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

Table of Content

Introduction
Curing Behavior of a Thermoset
Experimental Procedure
Results
Conclusion
About Lambient Technologies

Introduction

This article discusses the curing behavior of Sheet Molding Compound (SMC), presenting data for log(ion viscosity) and slope of log(ion viscosity) that represent the state of cure. A LT-451 Dielectric Cure Monitor was used for this purpose.

The plots reveal characteristic features, including maximum slope of log (ion viscosity), minimum ion viscosity, and the time to chosen end of cure. The test results are applicable to Bulk Molding Compounds (BMCs) as they are the bulk form of the same material used in SMCs.

Curing Behavior of a Thermoset

Ion viscosity is described as the frequency independent resistivity (ρDC). In most of the cases, ion viscosity varies in proportion to mechanical viscosity in the initial stage of curing and represents cure state in the latter stage of curing.

Ion viscosity obtained from data at a single frequency yields a characterization curve of the state of cure. Figures 1 and 2 depict the curing behavior of a typical thermoset with one temperature ramp step and one temperature hold step.

Figure 1. Typical ion viscosity behavior of a curing thermoset

Figure 2. Ion viscosity curve and slope of ion viscosity of a curing thermoset

Ion viscosity decreases initially with an increase in temperature due to melting of the thermoset. The increasing material temperature accelerates the reaction rate. After some time, the onset of crosslinking increases the ion viscosity despite temperature increase. This point is called the ion viscosity minimum, which also takes place at the time of minimum mechanical viscosity.

Subsequent to the minimum point, there is a continuous increment in ion viscosity until the concentration of unreacted monomers decreases with decelerating reaction rate. As a result, there is a decrease in the slop of ion viscosity until the completion of the curing process, at which point slope of the ion viscosity will be zero.

The dielectric cure curve is characterized by four critical points:

  • CP(1) is a user-defined level of ion viscosity to determine the start of material flow at the onset of cure.
  • CP(2) is ion viscosity minimum indicating the onset of crosslinking and corresponding increase in viscosity.
  • CP(3) is inflection point at which the crosslinking reaction begins to decelerate, and is generally used as a signpost that can be related to gelation.
  • CP(4) is a user-defined slope capable of defining the end of cure. The slope decreases with decelerating reaction rate.

Modifications in the resulting material beyond that point where mechanical measurement of viscosity is impossible can be observed with dielectric cure curve.

Experimental Procedure

The experimental procedure involved the application of SMC samples to Mini-Varicon sensors and subsequent curing in a laboratory press at 135°C, 145°C, and 155°C. Optimum excitation frequency for cure monitoring as identified in earlier tests was 10Hz.

Each sample’s dielectric properties were measured at 10Hz using an LT-451 Dielectric Cure Monitor for the duration of each test. Data acquisition and storage as well as post-analysis and presentation of the results can be done with the CureView software from Lambient Technology.

Results

Figures 3, 4, and 5 present the cure data of SMC samples at 135°C, 145°C, and 155°C, respectively. After averaging the data, they are filtered for noise reduction. The critical points characterizing each cure are presented in Table 1.

The slope of 0.25 to define CP(4) was selected randomly. In actual situation, a user has to determine the correct slope to represent the end of cure for the application.

Figure 3. 135°C SMC cure data at 10Hz. CureView data processing parameters: Data Averaging = 1, Slope Span = 3, Data Filtering = 0, Slope Filtering = 1, Slope Filtering Start Time = 0 minutes.

Figure 4. 145°C SMC cure data at 10Hz.

Figure 5. 155°C SMC cure data at 10Hz.

Table 1. Critical Points from SMC cure monitoring

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

As shown in Figure 6, the time taken for reaching each Critical Point is shorter for cures at elevated temperatures as expected for reactions that are thermally driven. In addition, the correlation between the Critical Point time and the cure temperature follows a well-defined line.

Figure 6. Critical Point time vs. cure temperature for SMC

The plot shown in Figure 6 does not have CP(1), which reveals when the SMC’s ion viscosity has reduced the user-selected value of 8.0, which was selected to represent the start of material flow. Since the flow time is an indication of heating time and not of curing, CP(1) has not been plotted for clarity.

The time taken for reaching the ion viscosity minimum (CP(2)) shortens by roughly 50s for every 10°C increment in processing temperature. The time taken for reaching CP(3) and CP(4) differ by a similar amount with temperature. This correlation follows an Arrhenius function over a broader temperature range.

Conclusion

With dielectric measurements, the curing characteristics of SMCs and BMCs can be observed in real time. In addition, Critical Points can be extracted to quantify reaction characteristics. The dielectric data reveal the straightforward relationship between temperature and curing rate.

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 materials provided by Lambient Technologies.

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

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