Dielectric cure monitoring is an AC electrical measurement of a polymer’s resistivity and relative permittivity, which are dielectric properties. For properly chosen frequencies, resistivity correlates with cure state. More specifically, before gelation the change in frequency independent resistivity, due to the flow of mobile ions, is often proportional to the change in mechanical viscosity. To emphasize this relationship, the term ion viscosity (IV) was coined as a synonym for frequency independent resistivity. A thermoset cures when monomers react to form polymer chains, which eventually develop a crosslinked network. The reaction is exothermic—generating heat—and may additionally be driven by the heat of a press or oven. A plot of log(ion viscosity) is a simple way to characterize the progress of cure. Dielectric cure monitoring, also known as dielectric analysis (DEA), complements more conventional thermal analysis techniques such as differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), and bridges the gap between laboratory and manufacturing applications. Dielectric analysis or cure monitoring requires a sensor in good contact with the material under test. If the sensor is reusable, it is typically embedded in a platen or mold, which has the advantage of reducing long-term costs over many thousands of tests. If the sensor is disposable, the material is placed on the sensor and after the test everything is either stored for purposes of documentation or discarded. With the sensor connected to a dielectric cure monitor, software controls the measurement process—acquiring, storing and processing the data. If necessary, the material is compressed for good contact with the sensor then heated to initiate cure. DEA has the advantage of allowing material tests under a wide variety of conditions, both in the laboratory, on the QA/QC bench or on the manufacturing floor. No other method has this versatility. Dielectric cure monitoring may be performed in an oven, on a hot plate, in a press or mold, in an autoclave or in an actual part being developed or manufactured. When embedded in a part or a large mass of material, the dielectric sensor can directly measure the effect of an exotherm on the rate of cure. In QA/QC, consistency can be quantified with the use of four Critical Points to characterize features of these curves: CP(1) Onset of flow (User defined IV level); CP(2) Minimum ion viscosity; CP(3) Maximum reaction (Maximum slope of IV); CP(4) End of cure (User defined slope of IV). Critical Point information may be saved in a database and analyzed as run charts; control charts with upper and lower control limits; histograms; or other charts useful for statistical quality control. Closed loop process control with DEA can be a powerful aid to productivity because the time to end of cure decreases with increasing temperature—and conversely increases with decreasing temperature—expected behavior for a thermally driven reaction. Dielectric cure monitoring is a simple electrical measurement that requires minimal sample preparation or skill to perform. In addition, the same sensors and measurement techniques may be used in research, quality control and manufacturing applications. Dielectric analysis correlates with measurements from more conventional laboratory tests, such as differential scanning calorimetry or dynamic mechanical analysis. As a result, DEA can act as the “go between” that brings information from the research lab to the manufacturing floor, and from the manufacturing floor to the manager responsible for product quality.
