Monitoring the cure itself

Process control in composites molding has come a long way in recent years, with the advent of often computerized, automated measurement and control of process inputs, such as heat, temperature and pressure, through the use of thermocouples and other in-situ monitoring devices. During part development, cure can be reliably characterized, in general terms, using these variables. Cure rates, however, vary from part to part, even when parts are meant to be identical. Many factors - the age of the prepreg or resin, the environmental conditions under which the part is layed up, variations in the ambient temperature within the autoclave or press, slight changes in ramp rates - can affect cure time. Without a way to directly measure the progress of crosslinking, fabricators of thermoset composites traditionally have had to build a safety margin into cure times in order to avoid the damage that premature demolding can do to tools and undercured parts. While this ensures that sufficient crosslinking occurs, it lengthens cycle time and processing costs beyond what might otherwise be required. Additionally, there is the risk of overcuring, which can reduce ductility, making a part brittle.

Today, however, a new generation of monitoring devices is taking measurements in real time that correlate directly to the actual physical and/or chemical state of the resin. These devices, therefore, determine when full crosslinking has occurred, enabling fabricators to pinpoint the actual cure endpoint for each part. With safety margins no longer required, companies using such devices have reported savings of 10 to 40 percent in cure duration, without producing parts that fail quality assurance tests due to over- or undercuring. The result is a shorter overall cycle time and a part with more predictable mechanical and performance properties.

Currently available to composites fabricators are cure monitoring devices that measure electrical and/or thermal characteristics of the resin. These include dielectric monitors, dipole monitors and heat flux monitors as well as proposed "smart"sensing systems that rely on fiber-optics.

Dielectric cure monitoring

Dielectric cure monitoring systems measure the extent of cure by measuring the conductivity of ions - small, polarized, otherwise insignificant amounts of impurities resident within resins. Ions tend to migrate toward an electrode of opposite polarity. Yet the ions' migration speed is limited by the viscosity of the resin. As crosslinking proceeds during cure, resin viscosity increases, with a corresponding reduction in the ion migration rate.

A dielectric device takes measurements by means of two electrodes, which are linked to the device by conductive cabling. The electrodes are typically embedded in the uncured matrix, and positioned a short distance apart, with a small resin-filled space in between them. The electrodes, are made from metal or ceramic materials and consist of either parallel plates, which can measure the bulk ionic conductivity through the thickness of a composite component; or interdigitated combs, in which two comb-shaped electrodes have their "teeth"interleaved such that a pair of teeth, one from each comb, serves as an electrode pair from which a localized reading of ion conductivity can be taken. The interdigitated comb is flat, and the signal passes in an arc perhaps 125 microns deep, from one electrode through the resin and back to the other electrode. The sensors can be embedded within the tool face, by machining the tool and flush-mounting the electrode on a non-critical surface, or within the part itself. Either mounting option provides the same accuracy, but the part-embedded sensors must be incorporated into the layup and are not reusable.

To measure ion conductivity, an AC voltage is applied between the two electrodes, using frequencies typically ranging from 0.1 Hz to 100 kHz. The amplitude and phase of the response frequency, generated within the sensor because of ion mobility, are used to calculate the ion conductivity. Some devices measure the decrease in ion conductivity and calculate the increase in viscosity. Other devices measure ion impedance or resistivity (the inverse of conductivity, which is proportional to viscosity). Thus, dielectric sensors can be used to relate ion conductivity to the onset of gelation and the minimum viscosity point as well as the glass transition temperature of the cured matrix. "Such information can be used to adjust compound formulations, mold closing rates, molding temperatures, and to optimize or troubleshoot molding parameters for a given molded part,"points out Tom Trexler, VP of sales and marketing for Signature Control Systems, Inc. (Denver, Colo.), a manufacturer of cure monitoring systems.

Dave Shepard, northeast regional sales manager of cure monitoring system manufacturer NETZSCH Instruments (Burlington, Mass.), formerly Micromet, notes that these systems have already established a successful track record. "Dielectric cure monitoring is routinely used [by compounders] as a quality control test of bulk and sheet molding compounds,"he reports. Its application to curing of finished composite components is emerging, he continues. NETZSCH, which sells its cure monitoring systems under the Micromet brand, has found significant interest in cure monitoring for developmental work on such components, to establish and then predict cure parameters. Finding the optimum cure cycle conventionally has required trial-and-error experimentation, process modeling, and off-line characterization of the cure state using techniques such as differential scanning calorimetry (DSC) or rheology dynamic spectroscopy (RDS). Such offline measurements are relatively costly and tedious, Shepard notes, and some of these techniques provide data only before or after gelation. By comparison, dielectric sensors can monitor the entire cure cycle, from liquid resin to fully crosslinked gel, and do so in real time.

NETZSCH also has supplied dielectric sensors for production applications related to composites. For example, the company's Micromet IDEX sensor is monitoring the cure state of FM-300 adhesive used to bond the titanium root fittings to the carbon/epoxy wing skins of F/A-18C/D and E/F aircraft. Previously, FM-300, a film adhesive from Cytec Engineered Materials Inc. (Tempe, Ariz.), was B-staged (partially cured) to adjust its viscosity prior to application by heating it in an oven for a predetermined amount of time. With this approach, out-time could not be accounted for, viscosity testing took two days, and the film would have to be re-staged in the oven if viscosity did not fall into the proper range. Film that was over-staged would have to be stripped from the titanium, a tedious and costly process. Using dielectric cure monitoring, prime contractor Boeing correlated dielectric measurements to offline rheological measurements. This correlation database can be used to determine when B-stage is reached for each batch of film adhesive, eliminating the variation as well as the need for off-line measurements of the old method. "The ability to make measurements in actual or simulated production environments, as well as in the laboratory, makes dielectric cure monitoring a very versatile technique,"Shepard concludes.

Dielectric cure monitoring systems also can be linked via software to existing process controls on the press or autoclave to automatically trigger end-cycle mechanisms (opening the press, shutting down the heating coil, etc.) when designated ion conductivity is reached. Such production applications already have been implemented, using Signature Control Systems' SmartTrac Intelligent Cure Control technology. This technology was recognized in 2005 by the Society of Plastics Engineers with the "Innovation in Thermoset Processing"award, presented to the company at the 51st annual Madison Thermoset Molding Conference (Madison, Wis.).

Since commercializing the technology in 2003, Signature Control Systems has installed more than 75 systems into molding facilities in North America and Europe. Since Meridian Automotive Systems’ (Allen Park, Mich.) installed a SmartTrac system at its Huntington, Ind. facility, average cure time has been reduced by 25 percent for a class B automotive cross-car beam fabricated from a compression molded SMC. A second system, at Meridian’s Salisbury, N.C. facility, reduced cure time by 20 percent on a Class A part made from SMC. In the case of the latter, SmartTrac triggers injection of an in-mold coating (IMC) as the SMC’s optimal cure state is reached, then monitors the cure of the IMC and triggers the press to open automatically when the whole part is fully cured, ensuring acceptable paintable surfaces.

SmartTrac systems also are being used with carbon composites, which represent a significant hurdle for dielectric technology because the reinforcement is highly conductive and can interfere with readings of ion conductivity. To overcome this obstacle, Signature’s system uses ceramic-coated sensors that maintain electrical isolation between sensor components and mold tool for most applications. The coating prevents shorting to the tool when a conductive fiber makes contact with the sensor face. In one recent application, the system helped determine the optimum cure profile for a resin transfer molded (RTM’d) component consisting of 60 percent carbon reinforcement in an epoxy matrix. SmartTrac electrodes were positioned at the RTM inlet and outlet ports, providing a simple means of monitoring resin flow. Tests were then conducted to identify the optimum cure temperature. Viscosity curves for a series of cure temperatures, produced by the SmartTrac system, were compared side by side to identify the optimum cure temperature. Once this temperature was determined, the customer reports, only one additional trial was needed to determine the required cure time. Without the cure monitoring system, many trials would have been needed, since the mold would have to be opened to check the state of cure. The overall process of determining the cure profile took less than half the time it would have required using traditional trial-and-error methods, the customer estimates. Signature has demonstrated its technology on composites with as much as 70 percent carbon content.

 For its carbon applications, NETZSCH deals with carbon’s tendency to interfere with measurement accuracy by offering a filtered version of its Micromet IDEX sensors. “A special glass filter cloth is used on top of the sensing area, which avoids a short circuit of the conductive carbon fiber,” explains Stephan Knappe, worldwide sales and applications support manager. “The resin can penetrate the cloth and comes into close contact with the sensing area, providing the curing curve.”

Dipole monitoring

While ionic conductivity is the predominant electrical characteristic available for dielectric measurement at lower frequencies, another approach to cure monitoring relies on monitoring pairs of polar molecules that rotate with respect to one another. Called dipoles, these pairs are part of the resin itself and dominate the resin’s electrical response at higher frequencies. One such system, the Time Domain Reflectometry (TDR) system from Material Sensing & Instrumentation Inc. (MSI, Lancaster, Pa.), expands functionality compared to all other cure monitoring methods by providing a quantitative measurement of percent of cure at any given time, including the later stages of cure.

The TDR system relies on relatively small electrodes, which are tuned to frequencies in the microwave band, ranging from 10 MHz to 10 GHz. TDR inputs a rapid voltage pulse containing a broad range of frequencies. Such frequencies create a dielectric “relaxation response” in rotating dipoles. The response is characterized by a parameter called the “dielectric constant,” which indicates the amount of and rate at which the dipoles absorb and then release electromagnetic energy. As crosslinking begins and the resin’s viscosity rises, the frequencies at which relaxation responses occur also change. Because resins contain a known concentration of dipoles, these responses can be related both qualitatively and quantitatively to the degree of cure. “The dipole spectrum provides a unique signature, detailing viscosity and percent cure,” summarizes Nat Hager, MSI president. “What we’re measuring is the concentration of unreacted molecules, which continues to decrease even at the later stages of cure.”

TDR cure monitoring was developed under a U.S. Army Small Business Innovation Research (SBIR) contract in the late 1990s. In that project, TDR was used in a closed-loop control system to apply pressure to a laminate at the optimal time in the cure cycle. The system also can be used to control temperature. The Boeing Co. (Chicago, Ill.) and Bell Helicopter (Fort Worth, Texas) tested the system for in-plant use, and investigations have since focused on nondestructive testing and structural health monitoring applications. TDR’s system, so far, is used primarily by concrete suppliers. Hager welcomes inquiries from the composites industry, noting that the successful track record MSI is establishing in the concrete market could easily be duplicated.

Measuring crosslinking voltage 

While dielectric sensors and the TDR approach measure the response of the resin to induced voltages, a third technology measures the micro-voltage produced by the crosslinking reaction, a previously undetected electrical phenomenon. The polymerization voltage produced is minute and was previously assumed to be electrical “noise,” not a measurable event. The voltage changes distinctively as the cure reaction proceeds, explains Tison Wyatt, president of Tison Technologies LLC (Skyland, N.C.) and the inventor of the firm’s patented AccuCure monitoring technique. “The voltage produced within the reaction is definable and predictable, allowing AccuCure to monitor all stages of the reaction and pinpoint the precise conclusion of the reaction,” he says.

Initial studies of the AccuCure method at the Mississippi Polymer Institute (Hattiesburg, Miss.) have demonstrated repeatable and consistent measurement of gel time for epoxies, polyesters and polyurethanes. “It’s like using a thermometer in a turkey,” quips Harold McNair, a chemistry professor at Virginia Tech University (Blacksburg, Va.). “The thing goes ‘bing’ and you know it’s time to take the turkey out of the oven. I was surprised at how simple it was, and at the magnitude of the response you get from the reaction.” With these endorsements from the research community, Tison Technologies is working with specific industries to apply the technology.

 

Heat flux monitoring

A fourth method focuses on the exothermic reaction in the polymer matrix. Thermoset crosslinking produces thermokinetic information that can be used to monitor cure. This is the theory behind the heat-flux monitoring system offered by Thermoflux Technologies S.A. (Yverdon-les-Bains, Switzerland). The company’s sensors monitor the amount of thermal energy exchanged between the tool and material being cured per unit time, a parameter called heat flux. The technology detects and analyzes the flow of both the input heat to the tool and the heat produced by the exothermic reaction. As a result, the cure endpoint can be identified.

An advantage of this technology arises because it need not be in direct contact with the resin. The sensors are located beneath the surface of the tool. They can detect resin flow in infusion applications, as the local temperature correlates to viscosity and shearing while the tool is filled. During cure, the sensors detect the heat generated by the reaction. As cure ends, the sensors detect stabilization of the thermal exchange. The system then automatically stops the cure cycle (e.g., by signaling the press to open).

Thermoflux systems have been applied to a broad cross-section of molding techniques, including autoclave processing, RTM, pultrusion and compression molding. In one application, the system’s ability to pinpoint the end of the cure cycle has resulted in decreased autoclave time for a 15-ply thermoplastic-reinforced epoxy helicopter component. With the possible input of other existing process signals, Thermoflux claims that its system provides a complete process monitoring and data-logging system, plus a ready-to-use automatic cure control solution.

Fiber-optic future?

An as yet uncommercialized approach to direct cure monitoring involves fiber-optic sensors, which are, today, the subject of intensive study and development efforts for a variety of composites monitoring applications. These “smart” devices can provide feedback for structural design improvements, structural health monitoring, strain sensing, damage detection, design analysis and intelligent control. Their relatively small size makes it possible to embed them in composite structures without altering the composite’s mechanical properties.
To monitor cure, a fiber-optic device can be arranged to send an infrared (IR) signal through a small portion of resin within the layup — energy which will be absorbed and then released by the resin’s chemical components — and then capture the spectral data. Because chemical bonds exhibit characteristic vibrational frequencies in the absorption and release of IR energy, the captured data can be analyzed to quantitatively determine the degree of crosslinking. However, while optical fibers are commodity items today, the analytical instruments that read the signals that those fibers carry is, so far, cost-prohibitive for general use. Industry watchers are predicting that fiber-optic cure monitoring will surge when structural health monitoring becomes more common, because the same fibers serve both roles.