Microstructure and Morphology of Carbon Blacks

26 Microstructure and Morphology of Carbon Blacks

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 22, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch026

L. L. BAN and W. M. HESS Cities Service Co., Petrochemicals Research, Drawer #4, Cranbury, N.J. 08512

Carbon blacks exist in many diverse morphological forms which make them ideal objects for electron microscope study. Because of the advent of high resolution electron microscopy, the microstructure of these carbons can also be viewed in a completely new light. X-Ray Diffraction Studies. Warren, from his atomic radial distribution studies, indicated that carbon blacks are not composed of three-dimensional graphite crystals (1). Instead, he proposed that they are composed of small graphite-like layers with the same atomic positions within the layers as in graphite. He further inferred that the layers are parallel but rotated around the C-axis. The theory of the diffraction of X-rays by random layer lattices developed by Warren (2), and applied by Biscoe and Warren to the study of carbon blacks (3) showed that these carbons produced (00.l) three-dimensional and (hk) twodimensional reflections. The position of the (00.l) reflections indicated that the layers were further apart than in graphite (e.g., 3.35Å vs. 3.6Å). Crystal size along the C-axis, Lc, was determined from the (00.l) reflections, notably from the (00.2) and (00.4), using the Scherrer equation. Crystal size in the plane of the layers, L , was derived from the (hk) (10) or (11) reflections. Major refinements now exist in analyzing the diffraction patterns of carbons and a large literature of the published findings exist (4,5). The recent developments in analyzing the atomic radial distribution of carbons indicate that the layers are much larger than one would obtain from the two-dimensional (hk) reflections (4). a

358

Deviney and O'Grady; Petroleum Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


26.

B A N A N D HESS

Carbon

Blacks

359

Electron Microscope Studies. Electron microscopy of carbons proceeded w i t h the assumption that the find i n g s of the X - r a y L and L measurements were correct. T h e e l e c t r o n m i c r o s c o p e waS u s e d t o l o c a t e these crystallites. One o f t h e f i r s t t o a t t e m p t t o resolve and l o c a t e c r y s t a l l i t e s i n c a r b o n b l a c k was H a l l (β), who f o r m e d d a r k f i e l d i m a g e s w i t h t h e ( 0 0 . 2 ) reflec tions. He r e p o r t e d t h a t t h e i m a g e s o f v e r y large t h e r m a l b l a c k p a r t i c l e s showed a c o n c e n t r i c orienta t i o n of c r y s t a l l i t e s , w i t h the g r a p h i t i c layers p a r a l l e l to the s u r f a c e . A t t h e t i m e H a l l was n o t able to determine whether a s i m i l a r orientation existed for small p a r t i c l e size carbon blacks. Later e l e c t r o n m i c r o s c o p e s t u d i e s on t r e a t e d t h e r m a l carbon b l a c k s ( e . g . o x i d a t i o n and h i g h t e m p e r a t u r e g r a p h i t i z a t i o n ) supported the f i n d i n g s of H a l l ( 7 ) · I t was not u n t i l the a p p l i c a t i o n of h i g h r e s o l u t i o n dark field m i c r o s c o p y , h o w e v e r , t h a t H a l l ' s h y p o t h e s i s was s h o w n t o be v a l i d f o r a l l t y p e s o f c a r b o n b l a c k i n t h e i r i n i t i a l untreated state (£, 9). High

Resolution

Electron Microscopy

U s i n g h i g h - r e s o l u t i o n ( l a t t i c e image) phase c o n t r a s t m i c r o s c o p y , t h e g r a p h i t i c l a y e r l a t t i c e was r e s o l v e d i n a heat t r e a t e d c a r b o n b l a c k ( 2 6 0 0 ° C ) by the i n t e r f e r e n c e o f t h e ( 0 0 . 2 ) d i f f r a c t e d beam w i t h t h e (000) beam ( F i g u r e 1) ( 1 0 ) . These images showed the e x t e n s i v e b e n d i n g and c o n t i n u i t y o f t h e layer planes. The i m a g i n g o f l a y e r p l a n e s i n e v e r y t y p e o f c a r b o n b l a c k f o l l o w e d t h i s i n i t i a l s t u d y ( 1 1 , 1 2 , 13) . I n F i g u r e s 2 and 3 , the o r i e n t a t i o n o f the graphitic layers is apparent. The l a y e r p l a n e s a p p e a r to orient around growth c e n t e r s . The e a r l i e r d a r k f i e l d images of t h e s e c a r b o n b l a c k s o n l y showed s m a l l segments of the c o n t i n u o u s l a y e r s t r u c t u r e (See F i g u r e s 4, 5). T h e s i z e o f t h e l a y e r s v a r i e s , b u t d o e s e x t e n d up t o s e v e r a l hundred Angstroms. A l s o , many o f t h e layers c o n t i n u e from one g r o w t h c e n t e r to the o t h e r . The shape o f t h e l a y e r s a p p e a r s t o be e i t h e r r i b b o n or d i s c , w h i c h are d i s t o r t e d and w a r p e d . The s i z e of the l a y e r s p e r p e n d i c u l a r to and at d i f f e r e n t a n g l e s o f the p r o j e c t e d i m a g e s can be as l a r g e as t h e projected l e n g t h s shown. A l s o , the s i z e o f the l a y e r s seems t o be i n d e p e n d e n t of the r a d i u s of the growth c e n t e r s . The a b s e n c e o f l a y e r d e t a i l c l o s e to and i n the center of the growth centers i n d i c a t e s that they are either h o l l o w or composed of d i s o r g a n i z e d carbon (possibly m i x e d w i t h some i n o r g a n i c i m p u r i t i e s ) . From these s t u d i e s , we c o n c l u d e t h a t t h e p r i m a r y m i c r o s t r u c t u r a l



Figure I.

High resolution lattice image of a graphitized (3000°C)

ISAF


B A N A N D HESS

Carbon Blacks


26.

Figure 2.

High resolution lattice image of an HAF


361

DERIVED


PETROLEUM

Figure 3.

High resolution lattice image of MT


CARBONS


Figure 4.

High résolution dark field (00.2) image of an HAF


PETROLEUM

DERIVED

CARBONS


364

Figure 5. High resolution darkfield(002) image of MT



26.

B A N AND H E S S

Carbon

Blacks

365

u n i t s are the l a r g e d i s t o r t e d g r a p h i t i c l a y e r s and not t h e s m a l l 3 - d i m e n s i o n a l " c r y s t a l l i t e s " as had b e e n p r o p o s e d by the X - r a y d i f f r a c t i o n s t u d i e s . The t h e r m a l b l a c k process tends to form s i n g l e p a r t i c l e s , w i t h one growth center. The f u r n a c e and c h a n n e l b l a c k processes predominately form i r r e g u l a r primary u n i t s w i t h m u l t i ple growth centers. These e x i s t i n a v a r i e t y of shapes w h i c h c a n be b r o a d l y g r o u p e d on t h e b a s i s o f s p h e r i c a l , c l u s t e r e d or f i b r o u s h a b i t s . These primary u n i t s can now be c o n s i d e r e d as s i n g l e g r a p h i t i c paracrystals w h i c h are composed of l a r g e d i s t o r t e d graphitic layers (11, 14). The d i f f e r e n c e s in microstructure, e.g. interl a y e r s p a c i n g s a n d s i z e o f l a y e r s , do n o t seem t o be s i g n i f i c a n t among d i f f e r e n t t y p e s o f f u r n a c e blacks (13). H o w e v e r , t h e r e a p p e a r s t o be a n a r r o w e r dist r i b u t i o n o f i n t e r l a y e r s p a c i n g s f o r the c h a n n e l and t h e r m a l b l a c k s t h a n f o r f u r n a c e b l a c k s (12^, 1 5 ) . D u r i n g the i n c i p i e n t s t a g e s of g r a p h i t i z a t i o n (9001400°C) the l a y e r planes i n carbon b l a c k s m a i n t a i n t h e i r o r i e n t a t i o n around the o r i g i n a l growth centers. The s i z e o f the l a y e r s does n o t a p p e a r t o c h a n g e , but new t y p e s o f d i s t o r t i o n s b e t w e e n l a y e r s and l a y e r groups are introduced (12, 15). Along with introduct i o n o f t h e s e new d i s t o r t i o n s , t h e b u l k o f t h e layers tend to form s t r a i g h t e r segments i n comparison to the untreated samples. The s t r a i g h t e n i n g o f the graphitic layers i n d i c a t e s that atomic rearrangement is taking p l a c e w i t h i n them. The l a r g e e x t e n t o f the layers seems t o r u l e out g r o w t h by j o i n i n g to n e i g h b o r i n g stacked layers. I f l a y e r growth does take p l a c e , it most l i k e l y o c c u r s between l a y e r s t h a t are p o s i t i o n e d e d g e o n , i . e . , c l o s e t o b e i n g i n t h e same p l a n e . The

F o r m a t i o n Of

Carbon

Blacks

From t h e i r m i c r o s t r u c t u r e and g e n e r a l m o r p h o l o g y , carbon b l a c k s appear to form from hydrocarbon droplets ( 1 6 , 17) w h i c h a r e most l i k e l y composed o f s m a l l a r o m a t i c l a y e r s and p o s s i b l y i n a l i q u i d crystalline phase. The f o r m a t i o n o f the p r i m a r y d r o p l e t s appears t o be a homogeneous p r o c e s s ( e . g . see the p r e s e n c e of growth c e n t e r s i n F i g u r e s 2 and 3 ) , w h i l e the encapsul a t i o n of d r o p l e t s by the a d d i t i o n of more aromatic layers after c o l l i s i o n has t a k e n p l a c e i s a heterogeneous process ( 1JB) . I t i s u n l i k e l y that the growth centers i n furnace blacks e x i s t i n other than a l i q u i d state before "structure" f o r m a t i o n , as i n d i c a t e d by some w o r k e r s ( 1 3 , 1 9 ) . The h i g h r e s o l u t i o n images i n d i c a t e t h a t m a t e r i a l flow has taken p l a c e between growth



366

PETROLEUM

DERIVED

CARBONS

c e n t e r s before the c o m p l e t i o n of c a r b o n i z a t i o n . Marsh suggested r e c e n t l y that the l i q u i d d r o p l e t s are isot r o p i c and o n l y i n the p r o c e s s of f u r t h e r pyrolysis d o t h e y f o r m l i q u i d c r y s t a l s (20)· However, Sweitzer and H e l l e r , i n t h e i r e x p e r i m e n t s w i t h t h e furnace p r o c e s s , showed t h a t the o i l y m a t t e r c o l l e c t e d before carbonization contained mostly large aromatic molecules (.17). T h i s f a c t i n d i c a t e s to us t h a t i n t h e liquid d r o p l e t s these a r o m a t i c m o l e c u l e s a l i g n p a r a l l e l to each other i n order to form the lowest free energy state. They must a l s o o r i e n t t a n g e n t i a l l y to the surface, since carbonization is very rapid after the c o l l i s i o n of the l i q u i d d r o p l e t s . During carbonizat i o n , the p o l y c y c l i c a r o m a t i c m o l e c u l e s can grow edgew i s e w i t h i n the p a r a l l e l l a y e r s by l o s i n g t h e i r edge v o l a t i l e components (e.g. hydrogen). The e x t e n t to which the molecules can combine i s probably governed by t h e i r shape and t h e speed o f c a r b o n i z a t i o n . After c a r b o n i z a t i o n , i t i s a l s o l i k e l y that the graphitic structure c o n t a i n s many i n t r a l a y e r v a c a n c i e s . Unf o r t u n a t e l y , because of the absence of three-dimens i o n a l o r d e r i n g of the l a y e r s ( t u r b o s t r a t i c rotation), t h e s e v a c a n c i e s c a n n o t be d e t e c t e d w i t h h i g h r e s o l u t i o n l a t t i c e imaging. I t was i n d i c a t e d e a r l i e r t h a t t h e f o r m a t i o n o f l i q u i d d r o p l e t s a p p e a r s t o be homogeneous condensation (143). H o w e v e r , one c a n a l s o assume t h a t t h e r e are hydrocarbon ions present i n the furnace flame which c a n a c t as n u c l e a t i n g c e n t e r s . In this case, condens a t i o n i s h e t e r o g e n e o u s (21) , w h i c h g r e a t l y complicates the e x p l a n a t i o n of the f o r m a t i o n of the l i q u i d droplets in hydrocarbon flames. Size

And Shape

Of

Primary

Paracrystals

The s i z e and s h a p e o f t h e p r i m a r y g r a p h i t i c parac r y s t a l s ( u n i t s ) o f a c a r b o n b l a c k are governed by the r e a c t o r c o n d i t i o n s under w h i c h i t i s made. Smaller primary u n i t s (high s u r f a c e area) are formed under higher temperature conditions with diminishing contact time. I r r e g u l a r shape ( e . g . h i g h s t r u c t u r e ) is favored by h i g h c o n c e n t r a t i o n s o f f e e d s t o c k to i n c r e a s e the t e n d e n c y f o r c o l l i s i o n and f u s i o n of the hydrocarbon d r o p l e t s p r i o r to c a r b o n i z a t i o n . The c h a r a c t e r i z a t i o n o f c a r b o n b l a c k u n i t s i z e and s h a p e c a n now be a c c o m p l i s h e d v e r y e f f i c i e n t l y by d i r e c t , automated e l e c t r o n m i c r o s c o p e image a n a l y s i s (22, 23). T h e p r i m a r y u n i t s may be r a p i d l y c l a s s i f i e d i n terms o f a number o f g e o m e t r i c p r o p e r t i e s s u c h as: 1. volume ( V ) , 2. l o n g e s t dimension ( L ) , 3. average


26.

BAN

AND

Carbon Blacks

HESS

w i d t h (W) , 4* p r o j e c t e d 2 - d i m e n s i o n a l a r e a (A) and 5· perimeter (P). By c o m b i n i n g some o f t h e s e m e a s u r e m e n t s , s h a p e i n f o r m a t i o n c a n be o b t a i n e d . Linear a n i e o m e t r y o r form f a c t o r (F) i s d e r i v e d from L/W, w h i l e 3-dimensional i r r e g u l a r i t y ( r e l a t i v e to a sphere) i s o b t a i n e d from P /V * 6π . The l a t t e r h a s shown a goo.d c o r r e l a t i o n w i t h t h e a b s o r p t i v e c a p a c i t y ( e . g . DBP) of d i f f e r e n t carbon b l a c k s (24). F i e l d s p e c i f i c image a n a l y s i s p r o v i d e s only a v e r a g e v a l u e s f o r the above d i m e n s i o n a l p r o p e r t i e s , w h i l e f e a t u r e s p e c i f i c a n a l y s i s enables comparison of d i f f e r e n t m e a s u r e m e n t s on i n d i v i d u a l b l a c k u n i t s . Feature s p e c i f i c analyses enable c l a s s i f i c a t i o n into d i f f e r e n t shape c a t e g o r i e s (e.g. s p h e r i c a l , e l l i p s o i d a l , f i b r o u s ) by means o f p a t t e r n r e c o g n i t i o n t e c h n i q u e s (22) . 3


367

2

Dry B l a c k M e a s u r e m e n t s . Dry b l a c k samples (as m a n u f a c t u r e d ) may be p r e p a r e d f o r i m a g e a n a l y s i s by u l t r a s o n i c d i s p e r s i o n i n c h l o r o f o r m , f o l l o w e d by d e p o s i t i o n onto specimen g r i d s coated with t h i n carbon sub strates (Figure 6). T y p i c a l m e a s u r e m e n t s on a v a r i e t y of r u b b e r grade c a r b o n b l a c k s are l i s t e d i n T a b l e I. U n i t s i z e ( v o l u m e ) shows a p a r t i c u l a r l y w i d e r a n g e from below 2 χ 10 nm f o r t h e f i n e s t b l a c k (N-110) t o a b o v e 100 χ 1 0 n m f o r some o f t h e c o a r s e r g r a d e s (e.g. N-990). L i n e a r a n i e o m e t r y v a r i e s from below 2.0 t o a b o v e 4.0. S u r f a c e a r e a (EMSA) i s d e r i v e d f r o m u n i t p e r i m e t e r as r e l a t e d t o v o l u m e and specific g r a v i t y (2_5) . T h i s type of s u r f a c e area determination i s p a r t i c u l a r l y u s e f u l i n t h a t i t c a n be e m p l o y e d t o m e a s u r e t h e b l a c k s u r f a c e a r e a as i t e x i s t s i n a r u b b e r compound. Other types of s u r f a c e area measure m e n t s ( e . g . n i t r o g e n a d s o r p t i o n o r t h e e a r l y EM p a r t i c l e s i z e model) are not capable of doing this. U n i t s u r f a c e a r e a d i s t r i b u t i o n a l i n f o r m a t i o n a l s o can be obtained. 5

5

3

3

M o r p h o l o g i c a l Breakdown. Whether or not c a r b o n b l a c k p r i m a r y u n i t s a c t u a l l y f r a c t u r e and r e d u c e t h e i r s i z e d u r i n g mixing i n t o d i f f e r e n t systems such a s e l a s t o m e r s h a s b e e n a s u b j e c t o f some c o n t r o v e r s y . G e s s l e r ' s s t u d i e s , b a s e d on b l a c k s e x t r a c t e d f r o m r u b b e r compounds, i n d i c a t e d s i g n i f i c a n t l y l o w e r o i l a d s o r p t i o n i n comparison to the dry s t a t e (26). B r e a k d o w n was g r e a t e r f o r h i g h s t r u c t u r e b l a c k s , and a l s o i n c r e a s e d i n the d i r e c t i o n of h i g h e r b l a c k l o a d ings. Gessler also hypothesized that a c t i v e s i t e s w e r e f o r m e d on t h e b l a c k s u r f a c e as a r e s u l t o f f r a c t u r e of p r i m a r y s t r u c t u r e .


DERIVED


PETROLEUM

Figure 6.

Ultrasonic dispersions of HAF carbon blacks


CARBONS


26.

BAN AND HESS

Carbon

Blacks

369

E x t e n s i v e m o r p h o l o g i c a l b r e a k d o w n was a l s o r e p o r t e d by M e d a l i a and Heckman b a s e d on e l e c t r o n microscopy of black-elastomer mixtures (27). Their work i n d i c a t e d higher breakdown for f i n e r tread blacks (ISAF) i n comparison to c o a r s e r grades (HAF), indep e n d e n t o f DBP l e v e l . Hess and c o w o r k e r s demonstrated significant m o r p h o l o g i c a l breakdown for a v a r i e t y of carbon b l a c k s e m p l o y i n g EM i m a g e a n a l y s i s o f m i c r o t o m e d v u l c a n i z a t e sections (28.). A l l b u t v e r y l o w DBP b l a c k s (e.g. N - 3 2 7 , N - 7 7 0 ) showed a l o s s i n u n i t l e n g t h r a n g i n g from about 30-40%. S t u d i e s on f o u r p o l y m e r systems ( N R , S B R , BR a n d S B R / B R ) w e r e i n c o n c l u s i v e i n r e s o l v ing breakdown d i f f e r e n c e s r e l a t e d to polymer type. V o e t , A b o y t e s a n d M a r s h (30) reported similar reduct i o n s i n u n i t l e n g t h i n t h e i r EM s t u d i e s o n h i g h s t r u c t u r e b l a c k s i n S B R ( 3_9 ) . However, t h e i r interp r e t a t i o n of the r e s u l t s d i f f e r e d q u i t e m a r k e d l y . They a t t r i b u t e d the change to a r e d u c t i o n of the soc a l l e d secondary carbon black structure caused by p h y s i c a l l y bonded b l a c k u n i t s , r a t h e r than actual f r a c t u r e of the p r i m a r y u n i t s . We d i s a g r e e w i t h this c o n c l u s i o n because of the magnitude of the changes t h a t have been r e c o r d e d . Ban, et a l . have also resolved actual primary unit fracture i n r u b b e r by means o f h i g h r e s o l u t i o n e l e c t r o n m i c r o s c o p y (3_0). It is d i f f i c u l t , however, to a c c u r a t e l y measure the t o t a l e x t e n t o f f r a c t u r e b e c a u s e one c a n n e v e r fully e l i m i n a t e m i c r o a g g l o m e r a t i o n of carbon b l a c k i n the dry state. T h e r e f o r e , r a t h e r than argue the semantics of what c o n s t i t u t e s the true primary s t a t e of a carbon b l a c k , i t a p p e a r s more r e a s o n a b l e to s p e c i f y the c o n d i t i o n s u n d e r w h i c h t h e s a m p l e was p r e p a r e d and measured, i . e . , u l t r a s o n i c d i s p e r s i o n of dry b l a c k , microtomed s e c t i o n of a p a r t i c u l a r v e h i c l e , etc. Improved Methods For A n a l y z i n g B l a c k s In Rubber. A b o u t 90% o r m o r e o f t h e w o r l d ' s t o t a l c a r b o n b l a c k p r o d u c t i o n (now i n t h e r a n g e o f 7 b i l l i o n p o u n d s ) is u t i l i z e d i n r u b b e r a p p l i c a t i o n s , much o f t h i s b e i n g automotive. It i s not s u r p r i s i n g then, that the m a j o r i t y o f the s t u d i e s on b l a c k s i n end use a p p l i c a t i o n s apply to rubber. Two n e w m e t h o d s f o r t h e a n a l y s i s o f b l a c k s i n e l a s t o m e r compounds were i n t r o d u c e d d u r i n g 1973 and 1974. These a r e the t h i n l a y e r p y r o l y s i s and d i s persed carbon gel techniques f o r s e p a r a t i n g the b l a c k from the polymer ( 3 1 , 3 0 ) . The t h i n l a y e r p y r o l y s i s t e c h n i q u e c a n be a p p l i e d t o e i t h e r v u l c a n i z a t e s or raw s t o c k s and i s a m o d i f i e d v e r s i o n o f the standard



370

PETROLEUM

DERIVED

CARBONS

ASTM p y r o l y s i s p r o c e d u r e f o r r u b b e r (32). Pyrolysis i s c a r r i e d o u t s l o w l y on m i c r o t o m e d s e c t i o n s ( 1 0 - 1 5 μ t h i c k ) o f t h e compound t o m i n i m i z e s i n t e r i n g due t o carbonization. H o w e v e r , t h e l a t t e r c a n n o t be e l i m i n a t e d c o m p l e t e l y and w e i g h t a v e r a g e dimensional p r o p e r t i e s were f o u n d t o be t h e m o s t s u i t a b l e f o r c h a r a c t e r i z i n g d i f f e r e n t b l a c k s i n comparison to the dry s t a t e (31). The m e t h o d a p p e a r s t o be t h e b e s t developed t o d a t e f o r i d e n t i f y i n g unknown c a r b o n b l a c k s i n v u l c a n i z a t e s and i s a l s o u s e f u l f o r c o m p a r ing the microagglomeration and n e t w o r k f o r m i n g c h a r a c t e r i s t i c s of d i f f e r e n t b l a c k s (25). The dis p e r s e d g e l p r o c e d u r e i s b a s e d on u l t r a s o n i c s e p a r a t i o n of the c a r b o n b l a c k p r i m a r y u n i t s from the f i n a l raw s t o c k i n a good s o l v e n t . T h i s i s g e n e r a l l y accom p l i s h e d u s i n g s a m p l e s p r e p a r e d by t h e p r o c e d u r e f o r m e a s u r i n g bound r u b b e r (e.g., o v e r n i g h t benzene s w e l l i n g o f s m a l l b i t s o f t h e compound i n w i r e baskets). The g e l i s r e a d i l y b r o k e n down u l t r a s o n i c a l l y and s e p a r a t e d i n t o t h e p r i m a r y b l a c k u n i t s t h a t e x i s t i n t h e m i x e d compound. A typical gel dispersion f o r a h i g h s t r u c t u r e HAF c a r b o n b l a c k , i n c o m p a r i s o n t o t h e d r y s t a t e , i s i l l u s t r a t e d i n F i g u r e 7. The g e l s p e c i m e n was p y r o l y z e d t o r e m o v e any r e s i d u a l p o l y m e r . A s i g n i f i c a n t r e d u c t i o n i n the u n i t s i z e of the b l a c k is apparent. The d i s p e r s e d g e l p r o c e d u r e t h u s f a r a p p e a r s t o be t h e m o s t a c c u r a t e f o r o b t a i n i n g b l a c k morphological information i n rubber. I t i s l e s s i n f l u e n c e d by microd i s p e r s i o n e f f e c t s i n c o m p a r i s o n to t h i n l a y e r p y r o l y s i s and i s n o t s u b j e c t t o t h e c o n t r a s t and o r i e n t a t i o n problems a s s o c i a t e d w i t h the d i r e c t microtome s e c t i o n i n g a p p r o a c h (2_8) . The r e l a t i v e b r e a k d o w n c h a r a c t e r i s t i c s (% r e t a i n e d average u n i t volume i n comparison to the dry s t a t e ) of a s e r i e s o f HAF b l a c k s o f v a r i e d DBP a b s o r p t i o n , a r e i l l u s t r a t e d i n F i g u r e 8. The b l a c k s a r e c o m p a r e d a t a 70 p h r l o a d i n g i n SBR-1712 and a 70/30 b l e n d o f SBR1712 and BR ( b o t h s y s t e m s c o n t a i n i n g 37.5 p h r o f o i l ) . Breakdown i n c r e a s e d i n the d i r e c t i o n of h i g h e r DBP a b s o r p t i o n a n d , f o r a l l b u t N-326, was h i g h e r i n BR/OEP. T h e p o l y m e r e f f e c t on b r e a k d o w n a p p e a r s t o be a t t r i b u t able to a higher black-polymer i n t e r a c t i o n with BR, i . e . , s t r o n g bonding to the polymer f a v o r s breakage o f t h e b l a c k u n i t s and l e s s t e n d e n c y t o w a r d m i c r o agglomeration (30). H o w e v e r , no c o n c l u s i v e r e l a t i o n s h i p b e t w e e n b l a c k b r e a k d o w n and i n t e r a c t i o n was e s t a b l i s h e d when a v a r i e t y o f c o n v e n t i o n a l and improved t r e a d b l a c k s w e r e c o m p a r e d i n t h i s BR/OEP f o r m a t i o n ( 2 5 ) .



BAN

A N D HESS

Figure 7.

Figure 8.

Carbon

Blacks

Morphological breakdown of EX-1 (155 DBP) in SBR-1500

Relative breakdown of HAF carbon blacks as a function of DBP absorption


PETROLEUM DERIVED CARBONS

372

T h e d i m e n s i o n a l c h a r a c t e r i s t i c s f o r some o f t h e s e b l a c k s are l i s t e d i n T a b l e I I , w h i c h can be compared to Table I (dry b l a c k s ) for the magnitude of the change·


Carbon

Black

Surface

Activity

Following its morphological properties, the surface a c t i v i t y o f a c a r b o n b l a c k i s i t s n e x t most i m p o r t a n t c h a r a c t e r i s t i c , b u t r e m a i n s somewhat vague in definition. Many d i f f e r e n t t y p e s o f o x y g e n f u n c t i o n a l g r o u p s have been i d e n t i f i e d on t h e b l a c k surface. However, t h e i r s i g n i f i c a n c e has been questiona b l e , w i t h the e x c e p t i o n of b l a c k usage i n c e r t a i n p o l a r p a i n t and i n k v e h i c l e s and i n l o w u n s a t u r a t i o n rubbers i n the presence of a chemical promotor (33, 34). The h e t e r o g e n e i t y of the surface g r a p h i t i c l a y e r structure i s known t o be i m p o r t a n t i n t h e b o n d i n g o f carbon b l a c k s to d i f f e r e n t p o l y m e r s , at l e a s t from a physical standpoint (35). However, d i r e c t high r e s o l u t i o n e l e c t r o n m i c r o s c o p y has not yet p r o v i d e d a s u i t able quantitative technique for measuring surface a c t i v i t y d i f f e r e n c e s among m o s t c o m m e r c i a l b l a c k s . On the other hand, h i g h temperature heat treatment ( p a r t i a l g r a p h i t i z a t i o n ) o f any c a r b o n b l a c k i s known to d r a s t i c a l l y l o w e r i t s s u r f a c e a c t i v i t y . Any changes i n b l a c k end use p e r f o r m a n c e as a r e s u l t o f heat t r e a t m e n t , were a s s o c i a t e d w i t h concomitant changes i n the g r a p h i t i c l a y e r s t r u c t u r e which c o u l d be r e s o l v e d by h i g h r e s o l u t i o n e l e c t r o n m i c r o s c o p y (9)· To d a t e , t h e m o s t s u c c e s s f u l m é t h o d e o f assessing carbon b l a c k surface a c t i v i t y or i n t e r a c t i o n are those r e l a t i n g to s p e c i f i c systems i n which the b l a c k has been mixed. A g a i n , most of the e f f o r t s have been d i r e c t e d at elastomer systems. The bound r u b b e r test measures t h a t p o r t i o n of the polymer t h a t has been insolubilized during mixing (3£, 37). Vulcanizate m o d u l u s c h a n g e s as a f u n c t i o n o f t h e p a r t i a l g r a p h i t i z a t i o n of a b l a c k have a l s o been used to d e f i n e its " a c t i v i t y index" (£, 38). D i r e c t s t u d i e s on b l a c k p o l y m e r a d h e s i o n h a v e b e e n c a r r i e d o u t b y m e a n s o f EM a n a l y s i s of e x t r a c t i o n r e p l i c a s of v u l c a n i z a t e s p e c i mene u n d e r s t r a i n (39). A l l of the above procedures are e i t h e r l i m i t e d i n t h e i r s e n s i t i v i t y or a r e a l s o i n f l u e n c e d by o t h e r factors that are not r e l a t e d to carbon b l a c k surface activity. A more d i r e c t measure of b l a c k i n t e r a c t i o n i n e l a s t o m e r s y s t e m s c a n now b e o b t a i n e d b y m e a s u r i n g the amount o f p o l y m e r r e m a i n i n g on the b l a c k surface i n t h e d i s p e r s e d c a r b o n g e l (,30) · The change i n t h e



26.

B A N A N D HESS

Carbon

Blacks

373

Figure 9. Dispersed carbon gel showing residual adsorbed polymer. High resolution electron micrographs.


374

P E T R O L E U M DERIVED CARBONS

MORPHULOC'.I.CAI. PROPERTIES OF RUBBER GRADE CARBON Jl_L_AÇJCS ~IN "DRY .STATE


ASTM NO.

A)

L(_Q»>

W ( «JL)

F

?2jy.

EMSA(M2/ .) 8

N-12 1 N - l 10 ISAF Imp. N-220

284 235 310 288

I 57 139 152 14 5

40.2 39. 5 41.7 40.9

3.91 3.51 3.65 3.55

8.41 6.71 6.55 6.25

134 144 119 119

N- J39 N-347 N-330 N-326

423 611 570 3 36

180 203 187 145

43. 1 49.2 51.2 47.9

3.99 4.13 3.65 3.02

8.61 9. 30 6.40 4.45

105 94 85 89

N-550 N-601 N-660 N-765 N-770 N-761 N-990

6888 4292 9142 11382 6327 801 7 1 1847

431 314 453 502 348 363 320

108.4 106.2 125.8 132.6 120.4 135. 1 174.6

3.98 2.96 3.60 3. 79 2.89 2.69 1.83

7. 52 3.65 6.15 6.21 3.41 2.82 1.10

41 37 32 32 29 24 11

(NUMBER

AVERAGE VALUES FOR DIMENSIONAL

MEASUREMENTS)

TABLE II MORPHOLOGICAL PROPERTIES OF IMPROVED AND CONVENTIONAL TREAD BLACKS IN A 70/30 S B R - 1712/BR COMPOUND ASTM· NO.

VxlO-3(n«3) 267

N-121

EMSA(M2/ .)

L(nn)

W(nn)

_F

144

37.4

3.86

141

3.8 2.5

£

T(ni

N-110

188

123

36.7

3.35

137

ISAF Imp.

252

134

39. 3

3.36

128

3.4

N-220

221

129

38.0

3.38

125

2.7

N-339

351

161

44.0

3.66

111

2.7

N-347

460

180

47.3

3.81

98

1.9

N-330

420

159

47.8

3.33

89

1.4

N-326

246

129

45.2

2.86

86

0.6

(NUMBER

AVERAGE VALUES FOR DIMENSIONAL

MEASUREMENTS)



26.

BAN AND HESS

Carbon

Blacks

375

c h o r d l e n g t h d i s t r i b u t i o n from image a n a l y s e s i s u s e d to o b t a i n t h e a v e r a g e p o l y m e r l a y e r t h i c k n e s s , T. T h e v a l u e of Τ i s c a l c u l a t e d as h a l f t h e d i f f e r e n c e i n t h e mean c h o r d ( a v e r a g e w i d t h ) o f t h e b l a c k u n i t s b e f o r e and a f t e r p y r o l y s i s . The d i s t r i b u t i o n o f Τ i s non u n i f o r m as i l l u s t r a t e d b y h i g h r e s o l u t i o n e l e c t r o n m i c r o g r a p h s i n F i g u r e 9 and a l s o from measured changes i n t h e s p e c i f i c shape o f t h e b l a c k u n i t s f o l l o w i n g p y r o l y s i s (2_5, 22) . The f r e q u e n c y % i n c r e a s e from s h a p e c a t e g o r y 2 ( e l l i p s o i d a l ) t o t y p e s 6, 7 a n d 8 ( f i b r o u s ) , f o l l o w i n g p y r o l y s i s , i s i n d i c a t i v e of s i g n i f i c a n t p o l y m e r r e m o v a l f r o m c o n c a v e r e g i o n s on the u n i t s u r f a c e s . A uniformly distributed surface polymer l a y e r would n o t cause any shape c a t e g o r y changes· The a v e r a g e v a l u e s o f Τ i n SBR a n d SBR/BR b l e n d s r a n g e f r o m a b o u t 0.5 nm f o r i n a c t i v e g r a p h i t i z e d b l a c k s ( a n d v e r y l o w DBP t y p e s ) t o a b o v e 3.0 nm f o r many o f t h e i m p r o v e d t r e a d g r a d e s ( T a b l e I I ) . Τ i n c r e a s e s (to a l i m i t i n g v a l u e ) as a f u n c t i o n o f i n c r e a s i n g b l a c k l o a d i n g and m i x i n g time and d e c r e a s i n g extender o i l content. M e a s u r a b l y h i g h e r v a l u e s were o b t a i n e d i n a 70/30 SBR-1712/BR b l e n d i n c o m p a r i s o n t o SBR-1712 a l o n e , b o t h s y s t e m s c o n t a i n i n g t h e same b l a c k and o i l c o n t e n t . Problems

That

Remain

A d e t a i l e d knowledge o f carbon b l a c k s u r f a c e interaction i s s t i l l lacking. A complete understand ing of surface microstructure i s necessary for a b e t t e r u n d e r s t a n d i n g o f i n t e r a c t i o n phenomena. A t the p r e s e n t t i m e , h i g h r e s o l u t i o n e l e c t r o n m i c r o s c o p y i s l i m i t e d t o p r o j e c t i o n s o f t h e m i c r o s t r u c t u r e and s u r f a c e d e t a i l c a n n o t y e t be r e s o l v e d i n t h e s e images. The f o r m a t i o n m e c h a n i s m o f c a r b o n b l a c k s i n h i g h temperatures furnaces i s s t i l l a c h a l l e n g i n g problem. We f e e l t h a t a c o m p l e t e u n d e r s t a n d i n g o f how c a r b o n b l a c k s f o r m w o u l d be a g r e a t a s s e t t o t h e i n d u s t r y b o t h s c i e n t i f i c a l l y and e c o n o m i c a l l y .

LITERATURE CITED 1. 2. 3. 4.

Warren, B.E., J . Chem. Phys., (1934), 2, 551. Warren, B.E., Phys. Rev., (1941), 59, 693. Biscoe, F. and Warren, B.E., J . Appl. Phys. (1942) 13, 364. Ergun, S., in Walker, P.L. ed., "Chemistry and Physics of Carbon", Vol. 3, p.211-288, Marcel Dekker, New York, 1968.


376

5. 6. 7.


8. 9. 10. 11. 12.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

PETROLEUM DERIVED CARBONS

Rouland, W., in Walker, P.L. ed., "Chemistry and Physics of Carbon", Vol. 4, p. 1-84, Marcel Dekker, New York, 1968. Hall, C . E . , J. Appl. Phys., (1948), 19, 271. Heckman, F.A., Rubber Chem. Technol., (1964), 17, 245. Hess, W.M. and Ban, L.L. in R. Uyeda ed., "Electron Microscopy", Vol. 1, p. 569, Maruzen, Tokyo, 1966. Hess, W.M., Ban, L.L., Eckert, F.J., and Chirico, V . E . , Rubber Chem. Technol., (1968), 41, 356. Heindenreich, R.D., Hess, W.M. and Ban, L.L., J . Appl. Crystl., (1968), 1, 1. Ban, L.L. and Hess, W.M., 9th Biennial Conf. of Carbon, Columbus, Ohio, Battelle's Laboratories: Defense Ceramic Inf. Center, p. 162-163, 1969. Ban, L.L., in M.W. Roberts and Thomas, J.M. ed., "Surface and Defect Properties of Solids", Vol. 1, p. 54-91, The Chemical Society (London), 1972. Marsh, P.Α., Voet, Α., Mullen, T.J., and Price, L.D., Carbon, (1971), 9, 797. Burgess, K.A., Scott, C.E. and Hess, W.M., Rubber Chem. Technol., (1971), 44, 230. Ban, L.L. and Hess, W.M., 10th Biennial Conf. of Carbon, Columbus, Ohio, Battelle's Laboratories: Defense Ceramic Inf. Center, p. 159-160, 1971. Grisdale, R.O., J . Appl. Physics, (1953), 24, 1082. Sweitzer, C.W. and Heller, G.L., Rubber World, (1956), 134, 855. Lahaye, J., Prado, G., and Donnet, J . B . , Carbon, (1974), 12, 27. Donnet, J . B . , Lahaye, J., Voet Α., and Prado, G., Carbon, (1974), 12, 212. Marsh, Η., Carbon, (1973), 11, 254. Pound, G.M., J. Phys. Chem. Ref. Data, (1972), 1, 119. Hess, W.M., McDonald, G.C. and Urban, Ε . , Rubber Chem. and Technol., Vol. 1 (1973), 46, 204. Hess, W.M. and McDonald, G.C., ASTM, STP-553, 1974. Ulmer, J.D., Hess, W.M. and Chirico, V . E . , Rubber Chem. and Technol., No. 3 (1974), 47, 629. Hess, W.M., Chirico, V . E . , Ban, L.L. and Ulmer, J.D., "The Influence of Black Morphology and Elastomer Interaction on Reinforcement Proper ties", Intl. Rubber Conf., Munich, Germany, Sept. 1974.



26.

BAN AND HESS

Carbon Blacks

377

26. Gessler, A.M., Rubber Chem. and Technol., (1970), 43, No. 5, 943. 27. Heckman, F.A., and Medalia, A . I . , J. of the IRI, (1969), 2 , No. 2, 66. 28. Hess, W.M., Chirico, V.E., and Burgess, K.A., Kautschuk u. Gummi Kunststoffe, (1973), 26, 344. 29. Voet, A, Aboytes, P. and Marsh, P.Α., Rubber Age (1969), 101, 78. 30. Ban, L.L., Hess, W.M., and Papazian, L . L . , Rubber Chem. and Technol., No. 3 (1974), 47, 858. 31. Hess, W.M., Chirico, V.E., and Burgess, K.A., "Morphological Characterization of Carbon Black in Elastomer Vulcanizates", Intl. Rubber Conf., Prague, Czechoslovakia, Sept., 1973. 32. Banminger, B.B. and Pon Hon, F . C . J . , Supplement to Back of ASTM Standards, Part 9, 1960. 33. Gessler, A.M., Rubber Age (1960), 87, 64. 34. Ford, W.E., Callan, J.E. and Hess, W.M., ibid., 92, (1963), No. 5, 738. 35. Sweitzer, C.W., Burgess, K.A. and Lyon, F., Rubber World (1961), 143, No. 5, 73. 36. Dannenberg, E.M., Rubber Age (1966), 98, No. 9, 82; No. 10, 81. 37. Gessler, A.M., ibid., (1969), 101, No. 12, 54. 38. Hess, W.M., Burgess, K.A., Lyon, F . , and Chirico, V.E., Kautschuk u. Gummi Kunststoffe (1968), 21, 689. 39. Hess, W.M., Lyon F. and Burgess, K.A., ibid., (1967), 20, No. 3, 135.


Microstructure and Morphology of Carbon Blacks

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