Chapter 16
Determination of Fiber—Matrix Adhesion and Acid—Base Interactions A. E. Bolvari and Thomas Carl Ward Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 15, 2016 | http://pubs.acs.org Publication Date: April 24, 1989 | doi: 10.1021/bk-1989-0391.ch016
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Department of Chemistry and Adhesion Science Center, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Inverse gas chromatography (IGC) was used to determine the dispersive and non-dispersive (acid/base) surface energies of carbon fibers. Further investigations were carried out on thermoplastic polymers using capillary column IGC. Substantiating information concerning the chemical composition of the fibers was obtained using X-ray photoelectron spectroscopy (XPS). A single fiber critical length test was used to correlate the interfacial adhesion of the fiber and polymer matrix to the nature of their surfaces.
T h e r m o p l a s t i c polymers t h a t have t h e n e c e s s a r y r e q u i r e m e n t s t o q u a l i f y as a m a t r i x i n composite s t r u c t u r a l components ( f o r example, s o l v e n t r e s i s t a n c e , h i g h modulus, h i g h g l a s s t r a n s i t i o n , and good f r a c t u r e energy) tend t o e x h i b i t poor a d h e s i o n t o carbon fibers. The weakness o f t h i s f i b e r - m a t r i x i n t e r f a c e r e s u l t s i n a composite t h a t may be u n a c c e p t a b l e i n i t s f i n a l p e r f o r m a n c e . The q u e s t i o n of why t h i s bond i s weak and c o n c e r n f o r q u a n t i f y i n g t h e r e l a t i o n s h i p with respect t o adhesion l e d t o the c u r r e n t investigation. There a r e f o u r g e n e r a l models f o r o f a d h e s i o n (1,2); These invoke d i f f u s i o n , e l e c t r o s t a t i c s , m e c h a n i c a l i n t e r l o c k i n g , o r a d s o r p t i o n p r o c e s s e s . The a d s o r p t i v e t h e o r y i s t h e one t h a t b e s t a p p l i e s t o the carbon f i b e r / t h e r m o p l a s t i c p h y s i c s . A c c o r d i n g t o the model, f o r maximum a d h e s i o n t o o c c u r , t h e a d h e s i v e roust come i n t o i n t i m a t e c o n t a c t w i t h t h e s u b s t r a t e . One can t h e n use thermodynamic p r i n c i p l e s t o d e f i n e t h e work o f a d h e s i o n , W^, a s t h e work r e q u i r e d t o s e p a r a t e a u n i t a r e a o f t h e two m a t e r i a l s i n o r d e r to c r e a t e two new s u r f a c e s . Fowkes {3) has championed t h e importance o f a c i d / b a s e i n t e r a c t i o n s i n a d h e s i o n and i n W . He p o s t u l a t e d t h a t t h e work o f a d h e s i o n , and t h e r e f o r e t h e f i n a l performance o f an a d h e s i v e bond, i s dominated o n l y by a c i d / b a s e and d i s p e r s i v e e n e r g i e s . Thus, i t becomes d e s i r a b l e t o q u a n t i f y t h e s e d i s p e r s i v e and n o n - d i s p e r s i v e ( a c i d / b a s e ) i n t e r a c t i o n s as a means 1
Address correspondence to this author. 0097-6156/89/0391-0217$06.00A) • 1989 American Chemical Society
Lloyd et al.; Inverse Gas Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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of i n v e s t i g a t i n g and p r e d i c t i n g t h e s t r e n g t h o f an i n t e r f a c e * O v e r a l l , a s a t i s f a c t o r y f i n a l performance o f a composite c r e a t e d from f i b e r s and t h e r m o p l a s t i c s must i n some way, a t l e a s t p a r t i a l l y , r e f l e c t V? and, t h e r e f o r e , a f a v o r a b l e m a t c h i n g o f t h e two t y p e s of c o n t r i b u t i n g i n t e r m o l e c u l a r e n e r g i e s * By lumping v a r i o u s d i p o l a r , hydrogen b o n d i n g and o t h e r t y p e s o f p o s s i b l e s p e c i f i c i n t e r a c t i o n s i n t o the acid/base category, a great s i m p l i f i c a t i o n r e s u l t s and n u m e r i c a l r e s u l t s emerge. The t e c h n i q u e o f i n v e r s e gas chromatography (IGC) has been p r e v i o u s l y used t o study t h e n a t u r e o f c a r b o n f i b e r s u r f a c e s ( 4 ) ; g r a t e f u l l y , t h e c u r r e n t a u t h o r s acknowledge t h e i m p o r t a n t e a r l i e r i n v e s t i g a t i o n s and f o l l o w e d t h e i r t e c h n i q u e s (See a l s o S c h u l t z , e t . a l * i n t h i s b o o k ) . X - r a y P h o t o e l e c t r o n S p e c t r o s c o p y (XPS) was used t o e l u c i d a t e c o n f i r m i n g i n f o r m a t i o n on t h e c h e m i c a l c o m p o s i t i o n on the c a r b o n f i b e r s u r f a c e s (5)• However, new methodology had t o be d e v e l o p e d t o s t u d y t h e s u r f a c e e n e r g e t i c s o f t h e r m o p l a s t i c polymers which i n v o l v e d u s i n g c a p i l l a r y column i n v e r s e gas chromatography (CIGC). Once t h e s u r f a c e e n e r g i e s and s u r f a c e c h e m i c a l components of v a r i o u s f i b e r s and m a t r i c e s were i n v e s t i g a t e d , t h e a d h e s i o n o f t h e s e f i b e r s t o t h e t h e r m o p l a s t i c r e s i n s were e v a l u a t e d . These d a t a were o b t a i n e d by p e r f o r m i n g a f i b e r c r i t i c a l l e n g t h t e s t ( 6 ) • A c o r r e l a t i o n between d i s p e r s i v e and a c i d / b a s e p r o p e r t i e s o f t h e f i b e r s and the q u a l i t y o f t h e f i b e r m a t r i x a d h e s i o n was t h e n possible.
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M a t e r i a l s and Methods Materials. Several precursor materials e x i s t f o r the production of c a r b o n f i b e r s ( 7 ) . However, most o f t h e p r e s e n t l y a v a i l a b l e c a r b o n f i b e r s a r e s y n t h e s i z e d from p o l y a c r y l o n i t r i l e (PAN) s i n c e t h e s e f i b e r s have t h e b e s t m e c h a n i c a l p r o p e r t i e s . F i v e PAN based c a r b o n f i b e r s were used i n t h i s s t u d y : 1. AU-4, u n t r e a t e d f i b e r from H e r c u l e s , 2. AS-4, s u r f a c e t r e a t e d f i b e r from H e r c u l e s , 3. XAS, s u r f a c e t r e a t e d f i b e r from D e x t e r H y s o l , 4. AU-4 t r e a t e d w i t h Z-6040, and 5. AS-4 t r e a t e d w i t h Z-6040. The Z-6040 i s a Dow C o r n i n g s i l a n e c o u p l i n g a g e n t , 3 glycidoxypropyltrimethoxysilane: (CH 0) SiCH CH CH OCH CH-CH 3
3
2
2
2
2
2
\) The Z-6040 t r e a t m e n t p r o c e d u r e i s o u t l i n e d below. AS-4 and XAS f i b e r s were commercial samples h a v i n g a p r o p r i e t a r y s u r f a c e t r e a t m e n t which was n o t a v a i l a b l e f o r t h i s s t u d y . These f i b e r s were n o t c o a t e d . The t h e r m o p l a s t i c r e s i n s t h a t were used f o r t h e a d h e s i o n s t u d i e s a r e c o n s i d e r e d t o be tough, n o t e a s i l y c r y s t a l l i z a b l e p o l y m e r s . They a r e as f o l l o w s , w i t h d e s i g n a t e d l a b e l s a s s i g n e d t o each s t r u c t u r e :
Lloyd et al.; Inverse Gas Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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Tg«190°C, o b t a i n e d from S c i e n t i f i c Polymer P r o d u c t s , r e f e r r e d t o a s "polysulfone";
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Tg»150°C, o b t a i n e d from S c i e n t i f i c Polymer P r o d u c t s , r e f e r r e d t o as
Tg«210°C, o b t a i n e d from G e n e r a l E l e c t r i c below as p o l y e t h e r i m i d e .
(ULTEM R1000),
identified
The o r g a n i c m o l e c u l e s o r "probes" used t o i n v e s t i g a t e t h e d i s p e r s i v e s u r f a c e e n e r g i e s o f t h e f i b e r s u r f a c e s were a s e r i e s o f n-alkanes. The probes used t o study t h e n o n - d i s p e r s i v e f o r c e s were chosen based on t h e i r a c i d i c o r b a s i c c h a r a c t e r as d e t e r m i n e d by Gutmann ( 8 ) . Gutmann has p r a c t i c a l l y d e f i n e d b a s i c i t y a s t h e donor number, DN, or e l e c t r o n - d o n o r c a p a b i l i t y i n t h e Lewis s e n s e . The donor s c a l e i s based on t h e v a l u e o f t h e m o l a r e n t h a l p y f o r t h e r e a c t i o n o f t h e e l e c t r o n donor w i t h a r e f e r e n c e a c c e p t o r , S b C l ^ . On t h e o t h e r hand, t h e a c c e p t o r number, AN, c h a r a c t e r i z e s t h e a c i d i t y o r e l e c t r o n a c c e p t o r c a p a b i l i t y o f a m a t e r i a l . I t i s based on t h e NMR c h e m i c a l s h i f t o f P c o n t a i n e d i n (C^H^J^PO when r e a c t i n g with the acceptor. Each probe s e l e c t e d had a known AN and DN i n o r d e r t o q u a n t i t a t i v e l y "sample" t h e r e s p e c t i v e s u r f a c e s i n v o l v e d i n the composite. Three probes were used t o s t u d y t h e f i b e r s u r f a c e s . C h l o r o f o r m (CHC1 ) was used as t h e a c i d i c probe and had an AN e q u a l t o 23.1 and DN e q u a l t o 0. T e t r a h y d r o f u r a n (THF) was used as t h e b a s i c probe w i t h AN e q u a l t o 8.0 and DN e q u a l t o 20.0. E t h y l a c e t a t e (EA) i s c o n s i d e r e d t o be amphoteric w i t h an AN e q u a l t o 9.3 and DN e q u a l t o 17.1. F o r t h e f i b e r i n v e s t i g a t i o n s CHC1-, THF and EA proved t o be s a t i s f a c t o r y from a c h r o m a t o g r a p h i c standpoint. The t h e r m o p l a s t i c s were i n v e s t i g a t e d by CIGC w e l l below t h e i r g l a s s t r a n s i t i o n s so t h a t s u r f a c e thermodynamics were dominant. Column temperatures were v a r i e d o v e r a 40C range i n o r d e r t o f i n d e n t h a l p i e s and e n t r o p i e s of i n t e r a c t i o n . When i n j e c t e d i n t o t h e c a p i l l a r y columns c o a t e d w i t h t h e t h e r m o p l a s t i c polymers, t h e t h r e e probes l i s t e d above e x h i b i t e d peaks which t a i l e d c o n t i n u o u s l y even a t t h e l o w e s t p o s s i b l e c o n c e n t r a t i o n s . F o r t h i s r e a s o n , weaker a c i d s and bases which e l u t e d as symmetric peaks were used t o study the polymer s u r f a c e s . S p e c i f i c a l l y , methylene c h l o r i d e (CH C1 ) w i t h an AN o f 20.4 and i t s DN i s e q u a l t o 0, and n i t r o m e t h a n e TN0 CH > h a v i n g an AN e q u a l t o 20.5 and DN e q u a l t o 2.7 were c h o s e n . These c h e m i c a l s were g o l d - l a b e l grade from A l d r i c h C h e m i c a l and s t o r e d over 4 A molecular s i e v e s before use. 2
2
3
P r e t r e a t m e n t w i t h Z-6040. A d i l u t e aqueous s o l u t i o n (0.5 wt% s i l a n e c o n c e n t r a t i o n ) was p r e p a r e d . The pH o f t h e water was
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a d j u s t e d t o 3.0 t o 4*5 w i t h 0.1% a c e t i c a c i d , and t h e s i l a n e was then added. The c a r b o n f i b e r s were d i p p e d i n t h i s s o l u t i o n f o r a p p r o x i m a t e l y 5 min. and then d r i e d a t 115°C t o remove t r a c e s o f methanol t h a t r e s u l t e d from t h e h y d r o l y s i s o f t h e m e t h o x y s i l a n e .
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X-ray P h o t o e l e c t r o n S p e c t r o s c o p y . XPS s p e c t r a o f t h e c a r b o n f i b e r s were r e c o r d e d on a P e r k i n Elmer PHI 5300 e l e c t r o n s p e c t r o p h o t o m e t e r w i t h a magnesium Ka_^ource o p e r a t i n g a t 250 mW. The o p e r a t i n g p r e s s u r e was 4 x 10 torr. The samples were p r e p a r e d by mounting the f i b e r s a c r o s s gaps i n m e t a l h o l d e r s . F i b e r C r i t i c a l Length Experiment. The aluminum coupon f i b e r c r i t i c a l l e n g t h t e s t (9) was used t o o b t a i n t h e c r i t i c a l l e n g t h data. Coupons o f a n n e a l e d A1100 aluminum measuring 2.5 x 15.2 cm were p r e p a r e d by wet s a n d i n g w i t h 400 g r i t sand p a p e r . The coupons were c o a t e d w i t h a p p r o x i m a t e l y 3 mL o f a 5 wt% polymer s o l u t i o n i n methylene c h l o r i d e s o l v e n t . The s o l v e n t was a l l o w e d t o e v a p o r a t e a t room temperature f o r 24 h o u r s . S i n g l e f i b e r s (3 o r 4 p e r coupon) were p l a c e d on t h e polymer f i l m p a r a l l e l t o t h e l o n g a x i s of t h e coupon. The f i b e r s were t h e n c o a t e d w i t h a n o t h e r 3 mL o f the polymer s o l u t i o n and a g a i n t h e s o l v e n t was a l l o w e d t o e v a p o r a t e . The coupons were a n n e a l e d a t 10°C above t h e Tg o f t h e polymer f o l l o w e d by 265°C f o r 8 h o u r s . A f t e r c o o l i n g , t h e samples were p l a c e d i n an I n s t r o n t e s t i n g machine and p u l l e d i n t e n s i o n t o 30% s t r a i n a t 25% p e r minute s t r a i n r a t e . The l e n g t h s o f broken f i b e r s were measured on a m i c r o s c o p e w i t h a micrometer s t a g e . Chromatographic C o n d i t i o n s . IGC measurements were c a r r i e d o u t u s i n g a H e w l e t t - P a c k a r d 5890 gas chromatograph e q u i p p e d w i t h a flame i o n i z a t i o n d e t e c t o r . A 1.0 m s t a i n l e s s s t e e l column w i t h an i n t e r n a l d i a m e t e r o f 4.4 mm was packed w i t h 8 t o 9 g o f c a r b o n f i b e r by p u l l i n g a p p r o x i m a t e l y t e n 1.0 m l o n g tows o f f i b e r t h r o u g h the column. Helium was used as t h e c a r r i e r gas and methane as t h e n o n - i n t e r a c t i n g marker. The f l o w r a t e was 13.5 mL/minute. The i n j e c t o r temperature was 200°C and t h e d e t e c t o r temperature was 250°C. The CIGC was performed w i t h 60 m f u s e d s i l i c a columns w i t h an i n t e r n a l d i a m e t e r o f 0.53 mm. Columns, s u p p l i e d by H e w l e t t P a c k a r d Avondale D i v i s i o n , were s t a t i c a l l y c o a t e d (10) w i t h a r e s u l t i n g f i l m t h i c k n e s s o f 2 t o 3 ym. The c a r r i e r gas was hydrogen a t a f l o w r a t e o f 5 t o 10 mL/minute. T h i s a n a l y s i s was automated by t h e use o f a H e w l e t t - P a c k a r d 19395A headspace sampler. A l l columns were c o n d i t i o n e d o v e r n i g h t a t 110°C p r i o r t o u s e . F o r g r e a t e r d e t a i l about t h e e x p e r i m e n t a l t e c h n i q u e s and p r o c e d u r e s , see t h e c h a p t e r by B o l v a r i , Ward, Koning, and Sheehy i n t h i s book. I n v e r s e Gas Chromatography. The IGC r e s u l t s f o l l o w e d from measuring t h e r e t e n t i o n t i m e s o f t h e probe m o l e c u l e s i n j e c t e d i n t o the columns packed w i t h t h e f i b e r o r c o a t e d w i t h t h e polymer. To measure t h e d i s p e r s i v e i n t e r a c t i o n s , t h e n o n - p o l a r n - a l k a n e p r o b e s were used. F o r the acid/base (or non-dispersive) i n t e r a c t i o n s of t h e f i b e r s , CHCl^, THF, and EA were u s e d . On t h e o t h e r hand, C H C 1 and n i t r o m e t h a n e were t h e n o n d i s p e r s i v e p r o b e s f o r t h e t h e r m o p l a s t i c s f o r r e a s o n s d i s c u s s e d above. 2
2
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(11):
n e t r e t e n t i o n volume, V^, was c a l c u l a t e d from
where t i s t h e r e t e n t i o n time o f t h e p r o b e , t i s t h e r e t e n t i o n time o f t h e n o n - i n t e r a c t i n g marker (methane), 8 i s t h e f l o w r a t e , and j i s a c o r r e c t i o n f a c t o r f o r gas c o m p r e s s i b i l i t y . The f o l l o w i n g r e l a t i o n s h i p was used t o c a l c u l a t e t h e d i s p e r s i v e component o f t h e s u r f a c e f r e e e n e r g y , y^ , f o r t h e f i b e r s (4):
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