Polymer Characterization - American Chemical Society

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Resin Cure C. Y - C . LEE and I. J. G O L D F A R B Wright-Patterson Air Force Base, Air Force Wright Aeronautical Laboratories/MLBP, Dayton, O H 45433

Torsion impregnated cloth analysis (TICA) is a forced torsion technique used to study resin mechanical behavior while the resin is being supported by glass cloth. A constant frequency or a multifrequency scan can be employed. The transition peak temperatures of the neat materials and the TICA specimens agree reasonably well. The isothermal curing TICA results of an epoxy are also compared with neat resin results. The gel peak observed in TICA measurements is concluded to be the result of resin—substrate interaction. The TICA technique has been able to distinguish between loss peaks due to molecular transitions, and those from kinetic effects based on their frequency dependency. Isocure state curves have been constructed using a two-step curing method, and a calibration method to interpolate partially cured Tg has been demonstrated with the technique.

V

AST DIFFERENCES I N STRESS LEVELS b e t w e e n a l i q u i d p o l y m e r and a

s o l i d p o l y m e r necessitate separate e x p e r i m e n t a l t e c h n i q u e s for t h e i r m e c h a n i c a l c h a r a c t e r i z a t i o n (I). T o characterize f u l l y a p o l y m e r t h r o u g h its v a r i o u s states o f m a t t e r , at l e a s t t w o d i f f e r e n t e x p e r i m e n t s h a v e to b e p e r f o r m e d . T h e d a t a c a n t h e n b e r e c o n s t r u c t e d to c o v e r t h e entire r e g i o n of interest. S i m i l a r p r o b l e m s are e n c o u n t e r e d i n s t u d y i n g c u r e r h e o l o g y o f t h e r m o s e t t i n g s y s t e m s . A l t h o u g h i t is r a t h e r s t r a i g h t f o r w a r d to s t u d y t h e c u r i n g i n t h e l i q u i d s t a t e , i t i s a l m o s t i m p o s s i b l e to f o l l o w the v i t r i f i c a t i o n process t h r o u g h c o n v e n t i o n a l techniques. 0065-2393/83/0203-0065$06.00/0 © 1983 A m e r i c a n C h e m i c a l Society

Craver; Polymer Characterization Advances in Chemistry; American Chemical Society: Washington, DC, 1983.

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POLYMER CHARACTERIZATION

M e c h a n i c a l properties of thermosetting resins d u r i n g cure t h r o u g h t h e v a r i o u s states o f m a t t e r h a v e b e e n s t u d i e d w i t h d i f f e r e n t t e c h n i q u e s (2, 3) t h a t a l l h a d o n e f e a t u r e i n c o m m o n : t h e r e s i n s t o b e s t u d i e d w e r e s u p p o r t e d b y i n e r t substrates. T o r s i o n i m p r e g n a t e d c l o t h a n a l y s i s ( T I C A ) i s a v a r i a t i o n o f s u c h t e c h n i q u e s (4). T h e r e s i n b e i n g s t u d i e d is i m p r e g n a t e d onto a glass c l o t h that is s u b s e q u e n t l y f o l d e d into the shape o f a rectangular bar. T h e s p e c i m e n is m o u n t e d o n t h e rheometrics m e c h a n i c a l spectrometer ( R M S ) a n d is s u b j e c t e d to a forced torsion analysis that measures d i r e c t l y t h e i n - p h a s e a n d o u t of-phase components o f the s p e c i m e n ' s response. T h e T I C A s a m p l e preparation is v e r y similar to that o f torsion b r a i d a n a l y s i s ( T B A ) (2). T h e s a m p l e a r r a n g e m e n t h a s t h e a d v a n t a g e of a l l o w i n g a continuing study of a resin w i t h the same s p e c i m e n t h r o u g h i t s v a r i o u s p h y s i c a l states: l i q u i d , r u b b e r , a n d g l a s s y . A l s o , this arrangement shares the advantage o f the r e l a t i v e l y s m a l l s a m p l e size r e q u i r e d , c o m p a r e d w i t h most neat-resin measurements. B e cause o f the presence o f the resin—substrate interaction, liquid-state results have to b e treated w i t h caution. O n l y the relative changes i n the r e s i n responses are o b t a i n e d b y this t e c h n i q u e . A b s o l u t e m o d u l u s values cannot b e extracted from the measurements without further d e v e l o p m e n t a l studies o f the technique. U n l i k e T B A , w h i c h is a free o s c i l l a t i o n t e c h n i q u e , T I C A u t i l i z e s the torsion feature o f the R M S . Instead o f r e l y i n g o n t h e frequency a n d the decay o f the o s c i l l a t i o n a m p l i t u d e to y i e l d the r i g i d i t y a n d loss tan δ i n f o r m a t i o n , T I C A measures d i r e c t l y t h e i n - p h a s e a n d out-ofphase c o m p o n e n t s o f the s p e c i m e n ' s response that c o r r e s p o n d to t h e storage a n d loss m o d u l u s . B e c a u s e o f the f o r c e d o s c i l l a t i o n m o d e , a constant frequency can b e u s e d throughout the entire T I C A e x p e r i m e n t , thus a v o i d i n g the c o m p l i c a t i o n of the c h a n g i n g f r e q u e n c y effect on the observed response. M o r e importantly, a multifrequency scan c a n b e e m p l o y e d so t h e f r e q u e n c y effect o f t h e r e s p o n s e c a n b e s t u d i e d as w e l l . T h i s chapter reviews t h e application o f the T I C A technique i n our laboratory to the p r o b l e m of studying the m e c h a n i c a l properties o f resins.

Experimental T h e sample preparation procedure had b e e n d e s c r i b e d i n d e t a i l else w h e r e (5). T h e r e s i n to b e s t u d i e d was d i s s o l v e d w i t h appropriate solvents and a fiber glass c l o t h about 10 c m w i d e was wetted w i t h the solution. After h a n g i n g i n an exhaust h o o d overnight, the c l o t h was evacuated for solvent ex traction at room temperature for 1 week before use. T h e c l o t h was cut into rectangular patches o f 10 X 7.5 c m . T h e s e w e r e folded into strips 10 c m l o n g and 1.25 c m w i d e , w i t h the cut edges f o l d e d i n s i d e the strips. T h r e e strips were stacked together to compose one T I C A s p e c i m e n .


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F i x t u r e s w e r e fabricated to h o l d the T I C A s p e c i m e n i n the sample chamber of the R M S . T h e fixtures were stainless steel plates m e a s u r i n g 17.8 X 12.6 x 2.3 m m . T h e cloth ends w e r e s a n d w i c h e d b y two fixture plates h e l d together b y a screw at the center. A l u m i n u m f o i l was u s e d to separate the fixture from the cloth so that the c u r e d r e s i n w o u l d b o n d to the f o i l instead of the fixtures. T h e r e s u l t i n g specimens h a d dimensions at both ends s i m i l a r to those of torsion bar specimens of the R M S , a n d w e r e m o u n t e d o n the sampleh o l d i n g chucks of the R M S the same way as the torsion bars. T h e specimens were h e l d at both ends b e t w e e n the servomotor a n d transducer of the R M S i n an e n v i r o n m e n t a l chamber. A s i n u s o i d a l strain was i n d u c e d b y the oscillatory m o t i o n of the servomotor. T h e resultant s i n u s o i d a l stress was detected w i t h the stress gauge, w h o s e output voltage was t a p p e d into a frequency analyzer. B y c o m p a r i n g stress a n d strain waveforms, the in-phase a n d out-of-phase re sponses of the s p e c i m e n were calculated. T h e e n v i r o n m e n t a l chamber was capable of a temperature range from - 1 5 0 to 400 °C.

Comparison with Thermoplastics Neat Properties (4) Polyphenylsulfone (Radel, U n i o n Carbide Corporation) had been c h a r a c t e r i z e d m e c h a n i c a l l y i n n e a t f o r m i n o u r l a b o r a t o r y (6) u s i n g a n R M S . T h e r e g i o n b e l o w a n d t h r o u g h T was characterized i n the tor sional bar mode, a n d above the T region was measured i n the parallel plates m o d e . T h e p o l y m e r was also characterized w i t h T I C A from - 150 t o 3 8 0 ° C . T h e T I C A r e s u l t i s s h o w n i n F i g u r e 1, a n d t h e p e a k m a x i m u m temperatures, together w i t h those from the neat r e s i n m e a s u r e m e n t s , a r e l i s t e d i n T a b l e I . T h e l o s s p e a k s at 1 7 0 ° C a r e d u e to k i n e t i c effects (see l a t e r d i s c u s s i o n s ) a n d a r e n o t i n c l u d e d i n t h e t a b l e . g

g

B e c a u s e of the p r e s e n c e of t h e glass c l o t h i n T I C A , a l l loss p e a k s d u e to m o l e c u l a r m o t i o n t r a n s i t i o n s w e r e s m a l l e r i n a m p l i t u d e , i n c o m p a r i s o n to t h e n e a t - f o r m r e s u l t s . T h e t e m p e r a t u r e a n d f r e q u e n c y d e p e n d e n c e of the t r a n s i t i o n a g r e e d w e l l i n the glassy a n d glass transition regions. L o w - f r e q u e n c y T I C A measurements gave very n o i s y l o s s c o m p o n e n t s , so t h e T I C A sub-T^ l o s s t r a n s i t i o n s i n t h e l o w frequency (T

B

β y

1 10 100 1 10 100 1 10 100 1 10 100

247 260 278 2 2 3 (228) 2 2 6 (231) 2 3 0 (237) 15 32 70 -120 -105 - 95

265 280 325 2 2 2 (228) 2 2 5 (232) 2 3 0 (237) —

44 58 -112 - 98

Note: A l l temperature values are in degrees Centigrade; all values refer to the loss modulus maxima except those in parentheses, which denote the values at the tan δ maxima. See Reference 6. a


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o f m a g n i t u d e as t h e r e s i n c h a n g e s f r o m a l i q u i d to a g l a s s . T h e r e a r e also t w o tan δ m a x i m a , f o l l o w e d b y a p r o m i n e n t b (out-of-phase) c o m p o n e n t p e a k ( F i g u r e 2). T h e t w o t a n δ p e a k s s h o u l d c o r r e s p o n d t o t h e gel a n d vitrification peaks c o m m o n l y

observed

i n s i m i l a r T B A ex

p e r i m e n t s . T h e b m a x i m u m i s a l s o a v i t r i f i c a t i o n p e a k , a n d , as e x p e c t e d , occurs later t h a n the c o r r e s p o n d i n g tan δ p e a k . T h i s result is j u s t t h e o p p o s i t e o f w h a t o n e n o r m a l l y o b s e r v e s at t h e g l a s s t r a n s i t i o n r e g i o n . I n the glass t r a n s i t i o n r e g i o n , the l o w - f r e q u e n c y r e s p o n s e

is

always l e a d i n g the trend of changes d u r i n g an increasing temperature s w e e p . F o r the i s o t h e r m a l c u r i n g , a g a i n the reverse is t r u e ; the h i g h f r e q u e n c y r e s p o n s e is l e a d i n g the t r e n d . Isothermal results from several c u r i n g temperatures can be

used

t o c o m p o s e a t i m e — t e m p e r a t u r e - t r a n s f o r m a t i o n ( T T T ) d i a g r a m (2). T B A results o n t h e same s y s t e m are a v a i l a b l e for c o m p a r i s o n . B o t h sets o f d a t a a r e s h o w n i n F i g u r e 3. T h e T I C A r e s u l t s a n d t h e T B A r e s u l t s a g r e e o n l y q u a l i t a t i v e l y , n o t q u a n t i t a t i v e l y . B o t h sets o f d a t a

I

I

I I

1 1

I

I

'

'

ι

» ι ι • » I

1,000

I

I

1,0000

TIME (SEC.) Figure 2. A typical TICA isothermal curing result of an epoxy resin. (Reproduced with permission from Ref. 4. Copyright 1981, Society of Plastics Engineers, Inc.)


L

70


ΙΟ

5

-

•

TBA

•

TBA

tgel

x TIC

tvit b

Δ

TIC

h

ο

TIC

tgel

X - b i m

-

-tan δ

ΙΟ

-Gel 2

—

I

«

120

140

160

180

200

220

T E M P E R A T U R E (°C)

Figure 3. Time—temperature—transformation TICA and TBA results. (Reproduced with Copyright 1981, Society of Plastics

diagram comparing permission from Ref. 4. Engineers, Inc.)

i n d i c a t e d t h a t at t h e e x p e r i m e n t a l t e m p e r a t u r e r a n g e , t h e s y s t e m w a s between T

gg

a n d T (o°) as d e f i n e d b y G i l l h a m (2). g

D a t a at t h e l o w e r t e m p e r a t u r e r e g i m e a g r e e d r e a s o n a b l y w e l l i n light of the difference b e t w e e n

the two experiments i n frequency

effect. I n t h e h i g h e r t e m p e r a t u r e r a n g e , t h e T I C A

times to peak

m a x i m a w e r e c o n s i s t e n t l y l o n g e r . T h e s e t i m e s are a t t r i b u t a b l e to t h e different heat-up procedures

employed b y the two techniques. I n

both measurements, the samples were m o u n t e d onto the instrument at r o o m t e m p e r a t u r e . T h e s a m p l e c h a m b e r t e m p e r a t u r e w a s t h e n r a i s e d to t h e e x p e r i m e n t a l v a l u e . F o r T B A , t h e t e m p e r a t u r e w a s r a i s e d at a r a t e o f a b o u t 1 2 ° C / m i n , a n d t i m e z e r o o f t h e e x p e r i m e n t w a s w h e n t h e final t e m p e r a t u r e w a s r e a c h e d . I n T I C A , t h e e x p e r i m e n t s s t a r t e d at 0 ° C , a n d t h e t e m p e r a t u r e w a s r a i s e d at a r a t e o f 1 2 0 ° C / m i n . T i m e zero was w h e n the temperature increase was initiated.


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S y s t e m a t i c errors are p r e s e n t i n b o t h p r o c e d u r e s , b u t t h e y are n e g l i g i b l e w h e n t h e r e a c t i o n t i m e i s l o n g i n c o m p a r i s o n to t h e h e a t - u p t i m e . W h e n t h e c o n d i t i o n is n o t s a t i s f i e d , as i n t h e h i g h t e m p e r a t u r e region, t h e n the T B A data w i l l be consistently l o w e r , a n d the T I C A data consistently higher, t h a n the real values. T h e isothermal cur i n g results show a l o n g time separation b e t w e e n the tan δ vitrification peak a n d the corresponding b m a x i m u m vitrification peak. B y isother m a l l y c u r i n g to t h e p e a k m a x i m u m at l o w t e m p e r a t u r e , t h e s u b s e q u e n t t h e r m a l scan w i l l y i e l d the c o r r e s p o n d i n g glass t r a n s i t i o n p e a k at t h e s a m e t e m p e r a t u r e i f t h e r e is n o a d d i t i o n a l c u r i n g d u r i n g the t e m p e r a t u r e scan. T h e s e results i n d i c a t e that the o b s e r v e d v i t r i f i c a t i o n p e a k s t r u l y c o r r e s p o n d to t h e g l a s s t r a n s i t i o n p e a k s . T h e l o n g t i m e s e p a r a t i o n b e t w e e n t h e t a n δ a n d b m a x i m a i s d u e to t h e q u e n c h i n g o f t h e r e a c t i o n r a t e at h i g h v i s c o s i t y l e v e l . F o l l o w i n g the c o n v e n t i o n u s e d for s i m i l a r T B A e x p e r i m e n t s , the f i r s t t a n δ p e a k i n T I C A i s o t h e r m a l e x p e r i m e n t s is r e f e r r e d to as a g e l p e a k . T h e p e a k is n o t r e l a t e d to t h e c o n v e n t i o n a l t h e o r y o f g e l a t i o n . T h e c o n v e n t i o n a l t h e o r y of g e l a t i o n is d e f i n e d b y a c r i t i c a l c o n v e r s i o n c o n d i t i o n a n d s h o u l d be i n d e p e n d e n t of the measurement tempera t u r e s . H o w e v e r , t h e T I C A g e l p e a k c a n b e s h i f t e d to h i g h e r t e m p e r a ture b y partial c u r i n g . A parallel plates study of the resin u n d e r i s o t h e r m a l c o n d i t i o n s f a i l e d to r e v e a l s i m i l a r p e a k s i n e i t h e r t h e G" o r the tan δ c o m p o n e n t . W e b e l i e v e that the g e l p e a k o b s e r v e d i n T I C A e x p e r i m e n t s is a c t u a l l y a r e s u l t o f t h e r e s i n - s u b s t r a t e i n t e r a c t i o n . Loss

Peaks

Due

to Kinetic

Effects

(7)

B e c a u s e of the m u l t i f r e q u e n c y scan c a p a b i l i t y of T I C A , loss peaks d u e to k i n e t i c effects c a n b e d i s t i n g u i s h e d f r o m t h o s e a r i s i n g f r o m molecular motion transitions. Because of transition energy barrier c o n s i d e r a t i o n , l o s s p e a k s d u e to m o l e c u l a r m o t i o n t r a n s i t i o n s s h o u l d have a frequency dependence, w i t h lower frequency peaks occurring at l o w e r t e m p e r a t u r e s . F o r t h e l o s s p e a k s d u e to k i n e t i c e f f e c t s t h a t a r e the results of a c h a n g e i n the glass t r a n s i t i o n t e m p e r a t u r e of the t e s t i n g s p e c i m e n d u r i n g the experiment, there w i l l be no frequency d e p e n d e n c y . S u c h effects h a v e b e e n o b s e r v e d i n t h e r m o p l a s t i c s , w h e r e t h e r e m o v a l of r e s i d u a l solvents d u r i n g the e x p e r i m e n t causes a change i n T , a n d also i n p a r t i a l l y c u r e d t h e r m o s e t t i n g systems w h e r e the a d d i t i o n a l cure raises the glass t r a n s i t i o n t e m p e r a t u r e . 9

F i g u r e 4 s h o w s a n e x a m p l e of a k i n e t i c effect loss peak. T h e p e a k m a x i m a t e m p e r a t u r e o f t h e p e a k s at 180 ° C a r e i n d e p e n d e n t o f f r e q u e n c y . A n o t h e r i m p o r t a n t c h a r a c t e r i s t i c o f k i n e t i c effect loss p e a k s is the reversal of frequency trend. A t the l o w e r temperature regime of the peak, the low-frequency response leads the t r e n d of changes i n the a, b , a n d t a n δ c o m p o n e n t s . A f t e r t h e p e a k m a x i m u m , t h e h i g h - f r e -


72


1

I

ι

100

200

ι

I

300

T E M P E R A T U R E °C

Figure 4. Loss peaks due to kinetic effects. Key: —, 100 radls; —, 10 radls; and , 1 radls. (Reproduced with permission from Ref. 4. Copyright 1981, Society of Plastics Engineers, Inc.) q u e n c y r e s p o n s e is l e a d i n g t h e t r e n d . O n r e s c a n n i n g , t h e k i n e t i c loss peaks should disappear. T h e results from such experiments can be interpreted b y

de

s c r i b i n g t h e e x p e r i m e n t as a f u n c t i o n o f t i m e . W i t h p r o p e r a s s u m p t i o n s , t h e r e s p o n s e s o f t h e s p e c i m e n c a n b e e x p r e s s e d as a f u n c t i o n o f a r e d u c e d parameter ( T - T ) . T h e changes i n the observed g

response

are t h e n a r e s u l t of the c h a n g e s i n the r e d u c e d p a r a m e t e r w h i c h is controlled b y the relative magnitudes of the thermal changes, a n d t h e k i n e t i c c h a n g e s , dTJdt. s i o n p o i n t , w h e n dT/dt

= dT ldt, g

dT/dt,

W i t h the i n t r o d u c t i o n o f a rate c o n v e r a n d a four-stage s c h e m e to d i s t i n

guish various conditions encountered d u r i n g the experiment, seem i n g l y complex experimental results can be understood qualitatively.


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Analysis

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lso-cure State Curves (8) T h e T T T d i a g r a m separates a t i m e - t e m p e r a t u r e c u r i n g p l o t into l i q u i d , rubbery, a n d glassy regions. T h e cure phase diagram concept is e x t e n d e d f u r t h e r b y t h e i n t r o d u c t i o n o f a n i s o - c u r e state c u r v e t h a t j o i n s a l l t h e p o i n t s t h a t r e p r e s e n t t h e s a m e c u r e state o n t h e c u r e p h a s e d i a g r a m (8). T h e s u p e r p o s i t i o n o f t h e c u r v e o n t h e T T T d i a g r a m w i l l a l l o w r e c o n s t r u c t i o n o f t h e t h e r m a l p r o p e r t i e s o f a p a r t i a l l y c u r e d state that m a y not b e o b t a i n a b l e b y d i r e c t t h e r m o s c a n e x p e r i m e n t s b e c a u s e of the complications of a d d i t i o n a l cure d u r i n g the m e a s u r e m e n t process. A n i s o - c u r e state c u r v e h a s b e e n c o n s t r u c t e d w i t h t h e T I C A t e c h nique utilizing a two-step c u r i n g procedure. A batch of T I C A specim e n s o f t h e s a m e p a r t i a l l y c u r e d state w e r e p r e p a r e d b y s u b j e c t i n g t h e m t o a n i d e n t i c a l c u r e h i s t o r y . T h e y w e r e t h e n p o s t - c u r e d at d i f f e r e n t t e m p e r a t u r e s t o m e a s u r e t h e t i m e r e q u i r e d to c u r e f u r t h e r to t h e state o f t h e b m a x i m u m . B y c o m p a r i n g t h e s e p o s t - c u r e r e s u l t s w i t h t h e isothermal c u r i n g results of v i r g i n specimens one can obtain the time r e q u i r e d t o c u r e t o t h e p a r t i a l l y c u r e d state o f i n t e r e s t at v a r i o u s t e m peratures. F i g u r e 5 i s t h e s u p e r p o s i t i o n o f a n i s o - c u r e state c u r v e a n d t h e T T T d i a g r a m o f a n e p o x y s y s t e m . T h e c u r e state r e p r e s e n t s a c u r e h i s t o r y o f 5 h at 1 3 5 ° C . A t h e r m o s c a n o f s u c h a s p e c i m e n y i e l d e d a k i n e t i c e f f e c t l o s s p e a k at a b o u t 168°, a n d a g l a s s t r a n s i t i o n at a b o u t 2 7 0 ° C , w h i c h i s t h e s a m e as t h e T o f t h e c o m p l e t e l y c u r e d s y s t e m . H o w e v e r , F i g u r e 5 i n d i c a t e s t h a t t h e c u r e state i n q u e s t i o n s h o u l d h a v e a T o f 158 ° C . I s o - c u r e state c u r v e s c a n b e a v e r y u s e f u l t o o l t o v i s u a l i z e t h e r m a l p r o p e r t y c h a n g e s as a f u n c t i o n o f c u r e . I f t h e T T T d i a g r a m i s a c c o m p a n i e d b y p h y s i c a l p r o p e r t y m e a s u r e m e n t s , e.g., v o l u m e a n d d i e l e c t r i c c o n s t a n t , a f a m i l y o f i s o - c u r e state c u r v e s , as a f u n c t i o n o f c u r e t i m e a n d t e m p e r a t u r e , w i l l a l l o w t h e e x t r a c t i o n o f i n f o r m a t i o n as to t h e manner i n w h i c h the p h y s i c a l properties change over a certain temp e r a t u r e r a n g e as t h e c u r e i s a d v a n c e d . O f c o u r s e , t h e c o n c e p t o f t h e i s o - c u r e state c u r v e o n t h e T T T d i a g r a m r e q u i r e s c e r t a i n a s s u m p t i o n s (8). T h e c h o i c e o f p a r a m e t e r s t o i d e n t i f y t h e c u r e state a n d t h e v a l i d i t y o f t h e c o n c e p t w i l l h a v e to b e c o n s i d e r e d f o r e a c h i n d i v i d u a l p o l y m e r i c s y s t e m , as w e l l as f o r e a c h i n d i v i d u a l p h y s i c a l p r o p e r t y o f i n terest. g

g

T Calibration Method (8) g

Because T I C A specimens with k n o w n T values can be prepared g

b y i s o t h e r m a l l y c u r i n g t h e s p e c i m e n at t h e t e m p e r a t u r e o f i n t e r e s t ,


74


TEMPERATURE

Figure 5. Superposition ature—transformation Ref. 8. Copyright

and

terminating the

of iso-cured state curve (—) on time—temperdiagram. (Reproduced with permission from 1981, Society of Plastics Engineers, Inc.)

cure

when

the

reached, p h y s i c a l properties vs. T

g

structed. W h e n the T

g

(°C)

vitrification

b

maximum

calibration curves can be

is

con-

i n f o r m a t i o n o f a p a r t i a l l y c u r e d s p e c i m e n is

n e e d e d , the p h y s i c a l properties of the u n k n o w n sample can be m e a sured, a n d the T

g

value can be interpolated through the calibration

curve. S u c h a c o n c e p t has b e e n d e m o n s t r a t e d w i t h the T I C A t e c h n i q u e b y m e a s u r i n g t h e p o s t - c u r e p r o p e r t i e s o f t h e s p e c i m e n s . T h e t i m e to b m a x i m u m , o r a s o f t e n i n g p a r a m e t e r , c a n b e u s e d as t h e c a l i b r a t i o n index. Samples of k n o w n T

g

w e r e p r e p a r e d as d e s c r i b e d , a n d t h e y

w e r e s u b j e c t e d to a h i g h t e m p e r a t u r e p o s t - c u r e w i t h t h e same e x p e r i m e n t a l p r o c e d u r e as i n i s o t h e r m a l c u r i n g e x p e r i m e n t s . T h e t i m e s to


3.

LEE A N D GOLDFARB

Torsion

Impregnated

Cloth

75

Analysis

1000

ë

800

CO

1

600

χ < 2

.o

ω

400

200

0 100

120

140

160

180

200

Tg°C Figure 6. Time to b maximum calibration permission from Ref. 8. Copyright 1981, neers, Inc.)

curve. (Reproduced Society of Plastics

with Engi

r e a c h t h e b m a x i m u m w e r e t h e n u s e d to c o r r e l a t e w i t h t h e T . F i g u r e 6 s h o w s t h e r e s u l t o f u s i n g t h e t i m e to b m a x i m u m as t h e c a l i b r a t i o n i n d e x . T h e s a m e e p o x y s y s t e m i n t h e i s o - c u r e state e x a m p l e g i v e n a b o v e w a s u s e d . F o r t h e state r e p r e s e n t i n g 5 h c u r e at 1 3 5 ° C , t h e m e t h o d s h o w e d a T o f 155 °C, a g r e e i n g r e a s o n a b l y w e l l w i t h the i s o - c u r e state e x a m p l e . 9

Q

T h e softening parameter was also extracted from the post-cure e x p e r i m e n t . T y p i c a l p o s t - c u r e b e h a v i o r i s s h o w n i n F i g u r e 7. T h e p a r a m e t e r R is d e f i n e d as h^h . T h e a m o u n t o f s o f t e n i n g at t h e i n i t i a l h e a t - u p as i n d i c a t e d b y h a n d h i s t h e h a r d e n i n g d u e t o a d d i t i o n a l c u r i n g . T h e r a t i o o f t h e t w o v a l u e s i s n e e d e d to m a k e c o n s i s t e n t c o m parisons b e t w e e n s p e c i m e n s . T h e ratio is l i k e n o r m a l i z i n g the o b served changes w i t h an internal standard. W h e n the additional c u r i n g rate is fast a n d t h e r e s o l u t i o n o f t h e t i m e to b m a x i m u m i s p o o r , t h e s o f t e n i n g p a r a m e t e r is a n a t t r a c t i v e a l t e r n a t i v e to y i e l d t h e p a r t i a l l y c u r e d T i n f o r m a t i o n . T h i s m e t h o d h a d b e e n u s e d to extract i n f o r m a t i o n o f T a d v a n c e m e n t as a f u n c t i o n o f c u r e t i m e , w h i c h is u n o b t a i n a b l e t h r o u g h c o n v e n t i o n a l t h e r m o s c a n e x p e r i m e n t s (9). A g a i n , t h e m e t h o d suffers t h e s a m e l i m i t a t i o n s as t h e i s o - c u r e state m e t h o d b e c a u s e o f t h e n e c e s s i t y to m a k e c e r t a i n a s s u m p t i o n s a b o u t t h e c u r e state, a n d i t s a p p l i c a b i l i t y s h o u l d b e c o n s i d e r e d for e a c h i n d i v i d u a l s y s t e m . x

u

2

9

g


76

POLYMER

0

500

1000

CHARACTERIZATION

1500

TIME (sec) Figure 7. Post-cure curves of a partially cured specimen. Either the time to h maximum or the ratio b y h i can he used as calibration index. (Reproduced with permission from Ref. 8. Copyright 1981, Society of Plastics Engineers, Inc.)

Literature Cited 1. Ferry, J. D. "Visoelastic Properties of Polymer"; Wiley: New York, 1970. 2. Gilham, J. K. Polym. Eng. Sci. 1979, 19, 676. 3. Senich, G. Α.; MacKnight, W. J. J. Appl. Polym. Sci. 1978, 22, 2633. 4. Lee, C. Y-C.; Goldfarb, I. J. Polym. Eng. Sci. 1981, 21, 390. 5. Lee, C. Y-C.; Goldfarb, I. J. AFWAL-TR-80-4159. 6. Lee, C. Y-C.; Henes, J. D.; Grossman, T. E . AFML-TR-79-4062. 7. Lee, C. Y-C.; Goldfarb, I. J. Polym. Eng. Sci. 1981, 21, 951. 8. Lee, C. Y-C.; Goldfarb, I. J. Polym. Eng. Sci. 1981, 21, 787. 9. Hedberg, F. L.; Lee, C. Y-C.; Goldfarb, I. J. AFWAL-TR-80-4179. RECEIVED for review October 14, 1981. ACCEPTED December 24, 1981.


Polymer Characterization - American Chemical Society

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