Isothermal Cure

Gel time values of the three systems measured as abrupt change in the slope of G (t) under isothermal curing conditions show that gelation occurs earlier in PWE system at all temperatures considered as shown in Table 11.27. ETPI behaves like a catalyst for the primary epoxy-amino reaction which dominates the cure until vitrihcation occurs. Dynamic mechanical analysis and dielectric spectroscopic analysis carried out by the authors also confirm the above conclusions. 

Modification of the FT-IR analysis techniques to analyze coatings under isothermal cure conditions provides the data needed to determine the rate constants for each reaction. An effective method to generate the rate constants from the experimental data has been found and will be described. 

As seen in Equation 4, the concentration of free isocyanate as a function of time is dependent on both rate constants. Measuring the concentration of the isocyanate as a function of time during isothermal cure provides a convenient method to determine both rate constants. 

FIGURE 4 EXPERIMENTAL DATA AND CALCULATED ISOTHERMAL CURE CURVES (A PREDICTEDD [BLOCKED ISOCYANATE], 0 EXPERIMENTAL [NCO], PREDICTED [NCO], + PREDICTED [CROSSLINKS])... 

In order to get experimental information about the concentration and temperature range where phase separation occurs, it is required to perform a series of isothermal curing experiments varying these parameters [49]. By interconnecting individual phase separation points, say 10% solvent at 40 °C is needed while 17% solvent has to be employed at 100 °C,a phase separation fine is constructed. In order to obtain the temperature and concentration ranges where phase separation occurs with a very few experiments, a gradient oven made out of aluminum, schematically shown in Fig. 8, was constructed. 

Fig. 26. Influence of isothermal curing temperature on the phase separation behavior oi macroporous epoxies prepared via CIPS with cyclohexane...

The isothermal cure kinetics of a series of TGMDA/DDS resin formulations were investigated over the temperature range 121°-187°C. Figure 4 illustrates data obtained for the resin TGMDA/DDS(25%) at 177°C. During the early stage of cure prior to the onset of... 

Figure 2. Log e" vs. time for the uncatalyzed epoxy during a 165C isothermal cure.

Figure 5. Log a (ionic diffusion) vs. a (degree of cure) for the uncatalyzed epoxy during isothermal cure.

Conversion of the mixed system to the B-stage cure condition to yield Isothermal cure kinetics. 

Isothermal cure of the B-stage neat resin to the C-stage cure. 

This equation is valid for temperatures between 55 and 105°C. For DSM resin, Kenny et al. [30] presents a linear dependence of a on the isothermal cure temperature lower than the glass transition temperature of the fully cured resin, given by Equation 2.33 ... 

Figure 3.19 Reaction paths undergoing phase separation during isothermal cure...

In situ frequency dependent electromagnetic-impedence measurements provide a sensitive, convenient, automated technique to monitor the changes in macroscopic cure processing properties and the advancement of the reaction in situ in the fabrication tool. This chapter discusses the instrumentation, theory, and several applications of the techniques, including isothermal cure, complex time—temperature cure, resin film infusion, thick laminates, and smart, automated control of the cure process. 

Figure 4.3 Log a versus degree of cure a during 12I°C isothermal cure...

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