Dielectric cure monitoring measures the conductance GMUT and capacitance CMUT of the Material Under Test between the electrodes of a sensor. These two quantities are then used to calculate the material properties of conductivity, resistivity and relative permittivity as they change with time. As a result, DEA can follow material state throughout cure. As shown in Figure 6-2, four Critical Points characterize the dielectric cure curves: - CP(1)—A user defined level of log(IV) to identify the onset of material flow. - CP(2)—Minimum ion viscosity, which closely corresponds to minimum mechanical viscosity, indicating when polymerization and increasing viscosity begin to dominate the material's behavior. - CP(3)—Maximum slope, which identifies the time of maximum reaction rate. The height of CP(3) is a relative measure of the reaction rate and CP(3) is often used as a signpost associated with gelation. - CP(4)—A user selected slope that can define the end of cure. The decreasing slope corresponds to the decreasing reaction rate. Critical Points in Quality Assurance/Quality Control In Quality Assurance/Quality Control (QA/QC) applications, samples of thermoset and composite products are tested regularly to confirm the curing behavior meets specifications. Figure 6-4 shows five such tests of sheet molding compound, with the close overlay of ion viscosity and slope curves indicating good consistency. But visual assessment is subjective. Objective representation of the cure is necessary for rigorous evaluation. By characterizing the ion viscosity and slope curves with Critical Points, suppliers of sheet molding compound (SMC), bulk molding compound (BMC), epoxy molding compound (EMC), and similar feedstocks can document the cure of outgoing products sample-to-sample and day-to-day. With this data they can make quantitative comparisons and perform rigorous statistical analysis to keep the process under control. Histograms like Figure 6-5 are valuable for determining the average and standard deviation of Critical Points, for understanding the statistics of the manufacturing process and for determining acceptable Pass/Fail criteria. Control limits are typically some multiple of the standard deviation about the average, and control charts can indicate when a test exceeds these limits. Run charts like Figure 6-6 for 100 SMC tests show the variability of curing behavior from batch-to-batch. Statistical Process Control software can automatically issue an alarm to warn the supplier of a batch that is out of specification. Long term drift in Critical Points could signify a developing problem, so tracking the cure of the outgoing product enables the foresight to maintain process control. Ultimately, consistency in the Critical Points allows the supplier to assure customers of consistency in their materials, which is critical for the manufacture of end products. Effect of temperature on cure rate and Critical Points Figure 6-7 overlays the log(IV) and slope curves for cures of epoxy molding compound (EMC) at four different temperatures, showing the sensitivity of dielectric cure monitoring to differences in process temperatures. Characterizing these curves with Critical Points allows objective evaluation of important moments during the cure: - CP(2) is the point of lowest mechanical viscosity, useful for identifying the optimum time to increase pressure for squeezing out voids, consolidating the layers of a laminate or filling a mold. - CP(3) indicates the point of fastest reaction. Although CP(3) is not the gel point, CP(3) is often used as a signpost associated with the gel point. The value of CP(3) is related to reaction rate and increases with process temperature. - CP(4) is the time to a user defined slope indicating end of cure. The reaction may continue at a very low level for considerable time before reaching a slope of zero, so for practical purposes a small, non-zero slope is usually selected, with a value depending on the needs of the application.
