). 

Figure 2.25. Vitrification times ACp as a function of the modulation frequency (from 0.01 to 1 Hz, logarithmic) for the quasi-isothermal cure of an epoxy-amine system at 80°C. Results...

Figure 3.7 Epoxy conversion versus reaction time at different cure temperatures fOj 160 °C (Oj 135 °C (+) 80 °C (A) 30 °C (—) diffusion model predictions. For (a) DGEBA-MCDEA (b) DGEBA-DDS. Arrows (4) indicate vitrification times. Reprinted with permission from E.G. Reydet, C.G. Riccardi, H. Sautereau and P.J. Pascault, Macromolecules, 1995, 28, 23, 7599 1995, American Chemical...

The data of Table 17 show the strong reaction enhancements of the specific catalysts impart under microwave heating to all of reactive systems examined. The gelation and vitrification times were lowered to one-eighth to one-tenth of those under hot-air heating with the same catalyst and its concentration. An ion-hopping conduction mechanism was recognized as the dominant source of the microwave absorption capacities of these catalysts [2]. 

Since the cure reaction is effectively quenched by vitrification, curing iso-thermally for times longer than the vitrification time is counterproductive. Further cure advancement is most effectively achieved by advancing the cure temperature, either by staging isothermaUy or through a postcure. 

Figure 11.9 LCPTTT diagram for DOMS/SAA stoichiometric mixtures. AH phase changes and physical changes are observed during isothermal cure. Light grey indicates the region where the material is isotropic, dark grey indicates smectic, and white indicates a biphasic structure. Filled circles are the gel times at each temperature and open circles are the vitrification times. Reproduced from reference 38.

Table 14.1 Summary of the calculated and experimentally determined vitrification times of an epoxy using Eq. (14.4)...

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