Theory of Dielectric Analysis and Cure Monitoring

Terms and Definitions: Here are some common terms and definitions used in the study of dielectric analysis.

Measuring Degree of Cure with DEA: Data from dielectric cure monitoring (DEA) correlate with glass transition temperatures (Tg) obtained from differential scanning calorimetry (DSC). In many cases a linear relationship exists between log (ion viscosity) and Tg. Dielectric measurements can be converted to Cure Index, which is a reproducible indicator of the state of cure. For some materials, Cure Index closely follows the degree of cure calculated by the DiBenedetto equation, which uses glass transition temperature information.

Dielectric Measurement Techniques: Dielectric instrumentation measures the conductance G (or resistance R) and capacitance C between a pair of electrodes at a given frequency. The Material Under Test (MUT) between a pair of electrodes can be modeled as a conductance in parallel with a capacitance.

Parallel Plate Measurements: Dielectric instrumentation measures the electrical properties of the Material Under Test (MUT) between a pair of electrodes, which can be modeled as a conductance in parallel with a capacitance.

Electrode Polarization and Boundary Layer Effects: When the incorrect model is used to determine dielectric properties, low frequency measurements of highly conductive materials may appear to have unusually low conductivity. This phenomenon is caused by electrode polarization, the accumulation of charge against the electrodes, which occurs when the material under test .... 

Electrochemical Modeling of AC and DC Cure Monitoring: Both AC and DC measurements can probe the cure state of thermosets and composites, so it is important to understand their characteristics when deciding which to use. During early through mid-cure, electrochemical reactions can affect DC results, while these effects are not evident in AC measurements. Consequently, DC measurements have limitations in accuracy for some portion of the cure, but at properly chosen frequencies AC results are accurate through the entire cure. Although the nature of the resin electrochemistry is unknown, a DC electrochemical resistance added to the model of a curing thermoset can reproduce the behavior of DC resistance measurements.

Electrical Modeling of Polymers: The dielectric—literally “two-electric”—properties of conductivity s, and permittivity e, arise from ionic current and dipole rotation in bulk material. For polymers, mobile ions are often due to impurities and additives, while dipoles result from the separation of charge on nonpolar bonds or across a molecule. When analyzing dielectric properties, it is possible and convenient to separate the influence of ions from dipoles to consider their individual effects.