Chemical and Rheological Changes during Cure in Resin-Transfer

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Chemical and Rheological Changes

Downloaded by GEORGETOWN UNIV on August 16, 2015 | http://pubs.acs.org Publication Date: May 5, 1990 | doi: 10.1021/ba-1990-0227.ch014

during Cure in Resin-Transfer Molding In Situ Monitoring D. E . Kranbuehl , M . S. Hoff , T. C. Hamilton , W. T. Clark , and W. T. Freeman 1

1

1

1

2

Department of Chemistry, College of William and Mary, Williamsburg, VA 23185 Langley Research Center, National Aeronautics and Space Administration, Hampton, VA 23665 1

2

Frequency-dependent electromagnetic sensing (FDEMS) is a convenient and sensitive technique for monitoring in situ infiltration and cure in the tool during the resin-transfer molding (RTM) process. The magnitude of thefluidityand viscosity as a function of the time and temperature, the time to infiltration at various ply depths, the effects of aging and elapsed time before infiltration of the RTM process, and monitoring of the cure cycle are four important areas where FDEMS is shown to significantly help in determining the RTM process procedure.

l\

N O N D E S T R U C T I V E IN SITU F R E Q U E N C Y - D E P E N D E N T impedance sensing

technique was reported previously (1-10) for measuring cure-processing properties of both thermoset and thermoplastic resins. The technique uses the frequency dependence of the resin's impedance to measure molecular ionic and dipolar diffusion rates. These molecular parameters can be used continuously throughout the process cycle to monitor the time and temperature dependence of events such as reaction onset, maximum flow, viscosity, gel, the buildup in modulus, evolution of volatiles, and reaction completion (1-10). The measurements can be made in a research environment to evaluate resin processing properties and in the manufacturing tool

0065-2393/90/0227-0249$06.0070 © 1990 American Chemical Society

In Polymer Characterization; Craver, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1990.

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POLYMER CHARACTERIZATION


o n t h e p l a n t floor ( I , 9-11). I n this chapter w e discuss p r e l i m i n a r y results o n t h e use o f frequency-dependent electromagnetic sensing ( F D E M S ) to m o n i t o r a n d d e t e r m i n e t h e fabrication process i n resin-transfer m o l d i n g (RTM). Resin-transfer m o l d i n g o f a d v a n c e d fiber-architecture materials p r o m ises to b e a cost-effective process for o b t a i n i n g composite parts w i t h exceptional strength. H o w e v e r , a large n u m b e r o f m a t e r i a l processing parameters m u s t b e o b s e r v e d , k n o w n , o r c o n t r o l l e d d u r i n g t h e resin-transfer m o l d i n g process. T h e s e parameters i n c l u d e t h e viscosity d u r i n g b o t h i m p r e g n a t i o n a n d c u r e . I n situ sensors that c a n observe these processing p r o p e r t i e s w i t h i n the R T M tool d u r i n g t h e fabrication process are essential. T h i s c h a p t e r w i l l discuss recent w o r k o n t h e u s e o f F D E M S techniques to m o n i t o r these p r o p e r t i e s i n t h e R T M tool. O u r objective is to use these sensing t e c h n i q u e s to address p r o b l e m s o f R T M scale-up for large c o m p l e x parts a n d to d e v e l o p a closed-loop, i n t e l l i g e n t , sensor-controlled R T M fabrication process.

Experimental Details Dynamic dielectric measurements were made with a Hewlett-Packard 4192A L F impedance analyzer controlled by a 9836 Hewlett-Packard computer. Measurements at frequencies from 50 to 5 X 10 Hz were taken at regular intervals during the cure cycle and converted to the complex permittivity, €* = e ' - te". Measurements were made with a geometry-independent Dek Dyne frequency-dependent electromagnetic sensor (Polymer Laboratories, Amherst, MA) (12) that was embedded in the resin. Dynamic mechanical measurements were made with a Rheometrics RDA-700 rheometer at 1.6 Hz and were used to compute the magnitude of the complex viscosity. All measurements were made on a Shell Epon diglycidyl ether of bisphenol A (DGEBA) resin-transfer molding resin, RSL 1282, with the aromatic amine curing agent, 9470. 6

Theory M e a s u r e m e n t s o f capacitance, C , a n d conductance, G , w e r e u s e d to calculate the c o m p l e x p e r m i t t i v i t y €* = e' - ie" w h e r e n

tt

material

^material

C is t h e g e o m e t r y - i n d e p e n d e n t capacitance, a n d / i s frequencies b e t w e e n 125 H z a n d 1 M H z . 0

/Q\

frequency,


at n i n e

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KRANBUEHL E T AL.

Chemical and Rheological Changes during Cure

251

T h i s calculation is possible w h e n u s i n g the D e k D y n e F D E M S p r o b e whose g e o m e t r y - i n d e p e n d e n t

capacitance, C , is invariant o v e r a l l meas0

u r e m e n t conditions. B o t h the real a n d the i m a g i n a r y parts of e* have an i o n i c a n d a d i p o l a r c o m p o n e n t . T h e d i p o l a r c o m p o n e n t arises from diffusion of b o u n d charge or m o l e c u l a r d i p o l e m o m e n t s . T h e d i p o l a r t e r m is generally the major c o m p o n e n t of the d i e l e c t r i c signal at h i g h frequencies a n d i n h i g h l y viscous m e d i a . T h e i o n i c c o m p o n e n t often dominates €* at l o w

frequencies,

l o w viscosities, a n d / o r h i g h e r temperatures. A n a l y s i s of the

frequency

d e p e n d e n c e of €* i n the h e r t z to m e g a h e r t z


range is, i n g e n e r a l , o p t i m u m for d e t e r m i n i n g b o t h the i o n i c m o b i l i t y - c o n d u c t i v i t y , a , a n d a m e a n d i p o l a r relaxation t i m e , T . T h e s e two p a rameters are d i r e c t l y r e l a t e d o n a m o l e c u l a r l e v e l to the rate of i o n i c translational diffusion a n d d i p o l a r rotational m o b i l i t y a n d t h e r e b y to changes i n the m o l e c u l a r structure of the r e s i n that reflect the reaction rate, changes i n viscosity, a n d the degree of cure.

Results and Discussion F i g u r e 1 is a plot of the l o g of the d i e l e c t r i c loss factor (e" X ay) of the R T M r e s i n scaled b y the frequency d u r i n g a m u l t i p l e r a m p - h o l d c u r e cycle. M e a s u r e m e n t s w e r e m a d e o v e r the of

X Co times

frequency

30

60

frequency

range of 125 H z to 1 M H z . Plots

are c o n v e n i e n t , as p r e v i o u s l y discussed (1, 2),

120

90

150

180

210

240

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300

TIME (minutes) Figure 1. Log (e" X (o) vs. time during multiple ramp-hold cure cycle 1 for the DGEBA epoxy resin. The frequencies were 125, 250, 500, 5 x 10 , 25 x 10 , 50 x 10 , 125 x 10 , 250 x 10 , 500 x 10 , and 1 x 10 Hz. 3

3

3

3

3

3

6


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POLYMER

CHARACTERIZATION


because overlapping lines indicate the frequencies and time and temperature periods during cure where e" X co is dominated by ionic diffusion. Nonoverlapping lines that exhibit a systematic series of peaks with frequency, time, and temperature can be used to determine a characteristic dipolar relaxation time. Figure 1 shows that the value of e" X w is dominated by ionic contributions throughout the first hold and at the low frequencies in the second hold. The low-frequency values of e" monitor the ionic mobility and thus reciprocally monitor changes in viscosity. Figure 1 that shows the resin goes through an ionic mobility maximum, that is, a viscosity minimum, at the beginning of the first hold and at the beginning of the second hold. Figure 2 displays the magnitude of the logarithmic complex viscosity t] (1.6 Hz) versus time and temperature for cure cycle 1. The times of occurrence of viscosity minima are in good agreement with the maxima in e" (ionic mobility). The relative change in log e" and log viscosity during the initial hold indicates that for a given temperature the ionic mobility is directly proportional to the reciprocal of the viscosity in this highly fluid region of cure. Dipolar relaxation times in the megahertz to kilohertz region are observed during the third hold. The occurrence of a peak in e" X w for a particular frequency (/) indicates the time during cure when the relaxation time T is equal to V i / . The occurrence of these relaxation times indicates the onset of the relaxation region that is associated with glass formation. Glass formation occurs as T becomes infinite. -200 180 160 140 HI

1 •»

h-

y « »

-J

18

36

* * * *

»

I

L_

54

72 90 108 126 144 162 180 TIME (MIN) Figure 2. Log complex viscosity, n, vs. time during multiple ramp-hold cure cycle 1.


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253

The approach of the values of e" to a constant value in the third and fourth holds indicates the time at which the reaction is approaching completion. This point is supported by the flatness of e" with time during the fourth hold. The ability of the sensors to monitor the effects of a change in the cure cycle is shown in Figures 3 and 4. Figure 3 displays the loss factor e" for a cure cycle in which the initial hold temperature is increased to 100 °C and the second hold to 135 °C. Comparison of Figures 1 and 3 shows that higher temperature holds significantly increase the magnitude of the first e" maxDownloaded by GEORGETOWN UNIV on August 16, 2015 | http://pubs.acs.org Publication Date: May 5, 1990 | doi: 10.1021/ba-1990-0227.ch014

imum, a result indicating a decrease in the viscosity. The position of the second viscosity minimum, (e" maximum) occurs 10 min earlier. In addition, the reaction advances more quickly as the dipolar a relaxation peaks begin to occur during the second hold, an event indicating that the approach to glass transition temperature (T ) occurs 1 h earlier. The reaction is more g

advanced and is setting up much more quickly. The time of occurrence of the first and second viscosity minima as shown in Figure 4 are in good agreement with the time of occurrence in the maxima in c" (ionic mobility). This finding is supported by the rapid buildup in viscosity occurring at 100 min as opposed to 160 min for cure cycle 1. The e" values show that the cure is essentially complete in the third hold. This result suggests that if a high T is desired, the third hold could g

be eliminated and one could proceed directly to the 175 °C hold. Next we examine the feasibility of using F D E M S techniques to measure

30

60

se

120

150

180

210

240

270

300

TIME (minutes) Figure 3. Log (e" X o)) vs. time during multiple ramp-hold cure cycle 2.


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POLYMER

4

CHARACTERIZATION

-

-?0B

3.4

-

-180

2.8

-

160 140

U

0)

X *

Q_ w

(E

120 100

i

1-


5

80

o

60

-L.2 40 -.8 -1.4

20 •

)

18 1

5

4

^IME^MtN ) 1

0

8

1

2

6

H

4

1

6

2

1

Figure 4. Log complex viscosity, r\, vs. time during a multiple ramp-hold cycle 2.

8

0

cure

and monitor the effects of individual component age on resin processing properties, the effect of layup time on resin processing properties, and on the ability of the sensor to monitor resin infiltration into the fiber architecture. Figure 5 is a plot of e" versus time and temperature during cure. The resin was mixed and held at room temperature for 1, 8, and 24 h before being cured. The sensor measurement output suggests that after 8 h the mixed resin begins to show signs of advancement. The value of e", which is reflecting ionic mobility and resin fluidity, is lower throughout the cure cycle following the onset of the first hold at 30 min. After 24 h, the mixed resin shows significantly less fluidity, that is, a higher viscosity. O n the basis of the F D E M S and viscosity measurements, the viscosity has increased over a factor of 3 in the high-flow 20- to 120-min portion of the cure cycle. Finally in the last and most important experiment, we demonstrate the ability of F D E M S to monitor both the impregnation and cure process including these processing properties continuously and in situ at various positions in R T M mold during fabrication. T h e R T M process involved placement of the R T M resin in the bottom of a mold (see Figure 6). A n eight-ply 8-Harness satin graphite cloth layup was placed over the resin. Sensors were placed at the second, fourth, sixth, and top plies. After 4 min the mold was closed in a press, and cure cycle 1 was used to impregnate and cure the resin-cloth layup.



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F i g u r e s 7 a n d 8 ( F i g u r e 8 is an enlargement of a p o r t i o n of F i g u r e 7) show that the w e i g h t of the top plate a n d the application of a v a c u u m to the c l o s e d m o l d w e r e sufficient to partially consolidate the c l o t h layers a n d cause infiltration of the r e s i n t h r o u g h the second p l y . N o f u r t h e r infiltration was o b s e r v e d u n t i l the t e m p e r a t u r e c y c l e was b e g u n . It took approximately 4 m i n for the r e s i n to flow from the second p l y to the top p l y . T h e differences i n the viscosity b e t w e e n the second a n d top p l i e s , as i n d i c a t e d b y the m a g n i t u d e of e" (125 H z ) , o c c u r o n l y i n the i n i t i a l h o l d a n d are b e l i e v e d to reflect s m a l l variations i n t e m p e r a t u r e d u e to the i m p r e g n a t i o n process. A pressure of 100 p s i was a p p l i e d at 57 m i n . F o r this r e s i n - c l o t h l a y u p , the F D E M S o u t p u t shows that the cure was u n i f o r m at a l l p l y positions throughout the r e m a i n d e r of the c u r e cycle. I n s u m m a r y , F D E M S sensing is a convenient a n d sensitive t e c h n i q u e for m o n i t o r i n g i n situ infiltration a n d cure i n the tool d u r i n g the resin-transfer m o l d i n g process. T h e m a g n i t u d e of the fluidity-viscosity as a f u n c t i o n of t i m e - t e m p e r a t u r e , the t i m e to infiltration at various p l y positions, the effects of v a r y i n g the t i m e - t e m p e r a t u r e cure cycle, the effects of aging a n d elapsed t i m e before infiltration o n the R T M process, as w e l l as m o n i t o r i n g the u n i formity of the i m p r e g n a t i o n c u r e process are i m p o r t a n t areas w h e r e the F D E M S sensing t e c h n i q u e can make a significant c o n t r i b u t i o n to the d e v e l o p m e n t of an R T M process.



Dielectric S e n s o r 1

Dielectric Sensor 2

Dielectric Sensor 3

Dielectric Sensor 4

Tool

Flow

*\

IIIH—Resin

Figure 6. A diagram of the RTM tool

! 11! .t!.!! It!! !I!T &

Pressure Plate

3 9 7

Kapton

nonporous Teflon

N - 1 0 breather


N

o

w 2

O

H SB

o

*0


14.


KRANBUEHL ET AL.

0

60

120 180 Time (min)

240

257

300

Figure 7. Comparison of log (e" x

Chemical and Rheological Changes during Cure in Resin-Transfer

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