Electrical Methods of Characterising Cure Processes in Resins

Summary

The major objective of research on electrical methods of characterising cure processes in resins is to develop on-line electrical instrumentation for non-destructive material evaluation, for determination of fabrication cycle window boundaries of thermosets and thermoplastics, and for on-line closed loop cure cycle control. The key to achieving this goal is to relate the chemistry of the cure cycle process to the dielectric properties of the polymer system by correlating the time, temperature and frequency dependence of electrical measurements with chemical characterisation measurements, i.e. viscosity, ultrasonics and differential scanning calorimetry. Measurements of geometry independent variables such as the complex permittivity (ɛ*) need to be made rapidly over a wide frequency range. In turn, the time-temperature and frequency dependence of the electrical measurements provide the basis for interpreting and understanding the signal in terms of the chemistry and physics of the resin system. Dynamic electrical measurements, together with chemical characterisation work can then be used to determine: resin quality, composition and age; cure cycle window boundaries; onset of flow and point of maximum flow; extent of reaction and completion of reaction; evolution of volatiles; T_g; crosslinking and molecular weight buildup.

In this chapter, as examples of the use of dynamic electrical measurements, the cure processes of the polyimides, LARC-160 and PMR-15, and the DGEBA type epoxy Epon 828 cured with agent U have been reviewed. The thermoplastic resins UDEL-P1700 (a polysulphone) and PPQ (a polyphenylquinoxaline) have also been examined over a broad temperature range. The use of the magnitude of the complex permittivity (ɛ*), the specific conductivity σ(ohm⁻¹ cm⁻¹) and a characteristic relaxation time τ to monitor the state of the material is discussed.