Determination of FiberâMatrix Adhesion and AcidâBase Interactions

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

1

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.

INVERSE GAS CHROMATOGRAPHY

218

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.

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

A

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 :


16. BOLVARI & WARD

Fiber-Matrix Adhesion andAcid-Base Interactions

219

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";


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


220

INVERSE GAS CHROMATOGRAPHY

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 .


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


16.

The

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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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Determination of FiberâMatrix Adhesion and AcidâBase Interactions

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