Measuring the Curing Behaviour of Sheet Molding Compound Using a Dielectric Cure Monitor

By Staff Writers

Topics Covered

LT 451 Dielectric Cure Monitor
Results and Discussion
Dielectric Cure Curve
About Lambient Technologies


Lambient Technologies’ LT 451 Dielectric Cure Monitor was used to observe the curing behavior of a Sheet Molding Compound (SMC). Bulk Molding Compound (BMC) is the same as SMC but is available in bulk form; hence the results of this study are also valid to BMC.

The information obtained from dielectric cure monitoring clearly demonstrates that Critical Points identify characteristic aspects of the cure such as maximum slope of Log (ion viscosity), minimum ion viscosity, and the time to a chosen end of cure. It also shows that as cure temperature increases, cure time decreases which is anticipated for a reaction driven by heat.

LT 451 Dielectric Cure Monitor

A compact, versatile instrument, the LT-451 Dielectric Cure Monitor is specifically designed for quality assurance/quality control, research and development, and also for production applications involving coatings, paints, composites, resins and other thermoset materials. The instrument is capable of making measurements at various frequencies, and offers excellent flexibility because of its frequency range of 0.001 Hz to 100 KHz. It can be used with a wide range of materials and acts as the reference standard for dielectric cure monitoring.

The LT-451 employs digital signal processing for superior rejection of noise. The highest data acquisition speed of the instrument is about 3 seconds per data point, thus making it ideal for use with rapid SMC cures.

In research and development applications, dielectric cure monitoring allows for control of the development process by measuring and recording cure and viscosity data; observation of changes in cure time and reaction rate with novel formulations; and observation of changes in cure time and reaction rate with different ramp and hold profiles or cure temperatures.

For quality assurance/quality control applications, dielectric cure monitoring allows for testing of outgoing product control to validate cure time and to observe batch-to-batch consistency; testing of incoming materials to validate freshness and cure time; and documentation of outgoing and incoming material quality for statistical process control (SPC).


First, samples of SMC were applied to Mini-Varicon sensors and then cured in a lab oven at about 135°C, 145°C and 155°C. The dielectric properties of the samples were measured using the LT-451 Dielectric Cure Monitor at an excitation frequency of 10Hz. CureView Data Acquisition software from Lambient Technology obtained and stored the information, and carried out presentation and post-analysis of the results. The CureView software extracts Critical Points only for data acquired at a single frequency; as a result the LT- 451 monitored the SMC samples at 10Hz.

Results and Discussion

Figures 1, 2 and 3 demonstrate data from the cures of SMC at 135°C, 145°C and 155°C, respectively, as determined by the LT-451 Dielectric Cure Monitor. This data was averaged and filtered to lessen noise. Figures 4 and 5 reveal the behaviors for a standard thermoset cure with one hold step and one ramp step. Initially, when there is increase in temperature, the ion viscosity tends to reduce as the thermoset continues to melt and becomes more fluid and less resistive.

Figure 1. 135°C SMC cure data with the LT-451.

Figure 2. 145°C SMC cure data with the LT-451.

Figure 3. 155°C SMC cure data with the LT-451.

Figure 4. Typical ion viscosity behavior of a curing thermoset.

Figure 5. Ion viscosity curve and slope of ion viscosity in a curing material.

As the material becomes hotter, the rate of reaction increases and at some point of time the decrease in ion viscosity due to melting is overcome by the increase in ion viscosity due to cross-linking. This point is referred as the ion viscosity minimum, which occurs during minimum mechanical viscosity. Following the minimum point, ion viscosity continuous to increase until the concentration of unreacted monomers reduces; as a result, the slope of ion viscosity also reduces and finally ion viscosity will have zero slope once the cure stops completely.

Dielectric Cure Curve

The dielectric cure curve is distinguished by four Critical Points. The CureView software automatically calculates the Critical Points that characterize the cures are illustrated in the table below. The user must verify the proper slope in order to specify end of cure for the application.

Table 1. Critical Points from SMC cure monitoring.

Critical Point LT-451 135°C cure LT-451 145°C cure LT-451 155°C cure
  Value Time Value Time Value Time
CP(1) Crit. Visc. 8.0 0.65 min (39 sec) 8.0 0.60 min (36 sec) 8.0 0.65 min (39 sec)
CP(2) Min. Visc. 7.38 4.17 min (250 sec) 7.39 3.42 min (205 sec) 7.60 2.48 min (149 sec)
CP(3) Max Slope Ion Visc. 1.86 6.23 min (374 sec) 3.65 5.01 min (301 sec) 3.67 4.03 min (242 sec)
CP(4) Critical Slope Ion Visc. 0.25 7.21 min (433 sec) 0.25 6.13 min (368 sec) 0.25 5.14 min (308 sec)

In figure 6, the trends as plotted are clear. As anticipated for reactions that are thermally driven, the time taken to reach each Critical Point is shorter for cures at increased temperatures. In addition, the relationship between the temperature of cure and the time to a Critical Point follow a distinct line.

Figure 6. Time to Critical Points vs. cure temperature for SMC.


The above procedure demonstrates the ability of the Lambient LT-451 Dielectric Cure Monitor to follow the cure and acquire Critical Points for quality assurance/quality control and research and development purposes. As anticipated, at higher temperature cures have faster reactions and reduced time to end of cure. At 145°C, the cure time reduces by about 60 seconds for a 10°C increase in cure temperature, a major change that denotes the value of dielectric cure monitoring for quality control or process monitoring.

About Lambient Technologies

Lambient Technologies LLC, based in Boston, Massachusetts USA, develops instruments, sensors and software for monitoring the dielectric properties of curing polymers. These properties provide insight into the chemistry, formulation, reaction rate, viscosity and cure state of epoxies, polystyrenes, polyurethanes, silicones, SMC, BMC and other thermoset materials.

Dielectric cure monitoring has wide application in Research and Development, Quality Assurance/Quality Control and manufacturing. Products from Lambient Technologies are designed for flexibility and ease of use, together they form an integrated system for studying polymers and optimizing manufacturing processes.

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

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

Date Added: Oct 3, 2013 | Updated: Oct 4, 2013
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