Dielectric Analysis (DEA)

Dielectric Analysis (DEA, also known as Dielectric Thermal Analysis (DETA), is a technique for monitoring changes in the viscosity and cure state of thermo-setting resins, adhesives, paints, composites and other kinds of polymers or organic substances by measuring variations in their dielectric properties.

Fundamental Principle

Fundamental Principle

The Ideal Tool for Understanding the Curing Process

The functional principle is consistent with that of an impedance measurement.

In a typical test, the sample is placed in contact with two electrodes (the dielectric sensor). When a sinusoidal voltage is applied, the charge carriers inside the sample are forced to move: positively charged particles migrate to the negative pole and vice versa. This movement results in a sinusoidal current with a phase shift.

A low voltage AC signal (input) is applied at one electrode. The response signal detected at the other electrode (output) is attenuated and phase shifted.

Dielectric sensor:

  • Alignment of dipoles
  • Mobility of ions

 

C  =  εr  C0
εr  =   εr’ -  i  εr’’

 

Dielectric Mechanism

Dielectric Mechanism

  • Electronic polarization  
    This happens to atoms if the applied electrical field has influence on the electron density around the nucleus

  • Atomic polarization  
    Deformation of electron clouds, formation of positive and   negative charged areas

  • Orientation polarization  
    Alignment of permanent and induced dipoles

  • Ionic dislocation polarization  
    Includes ion conductivity and conductivity via space charge and boundary layers
    → ion conductivity dominates at low frequencies 

Frequency:

Orientation polarization

Ionic dislocation polarization

  • At low frequencies, the overall conductivity can be made up of many different conduction mechanisms, but ionic conductivity is the most prevalent in moist materials.
  • εr'' is dominated by the influence of electrolytic conduction caused by free ions which exist in the presence of a solvent (usually water).
  • Ionic conductivity only introduces losses into a material. At low frequencies the effect of ionic conductivity is inversely proportional to frequency and appears as a 1/f slope of the εr'' curve.

 

Water molecules, for example, are permanent dipoles, which rotate to follow an alternating electric field.
These mechanisms are quite lossy – which explains why food heats in a microwave oven.       

Atomic and electronic mechanisms are relatively weak, and usually constant over the microwave region. Each dielectric mechanism has a characteristic “cutoff frequency.” As frequency increases, the slow mechanisms drop out in turn, leaving the faster ones to contribute to ε'. The loss factor (εr'') will correspondingly peak at each critical frequency. The magnitude and “cutoff frequency” of each mechanism is unique for different materials.

Ion Conductivity

Ion Viscosity at Curing Temperature

DEA Measurement Information

DEA Measurement Information

  • Ion viscosity
  • Flow behavior
  • Reactivity
  • Cure monitoring
  • Degree of cure
  • Glass transition temperature
  • Process monitoring and optimization
  • Diffusion properties
  • Aging
  • Decomposition effect

Recommended Literature

White Paper

A variety of questions may arise during the curing process for thermosetting resins. At which temperature, or after how much time, does the resin begin curing? How high is the reactivity? When is curing complete? The White Paper focuses on cure monitoring by means of Dielectric Analysis (DEA).