Constant rate curing

The standard process cycle for polymer matrix composites is a two-step cure cycle, as seen in Figure 8.1. In such cycles the temperature of the material is increased from room temperature to the first dwell temperature and this temperature is held constant for the first dwell period ( 1 hour). Afterward, the temperature is increased again to the second dwell temperature and held constant for the second dwell period (2-8 hours). After the second dwell, the part is cooled down to room temperature at a constant rate. Because there are two dwell periods, this type of cure cycle is referred to as a two-step cure cycle. The purpose of the first dwell is to allow gases (e.g., entrapped air, water vapor, or volatiles) to escape and to allow the matrix to flow, which leads to compaction of the part. Thus, the viscosity of the matrix must be low during the first dwell. Typical viscosity versus temperature profiles of polymer matrices show that as the temperature is increased, the viscosity of the polymer decreases until a minimum viscosity is reached. As the temperature is increased further, the polymer begins to cure rapidly and the viscosity increases dramatically. The first dwell temperature must be chosen judiciously so that the viscosity of the resin is low while the cure is kept to a minimum. 

The occurrence of self-acceleration during curing of epoxy resins and epoxy-based compounds was proven by rheokinetic and calorimetric methods.53 This phenomenon can be treated formally in terms of an induction period (when the reaction is very slow in the initial stage of a process), followed by a constant rate. However, it seems preferable to use a single kinetic equation incorporating the self-acceleration effect to describe reaction as a whole. Such a kinetic equation contains only a limited number of constants (K and co in Eq. (2.33)) and allows easy and unambiguous interpretation of their dependencies on process factors. 

In order to obtain the degree of cure and rate of curing, we must first measure the reaction. This is typically done using a differential scanning calorimeter (DSC) as explained in Chapter 2. Typically, several dynamic tests are performed, where the temperature is increased at a constant rate and heat release rate (Q) is measured until the conversion is finished. To obtain Qt we must calculate the area under the curve Q versus t. Figure 7.17 shows four dynamic tests for a liquid silicone rubber at heating rates of 10, 5, 2.5 and 1 K/min. The trapezoidal rule was used to integrate the four curves. As expected, the total heat Qt is the same (more or less) for all four tests. This is to be expected, since each curve was represented with approximately 400 data points. 

Figure 5. Calculated and experimental rate constant for curing of DGEBA with t,3-diaminopropane as a function of conversion. Arrows indicate conversion at which Tg was reached Tcure- 50 C, 60 C, O 70 C (Ref. [18]).

Temperature dependence of rate constant characterizing curing processes of epoxy and its composites at 2.17 vol.% nanosilica content (reprinted from Wear, vol. 253, M. Q. Zhang eta .. Effect of particle surface treatment on the tribological performance of epoxy based nanocomposites, p. 1086,2002, with permission from Elsevier). 

Softening and cure is examined with the help of a torsional pendulum modified with a braid (65), which supports thermosets such as phenoHcs and epoxies that change from a Hquid to a soHd on curing. Another method uses vibrating arms coupled to a scrim-supported sample to measure storage and loss moduH as a function of time and temperature. An isothermal analytical method for phenoHc resins provides data regarding rate constants and activation energies and allows prediction of cure characteristics under conditions of commercial use (47). 

Table 6 Vulcanization Rate Constant (k) and Optimum Cure Time (T90)...

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