How does resistivity change during curing?

Figure 1 depicts the change in resistivity as a function of glass transition temperature during isothermal curing. These data demonstrate the use of micromedia sensors for coating monitoring and process control.

Using the techniques described earlier, ionic conductivity (or its inverse, resistivity) can be obtained in real time by continuously monitoring a series of frequencies. It can then control the reaction by changing the temperature or pressure. There are several ways to use dielectric feedback. Some of these examples are as follows:

•The temperature can be kept constant or controlled until the desired viscosity is reached (medium measurement).

• Viscosity can be kept constant or changed at will by controlling temperature changes.

Fig.1 Ion resistivity data and Tg during isothermal epoxy-amine curing

1. Process control via dielectric feedback

Figure 2 shows an example of process-controlled curing of graphite epoxy resin using micromedia feedback. Using improved Micromet Instruments dielectric software and hardware, the control is realized on IBM PC. Curing is carried out on a heat press with computer controlled temperature. The sequence of process control software is as follows:

1. Heat and hold at 250° until log resistivity reaches 7.0 (allows degassing while preventing premature curing).

2. Maintain resistivity (viscosity) at 7.0 until 350°F (allows controlled cure, prevents second viscosity minimum).

3. Hold at 350°F until the dielectric reaction rate approaches zero (allow the reaction to complete).

4. Cool down and notify the operator that the cycle is complete.

Fig. 2 The process of controlling the curing of epoxy graphite by microdielectric feedback

2. Medium-thermal feedback process control

Curing in Figure 2 is fully controlled by media and thermal feedback. The micro-media sensor is in contact with a graphite epoxy material. Note that the secondary viscosity (resistivity) minima for these materials are completely eliminated by using viscosity control. This technique is useful for limiting excessive emissions from composite materials prone to this type of problem. Finally, detect the end point, stop the reaction, and save unnecessary over-curing time.