On-line cure monitoring and viscosity measurement of carbon fiber epoxy composite materials

Journal of Materials Processing Technology, 37 (1993) 405-416 Elsevier

405

On-line cure monitoring and viscosity measurement of carbon fiber epoxy composite materials J i n - S o o K i m a n d D a i Gil Lee

Department of Precision Engineering and Mechatronics, Korea Advanced Institute of Science and Technology, Kusung-dong, Yusung-ku, Taejon-shi, South Korea 305-701

Industrial S u m m a r y Cure monitoring is important for the quality control and improvement of the mechanical properties of thermosetting resin-matrix composite materials. Dielectrometry is the most promising technique among the techniques developed so far to monitor the cure of thermosetting resin-matrix composite materials. In this study, a new simple dielectric cure monitoring device which consists of a simple electric circuit, two comparators and a digital oscilloscope was developed to monitor the dielectric constants during the cure of the thermosetting resin-matrix composite materials. Using the device developed, the size- and shape-effects of the electrodes, and also the sensitivity in measuring the dielectric constants of parallel and series type electrodes, were investigated experimentally. Also, an insitu viscosity measuring device was developed to match the measured dielectric constants to the viscosity of the thermosetting resin of the composite material. The combined devices could monitor the temperature, cure rate and resin viscosity simultaneously during the cure of carbon fiber epoxy composite materials, from which the relationship between the cure rate and viscosity could be derived. It was revealed that the data obtained by this method could be used in the smart autoclave cure of the thermosetting resin-matrix composite materials.

1. Introduction T h e c u r i n g p r o c e s s of t h e r m o s e t t i n g r e s i n - m a t r i x c o m p o s i t e m a t e r i a l s involves simultaneous heat, mass and momentum transfer along with chemical reaction in a multiphase system with time-dependent material properties and b o u n d a r y c o n d i t i o n s [1 4]. T h e a m b i e n t p r e s s u r e a n d t e m p e r a t u r e d u r i n g t h e

Correspondence to: D.G. Lee, Department of Precision Engineering and Mechatronics, Korea Advanced Institute of Science and Technology, Kusung-dong, Yusung-ku, Taejon-shi, South Korea 305-701. 0924-0136/93/$06.00 ~ 1993 Elsevier Science Publishers B.V. All rights reserved.

406

J.-S. Kim and D.G. Lee~On-line cure monitoring

cure process of the composite materials must be controlled closely to yield a satisfactory product in the shortest time and on-line cure monitoring is important in order to be able to apply the appropriate pressure and temper~ ature at the right time. There are several cure-monitoring methods, such as differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA), infrared spectroscopy (IRS), optical techniques and dielectrometry [5--8]. From these methods, dielectrometry is considered as the most promising technique to monitor the cure during a production molding operation because it can monitor continuously the cure chemistry of the resin t h r o u g h o u t the process of going from monomeric liquid of varying viscosity to a cross-linked, insoluble, high-temperature solid. In the dielectrometry measurement, dielectric constants, such as the electric loss factor of the resin, are measured as a function of cure time. The i n s t r u m e n t a t i o n includes two electrodes which are embedded in the composite materials and are connected to an alternating electric field. Since the thermosetting resins in the composite materials are dielectric materials, the combination of these electrodes and the composites forms an electric capacitor. The charge accumulated in this capacitor depends on the ability of the dipoles and ions present in the polymer molecules to follow the alternating electric field at different stages of curing. The loss factor of a resin represents the energy expended in aligning its dipoles and moving its ions in accordance with the direction of the alternating field. Much research has been performed on dielectric measurements. Yalof demonstrated that the dielectric characteristics of polymers are related closely to the mechanical properties [9]. Hudson measured the dissipation factor which was independent of the electrode shape and found the gel point of resins by measuring the resin viscosity by a viscometer. Also he applied pressure before and after the gel point to obtain the best mechanical properties of the composite materials [10]. Bromberg applied dielectrometry to polyurethane foaming and pultrusion forming [11]. Day reduced multifrequency dielectric data into a single viscosity-related cure curve and used dielectrometry to control an autoclave under closed-loop control [12]. He also correlated the dielectric cure index to the degree of cure for 3501-6 graphite epoxy composite material [13]. The relationship between the dielectric property and the viscosity or the resin is very complicated and depends on the formulation of the resin, B-stage curing, the delivery and the storage period of the prepregs [14]. Therefore, in this paper, a simple dielectric measurement method was developed using a simple electric circuit and two electronic comparators although there have been several commercialized dielectrometry techniques available such as Micromet [15] and Dekdyne [16]. Also, a device to measure the on-line resin viscosity of composite materials by measuring the torque to drive a pin in the composite was developed because the insitu measurement of the viscosity of the resin during cure was important [17].

J.-S. Kim and D.G. Lee/On-line cure monitoring

407

To measure the temperature of the resin, a thermocouple was inserted near the electrodes and the signal from the thermocouple was input to a personal computer through an A/D converter. The combined apparatus, which consists of a thermocouple, the dielectric measuring device and the viscosity measuring device, could monitor the temperature, cure rate and resin viscosity simultaneously during the cure of the carbon fiber epoxy composite materials, from which the relationship between the cure rate and viscosity of the composite materials could be derived. It was revealed that the data obtained by this method could be used in the smart autoclave curing of composite materials. 2. E l e c t r i c c i r c u i t for t h e dielectric m e a s u r e m e n t To measure the dielectric properties of the composite material, the equivalent circuits, i.e. series or parallel arrangements of a resistor and a capacitor which at a single frequency have the same electrical characteristics as the capacitor formed by the electrodes and the dielectric material in question, must be constructed [18,19]. Since the order of the maximum dissipation factor during the cure of the epoxy resin matrix composite is around 1.0 [10] and the series and parallel circuits give the same order of the dissipation factor, the parallel electric circuit was employed in this work. Figure 1 shows the circuit for the measurement of the dissipation factor of the composite material. It consists of the equivalent resistor R m and capacitor Cm of the composite, and an additional resistor R1 and two comparators. The comparator produces a square wave of + 5 and 0 V according to the plus and minus voltages of the input alternating signal [20]. Applying the alternating voltage V with angular frequency (~ to this circuit, the phase difference ~b between V and Vm was measured by a digital oscilloscope and transferred to the personal computer (IBM PC AT).

Rm

Cm Vm

)

Fig. 1. Electric circuit for measuring the dissipation factor of the composite material. Rm and Cm are the equivalent resistance and capacitance of the composite material, respectively.

J.-S. Kim and D.G. Lee~On-line cure monitoring

408

The voltage V., across the composite material in F i g . I is expressed a s follows:

Vm={Rm(RI + Rm--ju)C.~RmR1)/[(RI + Rm)2 +((,)CmRmRI)2]I V

It~

i Vra/V[ =Rm/[(R 1 + R m ) 2 +(o,)CmRmR 1)2] 1:2

{:;~)

where j is the imaginary number (j2 = - 1 ) , From eqn. (1), the phase difference ~b is calculated as follows:

tan¢=-(oCmRmR 1/(R~ +Rm)

~3}

Using eqns. (2) and (3), Rm and Cm can be calculated as tbllows: Rm = {I Vm/Vl/[c°s 0--IVm/Vl] ~,R~ Cm

=

~4',.

sin d~/ogRllVm/VI

-

~5)

The dissipation factor D of the parallel circuit is expressed by the following equation [18]:

D= I/tgCmR.~

~6)

Therefore, the dissipation factor of the composite material is expressed from eqn. (6) using eqns. (4) and (5) as follows: D = [I

Vm/VI-cos qS]/sin q5

7~

Since the dielectric measurement circuit consists of two comparators a n d a digital oscilloscope, an error might be induced in measuring the phase difference of signals owing to the time lag or lead in passing through the

10 9

8

7 6

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2 [] t

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- - q - -

20

. . . . . .

•

25

-

-

-

T. . . . . . . . . . . . 30

35

Rm (Mn) Fig. 2. Errors in measuring the resistance of the e q u i v a l e n t circuit of the composite materials w h e n the frequency of the voltage is 1.0 kHz and R~ is 1.0 Mt~.

J.-S. Kim and D.G. Lee~On-line cure monitoring

409

10

9 8

7 6

5 4 D

3

• D D D

2

D

1

D D

[]

[]

0

i

5O

100

I

[7

150

O

D I

O

200

I

I

I

I

I

250

300

350

400

450

Cm

500

(pF)

Fig. 3. Errors in measuring the capacitance of the equivalent circuit of the composite materials when the frequency of the voltage is 1.0 kHz and R1 is 1.0 M~.

10 9 8

~

5

3 E3D 2

[]

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[]

[3 1

o

[]

o%

[]

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0 0

i

i

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i

0.5

1

1.5

2

2.5

Dissipation factor (D) Fig. 4. Errors in measuring the dissipation factor D of the equivalent circuit of the composite materials when the frequency of the voltage is 1.0 kHz and R1 is 1.0 Mfl. electronic components. In order to measure the accuracy of the circuit, circuits constructed by k n o w n capacitors and resistors were tested. In this test, Rm in eqn. (4) and Cm in eqn. (5) were calculated by measuring the phase difference ¢ and JVm/VI. Figures 2 and 3 show the measuring errors of Rm and Cm, respectively at 1000 Hz w h e n Rl is 1 MfL From these figures, the errors in m e a s u r i n g Rm and Cm were determined to be less than 5% in wide ranges. The measured dissipation factor D was calculated by eqn. (7) using measured

410

J.-S. Kim and D.G. Lee~On-line cure monitoring

v a l u e s IVm/VI a n d ¢ a n d c o m p a r e d to the v a l u e s c a l c u l a t e d f r o m eqn. (6) u s i n g k n o w n v a l u e s of Rm a n d Cm. F i g u r e 4 shows the e r r o r of d i s s i p a t i o n f a c t o r b e t w e e n the m e a s u r e d and the c a l c u l a t e d . Since the e r r o r in m e a s u r i n g the d i s s i p a t i o n f a c t o r in wide r a n g e was less t h a n 3%, this circuit was e m p l o y e d to m e a s u r e the d i s s i p a t i o n f a c t o r of the c a r b o n fiber e p o x y c o m p o s i t e m a t e r i a l s .

3. M e a s u r e m e n t of c o m p o s i t e resin viscosity T h e on-line v i s c o s i t y m e a s u r e m e n t of p o l y m e r i c c o m p o s i t e m a t e r i a l s d u r i n g c u r e is not e a s y b e c a u s e the resin v i s c o s i t y is d e p e n d e n t on the t e m p e r a t u r e and the d e g r e e of c u r e of the c o m p o s i t e m a t e r i a l s , w h i c h is a f u n c t i o n of the B-stage cure, the delivery and the s t o r a g e period of the prepeg. T h e v i s c o s i t y m u s t be t r a c e d d u r i n g c u r e to c o n t r o l the a u t o c l a v e b e c a u s e it affects the c o n s o l i d a t i o n of the c o m p o s i t e m a t e r i a l s . Since on-line m o n i t o r i n g of the v i s c o s i t y of the resin in the c o m p o s i t e is not feasible, it is u s u a l l y m e a s u r e d u s i n g a small s a m p l e t a k e n from the specimen. This m e t h o d c a n n o t s i m u l a t e the v i s c o s i t y d u r i n g the a c t u a l c u r e of the c o m p o s i t e b e c a u s e the c u r e environm e n t of the s a m p l e is not the s a m e as the real one. A n o t h e r m e t h o d to m e a s u r e the v i s c o s i t y is to use p u r e resin o b t a i n e d f r o m the p r e p e g m a n u f a c t u r e r or e x t r a c t e d from the prepeg. I n s t e a d , in this work, an insitu viscosity m e a s u r i n g t e c h n i q u e , w h i c h consists of the small d i a m e t e r t u b e with a pin located c o n c e n t r i c a l l y inside of the t u b e and a dc m o t o r to r o t a t e the pin, was developed. T h e s u r f a c e of the t u b e h a s m a n y small holes for the resin p e n e t r a t i o n . T h e t u b e was e m b e d d e d in the p r e p e g and the n e c e s s a r y t o r q u e of a d c m o t o r to drive in the pin was m e a s u r e d . The p o w e r o u t p u t Pm of the m o t o r is e x p r e s s e d by the following e q u a t i o n i2l J:

Pm= E L - I: R~,

(s~

w h e r e Va, I~ and R~ are the voltage, c u r r e n t and r e s i s t a n c e of the motor. respectively.

Ovell Vm/V Computer Thermo-

couple

(AT)

er

supply

Fig. 5. Schematic diagram of the device tbr the on-line monitoring of the temperature, the dielectric properties and the viscosity of the composite materials.

J.-S. K i m and D.G. Lee~On-line cure monitoring

411

When an inner cylinder rotates steadily in an outer concentric fixed cylinder the gap of which is filled with a Newtonian fluid, the necessary power P, to maintain the motion is expressed by the following equation [22]: P, = 4rctt r2o r[ h~ (r2o - r[ )(o,2.

(9)

where ro and r~ are the outer and inner radii of the concentric gap, it is the viscosity of the resin, and h is the el~.,bedded length of the pin. If the resin of the composite material is assumed to be a Newtonian fluid, equating Pm in eqn. (8) and P, in eqn. (9), yields the resin viscosity p by the following equation: tl = ( V~ Ia -- 12 Ra) (r2o -- ri2 ) / 4 ~ h r2o r 2 (o2

(10)

In order to calculate the viscosity using eqn. (10), the rotational speed t~m of the dc motor was measured with an encoder and was input to a personal computer (IBM PC AT) through an A/D converter. Figure 5 shows the schematic diagram of the device for on-line monitoring of the temperature, dielectric properties and viscosity of the composite material.

4. Experiments To measure the temperature, the dielectric property and the viscosity of the carbon fiber epoxy composite material, a small press where the pressure was adjusted by four springs was developed, as shown in Fig. 6. The outer and inner diameters of the steel tube for the viscosity measurement were 1.3 and 1.2 mm, respectively. The diameter of the pin inside of the tube was 1.0 mm. The press and the measuring equipments, except for the motor, were surrounded by an oven with a temperature controller. The carbon fiber epoxy composite materials used was SKI-USN 150, manufactured by the Sun Kyung Industry of Korea and having the same properties as T300/5208 of Amoco. The prepeg thickness was 0.15 mm. Since the carbon fiber is a conductive material, a peel ply was inserted between the composite material and the thin copper electrode. The thicknesses of the electrode and the peel ply were both 0.05 ram. In order to estimate the fringing effect of the electric flux of the electrode, three different size electrodes arranged in parallel were tested. Figure 7 shows the shape of the parallel type electrode, the location of the electrode, the peel ply and the composite materials. Figure 8 shows the dissipation factors of the three different electrodes when the thickness of the composite is 1.5 mm (10 ply thickness). From Fig. 8, it can be shown that the dissipation factor varies little when the thickness of the composite is small compared to the electrode size. Therefore, it may be concluded that the fringing effect of the electric flux can be neglected when the thickness of the composite material is small compared to the size of the electrode. However, the parallel type electrode is difficult to be implemented when the shape of the composite structure is complicated or

J.-S. Kim and D.G. Lee~On-line cure monitoring

412

y,/////////////////_//////, Oven- - Composite

i //////

Thermocouple

/////////////////~dT_////// Fig. 6. Device to m e a s u r e t h e t e m p e r a t u r e , the dielectric p r o p e r t i e s a n d t h e viscosity ~,f t h e composite material.

Copper Electrode

t_ . . . . . . . . . .

3f

Electrode (thiekness=O.O5mm) Peel Ply (thickness=O.OSmm)

~

10 Ply C a r b o n / E p o x y

~

.......... i

Prepreg

(thickness = 1 . 5 0 m m ) i

Fig. 7. S h a p e of t h e parallel-type electrodes and t h e i r h)cations in t h e prepegs.

160 2.5

Dissipation factor~ Temperature

0~

X

///

_

......... /1

/~

/

120

t

e

f2X2

Cm

4x4 Cm

0

i

I

z'o

40

--

i

- T - -

i

6o

8o

1oo

~

--

0

I~o

Time (rain) Fig. 8. V a r i a t i o n of t h e d i s s i p a t i o n factors m e a s u r e d by t h e parallel-type electrodes w.r_t:, the electrode areas.

J.-S. Kim and D.G. Lee~On-line cure monitoring

413

Irnm 40 Ply Carbon/Epoxy Prepreg

(Area

=

5 x 5

cm 2)

Sensor Lead Line-

(a) Fig. 9. Shape of the series-type electrodes, their sizes and their locations in the composite materials: (a) location of the electrodes; (b) 2 mm x 2 mm electrode; (c) 2 mm x 4 mm electrode; (d) 2 mm x 6 mm electrode.

200

1,0

0.9-

Dissipation f

0.8-

i £ o:

a

c

t

o

r

~

ea

- 1 5 0

0.7-

o

130

0.6 0.5-

a0

0.4-

.iczl

i E

0.3-

40

020.1 0

20

i

i

i

30

40

50

i

i

i

60 70 80 Time (min)

i

i

i

90 100 110

Fig. 10. Variation of the dissipation factors measured by the series-type electrodes w.r.t. electrode areas. t h i c k . S i n c e t h e s m a l l s e r i e s t y p e e l e c t r o d e is e a s i e r to be i m p l e m e n t e d and g i v e s l e s s d a m a g e to t h e c o m p o s i t e s t r u c t u r e w h e n it is l o c a t e d i n s i d e of t h e c o m p o s i t e m a t e r i a l , s e v e r a l s m a l l s e r i e s t y p e e l e c t r o d e s w e r e tested. In t h e t e s t of t h e s e r i e s t y p e e l e c t r o d e s , t h e d i e l e c t r i c p r o p e r t i e s of t h i c k c o m p o s i t e m a t e r i a l s c o m p o s e d o f 40 ply p r e p e g s w e r e t e s t e d . F i g u r e 9 s h o w s t h e s h a p e of t h e e l e c t r o d e and its l o c a t i o n in t h e c o m p o s i t e m a t e r i a l . F i g u r e 10 s h o w s t h e v a r i a t i o n of t h e d i s s i p a t i o n f a c t o r w.r.t, t h e a r e a o f t h e s e r i e s t y p e e l e c t r o d e . In Fig. 10, t h e d i s s i p a t i o n f a c t o r i n c r e a s e s as t h e e l e c t r o d e a r e a i n c r e a s e s w h e n t h e e l e c t r o d e gap is c o n s t a n t . F r o m t h i s r e s u l t , t h e e l e c t r o d e s i z e and s h a p e in

414

J.-S. K i m a n d D.G. Lee/On-line cure monitoring

0.8

1E5

140

i

0.7

120

g

Temperature

loo

o5 0

%

0.4 0.3

a 1o 20

O.l [

0 |5

30

45

Time

60

75

90

t05

(min)

Fig. 11. Curves of the viscosity, the temperature, and the dissipation factor of the (arb(m fiber epoxy composite materials during cure.

100

tt~ .~

9O

emporo
8O

t

)4~ 130

IE4

120

7O

r

cur

g~ _1oo

4O

t.

3O

100

7o g0

20 10

,

15

30

~

45

~

60

IO

i

T

75

90

40

i05

Time (min)

Fig. 12. Curves of' the cure index, the viscosity, and the temperature of the carbon hber epoxy composite materials during cure.

the series type m e a s u r e m e n t must be fixed to o b t a i n the same absolute vatues of the dielectric properties in every experiment. F i g u r e 11 shows the curves of the dissipation factors, t e m p e r a t u r e and viscosity, m e a s u r e d by the series ~ype electrodes when the area of the electrode was 2 m m × 6 ram. A l t h o u g h the dissipation f a c t o r c h a n g e s with the cure, due to the addit, ional t e m p e r a t u r e d e p e n d e n c e of the c o n d u c t i v i t y of the resin in the composite material, it does not relate directly to degree of cure. D a y [12] suggested the use of a cure index defined by the following e q u a t i o n to remove the t e m p e r a t u r e dependence: Cure i n d e x = ( L C ( T ) - L C ( u n c u r e d ) / [ L C ( 1 0 0 oVo c u r e d ) - L C ( u n c u r e d ) ] ~ x 100°/(, (11)

J.-S. Kim and D.G. Lee~On-line cure monitoring

415

w h e r e LC ( T ) r e p r e s e n t s the l o g a r i t h m i c c o n d u c t i v i t y at the g i v e n t e m p e r a t u r e T. H e p r o v e d t h a t the c u r e index was r e l a t e d to the degree of c u r e of the resin matrix. F i g u r e 12 shows t h a t the c u r e index o b t a i n e d by eqn. (11) i n c r e a s e s monot o n i c a l l y with the cure rate. The r e l a t i o n s h i p b e t w e e n the viscosity, tempera t u r e and c u r e index in Fig. 12 c a n be used in the closed-loop control of t h e r m o s e t t i n g r e s i n - m a t r i x c o m p o s i t e m a t e r i a l s , w h i c h is the m o s t i m p o r t a n t d a t a b a s e for the s m a r t a u t o c l a v e curing of t h e r m o s e t t i n g r e s i n - m a t r i x composite m a t e r i a l s .

5. Conclusions In this study, a new insitu m e a s u r i n g device of the dielectric p r o p e r t i e s and the v i s c o s i t y of t h e r m o s e t t i n g r e s i n - m a t r i x c o m p o s i t e m a t e r i a l s was developed and tested d u r i n g the c u r e of c a r b o n fiber epoxy c o m p o s i t e m a t e r i a l s . F r o m the r e s u l t s of the e x p e r i m e n t a l i n v e s t i g a t i o n s , the following c o n c l u s i o n s were drawn: (1) The a p p a r a t u s developed can m e a s u r e s i m u l t a n e o u s l y the dielectric properties, the t e m p e r a t u r e and the viscosity of the e p o x y resin d u r i n g the c u r e of c a r b o n fiber e p o x y c o m p o s i t e m a t e r i a l s . (2) The s e n s i t i v i t y of the parallel type electrodes in m e a s u r i n g the dielectric p r o p e r t i e s was n o t d e p e n d e n t on the electrode area, but t h a t of the series type e l e c t r o d e was d e p e n d e n t on the electrode area, w h i c h r e q u i r e d the use of fixed type electrodes to o b t a i n the a b s o l u t e dielectric p r o p e r t i e s of the resin m a t r i x in the c o m p o s i t e m a t e r i a l s . (3) T h e m e a s u r e d r e l a t i o n s h i p b e t w e e n the cure index, the t e m p e r a t u r e and the v i s c o s i t y could be used to o p e r a t e an a u t o c l a v e in closed-loop control, w h i c h is the m o s t i m p o r t a n t b a s e for the s m a r t m a n u f a c t u r i n g of c o m p o s i t e materials.

Acknowledgements The a u t h o r s would like to t h a n k K O S E F for financial s u p p o r t of this research.

References [1] L. Chiao and R.E. Lyon, A fundamental approach to resin cure kinetics, J. Compos. Mater., 24 (1990) 739 752. [2] C.Y.M. Tung and P.J. Dynes, Relationship between viscoelastic properties and gelation in thermosetting systems, J. Appl. Polym. Sci., 27 (1982) 569 574. [3] J.B. Enns and J.K. Gillham, Time-temperature-transformation (TTT) cure diagram: Modelling the cure behavior of thermosets, J. Appl. Polym. Sci., 28 (1983) 2567 2691. [4] M.R. Dusi, W.I. Lee, P.R. Ciriscioli and G.S. Springer, Cure kinetics and viscosity of fiberite 976 resin, J. Compos. Mater., 21 (1987) 243 261.

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J.-S. Kim and D.G. Lee~On-line cure monitoring

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