Vitrification and Devitrification During the Non-Isothermal Cure of a Thermoset

Curing of an epoxy resin causes the resin molecules to react and form a highly crosslinked network. In case the epoxy resin is heated at a considerably high rate momentary sample temperature will be higher than the system’s glass transition temperature.

After curing, the degree of cure, α, that was first 0 is now 1 and the glass transition temperature (T_g ) reaches its peak. If the system is heated at a low heating rate, the momentary glass temperature may equal the momentary sample temperature. When this happens the system changes to a glassy state or vitrifies due to the chemical reaction. On vitrification the molecules almost completely lose their mobility, curing abruptly slows down and actually stops. This process occurs in normal curing and also isothermal curing. In isothermal curing, curing is done at a temperature less than the glass peak transition temperature.

In non-isothermal curing, vitrification is followed by devitrification. Here the sample temperature is higher than the glass transition temperature. Here the molecules regain their mobility and the reaction gets completed.

The paper discusses a method to determine the frequency behavior of vitrification and devitrification in non-isothermal curing processes using TOPEM. By knowing the frequency behavior of vitrification and devitrification, the temperature dependence of the relaxation time is measured. This is of basic interest to understand the glass transition.

Experimental Details

Sample Preparation

The chemicals used to prepare the sample are listed below:

• Epoxy resin - diglycidylether of bisphenol-A (DGEBA) named Epon 828 manufactured by Shell Chemicals
• Hardener - polyoxypropylene diamine named Jeffamine D-230 manufactured by Hunstman

Stoichiometric mixtures of hardener and resin were prepared and samples of suitable mass mostly about 25 mg were weighed into 40-μL aluminum crucibles.

Methods

A Mettler Toledo DSC823e comprising an intracooler was used for the TOPEM experiments shown in this study. TOPEM is a temperature-modulated DSC method in which stochastic temperature pulses are superimposed on the underlying heating rate.

Conditions followed for implementing the method are listed below:

• The pulse amplitude is constant but the pulse length randomly varies within a specific range.
• The range and amplitude of the pulse can be set by the user.
• The temperature ranges for non-isothermal experiments were in the range 25-100°C or 25-125°C, based on the heating rate used for the measurement.
• The heating rates were 0.05, 0.032, 0.025 and 0.019 K/min.
• These heating rates are so low that it is easily possible to separate vitrification and devitrification.
• For comparison, an experiment was performed at 0.1 K/min. At this heating rate, vitrification does not take place. The amplitude of the temperature pulse was 0.1 K, and the pulse duration 15 to 30s.

Results

Figure 1. TOPEM® measurement curve (top), quasi-static and frequency-dependent heat capacity curves (middle) and the total heat flow curve (bottom).

The results obtained are listed below:

• Figure 1 shows a characteristic result of a TOPEM measurement performed at a heating rate of 0.019 K/min shown by the blue curve in the top diagram.
• The quasi-static heat capacity shown by the black curve in the middle diagram and the total, non-reversing and reversing heat flow curves are then calculated from this signal.
• The figure shows only the total heat flow shown by the green curve in the bottom diagram. The curve of the quasi-static heat capacity was utilized to determine the frequency-dependent heat capacity for the frequencies selected by the user (here curves for 16.7 and 66.7 mHz are shown).
• It is also important that the user defines tangents for assessment of the vitrification or devitrification process, which are shown by the black dotted lines in the middle diagram.
• Vitrification relates to the relatively abrupt decrease in the heat capacity after about 900 min and devitrification to the increase in the heat capacity from about 2200 min onward. It can be seen that vitrification takes place much more quickly than devitrification.
• The total heat flow curve shows the exothermic curing reaction that occurs during the first 1000 min (green curve, bottom).
• The experiment lasts about 70 hours at the heating rate chosen. The total measurement time for the TOPEM experiments done at the heating rates given above was about 240 hours.
• The total measurement time for experiments at five different frequencies and the same heating rates (including blank curves) is estimated to be about 2400 hours (i.e. more than 100 days).

Figure 2 displays the quasi-static heat capacity curves for the different heating rates as a function of temperature. The figure shows that the temperature at which vitrification takes place keeps decreasing as the heating rate is decreased. The curves were used to calculate the frequency-dependent heat capacities and to determine the corresponding vitrification and devitrification times and temperatures for various frequencies.

Figure 2. Quasi-static heat capacity as a function of temperature for different heating rates.

The vitrification and devitrification times can be converted to corresponding temperatures and the data obtained is usually summarized in a “Continuous-Heating-Transformation” (CHT) diagram. This diagram is analogous to the “Time-Temperature Transformation” (TTT) diagram used for isothermal reactions. The CHT diagram is shown in Figure 3.

Figure 3. CHT diagram for the DGEBA (resin) and polyoxypropylene diamine (hardener). 

The vitrification and devitrification temperatures are plotted against the corresponding vitrification and devitrification times for various frequencies. The continuous thin lines correspond to the temperature programs at various heating rates. The dotted curve is an approximate indication of the regions in which vitrification (lower branch) and devitrification (upper branch) occur. In contrast to the vitrification temperature, the devitrification temperature is least dependent on the heating rate. The diagram shows the effect of time, heating rate and frequency on vitrification and devitrification. This is further illustrated in Figures 4 and 5.

Figure 4. Enlarged section from the CHT diagram (see Figure 3) for vitrification.

Figure 5. Enlarged section from the CHT diagram (see Figure 3) for devitrification.

Conclusions

The vitrification and devitrification of systems curing under non-isothermal conditions can easily be studied using TOPEM. As compared to conventional temperature- modulated techniques (ADSC, IsoStep), the frequency dependence of the vitrification process can be investigated by TOPEM in a single measurement. The complete frequency range of the DSC is used. A “Continuous Heating Transformation” (CHT) diagram can be constructed with very few measurements at different heating rates. The diagram summarizes the dependence of vitrification and devitrification on heating rate, frequency and time.

This information has been sourced, reviewed and adapted from materials provided by Mettler Toledo - Thermal Analysis.

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