The Advantages and Disadvantages of AC and DC Cure Monitoring

Dielectric cure monitors use AC signals over a wide range of frequencies to quantify the electrical properties of a thermoset. The use of DC methods to measure the resistance can also offer information related to a material. However, this poses challenges and drawbacks that must be considered by the user.

Specifically, the electrode polarization (EP) phenomenon can cause distortion of DC resistance data and lead to misinterpretation of the state of cure.

Dielectric Cure Monitoring

Also called Dielectric Analysis (DEA), dielectric cure monitoring involves measuring the resistivity (ρ) and permittivity (ε′) of a polymer, which are the dielectric properties of a material. Resistivity itself includes a frequency-dependent (ρAC) component induced by the rotation of stationary dipoles and a frequency-independent (ρDC) component that occurs when mobile ions flow.

Frequency-independent resistivity is often referred to as DC resistivity, but it actually extends over a wider frequency range that includes DC (0 Hz). Since frequency-independent resistivity corresponds to the state of cure, it is a helpful material probe for polyurethanes, epoxies, sheet molding compounds (SMC), polystyrenes, bulk molding compounds (BMC), and other thermosets.

To highlight the relationship with mechanical viscosity, the term ion viscosity (IV), which is dependent on the mobility of ions, was created in the early 1980s as a synonym for frequency-independent resistivity. Ion viscosity can be defined as:

IV = ρDC (Eq. 1)

 

DC Resistance Measurements

AC measurements performed on thermosets can offer the complete range of information related to the cure state. Data offered by simpler DC methods are limited yet helpful. In general, resistance monitors are highly sensitive ohmmeters that make use of a DC voltage source to pass current through the material between a pair of electrodes.

Frequency-independent resistivity, which is also called ion viscosity, is different from resistance only by a scaling factor that relies on sensor geometry. Therefore, problems related to ion viscosity are equally applicable to resistance.

Although instruments that use DC measurements realize simplicity, they sacrifice flexibility, and perhaps accuracy, due to the drawbacks mentioned below:

  • DC measurements can only fetch DC resistance values
    • AC methods can measure capacitance, frequency-dependent resistance, and frequency-independent resistance
    • Frequency-independent resistance, which is also called ion viscosity, is the most suitable term for describing DC resistance
    • It is possible to measure frequency-independent resistance over a wider frequency range, including DC (0 Hz)
  • DC measurements may generate distorted data due to EP
  • DC measurements cannot be performed with release layers
    • Release layers are very thin insulating sheets that are used to inhibit the adhesion of the material with a platen or mold
    • Release layers block DC current and hinder DC resistance measurement
  • Systematic errors may occur during DC measurements
    • Thermal drifts, offset voltage drifts, and leakage currents in circuits cannot be differentiated from the actual DC signal

Electrode Polarization in AC and DC Measurements

Measuring frequency-independent resistance (ρDC) with DC signals is often not helpful. The EP phenomenon can lead to the formation of a blocking layer across sensor electrodes at the time of early cure, when the material is most conductive. This can result in unusually high apparent ion viscosities.1

Resistivity from AC measurements of 5-minute epoxy has been plotted in Figure 1. All data have been plotted against an axis marked ion viscosity and can be collectively termed ion viscosity.

Ion viscosity/resistivity during cure of 5-minute epoxy.

Figure 1. Ion viscosity/resistivity during cure of 5-minute epoxy. Image Credit: Lambient Technologies.

From Figure 1, three features are evident:

  • Curves that overlap or almost overlap, pointing toward the dominance of frequency-independent resistivity, or true ion viscosity
    • Due to the movement of mobile ions
    • Corresponds well with the state of cure
  • Curves that deviate, specifying the dominance of frequency-dependent resistivity
    • Due to the rotation of dipoles
    • Does not correspond well with the state of cure
  • Distortion of the 1-Hz and 10-Hz curves around 2 minutes caused by electrode polarization

At the start of cure, electrode polarization leads to substantial distortion in the 1-Hz data, which is illustrated in the expanded plot of Figure 2. This distortion leads to variations in the predicted single minimum in resistivity/ion viscosity to a peak with two local minima.

Data from 10-Hz measurements exhibit distortion to a considerably lower degree since there is a decrease in the boundary layer effect when the frequency increases. Moreover, measurements at even greater excitation frequencies, of 1 to 10 kHz, do not exhibit any distortion, and precisely determine the minimum ion viscosity.

Expanded ion viscosity/resistivity around the time of minimum viscosity.

Figure 2. Expanded ion viscosity/resistivity around the time of minimum viscosity. Image Credit: Lambient Technologies.

In several cases, information about the cure can be mathematically restored.1,2 Figure 3 illustrates how boundary layer correction—also known as electrode polarization (EP) correction—recovers the impacted data. Following EP correction, 1- and 10-Hz ion viscosities exhibit a proper minimum and correlate well with the higher frequency data.

Resistivity/ion viscosity with boundary layer (EP) correction.

Figure 3. Resistivity/ion viscosity with boundary layer (EP) correction. Image Credit: Lambient Technologies.

Once boundary layer correction is applied, only the 1-Hz ion viscosity can be used to observe the entire 5-minute epoxy cure, as illustrated in Figure 4.

1 Hz ion viscosity with boundary layer (EP) correction.

Figure 4. 1 Hz ion viscosity with boundary layer (EP) correction. Image Credit: Lambient Technologies.

Release Films with AC and DC Measurements

A sensor in a mold with release film is shown in Figure 5. Since release films are often produced from PTFE or other electrically insulating material, they have the ability to pass only AC signals, rendering it impossible to perform cure monitoring with DC methods. However, an appropriately engineered dielectric sensor enables AC cure monitoring through a release film.

Dielectric sensor with release film.

Figure 5. Dielectric sensor with release film. Image Credit: Lambient Technologies.

Figure 6 illustrates a 1″ Single Electrode dielectric sensor from Lambient Technologies mounted in a press platen and covered with Northern Composites HTF-621, a PTFE-based release film with a thickness of only 0.001″.

1? Single Electrode Sensor in press platen with HTF-621 release film.

Figure 6. 1″ Single Electrode Sensor in press platen with HTF-621 release film. Image Credit: Lambient Technologies.

The 100-Hz ion viscosity measured in the presence and absence of the HTF-621 release film at the time of bulk molding compound (BMC) curing has been compared in Figure 7. The curves are almost similar except during the minimum ion viscosity condition, when the measurements through the film are distorted by the boundary layer effect.

Comparison of BMC cure with and without release film, 100 Hz AC measurement.

Figure 7. Comparison of BMC cure with and without release film, 100 Hz AC measurement. Image Credit: Lambient Technologies.

In several cases, this distortion can be mathematically corrected and information related to the cure can be restored.1,2,3 Figure 8 illustrates how boundary layer correction recovers the impacted data. Following correction, ion viscosity measured in the presence of the release film accurately correlates with ion viscosity measured in the absence of the release film. It must be noted that for the two tests, the slight differences between the curves are mainly caused by differences in process temperatures.

Comparison of raw ion viscosity and ion viscosity with EP (boundary layer) correction.

Figure 8. Comparison of raw ion viscosity and ion viscosity with EP (boundary layer) correction. Image Credit: Lambient Technologies.

DC measurement of resistance is a simple technique to probe the state of cure, but it has the following drawbacks:

  • Cure state cannot be measured through release films or vacuum bags
  • Probable distortion of data caused by electrode polarization

DC measurements of BMC using the 1″ Single Electrode Sensor in the presence and absence of the HTF-621 release film have been compared in Figure 9. As a reference point, data obtained from the 100-Hz AC measurements have also been plotted.

Comparison of AC and DC measurements of ion viscosity.

Figure 9. Comparison of AC and DC measurements of ion viscosity. Image Credit: Lambient Technologies.

DC measurements, even in the absence of release film, exhibit considerable distortion during the minimum ion viscosity condition. The distortion of AC data can be corrected, but DC data can never be corrected. Moreover, it is not at all possible to perform DC measurements through the release film, which is signified by the high ion viscosity at the instrument’s measurement limit.

Conclusion

Although it is easy to perform DC measurements of resistance, they have drawbacks when compared to AC measurements of dielectric properties. While performing thermoset cure monitoring, electrode polarization might cause distortion of DC data and misinterpretation of the state of cure. Electrode polarization can also have an impact on low-frequency AC measurements, but added information obtained from dielectric properties enables the distorted data to be corrected.

DC methods necessitate direct contact with the composite or thermoset, and cannot perform measurement through release films and vacuum bags. Hence, AC measurements become specifically useful in the field of manufacturing. AC methods exhibit the potential to monitor cure through insulators, thereby enabling sensors beneath a release film to be conveniently used, which eliminates problems related to adhesion of material to the sensor.

In vacuum-assisted resin transfer molding (VARTM) and other similar processes, AC measurements through the vacuum bag prevent the introduction of breaks in the bag that could become a leakage source.

References

  1. Day, D.R.; Lewis, J.; Lee, H.L. and Senturia, S.D., Journal of Adhesion, V18, p.73 (1985)
  2. Lambient Technologies application note AN3.29, “Electrode Polarization and Boundary Layer Effects”
  3. Lambient Technologies application note AN3.19, “Electrode Polarization with AC and DC Cure Monitoring”

This information has been sourced, reviewed and adapted from materials provided by Lambient Technologies.

For more information on this source, please visit Lambient Technologies.

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