RP035 - Correlation of Dielectric Cure Index to Degree of Cure for 3501-6 Graphite Epoxy; Micromet Instruments, Inc., Dr. David R. Day and David D. Shepard
ABSTRACT
Dielectric sensing is currently the most promising technique for monitoring and controlling composite curing. Several sensors have been developed to withstand the rugged and abusive environments found in production presses and autoclaves. Much work has been done interpreting the dielectric signals and using them to control various processes in real time.
Measurements are typically taken at several frequencies (0.1Hz to 100KHz) over several orders of magnitude and the ionic conductivity is extracted from the response. During the course of the cure the ionic conductivity is a strong function of cure state and temperature. In this work we have measured the temperature dependence of the uncured and cured resin. The temperature dependence of the dielectric response during cure is then removed from the data. Finally, the dielectric cure index is compared to degree of cure data from both the Springer cure model and the Landuyt cure model. In all cases the cure index was very sensitive to small changes in cure state near the end of cure.
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RP039 - On the Use of Cure Modeling in Honeycomb Processing; Hexcel Corporaton., L. Chiao and P.W.Borris
ABSTRACT
A general cure model for honeycomb core based on physical principles has been applied to both aramid paper and a fiberglass fabric honeycomb coated with phenolic resin. The cure model was used to design cure cycles which are much shorter than those currently used in industry, but advance the resin to the same degree of cure, as verified by microdielectrometer experiments. Plant trials show that these modified cure cycles result in core with equivalent mechanical properties to those which go through the standard cure cycles.
The model was also used to predict cure reaction runaway (exotherm) behavior. This informaton is useful in analyzing regions of the honeycomb block where the airflow is poor and also situations of sudden airflow loss.
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RP041 - Dielectric Cure Monitoring During Composite Lamination; IBM Corporation, Jeffery Gotro
ABSTRACT
Oscillatory parallel plate rheometry is commonly used to measure the viscosity of neat resins during curing. The objective of this work was to use a dielectric sensor embedded in an epoxy/glass cloth prepreg layup to infer the viscosity profile during lamination. This method allowed qualitative characterization of the viscosity profile during lamination at different heating rates.
Correlations between the dielectric loss factor and the complex viscosity were established using simultaneous viscosity and dielectric measurements. Viscosity measurements during composite lamination would be virtually impossible. The dielectric method is simple and relatively inexpensive compared to the cost of most rheometers.
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RP044 - PMR-15: Aging Detection Using Dielectric Sensors; Micromet Instruments, Inc., David. D. Shepard
ABSTRACT
The aging of PMR-15 graphite reinforced prepregs prior to processing can affect the performance of the resin system. PMR-15 consists of three monomers blended together in anhydrous methanol which is prepregged and stored in the monomer stage. Out time can result in aging which will increase the viscosity and reduce the flow of the resin. This in turn may affect the thermal and mechanical properties of the cured part. Dielectric measurements can monitor the entire cure cycle of PMR-15 graphite prepregs in both research and production environments.
The progressive room temperature aging of the prepreg over a 31 day period was observed using a rugged, reusable dielectric sensor. During a typical cure cycle, major differences in the Ion Viscosity (resistivity) were observed prior to imidization in samples aged to various degrees. These differences were attributed primarily to solvent volatilization and monomer reaction. The more aged samples appeared to complete their final cross-linking reaction at an earlier time than the fresher samples.
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RP051 - A New Dielectric Instrument for In-Mold Statistical Process Control of RIM, SMC, and RTM; Micromet Instruments, Inc., Dr. David R. Day, Huan L. Lee, David D. Shepard, and Norman F. Sheppard
ABSTRACT
The usefulness and versatility of dielectric measurements in monitoring the curing behavior of thermosetting resins and composites has been widely demonstrated. The ability to make dielectric measurements in the process environment allows the technique to be used for research, process development, and production applications.
The ICAM-1000 is a new instrument designed for process monitoring and Statistical Process Control of short cycle time processes such as RIM, SMC, and RTM. The system utilizes high-speed dielectric, pressure, and temperature measurements to monitor the intrinsic and extrinsic properties of the resin during processing. Measurements are made through reusable sensors flush mounted in the mold. Software provides the ability for window analysis and provides output for SPC use. Examples of the application of the instrumentation in RIM, SMC, and RTM processes will be discussed.
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RP050 - Moisture Diffusion Monitoring of Polyetheretherketone (PEEK) with Microdielectric Sensors; Micromet Instruments, Inc., Dr. David R. Day, David D. Shepard, and Kelly J. Craven
ABSTRACT
Classical diffusion analysis involves weight uptake measurements as a function of time on thin polymeric samples. The use of Microdielectrometry to monitor the diffusion of moisture through polymers is a relatively new technique. Sensors have been developed which are sensitive to the small changes in permittivity (dielectric constant) of polymeric materials as water concentration (and thereby, mobile dipole concentration) increases. In addition, sensors exist which can withstand the high temperatures (greater than 350C) required for the consolidation of high temperature thermoplastics. The diffusion coefficient can be estimated through a simple Fickian diffusion model of the dynamic dielectric response. The Microdielectric sensors measure a very localized area of approximately 0.01cm into the material. This allows monitoring diffusion properties of thin films, or of many simultaneous and independent locations in a bulk material.
Previous work has studied the diffusion characteristics of epoxies and polyimides by this technique. This study examines the diffuson of water through a thin film of polyetheretherketone (PEEK) and compares it with published data obtained by weight uptake studies. The diffusion rate of moisture through amorphous PEEK is shown to be slightly faster than diffusion through more crystalline PEEK.
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RP056 - Automotive/Aerospace Synergism in Computer-Aided Composites Processing; Advanced Composites Engineering, GM Delco Products Division, Johnny Gentry
ABSTRACT
In-mold dielectric cure monitoring techniques pioneered by the aerospace industry were applied to the manufacturing process of LITEFLEX® springs. Real-time data was collected in-situ and used to interpret the progression of cure in an epoxy/fiberglass composite. This information was used to trigger mold pressurization in an attempt to lower void content and improve part appearance.
A microdielectric sensor placed on the mold surface collected temperature and conductivity data that was converted to a cure percentage scale (cure index). The optimum pressurization point for the epoxy system was at approximately 40% conversion. Although results varied somewhat depending on spring geometry, void contents were reduced to less than 1%, composite shear strength increased 10-15% and surface appearance was also significantly improved.
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RP070 - Computer-Aided Curing of Composites; McDonnell Aircraft Company
ABSTRACT
The objective of this project is to demonstrate an artificial intelligence based control system using hardened sensor systems and an advanced knowledge base in an advanced composite facility. To achieve this objective, the program was divided into three tasks.
I. Sensor development: we are developing sensor technologies to interrogate composite materials during processing to allow intelligent processing decisions to be made based on actual material states. The developed sensors will be production-hardened for real-time composite processing control in a manufacturing environment.
II. Control Rules and Strategies: we investigate three critical composite properties (void volume, thickness, and resin state). A thorough understanding of these properties, and how to control each one, is crucial to this program. They are the goals which must be met for each part produced by the intelligent controller. We add on-line modeling capabilities to the QPA control system. Finally, we develop and demonstrate new control rules and strategies.
III. Advanced Processing Demonstration: we demonstrate the intelligent control system on a stiffened wing box configuration that represents one of the leading design candidates for a future fighter aircraft wing. We study innovative processes to identify the best unit process for QPA applications and assess the effect of the QPA approach to the candidate unit process.
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RP083 - Cure Characterization of Thick Composite Parts Using Dielectric and Finite Difference Analysis, Micromet Instruments, Inc., David R. Day
ABSTRACT
Curing characteristics of thick composite parts have long been known to be a function of thickness and location within the parts. While cure state as a function of thickness has often been modelled, actual experimental verification has been difficult. In this work, disposable and permanently mounted dielectric sensors were used to characterize the cure in a thick composite part a various locations through the thickness in a simulated molding environment. Using established techniques, the dielectric and temperature information were combined to yield local cure state information for each sensor. The cure state was found to be significantly nonuniform through the thickness of the part. These observed cure state data are compared to model predictions.
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RP046 - Production Implementation of Fully Automated, Closed Loop Cure Control for Advanced Composite Structures; General Dynamics, Sean Johnson and Nancy Roberts
ABSTRACT
Economics of advanced composite part production requires development and use of the most aggressive cure cycles possible without sacrificing quality. As cure cycles are shortened and heating rates increase, tolerance windows for process parameters become increasingly narrow. These factors are intensified by condensation curing systems which generate large amounts of volatiles.
Management of the situation requires fully automated, closed loop process control and a fundamental understanding of the material system used for the application. No turnkey system for this application is currently available. General Dynamics Pomona Division (GD/PD) has developed an integrated closed loop control system which is now being proofed in production. Realization of this system will enable cure time reductions of nearly 50 percent, while increasing yield and maintaining quality.
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RP096 - A Microdielectric Analysis of Moisture Diffusion in Thin Epoxy/Amine films of Varying Cure State and Mix Ratio, Micromet Instruments, Inc. David Day, David Shepard, and Kelly Craven
ABSTRACT
This work shows that dielectric sensors can be used to monitor moisture diffusion in polymer materials ranging from composites to thin films. When the technique is used with thin films, diffusion coefficients and degree of moisture uptake can be determined very quickly. This paper describes preliminary moisture diffusion analysis carried out in thin epoxy films as a function of ultimate cure state and mix ratio of epoxide to amine components.
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RP114 - Application of Dielectric and Thermal Analysis to the Curing of Epoxy Resins, Toshiba Corporation, T.Nakano, S.Makishima, Y.Inoue, K.Goto
ABSTRACT
Upon studying the application of dielectric analysis (DEA) to techniques for non-destructive on-line measuring of the curing process of epoxy casting resin in molds, it was found that the ionic conduction portion derived from dielectric loss factor was closely correlated to glass transition temperature and resin viscosity. A new study was undertaken using DEA, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and curing volume shrinkage measurement to investigate in more detail the relation between the electrical conductivity of resins and the degree of cure during the curing process. A relation was quantitatively determined between the equivalent resistivity derived from dielectric loss factor and the degree of cure, viscoelasticity and volume shrinkage.
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RP118 - - Effect of Advancement of Epoxy Prepreg Processing - A Dielectric Analysis, Micromet Instruments, Inc., David R. Day and David D. Shepard
ABSTRACT
With the relatively recent trend toward adaptive process control during processing of structural composites, there is need for an understanding of the nature of variations during cure that are encountered. Part-to-part variations can arise because of differences in thermal exposure arising from part placement or part thickness, or from chemical differences due to formulation or advancement. Microdielectric sensors are particularly well suited for monitoring thermoset cures from initial point of flow, through the liquid region, and on into the solid state. In this study, microdielectric sensors are used to systematically study the influence of resin advancement on cure behavior. Relationships between advancement and critical cure phenomena such as flow temperature, point of viscosity minimum, and reaction end point are discussed.
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RP071 -The Curing of Epoxy/Amine Systems Viewed Through Microdielectrometry; University of Lyon, C.Mathieu, G.Boiteux, G.Seytre, Laboratory of Macromolecular Materials, INSA, M.Feve, J.P.Pascault, Technical University of Lodz, Poland, J.Ulanski, and Aerospatiale, P.Dublineau
ABSTRACT
Aim of this article is to show how microdielectrometry can improve the curing of epoxy resins by detecting in real-time critical events like gelation or vitrification. The isothermal curing of DGEBA/3DCM in stoichiometric proportions is followed with: electrical techniques of microdielectrometry and DC measurements, analysis of insolubles, dynamic mechanical analysis and viscosimetry. Electrical techniques have evidenced their own efficiency to detect chemical phenomena with parameters sensitive to the modification of the system during the network formation.
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RP112 - Processing and Properties of IM7/LARC-IAX2 Polyimide Composites, Lockheed Engineering & Sciences Company, T.H.Hou and A.C.Chang, NASA Langley Research Center, N.L. Johnston and T.L.St.Clair
ABSTRACT
LARC-IAX2 (Langley Research Center-Improved Adhesive eXperimental resin 2) aromatic polyimide, based on oxydiphthalic anhydride (ODPA), benzophenone tetracarboxylic acid dianhydride (BRDA), and 3,4'-oxydianiline (3,4'-ODA), was evaluated as a matrix for high performance composites. This polymer is a modified version of the baseline LARCIA polyimide made from ODPA and 3,4'-ODA. Two poly(amide acid) solutions end-capped with phthalic anhydride were synthesized in N-methypyrrolidone and N,N-dimethylacetiamide (DMAc) with ODPA to BTDA ratios of 4:1a and 3:1, respectively. Fully imidized films exhibited improved solvent resistance in acetone, meghylethylketone, toluene, DMAc, and chloroform compared to the baseline LARCIA film. Unidirectional prepregs were fabricated from both solutions using the Langley multi-purpose prepreg machine. A separate molding cycle (350C/250psi) was developed for each prepreg based upon solvent/volatile depletion characteristics. These cycles consistently yielded void-free consolidated laminates but required more severe processing conditions than those for the baseline material. Short beam shear strength and longitudinal flexural properties were measured at room temperature, 93, 150, and 177C. Notatbly, the flexural moduli were about 5% higher than those reported for the baseline composite. In addition, engineering properties such as fracture toughness, unnotched tensile strength, notched and unnotched compressive strengths and moduli, compression strength after impact and open hole compression strength were also measured.
The enhanced solvent resistance, more difficult processability, and higher flexural moduli compared to those for the baseline composites were attributed to an increase in polymer backbone stiffness due to the presence of BTDA. No differences in the physical and mechanical properties were observed between composites made in NMP and DMAc. The results have helped define a chemical approach for improving solvent resistance in thermoplastic polyimide materials.
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RP085 - Chemorheological and Dielectric Behavior of the Epoxy Matrix in a Carbon Fiber Prepreg, Dept. of Materials and Production Engineering, University of Naples, Italy, J.M.Kenny and A.Trivisano
ABSTRACT
A scientific approach to the problem of autoclave curing of thermoset composites includes modelling of the cure kinetics and viscosity of the matrix. The objective of the present study was to develop and test a chemorheological model for the commercial carbon fiber/epoxy prepreg Fibredux HTA/6376 with a toughened epoxy matrix based on TGMDA/DDS. The model for the cure kinetics takes into account the effect of vitrification prior to full cure at cure temperatures below the glass transition temperature of the fully cured epoxy. The rheology of the matrix was modelled applying a similar viscosity model as was previously verified for commercial epoxy based prepregs containing the TGMDA/DDS system. The cure kinetic model was verified by calorimetric measurements simulating different processing conditions. Predictions of the viscosity qualitatively agreed with dielectric measurements during prepreg cure of the inverse of the ionic conductivity in the epoxy matrix. In particular, predicted and measured times at which minimum viscosity occured showed excellent agreement, even for complicated cure cycles. Experimental measurements revealed that the investigated epoxy matrix shows incomplete cure, approximately 90%, after the cure cycle suggested by the prepreg manufacturer.
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RP047 - Monitoring Rheology During Cure of Prepreg in Autoclaves, Tonen Co. Ltd. Carbon Fiber Project, O.Watanabe, S.Murakami, H.Inoue
ABSTRACT
Several techniques have been used to establish the monitoring and control of prepreg curing in autoclaves. Dielectric analysis techniques were used in this paper. Prepregs were made using Tonen pitch-based Ultra High Modulus carbon fiber and three kinds of epoxy resins. These prepregs were cured in an autoclave and their dielectric properties monitored. The dielectric data were compared with DSC reaction conversion data. These data show a very high correlation and the results suggest that the optimum processing conditions for prepreg can be accurately determined by DEA.
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RP127 - Dielectric and Thermal Cure Characterization of Resins Used in Pultrusion, University of Mississippi, Reshma Shanku, James G. Vaughan, Jeffrey A. Roux
ABSTRACT
The ability to conduct on-line measurements of a resin's degree of cure is important to a pultruder for setting optimum pultrusion processing parameters. In this study, in-situ dielectric measurements were conducted to determine the degree of cure as a function of the pultrusion die axial position. Dielectric analysis utilizes a sensor which is inserted at the center of the impregnated fibers and travels along with the fibers through the heated die. The conductivity due to ionic movement in the resin is measured and a cure index is calculated from this data. The purpose of this study was to determine degrees of cure for pultruded epoxy/graphite, epoxy/glass and polyester/glass composites using dielectric measurements and to compare the on-line measured values with values obtained using differential scanning calorimetry (DSC) and with values predicted using a numerical heat transfer/chemical kinetic pultrusion model. A 2.54cm x 0.32cm profile was produced at a pull speed of 30.5cm/min for the epoxy composites and at 61cm/min for the polyester composite. DSC samples were taken from the composite close to the location of the embedded sensor to verify the values of degrees of cure obtained from the dielectric measurements.
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