Epoxy-amine systems isothermal cure

Calorimetric studies have depicted the impact of nanoparticles on isothermal curing of epoxy-amine system. Isothermal measurements done at 298 K using temperature-modulated differential scanning calorimetry are shown in Figure 9.15. The heat flow signal recorded during this measurement is directly proportional to the reaction rate of the curing process. It was foxmd that,... 

An analogous approach has been applied to the epoxy-amine system. The three sets of parameters were derived one set for the chemical rate equation [Eq. (14)], one set for the diffusion rate constant according to Eq. (26), and one set for the T — x relation [Eq. (27)]. As seen in Figure 2.22, the experimental and the calculated DF profiles agree very well for the quasi-isothermal cure at reaction temperatures ranging from 25 to 100°C. 

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...

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). 

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. 

Pig. 51. Permittivity change measured over a range of frequencies during the isothermal cure at 137°C of an epoxy—amine system (DGEBA/DDS) (120). 

Fig. 7. Tg versus In(time) for data at different cure temperatures A 100°C, 120°C, X 140°C, o 150°C,B 160°C, 180°C. Isothermal vitrification (Tg = T are) at each cure temperature is designated by an arrow. Same epoxy-amine system as in Figures 4-6. From Ref 40.

Wisanrakkit and Gillham (1991) present an excellent example of DSC isothermal method 2 for an epoxy-amine system (see Fig. 2.70). Figure 2.69 shows DSC scans for the uncured and partially cured thermoset, illustrating the measurement of both AH , and AH,es. Figure 2.71 shows conversion-ln(time) curves for the same epoxy for cure temperatures of 100-180 °C. Conversion was calculated from Eq. (2.80). Note that the curves are parallel during the first part of cure, and at the lower cure temperatures, less than 100% conversion is achieved. 

An investigation using differential scanning calorimetry was carried out under both isothermal and dynamic curing conditions to determine the cure kinetics of four epoxy/ amine resins. Various cure kinetic models were used to compare them with the results of the DSC results. Good fits were found, in good agreement with the experimental results for the resin systems. 22 refs. 

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.)... 

S L Maddox and J K Gillham, Isothermal aging of a fully cured epoxy-amine thermosetting system , J Appl Polym Sci 1997 64(1) 55-67. 

Figure 2.4. Comparison of the evolution of complex viscosity with conversion for the isothermal cure of an epoxy(/ = 2)-amine(/ = 4) system at 60° C and of an unsaturated polyester at 40°C (logarithmic scale).

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 ejq)erimental MTDSC observations on anhydride-cured and amine-cured epoxies, described in the previous section, will now be modelled to illustrate the benefits of the technique to obtain a quantitative law of cure kinetics for such thermosetting systems. Because cure kinetics are often complicated by diffusion limitations and/or mobility restrictions, the effect of diffusion has to be incorporated into the overall reaction rate law. For this purpose, both heat capacity and non-reversing heat flow signals for quasi-isothermal and non-isothermal cure experiments are used. 

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 and to compare the extension of the region of vitrification or restricted mobility, the mobility factor is plotted as a function of the reduced time, equal to the ratio tltDF o.s, for the quasi-isothermal cure at several temperatures for the anhydride and amine-epoxy system (Figures 2.33a and b). As seen from Figure 2.33, all the experimental profiles... 

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. 

Isothermal Method 1. This method capitalizes on the ability of DSC to simultaneously monitor both the conversion and the rate of conversion over the entire course of the cure reaction. This allows direct use of derivative forms of the rate equation, such as Eq. (2.86), which are necessary for kinetic analysis of autocatalytic reactions such as epoxy-amine. Experimentally this method is well suited to autocatalytic reactions that do not reach maximum rate until later in the reaction after the instrument has achieved thermal equilibrium. Even so, at high temperatures a significant portion of the reaction can take place before the calorimeter equilibrates and go unrecorded. Widmann (1975) and Barton (1983) have proposed a means to correct for such unrecorded heat by rerunning the experiment on the reacted sample, under the same conditions, to obtain an estimate of the true baseline and the unrecorded heat that should be added to the measured heat, as illustrated in Fig. 2.68. Note that this system appears to follow nth-order kinetics where the maximum reaction rate occurs at f = 0. For the sample shown, Widmann reports that 5% of goes... 

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