Mechanism of Nucleation and Growth of Carbon Black - ACS

24 Mechanism of Nucleation and Growth of Carbon Black

Downloaded by CORNELL UNIV on September 7, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch024

J. LAHAYE and G. PRADO Centre de Recherches sur la Physico-Chimie des Surfaces Solides, 24 Avenue du Président Kennedy, 68200 Mulhouse, France

Carbon blacks may be obtained from hydrocarbons either by pyrolysis or by partial combustion. The main question in the understanding of carbon black formation is to determine how hydrocarbon molecules which may be as simple as methane or benzene can, after vaporisation, lead in a few hundredths of a second to solid particles containing several tens of thousands of carbon atoms. Besides the chemical transformation of initial hydrocarbon, a phase change from gaseous to solid occurs, either directly or via a liquid phase. Various aspects of the subject have been reviewed during the last ten years (1-4). In a forthcoming comprehensive review (5), the nucleation and growth aspects of carbon formation are emphasized. The present paper will include the description of: the nature of the nuclei precursors; the kind of nuclei formed (liquid or solid) and the type of nucleation responsible for their formation; the growth of nuclei and their association during growth. Nature of Nuclei Precursors 1. Carbon Blacks Produced in Flames. In the oxidation zone of a flame, concurrently with oxidation reactions and formation of ions, large hydrocarbon molecules are produced: polyacetylenes, polycyclic aromatics with or without side chains (6-9). The work of Homann and Wagner (6-10) is particularly relevant to the role of these large hydrocarbon molecules in the formation of soot. The authors studied a diffusion acetylene flame in which carbon black particles were formed. By direct probing in the flame and by extraction of the products adsorbed on soot, they could determine two types of aromatics: a. Polycyclic aromatics without side chains such as: naphthalene(128 mass units), acenaphthylene (154), phenanthrene (178), pyrene (202) and coronene (300). 335

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

PETROLEUM DERIVED CARBONS

336


b. P o l y c y c l i c aromatics w i t h s i d e chains. T h e i r molecular masses range from 150 up to 550. Contrary to the case of polyaromatics w i t h s i d e chains, i t appears that polyaromatics without s i d e chains d i r e c t l y taken from the flame are i d e n t i c a l to those e x t r a c t e d from the soot sampled at some d i s t a n c e from the o x i d a t i o n zone. T h e i r concen t r a t i o n s i n c r e a s e s t e a d i l y f o l l o w i n g the o x i d a t i o n zone without going through any maximum. Therefore, the authors c o n s i d e r that polyaromatic hydrocarbons w i t h s i d e chains are p r e c u r s o r s of carbon b l a c k p a r t i c l e s w h i l e those without s i d e chains are by products. These intermediate species may have two o r i g i n s : a. The p o l y m e r i z a t i o n of acetylene o r other unsaturated compounds, b. The formation of benzyne type r a d i c a l s . Homann and Wagner consider that i n the r a d i c a l - r i c h o x i d a t i o n zone, p o l y a c e t y l e n e s (Figure 1) are formed by r e a c t i o n s such as:

I f a "C^H or any other r a d i c a l attacks a p o l y a c e t y l e n e molecule, the p r o b a b i l i t y that i t w i l l c o l l i d e w i t h one end decreases w i t h i n c r e a s i n g c h a i n l e n g t h ; thus, branched macroradicals are formed. These r a d i c a l s go on adding p o l y a c e t y l e n e s and r i n g c l o s u r e s may occur. F i e l d s and Meyerson (11) i n v e s t i g a t e d the p y r o l y s i s of aromatic compounds, u n f o r t u n a t e l y at a r e l a t i v e l y low temperature (700°C). They n o t i c e d that the formation of benzyne i s almost as u n i v e r s a l as that of acetylene i n the p y r o l y s i s of a l i p h a t i c compounds. I t appears that only a f r a c t i o n of aromatic n u c l e i i n v o l v e s s c i s s i o n of C-C bonds.

INSERTION

BENZYNE

Η

C III

c 1-4 ADDITION ( F i e l d s and Meyerson)


Η

24.

LAHAYE AND PRADO

Carbon Black

337


In the case of benzene fiâmes» the formation of benzyne as intermediate might make i t c l e a r why the p o l y c y c l i c aromatic hydrocarbons are formed i n c o n c e n t r a t i o n s about 100 times l a r g e r than from a l i p h a t i c f u e l f o r the same carbon to oxygen r a t i o i n the unburnt gases. Whatever the mechanism, p o l y n u c l e a r s p e c i e s , some w i t h s i d e chains, are formed i n the o x i d a t i o n zone of a flame. What about systems i n which carbon b l a c k s are formed under c o n d i t i o n s of pure p y r o l y s i s (thermal b l a c k s ) ? 2. Carbon Blacks Produced by P y r o l y s i s of Hydrocarbons. The d i f f e r e n t intermediates proposed can be c l a s s i f i e d i n t o two categories (5): a. unsaturated a l i p h a t i c or aromatic molecules. b. carbon vapor r e s u l t i n g from the dehydrogenation of i n i t i a l hydrocarbons. Palmer (12) estimates that p o l y m e r i z a t i o n r e a c t i o n s produce c y c l i c molecules, p a r t l y aromatic, as intermediates i n carbon formation. In studying the thermal decomposition of benzene, Prado (13) measured the y i e l d s of carbon and t a r as a f u n c t i o n of the r e a c t i o n time (Figure 2 ) . I t appears that when the r e a c t i o n time i s i n c r e a s i n g the t a r y i e l d reaches a maximum at the beginning of the carbon b l a c k formation, and r a p i d l y decreases t h e r e a f t e r . There i s no longer doubt that under u s u a l p y r o l y s i s c o n d i t i o n s , the macromolecules formed i n the gas phase are intermediates i n carbon formation. To our knowledge, there i s no p u b l i s h e d work which demonstrates e x p e r i m e n t a l l y that carbon vapor i s an intermediate i n gas phase carbon formation, except the p r o d u c t i o n of d i v i d e d carbon by evaporation of graphite (14) and subsequent condensation, which does not concern carbon b l a c k p r o d u c t i o n . N u c l e a t i o n and Growth Phenomena At the end of the o x i d a t i o n zone of a flame or i n a thermal system at very small residence times, l a r g e polyaromatic molecules (to which we w i l l r e f e r as macromolecules) are present. They w i l l form the primary n u c l e i . What type of n u c l e a t i o n i s r e s p o n s i b l e f o r t h e i r formation? How are they going to grow? 1. Carbon Blacks Produced i n Thermal Systems. a. Theory i n v o l v i n g s o l i d n u c l e i . In 1971, Samkhan et a l . (15), r e f e r r i n g to the work of K a r g i n e t a l . (14) gave a mathemat i c a l o u t l i n e and equations f o r c a l c u l a t i o n of the rate of carbon b l a c k formation during the thermal decomposition of hydrocarbons. They i d e n t i f i e d the n u c l e i of the new phase as carbon b l a c k c r y s t a l l i t e s obtained by condensation of carbon vapor. A c t u a l l y , we know that i n the case of thermal b l a c k s , they are not c r y s t a l l i t e s but large c o n c e n t r i c l a y e r planes (16, 17). I t i s d i f f i c u l t to assume that carbon l a y e r s of the c r y s t a l l i t e s have enough m o b i l i t y to be able to a l i g n i n extended l a y e r s as large




338

Figure

2.



24.

LAHAYE AND PRADO

339

Cdfbon Black

and w e l l organized as those encountered i n thermal b l a c k s . b. Theory i n v o l v i n g l i q u i d n u c l e i : Lahaye e t a l . (18, 19) s t u d i e d the mechanism of carbon b l a c k formation by thermal decomposition i n a flow system of benzene d i l u t e d i n a stream of n i t r o g e n . The carbon b l a c k p a r t i c l e s were c o l l e c t e d at the end of the heated tube. The number and the s i z e of the p a r t i c l e s have been determined f o r d i f f e r e n t molal f r a c t i o n s of hydrocarbon, residence times and temperatures. The number of p a r t i c l e s gives i n f o r m a t i o n on the n u c l e a t i o n step and the s i z e e s s e n t i a l l y on the growth step. One important r e s u l t of t h i s study i s the o b s e r v a t i o n of the constancy of the number of n u c l e i when the molal f r a c t i o n (Table I) and the residence time (Table II) vary, f o r a given temperature (as a f i r s t approximation each nucleus i s assumed to l e a d to one carbon b l a c k p a r t i c l e ) . Without going i n t o d e t a i l s of the e x p l a n a t i o n of these r e s u l t s (5,13,19) worked out with the help of the c l a s s i c a l theory of n u c l e a t i o n (20-22), they can be explained as f o l l o w s : The i n i t i a l hydrocarbon i s transformed by gas phase r e a c t i o n s i n t o macromolecules as already mentioned. The p a r t i a l pressure of macromolecules i n c r e a s e s w i t h r e a c t i o n time u n t i l s u p e r s a t u r a t i o n i s h i g h enough to induce the condensation of macromolecules i n t o l i q u i d m i c r o d r o p l e t s ( c r i t i c a l n u c l e i ) . Since no a d d i t i o n a l n u c l e i are formed, we may conclude that s u p e r s a t u r a t i o n i s not high enough any more to induce the formation of new n u c l e i . T h i s means that the r a t e of formation of macromolecules i s s m a l l e r than the r a t e of growth of the n u c l e i present. I f the c r i t i c a l n u c l e i were m i c r o c r y s t a l l i t e s i n the f i r s t s t age of growth, they would y i e l d c r y s t a l l i t e s which could develop i n two d i f f e r e n t ways : a) growth and formation of an a n i s o t r o p i c macroscopic c r y s t a l l i n e s t r u c t u r e , which has never been observed i n such a systems; and b) c o l l i s i o n and agglomeration thus forming a p a r t i c l e . As p r e v i o u s l y p o i n t e d out, i t i s d i f f i c u l t to imagine the r e o r g a n i z a t i o n of i n d i v i d u a l c r y s t a l l i t e s i n t o large con c e n t r i c carbon l a y e r s . Table I Number of n u c l e i Ν formed per cc of gas TPN as a f u n c t i o n of the i n i t i a l molal f r a c t i o n of benzene [ Β ] t - 0.5 s Τ - 1100°C 0

(%) NxlO"

[B]o

1 0

1 0.78

2 0.79

4 0.83

6 0.87

8 0.72

Table I I Number of n u c l e i Ν formed per cc of gas f o r d i f f e r e n t r e a c t i o n time t [ B ] - 6Z Τ - 1100°C

10 0.72

(TPN)

0

t (s) Nxl0~

1 U

0.1 0.87

0.3 0.91

0.5 0.87

1.0 0.75




340

E l e c t r o n microscopy of samples c o l l e c t e d at very low residence time, w h i l e not b e i n g a d i r e c t proof of the e x i s t e n c e of l i q u i d i n t e r m e d i a t e s , i s a good i l l u s t r a t i o n of the l i q u i d s t a t e of the n u c l e i . The l i q u i d n u c l e i grow by adding macromolecules which continue to be produced. The k i n e t i c s of n u c l e a r growth i s i n agreement w i t h t h i s d e s c r i p t i o n of the phenomena. The l i q u i d n u c l e i are p y r o l y z e d i n t o s o l i d p a r t i c l e s (dehydrogenation). À s t a t i s t i c a l study of the diameter d i s t r i b u t i o n curves of carbon b l a c k samples gives a d d i t i o n a l i n f o r m a t i o n on n u c l e a t i o n and growth of the p a r t i c l e s formed i n the experiments c a r r i e d out by Prado (13). G e n e r a l l y , d i s p e r s e d systems f o l l o w a s t a t i s t i c a l s i z e d i s t r i b u t i o n law which i s e i t h e r normal(Gaussian) or lognormal (23). The d i s t r i b u t i o n law can be c h a r a c t e r i z e d by the parameters D^, Dg, σΑ, and og which are the a r i t h m e t i c and geometric average diameters, the a r i t h m e t i c and geometric gtandard d e v i a t i o n s , r e s p e c t i v e l y . For each d i s t r i b u t i o n curve, a χ test was c a r r i e d out by assuming the d i s t r i b u t i o n to be e i t h e r normal (x§) or lognormal (χ L.N.). E x p e r i m e n t a l l y , the f o l l o w i n g has been shown: 2

a. The s i z e d i s t r i b u t i o n s are n e i t h e r p e r f e c t l y normal nor p e r f e c t l y lognormal. b. The nearer the sample approaches the c o n d i t i o n at the moment o f n u c l e a t i o n , the c l o s e r the curve resembles a normal d i s t r i b u t i o n ; c o n v e r s e l y , the more s i g n i f i c a n t the p a r t i c l e growth, the c l o s e r the curve resembles a lognormal d i s t r i b u t i o n . c. For samples obtained under d i f f e r e n t c o n d i t i o n s of molal f r a c t i o n s and temperature and Log ag remain constant, w i t h i n experimental e r r o r (Table ° I I I ) . Wersborg, Howard and Williams (24) found an i d e n t i c a l r e s u l t f o r carbon b l a c k samples formed i n a f l a t acetylene-oxygen flame. What i s the meaning of the constancy of 2à and i n °g ? A s u f f i c i e n t p r e r e q u i s i t e f o r these two r e l a t i o n s ^ to be constant i s t h a t , on a s t a t i s t i c a l p o i n t of view, each p a r t i c l e grows p r o p o r t i o n a t e l y to i t s diameter (D ( t j ) - k D ( t 2 ) ) . I t has been shown (19) that i t w i l l be the case i f i n each elemental volume ( " s l i c e ^ o f the gas stream) where N£ n u c l e i appear, a l l of the p a r t i c l e s have the same diameter a f t e r growth. N u c l e a t i o n b e i n g a random phenomenon, N£ i s randomly d i s t r i b u t e d . I t means that the d i s t r i b u t i o n of p a r t i c l e diameters of a given sample i s not the r e s u l t of a random d i s t r i b u t i o n of the s i z e of the p a r t i c l e s i n an element volume but of the random d i s t r i b u t i o n of t h e i r numbers i n the d i f f e r e n t elemental volumes. An evident consequence of t h i s theory i s t h a t , i n each volume of the gas stream, p a r t i c l e s have a l l the same s i z e and, t h e r e f o r e , by c o l l i s i o n they form aggregates c o n t a i n i n g p a r t i c l e s w i t h the same s i z e , even i f the e n t i r e sample i s very p o l y d i s p e r s e d . T h i s consequence i s i n good agreement w i t h o b s e r v a t i o n c a r r i e d out w i t h i n d u s t r i a l carbon b l a c k s as w e l l as w i t h carbon b l a c k s prepared on a laboratory scale. Another consequence seems to be i n c o n t r a d i c t i o n w i t h some of the r e s u l t s obtained. Indeed, i t may be d e r i v e d that the growth of 0



24.

LAHAYE

A N D PRADO

Carbon Black

341

the p a r t i c l e s p r o p o r t i o n a l to t h e i r diameters m a i n t a i n s the n o r m a l i t y o f t h e s i z e d i s t r i b u t i o n ( 1 9 ) . The d i s t o r t i o n o f t h e d i s t r i b u t i o n c u r v e s a p p e a r s as a s e c o n d a r y phenomenon w h i c h w i l l be shown t o be a p a r t i a l c o a l e s c e n c e o r a s s o c i a t i o n b e t w e e n m i c r o d r o p l e t s or growing s o l i d p a r t i c l e s (25). I n c a r b o n - b l a c k - p r o d u c i n g s y s t e m t h e mean f r e e p a t h o f n u c l e i i s s e v e r a l orders o f magnitude l a r g e than the n u c l e i d i a m e t e r ; t h e r e f o r e , t h e i n t e r a c t i o n s b e t w e e n n u c l e i a r e n e g l i g i b l e and t h e n u c l e i p o p u l a t i o n c a n be c o n s i d e r e d as a M a x w e l l gas ( 2 5 , 2 6 ) . The number o f p a r t i c l e s Ν o f t h e s y s t e m as a f u n c t i o n o f t h e d u r a t i o n o f a g g r e g a t i o n t c a n be computed u s i n g t h e k i n e t i c theory of g a s e s . The f o l l o w i n g e x p r e s s i o n has b e e n e s t a b l i s h e d : Ν - No (1 + 8 χ l O - ^ N o S / O T l ^ C o ^ c t r / ^ where No * number o f p a r t i c l e s p e r c c b e f o r e a g g r e g a t i o n Τ » T e m p e r a t u r e i n °K c • sticking coefficient C o - number o f b e n z e n e m o l e c u l e s g i v i n g c a r b o n b l a c k ( p e r c c ) . The e x a c t v a l u e o f t h e s t i c k i n g c o e f f i c i e n t i s n o t known, and c t has b e e n u s e d ' as a v a r i a b l e o f t h e e x p r e s s i o n o f N . Each d i s t r i b u t i o n curve b e i n g c h a r a c t e r i z e d by a X t e s t , the s m a l l e r the r a t i o X^LN the c l o s e r the curve resembles a lognormal distribution. χ^Ν The v a l u e o f Ν has b e e n computed f r o m a p u r e l y n o r m a l i n i t i a l p o p u l a t i o n . The r a t i o x ^ L N / x ^ f l d t h e v a r i a t i o n o f Ν have b e e n computed when c t v a r i e s f r o m 10" 2 t o 1 0 " ^ s , b e f o r e and a f t e r growth (Table I V ) . I n b o t h c a s e s , f o r v a l u e s o f c t below 10~^s, the diameter d i s t r i b u t i o n tends towards a l o g n o r m a l d i s t r i b u t i o n , t h o u g h t h e p e r c e n t a g e o f a s s o c i a t i o n i s l e s s o r e q u a l t o 20% (20% i s t h e e x p e r i m e n t a l u n c e r t a i n t y i n t h e d e t e r m i n a t i o n o f t h e number o f p a r t i c l e s f o r m e d , i n L a h a y e and P r a d o ' s w o r k ) . 6

2

a

n

Table I I I S t a t i s t i c a l p a r t i c l e parameters c a l c u l a t e d from e x p e r i m e n t a l data 0 9 ) (B)o (%) t (s) Τ (°C) D a (1) a t (A) ϋ /σ I n Η:Α ) ADa (A°) D /o 4 χ2^/ 2 A

ο

A

l

n

χ

Ν

10-3 2.6 151.4 0.0 30.0 5.0 0.208 104.5

Before growth 5xl0~ 8xl011.4 16.9 156.4 159.9 °·° °·° 30.3 30.4 5.16 5.25 0.199 0.193 1.5 0.60 3

3

2

10" 20.1 162.2 °·° 30.5 5.32 0.189 0.47

A f t e r growth 3

10" ct (s) 2.6 % association 1517 ( \ 1517

5x10-3 11.4 1575 !!>/:>

8x10" 16.9 1614

Δ3Μ(Α° )

1350 1350

1350 1350

D

A

A

1350 1350

A

, 2 * (Α°ϊ /.. Δη. „ (A°) ' Όα/ °Α £oi

X

X

2

LN/ 2N X

294.5 294.5 5.15 O".204 0.204 16.7

277.2 277.2 5.68 o'.lh 0.181 1.82

3

10" 20. I1639 UJ* l1350 ^u

267.3 zo/.J 6.04 0.169 0.169 1.28

261.5 6.27 0.162 0.16S 1.12

2. Carbon blacks produced i n flames a. Nucleation step : Three separate theories have been developed to e x p l a i n carbon n u c l e i formation i n the p y r o l y s i s zone (yellow colored) of a flame namely -"chemical n u c l e a t i o n " - , p h y s i c a l n u c l e a t i o n on ions and homogeneous p h y s i c a l n u c l e a t i o n . b. Chemical n u c l e a t i o n : "Chemical n u c l e a t i o n " r e f e r s to the progressive transformation by chemical r e a c t i o n of the polynuclear molecules i n t o more and more condensed ones, reaching



24.

L A H A Y E AND PRADO

Cafbotl Black

343

the carbon n u c l e u s i n the f i n a l s t e p s . T h i s mechanism, advanced by Homann and Wagner ( 6 ) , does n o t e x p l a i n t h e d i s c o n t i n u o u s c h a r a c t e r o f c a r b o n f o r m a t i o n . These a u t h o r s t h e m s e l v e s o b s e r v e d t h a t a f t e r t h e a p p e a r a n c e o f c a r b o n "no new n u c l e i w e r e f o r m e d , a l t h o u g h r e l a t i v e l y l a r g e q u a n t i t i e s o f p o l y a c e t y l e n e s were s t i l l p r e s e n t " . F u r t h e r m o r e , t h e r e i s a l a r g e gap b e t w e e n t h e l a r g e r m o l e c u l e s i d e n t i f i e d ( w i t h a mass o f about 600) and t h e s m a l l e r c a r b o n p a r t i c l e s w i t h masses o f a b o u t 4 0 , 0 0 0 f o r p a r t i c l e s o f a b o u t 40 Â d i a m e t e r . A s e q u e n c e o f r a d i c a l c h e m i c a l r e a c t i o n s c a n h a r d l y e x p l a i n t h i s d i s c o n t i n u i t y . Thus, i t appears t h a t chemical n u c l e a t i o n c a n n o t be r e s p o n s i b l e f o r t h e phenomena o b s e r v e d . c . P h y s i c a l n u c l e a t i o n on i o n s : I t has b e e n shown t h a t i o n s a r e formed i n t h e o x i d a t i o n z o n e . Many a u t h o r s i n d i c a t e t h a t some p a r t i c l e s are e l e c t r i c a l l y charged. I n p a r t i c u l a r , Weinberg et a l . (27) s t u d i e d t h e i n f l u e n c e o f e l e c t r i c a l f i e l d s on t h e f o r m a t i o n of carbon i n flames. The a u t h o r s p o i n t e d o u t t h a t i n c h a n g i n g t h e d i r e c t i o n o f t h e e l e c t r i c a l f i e l d , the d i r e c t i o n o f m i g r a t i o n o f the p a r t i c l e s a l s o c h a n g e d . They c o n c l u d e d t h a t t h e p o s i t i v e i o n s p r e s e n t i n t h e f l a m e p r o b a b l y a c t as n u c l e i f o r c a r b o n f o r m a t i o n . An e l e c t r i c a l f i e l d , however, v e r y q u i c k l y c a r r i e s the ions out o f the o x i d a t i o n zone, p r e v e n t i n g t h e i r e v e n t u a l n e u t r a l i s a t i o n . Under these c o n d i t i o n s , the i o n c o n c e n t r a t i o n i n the p y r o l y s i s zone becomes much h i g h e r , s t r o n g l y c h a n g i n g t h e c o n d i t i o n s o f n u c l e a t i o n . Moreover, the carbon p a r t i c l e s form a g g r e g a t e s r e q u i r i n g o n l y one o f i t s p a r t i c l e s t o be c h a r g e d t o m i g r a t e i n the gaseous f l u x . T h e r e f o r e , i t i s not e v i d e n t t h a t a l l p a r t i c l e s n u c l e a t e on i o n s . More r e c e n t l y , Howard e t a l . (24) measured the f r a c t i o n o f p a r t i c l e s charged i n a ρ r e m i x e d flame o f a c e t y l e n e . T h e i r a v e r a g e number f r a c t i o n i s 30%. I f i o n i c n u c l e a t i o n i s the predominant n u c l e a t i o n mechanism, t h e m e a s u r e d 70% a v e r a g e f r a c t i o n o f n e u t r a l p a r t i c l e s r e q u i r e s t h a t t h e c h a r g e s be r a p i d l y n e u t r a l i s e d u n d e r t h e i n v e s t i g a t e d flame c o n d i t i o n . I n t h e c a s e o f f l a m e s , i o n i c n u c l e a t i o n may be i m p o r t a n t , but a f i r m c o n c l u s i o n i s however, not y e t p o s s i b l e . d . Homogeneous n u c l e a t i o n : Homann and Wagner (6) n o t i c e d an i n c r e a s e o f the p o l y a r o m a t i c molecules c o n c e n t r a t i o n at the b e g i n n i n g o f t h e zone o f s o o t f o r m a t i o n . The s u p e r s a t u r a t i o n o f t h e s e s p e c i e s i n c r e a s e s and when i t i s h i g h e n o u g h , c o n d e n s a t i o n may o c c u r a c c o r d i n g t o homogeneous p h y s i c a l p r o c e s s . T h i s t h e o r y d e v e l o p e d f o r t h e c a s e o f t h e r m a l s y s t e m s (19) a l l o w s t h e e x p l a n a t i o n o f the f o r m a t i o n o f soot p a r t i c l e s i n flames through t h e i n t e r m e d i a t e f o r m o f d r o p l e t s w h i c h , a f t e r g r o w t h and c h e m i c a l t r a n s f o r m a t i o n , g i v e s o l i d c a r b o n p a r t i c l e s . U l r i c h (28) c o n s i d e r s t h a t t h i s mechanism o f a p p e a r a n c e o f s o l i d s i n f l a m e s i s g e n e r a l ( s i l i c a , carbon b l a c k , e t c . ) . I t i s noteworthy that t h i s theory, r e c e n t l y p r o v e n f o r t h e c a s e o f t h e r m a l s y s t e m s , h a d b e e n assumed a l o n g t i m e ago : i t i s t h e w e l l - k n o w n " o i l d r o p l e t t h e o r y " (29,30).



344

PETROLEUM

DERIVED

CARBONS

e. Growth step : I t has been known f o r many years (31) that an i n d i v i d u a l carbon b l a c k p a r t i c l e u s u a l l y has s e v e r a l ^ c e n t e r s " . I n v e s t i g a t i o n s by phase c o n t r a s t e l e c t r o n microscopy (17) s u s t a i n t h i s observation. I t i s a c t u a l l y impossible to exclude the p o s s i b i l i t y f o r the " c e n t e r s " to be g r a p h i t i z a t i o n centers formed during the transformation of a s i n g l e l i q u i d d r o p l e t i n t o a s o l i d p a r t i c l e . I t appears more l i k e l y that they are growth centers corresponding to primary p a r t i c l e s which subsequently a s s o c i a t e d . The presence of p a r t i a l l y coalesced p a r t i c l e s observed i n thermal b l a c k samples ( 1 3 , 2 5 ) agrees w i t h t h i s hypothesis. Moreover, p a r t i c l e aggregation i s much more important f o r carbon b l a c k s formed i n flames than f o r thermal b l a c k s . Any t h e o r e t i c a l c a l c u l a t i o n of p a r t i c l e growth must take i n t o account the phenomena of d e p o s i t on the p a r t i c l e surface and of a s s o c i a t i o n . In that r e s p e c t , Wersborg's model i s i n t e r e s t i n g (24). I t includes n u c l e a t i o n , c o a g u l a t i o n ( a s s o c i a t i o n ) and surface growth. For p r a c t i c a l purposes the r a t e of n u c l e a t i o n i s d e f i n e d as the r a t e of appearance of the s m a l l e s t observable p a r t i c l e s ( 1 5 Â ) . Experimentally c o a g u l a t i o n r a t e constants were obtained by assuming a monodisperse system and expressing the c o a g u l a t i o n r a t e according to the o v e r s i m p l i f i e d Smoluchoski s (32) theory. f

A f a i r l y good agreement w i t h the equations was observed. I t appears that the main i n t e r e s t of t h i s study i s the p o s s i b i l i t y of d i f f e r e n t i a t i n g the n u c l e a t i o n zone from the growth zone. I t shows a l s o c l e a r l y both ways of growth of a p a r t i c l e i n flames. In other i n v e s t i g a t i o n s , systems were s t u d i e d i n which one type of growth i s predominant, h i d i n g the i n f l u e n c e of the other. In t h i s way, U l r i c h (28), r e f e r r i n g to the c a l c u l a t i o n of Brock and Hidy ( 3 3 - 3 6 ) e s t a b l i s h e d the equations d e s c r i b i n g a s s o c i a t i o n process f o r s i l i c a formation i n flames. For carbon b l a c k s , the problem becomes somewhat more complicated s i n c e there i s more than one r e a c t i o n and macromolecules continue b e i n g produced a f t e r n u c l e a t i o n . T h i s l e a d s , as already p o i n t e d out, to s u r f a c e d e p o s i t s . For thermal b l a c k s ( 1 3 , 1 8 , 1 9 ) surface growth predominates and f o r carbons formed i n flames a s s o c i a t i o n phenomena predominates. G i l y z a e t d i n o v (37) a l s o considered the k i n e t i c s of formation, growth and c o a g u l a t i o n of growing carbon b l a c k p a r t i c l e s by means of the a c t i v e c o l l i s i o n theory. He concluded that c o a g u l a t i o n of growing p a r t i c l e s of carbon b l a c k a e r o s o l s has a s i g n i f i c a n t r o l e . The i n f l u e n c e of e l e c t r i c a l f o r c e s on p a r t i c l e growth (24, 38) w i l l not be d i s c u s s e d i n the present review. As n o t i c e d by U l r i c h (39), the evidence used to support i o n i c e f f e c t s i n carbon b l a c k formation can be a p p l i e d with equal weight to support a theory of growth through random c o l l i s i o n s . Thus f o r example, extensive work by V o i d (40), Medalia (41), Sutherland (42) and Ravey (43) u s i n g computer s i m u l a t i o n has shown that aggregates s y n t h e s i z e d through random s t i c k i n g c o l l i s i o n resemble the c h a i n l i k e carbon b l a c k aggregates observed by e l e c t r o n microscopy. Therefore, i o n i z a t i o n may not play a key r o l e i n carbon b l a c k


24.

LAHAYE

A N D PRADO

Carbon Black

345

growth, but i t may i n h i b i t growth and agglomeration somewhat when the i o n p a r t i c l e number becomes l a r g e .


CONCLUSIONS Carbon b l a c k s may be produced by thermal treatment o f hydro carbons i n the absence of oxygen as w e l l as by t h e i r p a r t i a l combustion. In the case of thermal systems, i t appears t o be w e l l e s t a b l i s h e d a t present that the i n i t i a l hydrocarbon i s b e i n g transformed by complex r e a c t i o n s i n t o l a r g e polyaromatic molecules which condense i n t o l i q u i d m i c r o d r o p l e t s , p r e c u r s o r s of the u l t i m a t e carbon b l a c k p a r t i c l e s . The growth c o n s i s t s o f s i m u l t a neous a s s o c i a t i o n s of small n u c l e i and deposit of l a r g e p o l y aromatic molecules at t h e i r s u r f a c e . F o r thermal b l a c k s the deposit i s the main growth step. Some a s s o c i a t i o n s which occur have the e f f e c t of transformation of the p a r t i c l e s i z e d i s t r i b u t i o n from normal (Gaussian), found immediately a f t e r n u c l e a t i o n , to a lognormal d i s t r i b u t i o n . In the case of carbon b l a c k s produced i n flames, the general mechanism of carbon b l a c k formation as i n d i c a t e d f o r thermal systems appear to be v a l i d . The intermediate s p e c i e s between the i n i t i a l hydrocarbon and the large aromatic molecules are p o l y acetylenes. F o r benzene and aromatic hydrocarbons i n g e n e r a l , the benzyne r a d i c a l might be an important intermediate. A s s o c i a t i o n o f p a r t i c l e s appears much more important i n flames than i n thermal systems. According t o some i n v e s t i g a t o r s , the ions present i n flames play an important r o l e i n the n u c l e a t i o n o f l i q u i d m i c r o d r o p l e t s and i n growth and formation o f aggregates. A d d i t i o n a l work i s r e q u i r e d to e s t a b l i s h the s i g n i f i c a n c e o f n u c l e a t i o n on i o n s .

Literature Cited (1) Palmer, H.B., and Cullis, C.F., Chemistry and Physics of Carbon, Vol. 1 (P.L. Walker, Jr., Ed.) Dekker, New York, (1965), p. 265. (2) Donnet, J.B., "Les Carbones", Masson Ed. Paris, Tome II, (1965), 712. (3) Feugier, Α., Rev. Inst. Fr. Petr., (1969), XXIV, 11, 1374. (4) Gaydon, A.G., and Wolfhard, H.G., "Flames" 3rd edition, Chapman and Hall, Ed., London, (1970). (5) Lahaye, J., and Prado, G., Chemistry and Physics of Carbon, Vol. 13 (P.L. Walker, Jr., Ed.) Dekker, New York (to be published). (6) Homann, K.H., and Wagner, H.G., 11th Symp. (Intern.) on Combustion. The Combustion Institute, Pittsburgh, Pa., (1967), p. 371. (7) Palmer, H.B., Voet, Α., and Lahaye, J., Carbon (1968), 6, 65. (8) Ray, S.K., and Long, R., Combust. Flame, (1964), 8, 139; ibid., (1968), 12(3), 226.



346


(9) Crittenden, B.D., and Long, R., ibid., (1973), 20, 359. (10) Homann, K.H., ibid., (1967), 11, 265. (11) Fields, E.K., and Meyerson, S., Accounts of Chemical Research, (1969), Vol. 2, 9, 273. (12) Palmer, H.B., J. Chim. Phys., April (Special Issue), (1969), p. 87. (13) Prado, G., Ph. D. Dissertation Thesis, C.U.H.R. and U.L.P. of Strasbourg (1972). (14) Kargin, V.A., Berestneva, Z. Ya., Safronov, N.Ya., and Zhilkina, V.I., Kolloid. Zh., (1967), 29(3), 342. (15) Samkhan, I.I., Tsvetkov, Yu.V., Petrunichev, V.A., and Glushko, I.K., ibid., (1971), 33(6), 885. (16) Ban, L.L., "Surface and Defect Properties of Solids",Roberts, M.W., and Thomas, J.M., ed., (1972), Vol. 1, p. 54, Chem.Soc. (17) Marsh, P.Α., Voet, Α., Mullens, T.J., and Price, L.D., Carbon, (1971), 9, 797. (18) Prado, G., and Lahaye, J., C.R. Acad. Sci. Paris (1972), 274, 569; ibid., (1972), 274, 1880. (19) Lahaye, J., Prado, G., and Donnet, J.B., Carbon, (1974), 12, 27-35. (20) Volmer, Μ., and Weber, Α., Ζ. Physik. Chem. Leipzig, (1926), 119, 277. (21) Becker, R., and Doering, W., Ann. Phys., (1935), 24, 719. (22) Zel'dovitch, J.B., J. Exp. Theoret. Phys., (1942), 12, 525 ; Act. Phys. Chem. SSSR, (1943), 18, 1. (23) Smith, J.E., and Jordan, M.L., J. Coll. Sci., (1964), 19(6), 540. (24) Wersborg, B.L., Howard, J.B., and Williams, G.C., 14th Symp. (Intern.) on Combustion. The Combustion Institute, (1973) p. 929. (25) Prado, G., and Lahaye, J., J. Chim. Phys. (1975), 4, 483. (26) Benson, S.W., "The Foundations of Chemical Kinetics", GrawHill Book Company, Inc., (1960), p. 135. (27) Place, E.R., and Weinberg, F.J., 11th Symp. (Intern) on Combustion. The Combustion Institute, (1967), p. 245. (28) Ulrich, G.D., Comb. Sci. Techn. (1971), 4, 47. (29) Parker, W.G., and Wolfhard, H.G., J. Chem. Soc., (1950), 2038. (30) Sweitzer, C.W., and Heller, G.L., Rubber World, (1956), 134, 855. (31) Donnet, J.B., Bouland, J.C., and Jaeger, J., C.R. Acad. Sci. Paris, (1963), 256, 5340. (32) Smoluchowski, Μ., Z. Phys. Chem., (1917), 92, 129. (33) Brock, J.R., and Hidy, G.M., J. Appl. Phys., (1965), 36, 1957. (34) Hidy, G.M., J. Colloid Sci., (1965), 20, 123. (35) Hidy, G.M., and Brock, J.R., ibid., (1965), 20, 447. (36) Hidy, G.M., and Lilly, D.K., ibid., (1965), 20, 867. (37) Gilyazetdinov, L., Zh. Fiz. Khim., (1970), 44(7), 1828. (38) Wersborg, B.L., Howard, J.B., and Williams, G.C., Technical Report, Dept. of Chem. Eng. Massachusetts Inst, of Techn., Cambridge, December (1972).


24.

LAHAYE AND PRADO

Carbon Black

347


(39) Ulrich, G.D., 12th Symp. (Intern.) on Combustion. The Combustion Institute, Pittsburgh, (1969), p. 884. (40) Void, M.J., J. Colloid Sci., (1963), 18, 684. (41) Medalia, A.I., ibid., (1967), 24, 393; Carbon, (1969), 7, 567. (42) Sutherland, D.N., J. Colloid Interf. Sci., (1967), 25, 373. (43) Ravey, J.C., Ph. D. Dissertation Thesis Nancy (France) (1973).


Mechanism of Nucleation and Growth of Carbon Black - ACS

Recommend Documents