Fifty years of research and progress on carbon black

Carbon,

Pergamon

Vol. 32, No. 7, pp. 1305-1310, 1994 Copyright 0 1994 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0008.6223/941F6.M) + .OO

FIFTY YEARS OF RESEARCH AND PROGRESS ON CARBON BLACK JEAN-BAPTISTE DONNET Laboratoire de Chimie Physique, Ecofe Nationale Superieure de Chimie de M&house and Centre de Recherches sur la Physico-Chimie des Surfaces SolidesKNRS, Mulhouse, France (Received 6 December

Key Words-Carbon

1993; accepted 7 December

black, structure, surface properties, reinforcement,

It is certainly rewarding to look back at the last half century and try to summarize what concepts were considered as established, and from there on discuss the progress and developments so as to assess the present state of knowledge. I started my laboratory forty years ago (exactly, the 3rd of February 1953) when I was hired as Professor at the School of Chemistry in Mulhouse after six years devoted to my thesis (1947-1952), during which 1 already worked with carbon black, which I chose as a physical mode1 for rigid “spherical particles”[ I]. Excelient review papers where published between 1949 and 196412-51 and it is quite clear that several points were already firmly established. The electron transmission microscope discovered by Ruska in 1939 and made operative by Siemens in the forties was, from the very beginning, extensiveIy used to observe carbon black; extremely good pictures were already published by the late thirties and during the fifties when extensive studies were made[6,7]. Figure 1 reproduces one of the these picture@]. From there on, the approximately spherical shape of more or less aggregated individual particles, whose dimensions where strongly dependent on the process parameters, could not be doubted. In the early forties, the X-ray diffraction diagram of carbon black was also observed[6,7] and in the fifties the “crystalline character” of carbon black was evidenced by Warren[9], Alexander et a/.[lO,l I] and Austin et a1.[12,13], by analytical treatment of the X-ray spectrograms. It was then possible to deduce the data published by Austin ef al. 112,131, and widely discussed in the carbon black community. It was clearly shown that the crystalline parameters so obtained, and although representing average values which were tied to a simplified treatment of X-ray experimental data, nevertheless truly existed and were strongly dependent on the carbon black species (channel, thermal, furnace, acetylene , , . ) and on the process parameters as shown in Table 1, reproduced from Austin et a/.[12,13]. It is amazing also to read again the papers published more than forty years ago trying to relate oxygen and hydrogen content together or to pH, oil adsorption, extrusion shrinkage[3]; the DBP concept was already there. CAR 32:7-F

1305

1993)

scanning tunneling microscopy.

The surface chemistry was also largely started, hydrogen and oxygen content were deeply studied, and Studebaker[3] already described carbon black as “being somewhat like a series of degraded polycyclic aromatic hydrocarbons at various states of oxidation” and “to the extent that carbon black resembles these polycyclic hydrocarbons, we could expect a high order of reactivity.” This concept was useful when the theory concerning the nucleation and growth of carbon black particles was established[l4].

Fig. 1. Electron micrograph of oil furnace blacks: (a) highstructure, (b) low-structure (from@]).

1306

J.-B. DONNET Table 1. X-ray analysis (from Austin[lS]) Crystallite dimensions, A L, (breadth)

Carbon per cent

(L, (height)

(ll)

(002)

Random

In parallel layers

30 23 19 19 21 30 20 16 18 18 18 19 18 18 18 19 14

20 18 14 14 18 27 16 15 18 18 18 15 14 12 13 17 11

17.3 17.6 15.5 14.8 14.8 24.6 13.7 14.4 12.7 12.3 11.6 13.0 13.4 13.7 12.7 12.0 12.7

9 13 9 13 10 11 6 18 23 23 19 21 16 17 13 9 26

78 62 68 64 72 78 69 60 58 54 64 45 52 61 60 67 55

13 25 23 23 18 11 25 22 19 23 17 34 32 22 27 24 19

15 16 15.5

1s 14 12

12.7 14.4 13.0

31 17 31

36 52 37

33 31 32

Trademark

(10)

Rubber-grade blacks Thermax P-33 Pelletex Sterling V Kosmos 40 Shawinigan Statex B Philblack A Philblack 0 Philblack 1 Philblack E Vulcan SC Spheron 9 Spheron 6 Spheron 4 Spheron C CK-4 Color btacks Elf 0 Mogul A Mogul

in single layers

Probability of number of parallel layers 3

4

5

.l

.9 1.0 .4 .2 .2

.6 .8 .8 * .l .4 .s .7 .3 .2 .l .4 .6 .4 .4 .3

.9 .9 .6 .5 .3 .7 .8 .9 .6 .4 .6 .6 .9 .7

.l

.l

*Approx. mean number of layers equals 7.

The nature of the oxygen-containing groups was also the focus of intense activity and there were papers on the possible existence of -COOH and -OH groups, by ViIlars in 1948[15]. Studebaker, et a!.[161 in 1956 showed that the presence of active hydrogen can be evidenced by the Grignard reagent, and the diazomethane reaction with oxygen-containing groups on the carbon surface indicated the presence of labile hydrogen and carboxylic groups. Study of the chemical surface functions of carbon black has been very active. It has been reviewed and was considered almost completed[~7-201 (however, recent signals indicate that this field should not be considered exhausted). Anyway, Watson in 1956[21,22] postulated a radical acceptor character as did Garten and Sutherland[23,24] in 19.54-1957, and Szwarc[25], in 1956. These observations were the basis for one of the theories of reinforcement which were so deeply worked on and disputed during the next twenty years. Let us recall here the names of Blanchard and Parkinson (chemical reinforcement~26]), Payne (filler networking theory~27,28]), Medalia (rubber o~clusionI29]), Gessler (free radicals[30,31]), Smallwood (hydrodynamic effect[32]), Wolff et al. (filler effect and effectiveness factor[33,34]), Dannenberg (molecular slippage theory[35,36]) and Gerspacher (filler-filler interaction[37-391). This paragraph should be completed by the excellent analysis of the dynamic properties given by Medalia[40], and more recently in depth by several chapters of books about carbon black[41,42]. Concerning the modelling of the carbon black particle internaf organization, the first step is undoubt-

Sweitzcrct Heller (I 956)

DOMetet SChUltZ(1965)

Dortnctet Bouland (1963)

IIeekmanet Ihrdling (19GG)

lless. Banet Heidenreich (1968) Fig. 2. Carbon black mod~llisation.

Fifty years of research

and progress

edly due to the X-ray data of Riley[43] as early as 1939. The systematic studies of oxidation followed by transmission electron microscopy gave results[44-481 that supported a clear improvement of this simplified view. Heckman and Harling[49] confirmed our results and proposed a more elaborated model, which was further improved by the systematic use of high resolution electron microscopy under the guidance of Hess[SO]. This model “saga” ended in 1968 as illustrated by Figure 2. The most recent progress is due to new methods, namely Inverse Gas Chromatography (ICC) and Scanning Tunneling Microscopy (STM). Inverse gas chromatography, introduced in our laboratory by Saint-Flour and Papirer[S 11, is a very simple powerful adsorption method giving access to the thermodynamic data and the surface energy of a solid surface. Although application to carbon black encountered real difficulties at the very beginning[52,53], because the method of infinite dilution (extremely small concentration of the probe) was used, it was rapidly evident that our hypothesis concerning the preferred detection of the most active sites was correct, and the

1307

black

Table 2. Average value of adsorption energy (F) of and dispersive component (rf) of surface energy of carbon blacks N...x NllOx N234x N330x N762x

Area

E, kJ/mol

rz’, mJ/m’

0.1127 0.0629 0.0432 0.0179

27.2 26.3 25.4 24.8

87.6 84.8 82.0 80.4

recent systematic use of the finite dilution method not only proved it, but enlarged the possibilities of IGC. Systematic studies of carbon black by IGC were made by Wolff et a/.[54] in correlation with its reinforcing properties. We have shown in Mulhouse not only that ICC was extremely useful to study modification of the surface, but also that by finite concentration studies we could reach data that fit perfectly with the limiting values of the surface energy of pure graphite[55,56]. Table 2 reports the average value F (kJ/mol) of the adsorption energy and the surface energy r,* (mJ/m’)

N234x - 1Onmxl Onm

NllOx - 15nmxl5nm

N3!3Ox - 6nmx6nm Fig. 3. Scanning

on carbon

N762x - lSnmxl5nm tunnelling

microscopy

of carbon

blacks.

1308

J.-B. DONNET Table 3. Compounding

recipe

SBR (solution)* Carbon Black ZnO Stearic acid PPD (anti-oxidant)? Sulfur CBS$

100 50 3.0 1.5 1.0 1.1 1.1

*SBR: Butadiene/Styrene 14/26 tPPD: N 1,3-dimethylbutyl N’-phenyl paraphenylene diamine $CBS: Cyclohexylbenzothiazolsulphenamide

for representative carbon blacks. These values are quite close and the surface energy is not changed appreciably by chemical treatment (reduction, oxidation, grafting). The value of r,d is then in the range of 80 mJ/m2 f 5’70, almost independent of the density of active sites, which is related to the process parameters and carbon black grade. As observed at the beginning of our studies[52,53], IGC is instrumental for classification of carbon black by the infinite dilution method.

Table 4. Mechanical

Fig. 4. Carbon black model.

Our recent results with STM gave access to the surface organization of carbon black at the atomic range[55-611. These results are illustrated by Fig. 3; they are in good agreement with the recent findings of Schlogl[62].

properties

of the green compound

vsc., Carbon

black*

Energy

N234 N234x N234xL N234XHh N234xHhNaOE,C1 N234xHhNaOEtCS N234G,,,.c N234G,,., N234Gzsc,,ac N234G,,,.,

1460 1350 1350 1400 1420 1530 -

Table 5. Mechanical

black

N234 N234x N234xL N234XHh N234xHhNaoEtC, N234xHhNaoEtC6 N234G,,cc.c N234G,,., N234G,,,., N234G,,,.c N~~~XOK~SZO~

Bound Rubber, %

115 109 114 112 97 104 100 100 100 100 125

31 30 32 32 18 25 7 2 3 5 35

Original carbon black 48 hours toluene extracted N234 72 hours water extracted N234x Chemically reduced N234x by LiAlH, Chemically reduced N234x by LiAlH, and Cl and C6 grafted N234 Graphitized 30 min. at 1700, 2000, 2300, 2700°C N234x oxidized 12 hours by K&Os.

*N234: N234x: N234xL: N234xHh: N234xHh,,,,,C,: N234G, SC: N234xOKzszo,:

Carbon

156 155 157 158 156 160 152 153 152 153 164

1630 1440 1460 1580

N~~~X~K,S,O,

Moony

Trinatr “C

properties

of the vulcanized

compound

ElO, MPa

ElOO, MPa

E300, MPa

loss at 10%

A r”pl %

%pt MPa

Energy J.rnm3

5.13 5.47 5.71 5.25 4.51 4.99 4.71 5.08 4.88 4.80 5.64

4.04 3.78 3.86 3.53 2.92 3.34 2.24 2.24 2.14 2.10 4.10

9.20 8.62 8.53 8.28 6.27 1.85 3.16 2.96 2.84 2.71 7.20

31 30 32 28 27 28 33 33 34 33 31

170 790 740 800 870 770 780 750 750 740 640

244 248 246 255 265 229 112 96 88 86 110

137 141 137 139 140 121 52 45 41 39 62

Fifty

yean

of research

and

progress

on carbon

13OY

black

MYPa 10

t

-d-

reference

-Cl

grafted

To elangatinn 0

so

IW

I

I

I

I50

200

250

+

?Ofl

Fix. 5. Stress-strain curves of SDS rubber filled with initial and modified carbon black samples.

The quantochemical simulations by HOMO calculations supported the hypothesis that the surface organization could adequately be represented by quasigraphitic scales whose edges were characteristic of the growth of aromatic systems. The deposition of these scales during the formation process results in approximately spherical particles modelled in Fig. 4. Moreover, the condition of nucleation and growth led us to propose a possible mechanism involving fullerenes according to the Kroto[63] proposal for soot formation.

Macromolecular ChGlS

The surface roughness due to the scale’s edges may be involved in the aggregation’s primary mechanism. On the other hand we recently studied again, very extensively, the influence of chemical modifications on carbon black behavior in compounding[55,56]. Table 4 and 5 and Fig. 5 summarize our findings. We see again that chemical treatment, with the exception of oxidation for the low extensions, did not improve the reinforcing character of carbon black. (The compounding recipe is given in Table 3 nith the carbon black sample indexing.) Finally, we think that our model permits a very rational explanation of the reinforcing effect, as illustrated by F‘ig. 6. If an elastomeric chain is “wetting” the surface, and is then submitted to any mechanical strain, the elements of the chain which are (be it mechanicalIy or physicochemically “wetting”) in interact ion with the edges of the scales will need energy to be removed, for “dewetting.” As soon as the strain is cut off, wetting and interaction at the edges may take place again. Such a mechanism, very similar to Dannenberg’s molecular slippage, seems to be able to explain many physical properties of the filled compounds. There remain, however, many unexplained aspects of carbon black concerning its formation mechanism, physicochemistry, and behavior. Scientific knowledge has an ever-moving frontier, and every new acquisition starts new- questions. Let us make an appointment for the next International

Conference

on Carbon

Black!

REFERENCES Fig. 6. Schematic representation of the conformauon of macromolecular chains on the surface of a carbon black particle.

I. J.-B. Donnet, .J. Polym. Sci. 12, 53 (1954). 2. W. R. Smith, H~cycto~rdia of Chemical Tdtnolog), Editmn, Vol. 3, page 48 (1949).

2nd

1310

J.-B. DONNET

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p. 220, 290, M. Dekker, New York (1993). This article is being published without the benefit of the author’s which were not available at press time.

review of the proofs,