Dielectric Cure Monitoring and its Applications

Huan Lee Co-founder of Lambient Technologies LLC, talks to AZoM about dielectric cure monitoring and its applications

How is dielectric cure monitoring transforming the way Lambient Technologies operates its business across its range of customers?

There's a couple of ways to look at this question. The simplest may be to consider how dielectric cure monitoring is useful for companies that manufacture raw resins; for companies that combine raw resins with chopped fiberglass to make sheet/bulk molding compounds (SMC/BMC) or carbon/fiberglass fabrics to make prepregs; and for companies that use thermosets to make end products.

Lambient Technologies followed this trail by first developing the LT-451, an instrument with the flexibility and accuracy that addresses the research and development needs of raw resin manufacturers. Then we developed the LTF-631, a cost effective instrument to serve the functions of quality assurance/quality control, which are more important for SMC/BMC or prepregs. Finally in the next year we plan to release an instrument that can be easily integrated into manufacturing lines for end user applications.

Across the functions of R&D, QA/QC and manufacturing, dielectric cure monitoring has the advantage of using the same measurement and sensors.  Given this commonality, we've decided to create a range of instruments and tailor their functions and prices to be most suitable for each market.

How does dielectric cure monitoring work?

Dielectric cure monitoring works by measuring the electrical resistance of a curing material.  Under the influence of the electric field of a dielectric sensor, ions flow through the resin under test.

As the material cures, more and more of the molecules within it bond to each other, growing a cross-linked network that increases mechanical viscosity and at the same time restricts the flow of these ions, which in turn increases resistance.

In the early part of cure, resistance tracks viscosity; this correspondence has given rise to the term ion viscosity, which is simply another name for electrical resistivity, the material property that determines resistance.

Even after the resin has become rigid and viscosity is infinite, resistance—or ion viscosity—continues to increase and dielectric measurements can still observe the advancing cure.

What parameters are measured using dielectric cure monitoring to assist in the quality assurance of products?

Continuous dielectric measurements produce a curve that shows how ion viscosity changes as a material cures. The shape of this curve can be distilled to four Critical Points that identify distinct events and characterize the cure, as shown in Figure 1:

• CP(1)—A user defined level of ion viscosity that can be used to identify the onset of material flow at the beginning of cure.
• CP(2)—Minimum ion viscosity, which typically also corresponds to the minimum mechanical viscosity. Identifying the time of minimum mechanical viscosity is useful for knowing when to apply pressure to a mold to squeeze out voids.
• CP(3)—Inflection point of ion viscosity, the time of maximum rate of change (slope) of ion viscosity, which identifies when the cross-linking reaction begins to slow. CP(3) is sometimes used to measure the maximum rate of reaction and can be a signpost to indicate a mechanical event called gelation.
• CP(4)—A user defined rate of change (slope) for ion viscosity that can identify the end of cure. The decreasing slope corresponds to a decreasing reaction rate.  Note that dielectric cure monitoring continues to reveal changes in the evolving material past the time when mechanical measurement of viscosity is not possible.

Figure 1. Typical cure curve.

Consistent quality of a thermoset product results in consistent values of these Critical Points.  Aging of the material, deviant processing conditions or different formulations can change the ion viscosity curve, so monitoring Critical Points is a way to quickly and easily identify problems.

How is this technology used in manufacturing and for what applications?

In the development of raw resins and thermosets, dielectric cure monitoring allows a researcher to see how the material cures, how fast it cures in response to different formulations, how the reaction responds to the additions of catalysts or additives, and how the reaction rate changes at different temperatures.

For the manufacturers of SMC/BMC and prepregs, dielectric cure monitoring is largely used to check consistency of the product, as assurance to their customers that these products will cure as expected.

The most interesting manufacturing applications are often with the ultimate end users of thermosets and polymers. Many aerospace projects use composite materials because they are very light and very strong. The Boeing 777 used carbon filled composites for the vertical and horizontal tails and the floor beams of the passenger compartment. The Boeing 787 is the first production aircraft with one-piece composite barrel sections that replace multiple aluminum sheets and some 50,000 fasteners.

In aerospace applications, different sections of single, large composite parts can cure at different rates because of varying thicknesses and thermal conditions. Several current aerospace projects around the world use dielectric cure monitoring to control the manufacture of large components. Dielectric cure monitoring provides information for adjusting the process temperature, therefore ensuring that a large part cures uniformly.

The growing field of commercial space ventures has many opportunities for dielectric cure monitoring, though to my knowledge none of the players have yet adopted this technology. Spacecraft components such as fuselages and heat shields use composites because of their unique combination of high strength and low weight.

Even more than for aircraft, the safety requirements for spacecraft are paramount and dielectric cure monitoring can document that a life and mission critical component was manufactured to specification.

What software and sensor applications are integrated into the dielectric cure monitoring system to enhance its features?

Our CureView software is designed to make dielectric measurements with our various instruments and can set the temperature of our LTP-350 MicroPress to initiate a series of ramps and holds.  As a result an entire test system can operate under software control, making the use of dielectric cure monitoring as simple as possible for sample testing.

One particularly useful function of CureView is the ability to overlay data from different cures to compare ion viscosity curves. This ability makes it much easier to check the consistency of results for QA/QC applications. The software can also automatically extract Critical Points for quantitative comparison of the cures.

Does Lambient Technologies plan on developing its range of dielectric cure monitoring technology to enhance its functions and capabilities?

Next year we plan to release a new product called the Dielectric Channel, which will simplify the process of making dielectric measurements and extracting useful data. The goal is to make dielectric measurements very cost effective and as easy as a thermocouple to integrate into a manufacturing process.

Has this technology been used in any recent material science research efforts and do you have a case study to demonstrate this?

A current research program at the University of Houston is using dielectric cure monitoring to study the manufacture of wind turbine blades. We don't have a case study yet, but shortly plan to write one about this project.

Also, in the aftermath of loss of the space shuttle Columbia, NASA was using dielectric cure monitoring to study adhesives for repair kits to patch the heat resistant tiles on the space shuttle while in orbit.  I'd like to have a case study for this project, but the shuttle fleet is now retired and I'd have to track down the researcher.

How has this technology helped transform the way end-users manage quality control during manufacturing of thermoset materials?

Several of our customers are sheet molding compound (SMC) or bulk molding compound (BMC) manufacturers that test their materials very frequently—sometimes daily—to confirm and document curing behavior. When their fresh product is processed at 150 °C, for example, they want to know that the cure time is what they expect, or else they want to see problems right away.

The ability to measure cure simply, automatically and objectively is very important. Dielectric cure monitoring requires only a small sample, and the setup is easy. A technician places the sample on a sensor, applies heat and/or pressure then starts the CureView data acquisition software.

The result is a repeatable data curve that gives insight into the behavior of the reaction during the entire cure. The test is easy and inexpensive enough to perform every day. As a result, the cost of quality control decreases while at the same time companies can be more vigilant about the quality of their product.

In the past, and even now, it is very common for manufacturers to test cure times by having a technician measure a known quantity of raw resin and then stir it on a hot plate. The technician then starts a stopwatch and notes the time when the material stiffens enough to draw a string, somewhat like making candy. This method is very subjective and suffers from considerable variation with different technicians.

Of course, with SMC or BMC such stir tests are not possible. An alternative, such as a spiral flow test, forces uncured material through a heated spiral channel. As the material flows, it cures, becomes more viscous and eventually stops. Someone then measures the length at which the material has stopped flowing.

At best this method crudely indicates the time to cure, but requires considerable apparatus, is labor intensive and—like the stir test—gives no information at all about the reaction during entire cure process.

Where can we find further information?

You can find further information at the Lambient Technologies web site.

About Huan Lee

Huan Lee is an electronics engineer with more than 30 years’ 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.