Testing Repeatability and Temperature Effects on Cure Rates in Carbon Fiber Prepreg

Table of Contents

Introduction
Experimental Procedure
Reproducibility of Measurements
Process temperature Effect on Cure Rate
Curing Characteristics of a Thermoset
Conclusion
About Lambient Technologies

Introduction

This article discusses the testing of carbon fiber reinforced prepreg (CFRP) samples obtained from a single batch for reproducibility and the effect temperature has on the cure rate.

Experimental Procedure

The Ceramicomb-1” sensor embedded into a press’ lower heater platen (Figure 1) was used for testing the samples. A laboratory grade filter paper sheet was placed over the sensor to avoid short circuiting of electrodes by carbon fibers, while allowing the resin to flow.

This was followed by placing two 1” x 1” CFRP layers over the filter paper. Heat and pressure was applied to the lay-up by the press during each test.

Figure 1. Ceramicomb-1” reusable sensor embedded in press platen

The dielectric properties of the samples were measured by an LT-451 Dielectric Cure Monitor at 100Hz and 1.0KHz excitation frequencies. Lambient Technologies’ CureView software was used for data acquisition and storage as well as for post-analysis and presentation of the results.

Since extraction of Critical Points (CPs) by the CureView software can only for data acquired at a single frequency, an optimum single frequency needs to be determined by performing measurements initially at multiple frequencies. In the successive tests, CureView is able to automatically extract CPs and characterize the cure.

Reproducibility of Measurements

The results from one of five tests on fresh CFRP under same conditions are presented in Figure 2. The minimum of the log(ion viscosity) curve as well as the time of minimum mechanical viscosity takes place at the beginning of the test.

This point is CP(2), indicating the occurrence of minimum viscosity due to increasing viscosity caused by softening of the material. The domination of the reaction is from the very beginning at 120°C.

Figure 2. CFRP cure at 120 °C, 100Hz and 1kHz data

The occurrence of maximum slope at roughly 15 minutes represents the time of optimum reaction rate, which is CP(3). The end of cure (CP(4)) is user-defined based on the application requirements. CP(3) is also identified with gelation by some users. However, gelation has no dielectric indicator as it is mechanical event.

Log (ion viscosity) and slop curves are shown in Figures 3 and 4 for the 100Hz data from six tests. The repeatability and range of variation are revealed by overlaying the data on top of each other.

Figure 3. Log(ion viscosity) from cures of six samples of CFRP

Figure 4. Slope from cures of six samples of CFRP

Process temperature Effect on Cure Rate

The test results obtained at 120, 135, 150, and 165°C are presented in Figures 5 through 8.

Figure 5. CFRP cure at 120°C, 100Hz and 1kHz data

Figure 6. CFRP cure at 135°C, 100Hz and 1kHz data

Figure 7. CFRP cure at 150°C, 100Hz and 1kHz data

Figure 8. CFRP cure at 165°C, 100Hz and 1kHz data

Figure 9 overlays log (ion viscosity) data for the cures at 120, 135, 150, and 165°C and Figure 10 overlays slope data for these cures. Curing rates are higher at elevated temperatures, but CPs need to be extracted in order to quantify this correlation.

Figure 9. Log(IV) curves from isothermal cures at different temperatures

Figure 10. Slope curves from isothermal cures at different temperatures

The time taken to reach CP(3) and the level of this slope are summarized in Table 1, showing an Arrhenius correlation between temperature and reaction rate. The time taken to complete the curing (CP(4)) is also presented in Table 1. As predicted, the time to reach CP(4) shortens with increasing cure temperature (Figure 11).

Table 1. Effect of temperature on Critical Points

Lamp Lamp turn-on time CP(2) Min. Visc. CP(3) Max Slope CP(4) Crit. Slope
Value Time (min) Value Time (min) Value Time (min) Value Time (min)
120 --- --- --- --- 8.96E-02 7.282 5.00E-02 26.453
135 --- --- --- --- 2.90E-01 2.314 5.00E-02 16.603
150 --- --- --- --- 4.58E-01 0.782 5.00E-02 13.612
165 --- --- --- --- 6.50E-01 0.576 5.00E-02 11.970

Figure 11. Time to Critical Points 3 and 4 vs. cure temperature

Curing Characteristics 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 12 and 13 depict the curing behavior of a typical thermoset with one temperature ramp step and one temperature hold step.

Figure 12. Typical ion viscosity behavior of a curing thermoset

Figure 13. 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

Conclusion

The curing behaviour of thermosets can be observed in real time using dielectric measurements. In adidition, CPs can be extracted to measure reaction characteristics.

The CFRP data presented in this article demonstrate the consistency and repeatability of the results from sensor to sensor. Dielectric cure monitoring at different temperatures reveal the relationship between the process 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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