Huan Lee, Co-Founder of Lambient Technologies LLC, talks to AZoM.com about Dielectric Cure Monitoring and how it is used during the manufacturing process.
Could you please explain to our readers what Dielectric Cure Monitoring (DEA) is?
Dielectric cure monitoring, also called “Dielectric Analysis” or DEA, is a technique that relates the electrical properties of thermosets and polymers to physical properties such as viscosity, modulus and cure state.
Dielectric cure monitoring uses sensors and electrical measurements to obtain the resistivity and permittivity of a material. The frequency independent resistivity, or DC resistivity, is often proportional to viscosity before gelation and to modulus after gelation, so DC resistivity is the most useful property for observing cure.
What kind of data is obtained from DEA, and how is this data analyzed?
The relationship between viscosity and DC resistivity, as shown in Figure 1, has led to use of the term “ion viscosity” in place of DC resistivity.
Figure 1. Typical cure information from dielectric cure monitoring
Figure 1 shows the typical cure of a thermoset or composite material. The process temperature increases until it reaches a desired value. During this time the material softens and flows because its viscosity decreases. As viscosity decreases, the electrical resistivity (“ion viscosity”) also decreases. The user may define an ion viscosity level to indicate the onset of flow. As temperature increases, however, the reaction rate increases, causing polymerization and eventually crosslinking.
At some point the reaction dominates; increasing viscosity from the growing molecular network overcomes decreasing viscosity from elevated temperature, then overall viscosity begins to increase.
Figure 2. Ion viscosity and slope of ion viscosity from dielectric cure monitoring
The slope of ion viscosity, here called “slope” for brevity, and shown in Figure 2, can also provide useful information. The slope indicates the reaction rate, and the peak of the slope is the point of maximum reaction rate.
Finally, cure ends when the reaction stops and the material undergoes no further physical or electrical change. In reality, this point may take a long time to reach, and for practical reasons the user may choose to define a non-zero slope to indicate the end of cure. What is the value of that slope? It depends on the nature of the application and the desired properties of the material at the “end of cure.”
So we can characterize the dielectric cure curve by four Critical Points:
- CP(1) - a user defined level of ion viscosity, which can identify the onset of material flow at the beginning of cure.
- CP(2) - ion viscosity minimum, which typically also corresponds to the mechanical viscosity minimum. This Critical Point indicates the time the accelerating reaction dominates behavior of the system. The proportionality between ion viscosity and viscosity has proven to be useful in thermoset processing. Knowing when a material has reached the viscosity minimum, for example, allows optimum application of pressure to compress a laminate or to extract air bubbles from a molded part.
- CP(3) - maximum slope or inflection point, which identifies when the crosslinking reaction begins to slow. CP(3) is often used as a signpost that can be associated with gelation.
- CP(4) - a user determined slope that can define the end of cure. The decreasing slope corresponds to the decreasing reaction rate. Note that dielectric cure monitoring continues to reveal changes in the evolving material past the point when mechanical measurement of viscosity is not possible.
Critical Points characterize the cure of a thermoset and allow easy comparison of different cure curves. Dielectric cure monitoring has the advantage of being a simple electrical measurement, which requires minimal sample preparation or skill to perform. In addition, the same sensors and measurement techniques may be used in the laboratory or on the manufacturing floor.
Results from dielectric cure monitoring correlate with results from more conventional, purely laboratory, tests, such as dynamic mechanical analysis (DMA) or differential scanning calorimetry (DSC). As a result, DEA can act as the “go between” that brings information from the manufacturing environment to the manager responsible for product quality.
How can DEA facilitate Research & Development (R&D) and materials formulation?
Since R&D drives the manufacturing process, I think it’s good to start here. So imagine that you’re working on a new thermoset, or a new formulation of a thermoset. How do you learn what happens when you heat it? When is the best time to apply pressure to squeeze out voids? How fast does it react at 120 C? 150 C? 180 C? You could cure a sample for some time and then test it. Maybe it has reached the end of cure and maybe not. How do you know whether you are processing the material longer than necessary?
Differential scanning calorimetry (DSC), one method for studying these materials, measures glass transition temperature (Tg), which changes with cure state. For a particular epoxy, Figure 3 shows Tg measured with DSC and compared with results from dielectric cure monitoring.
Each DSC data point requires curing the material to a chosen time, quenching the sample to stop cure and then performing the DSC analysis. This test must be repeated at multiple points during processing to obtain enough data to see the cure curve—a very tedious and repetitive task. In contrast, the cure curve of Figure 3 from dielectric cure monitoring was obtained from a single test.
Glass transition temperatures from dielectric measurements come from a calculation that yields a quantity called Cure Index, and in this case happen to overlay DSC data very well. Furthermore, the DC resistivity (also known as ion viscosity) provides information about viscosity and modulus, which DSC cannot do.
Ion viscosity shows the time of minimum viscosity, the time of maximum reaction rate and the end of cure. All this information is available quickly and in real time, in contrast to the delay between process and test for DSC.
Figure 3. Data from DEA and DSC tests.
From: “Dielectric Properties of Polymeric Materials,” D.R. Day, Micromet Instruments, 1987 (Figure has been redrawn for clarity)
Even if DEA and DSC data do not superimpose quite so neatly as in Figure 3, there is still a direct correspondence between DEA and DSC measurements. So one can use dielectric cure monitoring to very quickly evaluate the progress of cure under given conditions, change those conditions, see what happens and change the conditions again as often as necessary. Sample preparation for DEA is very simple—apply material to a sensor and heat it. After using DEA for rapid iterations to reach a final formulation or process, then DSC may be performed to verify thermal-physical properties. The result is a great savings of time, effort and expense.
Dynamic Mechanical Analysis (DMA) is a second common technique for studying thermoset cure. Depending on the operating mode, DMA can measure certain moduli for either the early part of cure or the later part of cure. DMA is a direct measure of mechanical properties such as viscosity or modulus, but a single mode usually won’t work for the entire cure. Furthermore, some DMA methods require careful sample preparation for consistent results.
Here dielectric cure monitoring can supplement DMA. Ion viscosity is often directly proportional to viscosity before gelation and to modulus after gelation. With proper frequency selection, DEA can measure electrical properties that directly relate to mechanical properties during the entire cure.
In fact, the overlap between DEA and DMA data can be used to good advantage and at least two major manufacturers of thermal analysis instruments offer combined DMA-DEA test cells. Simultaneous DMA-DEA tests extend the portion of cure time during which mechanical properties can be measured or inferred.
Again, dielectric cure monitoring can be used to easily evaluate the preliminary formulations or processes, allowing rapid iterations to achieve a desired result. At the end of development, DMA can then be used to verify mechanical properties.
Keep in mind that DEA, DSC and DMA each measures different material properties. DEA doesn’t replace either DSC or DMA, but instead compliments them. In R&D or process development, DEA has the advantage of very simple sample preparation and the ability to make measurements during the entire cure in real time. Dielectric cure monitoring can accelerate R&D by deferring the need to make laborious DSC or DMA tests until near end of development.
Figure 4. Dielectric Analysis (DEA) enables rapid feedback in the process development cycle
How can DEA be carried out?
Dielectric analysis or cure monitoring requires a sensor that is 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 thrown away.
After connecting the sensor to dielectric measurement instrumentation, software controls the measurement process—acquiring, storing and processing the data. If necessary, the material is compressed for good contact with the sensor and then heated to initiate cure.
Under what conditions is DEA carried out?
DEA has the advantage of allowing material tests in a wide variety of conditions, both in the laboratory, the QA/QC bench or 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 contrast, DMA is confined to a laboratory. If the sample is liquid, it must be tested in a special cell or impregnated in a matrix of some kind. If the sample is solid, it must be prepared with a specific geometric configuration. DSC is similarly limited to a laboratory, and the sample to a tiny DSC pan—hardly mimicking actual process conditions.
How is DEA used in conjunction with a manufacturing process?
Right now, in the manufacturing of composites, parts are cured using a recipe for time and temperature. It’s like baking a cake at 175 °C for 30 minutes—it might be done at the end of that time or it might not. You have to test the cake before removing it from the oven. If it’s not done, you continue baking it and test it again later. If you can’t test your cake, your only choice is to bake it longer, maybe for 60 minutes or more until you’re sure it’s done—but then it might be burnt.
DEA currently is most often used to confirm that parts are made consistently. For example, the nominal cure of an automobile body panel made of sheet molding compound (SMC) might look like that of Figure 5. By extracting and comparing Critical Points, the cure of every panel can be judged against this nominal curve. Results for each panel can be recorded for statistical quality control (SQC). Deviations beyond defined limits indicate that something in the curing process has drifted and information from the cure is available to correct the problem. Thus part quality is assured.
Figure 5. Typical sheet molding compound cure
For highly critical parts such as composite aircraft or spacecraft components, every step in manufacturing is documented, both to record that the part is made according to specification and for analysis in the event of failure.
Many manufacturers measure temperature of the part as a very indirect and inaccurate way to infer the progress of cure. DEA, however, measures ion viscosity, which directly indicates cure state. So dielectric cure monitoring is valuable for documentation because no other technique can observe cure state in manufacturing and in real-time.
What aspects of the manufacturing of composite components do you believe DEA can be used to improve?
I think productivity can benefit most immediately from dielectric cure monitoring, especially for high value components like wind turbine blades. Right now these blades, which can be more than 50 meters long, are fabricated in a mold. The thickness of the blade, which affects the exotherm, and the rate of cure both vary along its length. Manufacturers use experience and guesswork to decide how long to cure the blade before removing it from the mold, but they walk a fine line. Remove a blade too soon, and it can crack because it isn’t stiff enough. Remove a blade later than necessary, and throughput is reduced.
Dielectric sensors could be installed in the mold at key locations, or perhaps every five meters along its length, for example. Dielectric cure monitoring can determine when cure along the entire part has reached a desired point. Only at that time, not sooner and not later, can the wind turbine blade be removed from its mold. I don’t know the value of a blade, but imagine how profitability might increase if a factory ships as little as one or two more blades a week.
Related to productivity is the possibility of closed loop process control. Figure 6 shows how Critical Points vary with temperature for a sample of sheet molding compound. As expected, the time to end of cure (Critical Point 4) decreases with increasing temperature, in this case a decrease of about 15% for a 10 °C increase in temperature.
So imagine a large composite structure, which cures at different rates at different points. Perhaps this part is a wind turbine blade. Perhaps it’s a composite beam for the bridge construction. If the various sections of this structure have independent heaters, then dielectric measurements can provide feedback for a control system. This system can adjust temperatures so all sections cure at a uniform rate for optimum throughput.
Figure 6. Variation of Critical Points with temperature for a sample of sheet molding compound
Can you give any examples/case studies where a manufacturer has used DEA?
Manufacturers of SMC products use timers to determine when to demold parts formed in their presses. This is standard practice, and must allow for normal variation in process temperature and other factors that affect the rate of cure.
To be conservative, the demold time is chosen to guarantee that all parts at the very least reach a given cure state, with the result that some parts may be cured longer than necessary.
Some time ago the manufacturing engineer at one such company studied the productivity when dielectric measurements indicated parts had reached a defined end of cure. Based on the detection of end of cure, the dielectric instrument issued a signal to open the press automatically.
Compared to the press timer setting of 70 seconds, dielectric cure monitoring reduced average production cycle time by 10 seconds. This cycle time improvement would have saved $70,000/year/press in labor costs alone.
About Huan Lee
Huan Lee is an electronics engineer with more than 30 years of experience designing instrumentation for the composites industry.
He has graduate degrees from the Massachusetts Institute of Technology, where he was part of a research group that worked on dielectric cure monitoring under contracts from NASA and other government agencies.
He is also a co-founder of Micromet Instruments, which first commercialized the technology developed at MIT.
In 2009 he co-founded Lambient Technologies LLC with Stephen Pomeroy to advance the use of dielectric cure monitoring for thermosets and composites.
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