ID 63 CURE MONITORING OF THERMOSETTING RESIN COMPOSITES BY LACOMTECH DIELECTROMETRY Jae Wook Kwon and Dai Gil Lee Departent of Mechanical Engineering, Korea Advanced Institute of Science and Technology, ME322, Kusong-dong, Yusong-gu, Taejon-shi, Korea 35-7 SUMMARY: The properties of therosetting resin coposites are dependent on the degree of cure and consolidation quality. During the consolidation process of therosetting resin coposites, the viscosity of the resin of coposite has a doinant role for unifor and quality products. In this study, the dissipation factor that is a function of viscosity was easured during the cure process of therosetting resin coposites by the newly developed Lacotech dielectroetry apparatus and sensors. Using the easured dissipation factor, the relationship between the dissipation factor and degree of cure with respect to environental teperature was investigated. KEYWORDS: Dielectroetry, Cure Monitoring, Dissipation Factor INTRODUCTION The cure process of therosetting resin atrix fiber coposites accopanies not only cheical reaction in a ultiphase but also physical oveent of heat and ass [-3], which is dependent on resin viscosity. The cheical reactions of therosetting polyeric aterials are exotheric by the cross link between onoers with coplex tendencies. Since the conductivities of polyeric aterials are usually lower than those of etallic aterials, deforation and distortion in coposite products after curing occur due to teperature gradient. Therefore, the on-line easureent and process control of the cure process are iportant for unifor and quality coposite products: the control of abient teperature and pressure through on-line cure onitoring yields reliable coposite products efficiently in short tie. Much research has been perfored in this area. Ki and Lee investigated the relationship between the dissipation factor and the viscosity of coposites, and odeled the degree of cure as function of dissipation factor for on-line cure onitoring [4, 5]. Stephan et al. perfored on-line real-tie dielectric easureent during copression olding for coposites and controlled the process teperature and pressure [6]. In this paper, a new sensor and on-line cure onitoring syste using dielectroetry were developed and their perforance were copared with the previous systes. Also, the dielectric constant and teperature during the cure of coposites were easured.
MEASUREMENT OF DISSIPATION FACTOR The Lacotech dielectroetry consists of an electric circuit and several sensors with the shape of two planar inter-digital type electrodes as shown in Fig.. When the sensors are ebedded in coposite aterials and connected to an alternating electric field, the cobination of the electrodes and the coposites fors a capacitor because the therosetting resins in coposite aterials are dielectric. The charge accuulated in the capacitor depends on the obility of dipoles and ions present in the resin to follow the alternating electric field and varies with the stage of cure. The degree of cure is related with dipoles and ions oveents. They have high obility when the resin is uncured, but they ove little when the resin is fully cured. The degree of the oveent can be expressed by the dissipation factor D that represents the ratio of energy loss by oveents of dipoles and ions to supplied energy. Using the dielectroetry sensor that is odeled as a parallel circuit of resistance R and capacitance C as shown in Fig. 2, the dissipation factor of equivalent circuit can be obtained as follows. D I I R R C = = = = () C I I C R ωr C where ω is the angular frequency of alternating current. EXPERIMENTAL SETUP In order to increase the sensitivity of signals of dielectroetry sensors, the Lacotech dielectroetry eploys the Wheatstone bridge circuit as shown in Fig. 3. The accuracy of Wheatstone bridge circuit becoes higher as the aplitude of output signal approaches zero. The relationship between the alternating input signal i and the output signal o is expressed as follows. o i = = (2) + 2 2( + ) 9 I R I C 25 R C Sectional view µ µ 5 µ Electrode Substrate 5 µ Fig. Shape of the dielectroetric sensor Fig. 2 Equivalent circuit of coposite aterials.
Equivalent circuit of a dielectric sensor R R C C2 C C2 o R2 R2 i Fig. 3 Electric circuit eploying Wheatstone bridge for easuring the dissipation factor. where and represent the ipedance of the sensor and the eleent of circuit, respectively. o 2 + i = o 2 i 2 I[ ] R = C = (3) 2 Re[ ] ω Using Equation (3), the dissipation factor can be obtained fro Equation (). Since R and C change according to the degree of cure, a variable resistance and a condenser were used for R and C to ake the output signal of Wheatstone bridge circuit zero. In this anner, the dissipation factor was obtained with respect to abient teperature. Also the process variables such as degree of cure and teperature were related statistically to the dissipation factor. Since the distance between dielectroetry instruent and coposite anufacturing apparatus such autoclave is usually not short, long lead wires for signal transfer are Equivalent circuit of a dielectric sensor Duy sensor Equivalent circuit of a dielectric sensor Float capacitance C w Float capacitance C w Fig. 4 Electric circuit eploying Wheatstone bridge and long lead wire for easuring the dissipation factor. Fig. 5 Electric circuit eploying Wheatstone bridge, long lead wire and duy sensor for easuring the dissipation factor.
necessary as shown in Fig. 4, which induces float capacitance. Also, if the coposite anufacturing apparatus uses electricity, the dielectroetry easureent ay be affected by environental electric field that causes error and nonlinearity. In this case, dielectroetry sensors should be shielded fro environental electrical field. If the shielding of lead wires is not easy, the effect of environental electric field ust be reduced. In order to reduce the float capacitance and environental effect, a duy sensor was substituted for an eleent of Wheatstone bridge whose wire length was equal to that of the dielectroetry sensor as shown in Fig. 5. The duy sensor located close to the dielectroetry sensor will be exposed to the sae environents. Then Equation (2) can be rewritten as follows. o i = = 2 ( + ) + w {( + ) + } + {( + ) + } ( ) + ( ) {( + ) + 2 + ( + )} w w w 2 (4) where and represent the change of and by the environent effect, respectively. Since the ipedance and its change of the duy sensor are alost equal to and of the dielectroetry sensor, respectively, the nuerator of Equation (4) is very sall, which akes the error and the nonlinearity very sall. In order to reduce the float capacitance of the lead wire, two wires were twisted each other 5 turns per eter. Float capacitance of the twisted wire and the noral wire were 4 and 7 pf/, respectively, while the sensor capacitance itself was about pf. Since the lead wire was usually 2 ~ 3 long, the float capacitance of coon wire were hundreds pf larger than that of the twisted wire. The float capacitance was reduced by eploying the twisted wire. To easure the dielectroetry up to 2 C, ariable resistance & capacitance Sensor connector Duy sensor connector Therocouple connector A/D board output Function generator ( Hz ~ MHz) Power supply Fig. 6 Newly developed Lacotech dielectroetry apparatus.
Measurand Electrode Substrate Electrode Substrate Measurand Fig. 7 Finite eleent odel of the electric field for the dielectroetric sensor. the heat resistant cable covered by Teflon was used. Also the cable was shielded to reduce environental electric field. Fig. 6 shows the photograph of the developed cure onitoring apparatus that easures teperatures and dielectric constants siultaneously at 6 channels. DIELECTROMETRY SENSOR The planar inter-digital dielectroetry sensor used in the experient was coposed of a substrate and two electrodes with opposite polarity on the sae plane. Since this sensor shape increases the adjacent area between the electrodes in the unit area of parallel plates, the capacitance of the dielectroetry sensor is increased [, 7]. The electrodes were fabricated by photolithographic etching of copper fil. The substrate was ade of polyiide fil for high teperature use. The thicknesses of the polyiide base and the copper fil were equally 5 µ. As shown in Fig., the sensor area, the width of the electrode and the distance between the electrodes were 25 9, µ and µ, respectively. The electric field governed by Poisson s equation as depicted in Equation (5), was analyzed to obtain the capacitance of the dielectroetry sensor. ( ε ) = ρ (5) where ε, and ρ represent the dielectric constant, electric potential and charge density, respectively. In this study, a coercial finite eleent analysis package was used for calculation of the sensor capacitance. Due to the syetry and repetition of electrodes, the sensor was odeled as shown in Fig. 7. The value of charge Q was calculated by ultiplying volues of eleents by their charge densities obtained fro the finite eleent analysis. Then the sensor capacitance was obtained fro the relation of capacitance C
(a) (b) Fig. 8 Analysis of electric field for the dielectroetric sensor (a) Electric field line (b) Charge density. = Q /. Fig. 8 shows the electric field line and the charge density. Fig. 9 shows the sensor capacitance w.r.t. the ratio of dielectric constant ε of the easurand to that of vacuu ε. When the easurand was air, the sensor capacitance was 25 pf. When the dielectric constant of easurand was ε, the sensor capacitance was 52 pf. In order to verify the accuracy of the calculated results, the capacitances of dielectroetry sensor were easured using the air (.ε ) and the silicon oil of known dielectric constant (3.5ε ) as the easurands. The sensor capacitances were 24 pf and 5 pf, respectively; whose errors were less than 4 %. 25 Capacitance (pf) 2 5 5 5 5 2 ε / ε Fig. 9 Capacitance of dielectroetric sensor w.r.t. the dielectric constant of easurand. MEASUREMENT Unidirectional glass fiber epoxy prepreg whose diensions were 3 3.2 was used to easure the dissipation factor during autoclave vacuu bag degassing olding process. The dielectroetry sensor and therocouple were inserted between the iddle plies of the specien. The specien was cured at 25 C under. MPa. The dissipation factor and teperature of the specien were easured using both the conventional and the newly
developed apparatuses. Fig. (a), (b) and (c) show the test results when the circuits of Fig. 3 (case a), Fig. 4 (case b) and Fig. 5 (case c) were used, respectively. The values of R 2, C 2 and frequency of sinusoidal wave for the stable set-up of circuit were MΩ, pf and khz, respectively. In Fig., the dissipation factors of case (a) and case (b) were.25 and.2 at their start points, respectively. Since case (b) had long lead wires that countervailed the float capacitance of the wire, it had a lower dissipation factor than (a). Case (c) shows noise reduction copared with cases (a) and (b) because a duy sensor was ipleented to the case (c), which reduced the effects by the environental teperature and electric field as well as the coon ode noise. While, in case of (b), in equation (4) was not eliinated because was nonexistent. Fro the experient, it was found that the new Lacotech dielectroetry apparatus successfully easured stable signals with less noise. Also it was found that the apparatus was easier to obtain the start and end points of cure reaction and to relate dissipation factors to the environental teperature and degree of cure. CONCLUSION In this study, the cure onitoring using during the cure of theroset atrix fiber coposites was investigated by the newly developed Lacotech dielectroetry.5 5.5 5.4.3.2 Teperature 2 9 6 Teperature ( C).4.3.2 Teperature 2 9 6 Teperature ( C). 3. 3 2 3 4 2 3 4 Tie (in) (a) Tie (in) (b).5 5.4.3.2. Teperature 2 3 4 2 9 6 3 Teperature ( C) Fig. s and teperatures easured by the circuit in (a) Fig. 3. (b) Fig. 4. (c) Fig. 5. Tie (in) (c)
apparatus eploying Wheatstone bridge circuit. Also new dielectroetry sensors incorporating duy sensor were used in order to iprove the accuracy of easureent by reducing the float capacitance, effects of environental teperature and electric field. Fro the test, it was found that the stable dissipation factor could be obtained by the newly developed syste, which could be used to control process variables such as teperature, pressure and anufacturing tie. REFERENCES. Ki, J.S. and Lee, D.G., On-Line Cure Monitoring and iscosity Measureent of Carbon Fiber Epoxy Coposite Materials, J. of Materials Processing Tech., 993, ol. 37. 2. Twobly, B. and Shepard, D.D., Siultaneous Dynaic-Mechanical Analysis and Dielectric Analysis of Polyers (DMA-DEA), Instruentation Sci. Tech., 994, ol. 22, No. 3. 3. Fournier, J., Willias, G., Dutch, C. and Aldridge, G.A., Changes in Molecular Dynaics during Bulk Polyerization of an Epoxide-Aine Syste As Studied by Dielectric Relaxation Spectroscopy, Macroolecules, 996, ol. 29, No. 22. 4. Ki, J.S. and Lee, D.G., Measureent of the Degree of Cure of Carbon Fiber Epoxy Coposite Materials, J. of Coposite Materials, 996, ol. 3, No. 3. 5. Ki, J.S. and Lee, D.G., Analysis of Dielectric Sensors for the Cure Monitoring of Resin Matrix Coposite Materials, Sensors and Actuators B, 996, ol. 3. 6. Stephan, F., Duteurtre, X. and Fit, A., In-Process Control of Epoxy Coposite by Microdielectroetric Analysis. Part II: On-Line Real-Tie Dielectric Measureents During a Copression Molding Process, Polyer Eng. and Sci., 998, ol. 38, No. 9. 7. Alig, I. and Jenninger, W., Curing Kinetics of Phase Separating Epoxy Therosets Studied by Dielectric and Calorietric Investigations: A siple Model for the Coplex Dielectric Perittivity, J. of Polyer Science: Part B : Polyer Phys, 998, ol. 36.