Chemical modifications and surface properties of carbon blacks

Carbon Vol. 34,No. 12,pp. 1521-1529,1996 Copyright 0 1996ElsevierScienceLtd Printed in Great Britain.All rights reserved ooO8-6223/96 $15.00+ 0.00

Pergamon PII: MOOS-6223(96)001034

CHEMICAL

MODIFICATIONS AND SURFACE PROPERTIES OF CARBON BLACKS

E. PAPIRER,‘** R. LACROIX~ and J.-B. DONNET~ “Institut de Chimie des Surfaces et Interfaces (CNRS), 15 rue Jean Starcky, BP 2488, 68057 Mulhouse, ‘Ecole Nationale

Superieure

France de Chimie, 3 rue Alfred Werner, 68057, Mulhouse,

France

(Received 9 April 1996; accepted in revised form 31 May 1996) Abstract-Five carbon blacks, differing in origin or specific surface area, were submitted to a series of chemical treatments: moderate oxidation with potassium persulfate, severe air oxidation, halogenation with chlorine or bromine, and grafting of short (ethyl, butyl, pentyl) or long (lauroic) alkyl chains in order to modify their surface properties and to compare their reactivity in relation to their atomic structure. Various techniques such as elemental analysis, active surface-area determination and inverse gas chromatography were used to assess the consequences of the modifications. In particular, inverse gas chromatography at infinite dilution is shown to detect essentially the most active sites, and allows monitoring of the variation of surface properties upon chemical modification of the blacks. Copyright 0 1996 Elsevier Science Ltd Key Words-Carbon

black, oxidation, halogenation, grafting, inverse gas chromatography.

1. INTRODUCTION

ridges of the scales are certainly the location of very active sites, comparable to the structural defects of mineral oxides having catalytic properties. Possibly, the surface chemical groups (H, oxygenated groups) are situated in that region. The object of this paper is therefore to re-examine carbon-black reactivity (modification) in the light of recent progress concerning surface structure at atomic resolution, and also because of the possibility of examining these objects using the highly sensitive inverse gas chromatography technique.

Carbon blacks have received much scientific and technological attention because, on the one hand, they are irreplaceable elastomer fillers allowing the obtention of high-performance tires and, on the other hand, they are fascinating objects for physicochemical surface investigations. Since they are produced in such a variety of specifications, having small or larger particle sizes and degrees of aggregation (structure), having more or less amounts of heteroatoms on their surface, we thought that it would be of interest to select a series of samples and to submit them to the same treatments and surface analyses, so as to learn more not only about their respective chemical surface reactivity, but also about their potential to interact physically with their environment, i.e. about their surface energy characteristics. In recent years, a decisive step was made after the development of near-field microscopies, in particular tunnelling electron microscopy, which applies well to electricity-conducting carbon materials [ 1,2]. Quite unexpected atomic structures could be revealed on the carbon black surface, which can no longer be considered as a smooth surface made of polycondensed aromatic layers, so-called “microcrystallites” linked together by amorphous carbon material, but rather as a structured and rough surface at the atomic level. Presently, one imagines the carbon black surface as being composed of superimposed polyaromatic scales, of small extension, having dimensions in relation with their surface area. This model underlines the most complex and heterogeneous nature of the carbon surface. The edge

2. EXPERIMENTAL

2.1 Choice of carbon blacks For this study, five carbon blacks (from Degussa, Germany) were selected. These samples had different characteristics as can be seen from Table 1. XE-2 is a highly conducting carbon black appearing as hollow shells exhibiting a high surface area. N115, N326 and N772 are furnace blacks having variable dimensions and “structure” (measured by DBP adsorption). N990 was obtained through the thermal process. The elemental compositions of these samples,

Table 1. Characteristics of selected carbon blacks

Printex XE-2 Corax N115 Corax N326 Corax N772 MT N990

*Corresponding author. 1521

Mean particle size (nm)

Specific surface area (m* g-r)

DBP index (ml/tC@ g)

30 25 41 320

1000 145 83 29 8

370 114 72 65 43

E. PAPIRERet al.

1522

before and after a four day toluene extraction in a Soxhlet apparatus, are shown in Table 2. The compositions of the blacks are comparable. The oxygen content increases with increasing surface area, yet this no longer holds when calculating the amount of oxygen per unit of surface area. Finally, solvent extraction does not significantly alter the global composition of the samples. However, even small variations may induce changes of the surface properties, as will be seen later, since obviously those changes solely concern the outermost surface layer of the carbons.

2.2 Surface modljication

procedures

Carbon blacks were submitted to a series of surface modification procedures in order to elucidate their structure or to try to adapt their surface properties for special applications [ 31. In this instance, samples were prepared and modified under well-controlled chemical conditions before their characterisation was envisaged. Essentially, three types of treatments were performed: oxidation, halogenation and grafting of short alkyl chains. 2.3 Oxidation of carbon blacks Two types of oxidation were performed: a moderate treatment principally intended to limit the modification to the outer surface region, and a more severe treatment, possibly generating micropores. In the first case, the samples were treated with an aqueous solution of potassium persulfate, at room temperature, under constant stirring, for 24 hours. Thereafter, the carbon was filtrated and purified with water following the method of Puri et al. [4]. In the second case, the samples were air-oxidised at temperatures between 300 (for XE-2) and 500°C for N990 so as to control somewhat the reaction kinetics. One advantage of this method [ 51 is to avoid the purification step after treatment. 2.4 Halogenation of carbon blacks Electron spectroscopy was recently applied for the examination of brominated [6], chlorinated [ 71 and fluorinated [S] carbon black samples also described in this paper. Various bromine and chlorine bonds were detected and evaluated quantitatively. Table 2. Elemental analysis of original extracted carbon blacks

XE-2 original XE-2 extracted N115 original N115 extracted N326 original N326 extracted N772 original N772 extracted N990 original N990 extracted

%C

%0

97.2 96.1 91.2 97.4 96.6 98.1 98.7 98.9 98.9 98.1

0.76 0.13 0.85 1.05 0.74 0.64 0.27 0.24 0.10

0.09

and

toluene-

%H

%N

%S

0.2 0.2 0.2 0.2 0.2 0.3 0.2 0.3 0.6 0.5

0.1 0.1

0.18 0.33 0.75 0.72 0.51 0.93 0.53 0.55 0.37 0.7

0.15

0.1 0.1

0.1 0.1 0.1 0.1 0.2

According to the reaction conditions, the chlorination and bromination may go beyond the surface layer into the inner part of the carbon particles. Fluorination is quite complex and leads to unstable modified samples that may not be analysed by IGC, and will therefore be ignored in this paper. 2.4.1 Bromination of carbon blacks Among the variety of methods tested [9], treatment with bromine vapour at 450°C gave the most reproducible results. Carbon blacks were first introduced in a quartz boat, itself placed in a quartz tube which was flushed with N, at 100°C so as to desorb water and adsorbed oxygen. Thereafter, Br, vapour was carried over the carbon sample heated at 500°C for 1 hour. Finally, the brominated sample was cooled under an N, blanket. Thus, all physically adsorbed Br, was eliminated. 2.4.2 Chlorination of carbon blacks The best results, judging from the total amount of fixed chlorine, was achieved when combining two procedures: chlorine at 450°C in conjunction with a reaction with Ccl,. Practically, a nitrogen stream is mixed with Cl, and bubbled through carbon tetrachloride, and this mix is then carried over the carbon black heated at 450°C using a procedure described in the literature [lo].

2.5 Grafting of alkyl chains Whereas the previously described treatments [ 111 principally aim at an increase of the surface reactivity, the fixation of short alkyl chains is supposed to deactivate the carbon surface by chemically blocking active surface sites. Again, several attempts were made in this direction. 2.5.1 Anionic grafting The principle of the treatment leading to a C-C bond formation is illustrated by the following reaction schemes: C-H+RLi-+C-Li+RH C-Li+R’I+C-R’+LiI Carbon blacks (N115, N990), dried at 12O”C, were suspended in a butyllithium cyclohexane solution, under an inert atmosphere (NJ. After 1 hour, iodides (methyl, pentyl) were introduced in the reaction mix in equimolecular amounts to butyllithium. Finally, after 1 hour, the treated blacks were recovered by filtration, solvent-rinsed and water-extracted. 2.5.2 Free radical grafting The carbon black samples were mixed with a benzene solution of lauroyl peroxide which was refluxed for 18 hours under nitrogen so as to avoid additional carbon oxidation. The thermal decomposition of lauroyl peroxide leads to free radicals, either [CH3(CH,)&02]* or [CH,(CH,),,]*, which fix on the carbon surface. After treatment, the reaction mix was poured into an aqueous solution of methanol so as to flocculate the carbon particles. However, recovery was only partial (20% of the initial amount of carbon black) since the modified carbon black suspension was most stable. Finally, the product was extracted with methanol in

Chemical modifications and surface properties of carbon blacks order to eliminate remaining polar degradation products eventually adsorbed on the carbon blacks. 2.6 Inverse gas chromatography (IGC) IGC of carbon blacks has become a most valuable method for the evaluation of their surface properties in terms of surface energy [ 12-151. The principles, advantages and restrictions of IGC have been summarised in review papers [ 151 and will not be developed further here. Two main values may be extracted from IGC results: the dispersive component yt of the surface energy of carbon blacks, and a specific interaction parameter I,,. yp expresses the potential of a solid to undergo London- or dispersivetype interactions. These interactions are also termed “non-specific” because they will always take place when two partners are brought into contact. Isp, however, determines the potential of the surface to exchange specific interactions, such as polar, acid/base, hydrogen bonding, etc., i.e. all types other than dispersive interactions. Practically, yp is estimated from the retention time (t,) or retention volume (V,) of a series of n-alkanes (C,-C,,) injected in the GC column, which is filled with carbon black. yt is experimentally calculated from the slope of the straight line connecting log t, (or the variation of standard free-energy adsorption) to the number of carbon atoms of the series of n-alkanes injected in the GC column. This allows us to determine the free energy of adsorption of a -CH,group, i.e. AG:“z, from which yt is computed according to Dorris and Gray [16], and is calculated by application of the following expression: yd =

s

(AG:HZ)2 4~CH2(Na)2

where N is Avogadro’s number, n is the area of a -CH,group (0.06 nm’), and yCH, is the surface energy of a solid made entirely of -CH,groups, i.e. poly(ethylene). Hence everything is known (a, ycHZ, N) or measurable (AGz”z), except yp. For I,, measurements, one employs polar solutes. I,, is determined from the plot relating RTlog I/, to the vapour pressure at saturation of the injected solutes, as shown in Fig. 1. The points corresponding to alkanes define the “alkane line”, where I,, of ether is given by the distance separating the representative point of ether from the alkane line. Experimentally, a commercial GC is used, fitted with a highly sensitive flame ionisation detector, a stainless steel column having a length of 30 cm and an inner diameter of 0.125 in. Helium was used as a vector gas, with a flow rate of 20-30ml min-‘. The temperature of both injector, detector and column was fixed at 150°C. Table 3 indicates the yp values determined for the various carbon samples and the experimental errors. It is apparent that the errors are related to the extent of the surface area. However, XE-2 possibly represents a special case given its very peculiar morphology. The estimation of I,, using GC probes like

1523

chloroform, acetone or ether is most delicate, since steric effects (lack of access to the surface sites) obscure the interpretation. Consequently, only a few results will be quoted.

2.7 Determination

of active surface area (ASA)

The measurement of ASA was pioneered on graphites by Vastola et al. [17] and is defined as the chemically most reactive part of the carbon surface, which most probably corresponds to the peripheral or border surface of the polyaromatic layers that constitute the carbon materials. This method has been adapted by Ehrburger et al. [lS] to the study of carbon blacks. The principle of the method is apparently simple [ 191. The sample is first cleaned under high vacuum ( 10m6 Torr) and heated up to 900°C so as to eliminate hydrocarbon surface contaminants and oxygenated surface groups. Thereafter, still under vacuum, the sample is cooled down to 300°C. A small amount of oxygen, diminishing the pressure to a value of 0.5 Torr, is introduced in the reactor and allowed to react for 10 hours. After elimination of the gaseous reaction mix, the sample is again heated under vacuum up to 900°C and the oxygenated volatiles (CO, COZ, H,O) analysed by mass spectrometry. This allows us, knowing the section of an oxygen atom, to calculate the corresponding area of oxygen atoms occupied on the carbon surface, i.e. ASA.

3. RESULTS AND DISCUSSION

3.1 Surface properties of initial carbon blacks The -& values of the various samples determined by IGC at infinite dilution and shown in Table 3, were determined at 150°C. These values are rather high when compared with the yf of graphite evaluated from water adsorption isotherms at 20°C [20] (i.e. 96.8 mJ mm*), or of carbon blacks. Smaller values are observed where starting from adsorption isotherms and spreading pressure calculations [21]. It is also apparent that there exists a quite linear relationship between rf and the specific surface areas of the blacks, confirming earlier results [12]. However, non-linear relationships were evidenced by Lopez-Garzon et al. [22], but considering IGC results at finite concentration conditions. To explain the high yf values, it is necessary to recall the principle of the IGC measurement. Only tiny amounts of alkanes are injected in the GC column containing the carbon black, amounts which are insufficient to cover the surface with a monolayer of adsorbed alkane probes. Hence, the answer IGC will deliver will concern the most active sites, which retain the probe molecules most strongly. In other words, using IGC at infinite dilution conditions will provide distorted and partial information on the surface energy characteristics of the blacks. Possibly the sites responsible for this observation are the edges of the scales evidenced by TEM. Preliminary molecular simula-

E. PAPIRERet

1524

al.

e 14 a, .E” 12-3 - lO-3

8--

2

6--

ether q

ISP

4 -2 3.4

3.6

3.8

Fig. 1. Determination

Error (mJ m-‘)

yf (mJ me2)

188 60 39 13 3

381 342 288 153 121

tions of alkane adsorptions on graphite-presenting step defects are in favour of this assumption. It is also probable that these defects may be quantified by ASA measurements, since they also correspond to the most reactive chemical sites. Table 4 compares the “apparent” yp values with the ASA values. It can be seen a correlation indeed exists between these two quantities. It is also possible to compute the “reduced” yt value supposing that only ASA contributes to yt. Doing so lowers yt to a value that is not realistic. When ignoring the case of XE-2, which is actually a very special sample, and plotting the reduced yp versus the specific surface area of the other blacks, one records a smooth curve that deviates from linearity for the fine-particle samples. This is understandable since the ASA determinations are not absolute. In fact, one needs to make the choice of the reoxidation temperature (here 300°C) of the cleaned surface. The rate of oxidation will depend on temperature, but also on the extent of surface area. This causes difficulties when comparing samples of quite different surface areas. Nevertheless, one might safely conclude that the unexpected high values of the “apparent” Table 4. Relation between yt and active surface area (ASA) of carbon blacks

XE-2 N115 N326 N772 N990

4.4

4.6

PO

of the specific interaction parameter (I,,) on persulfate-oxidised N772.

Table 3. Dispersive component (yf) of the surface energy of carbon blacks

XE-2 N115 N326 N772 N990

4.2

4.0 bJ

d Ys (mJ me*)

ASA (m2 K’)

A&S

381 342 288 153 125

24 8.8 3.3 1.1 0.35

2.4 6.1 3.7 3.7 4.4

rf are connected with the active zones, i.e. with the periphery of the polyaromatic scales. The carbon residue of the ASA measurement constitutes a “cleaned” carbon black sample, and hence it was appropriate to check the influence of such a treatment on yt. As an example, solvent-extracted N990 exhibits a yt of 125-t 14 mJ m-‘. The same sample, from the ASA measurement, now has a yp of 530f290 mJ m-‘. Even though the experimental error is important, it seems that “cleaning” of this sample dramatically enhances its apparent yf. This is accounted for by the evacuation of organic impurities which accumulate in the surface asperities, giving rise to very efficient alkane adsorption centers.

3.2 Surface properties of oxidised samples 3.2.1 Persulfate oxidised samples This treatment was intended to limit the oxidation to the surface of the carbon particles. Table 5 shows that the treatment was efficient since the oxygen content has increased after treatment. Globally, the oxygen contents are multiplied by a factor of three and depend clearly on the extent of surface area. When calculating the oxygen coverage roughly (taking 0.073 nm for the covalent radius of oxygen), it appears that the samples having the larger particle sizes are also those having the higher oxygen coverages. Figure 2 compares the oxygen contents and the ASA values determined on the initial blacks. Again ignoring XE-2, for reasons discussed earlier, a linear relation is observed. However, before making a general statement, supplementary carbon samples should be examined. Table 6 contains the results of the IGC measureTable 5. Elemental analysis of persulfate-oxidized blacks

rP/ASA 9 21 11 6 5.5

XE-2 Nll5 N326 N772 N990

carbon

%O

% surface coverage

% 0 initial

3.95 3.75 1.53 0.91 0.32

2.5 16.3 10.8 19.8 25.2

0.73 1.05 0.64 0.24 0.09

Chemical modifications and surface properties of carbon blacks

0

2

4

1525

8

6

A.S.A. (m*/g) Fig. 2. Relationship between 0 contents of persulfate-oxidised carbon blacks and ASA of the corresponding

non-oxidised

samples. Table 6. yf of persulfate-oxidized

(rnZ:-l) XE-2 XE-2 oxyd. N115 N115 oxyd. N326 N326 oxyd. N772 N772 oxyd. N990 N990 oxyd.

1000 145 83 29 8

carbon d Y. (mJ m-‘) 381 354 342 320 288 261 153 109 121 79

ments. Oxidation is accompanied by a decrease of the yt values. For high surface area samples, this decrease is not very marked given the measurement errors. For lower surface area blacks, the diminution of yt is significant. However, these samples are those having the highest oxygen surface coverage. The diminution of yf may be accounted for by the “pollution” with polar oxygenated functional groups of the most efficient adsorption sites, i.e. the lateral surfaces generated by the polyaromatic scales. That such groups form is indicated by the strong increase of ZSp,using a Lewis base probe (diethylether) or an amphoteric probe (acetone), i.e. a probe capable of both acid and base interactions. Whereas nonoxidised samples hardly interact with the IGC probes, treated samples exchange strong interactions with both types of probes, suggesting that oxidation under mild chemical and temperature conditions generates essentially acidic groups on the carbon surface. 3.2.2 Air oxidised samples Table 7 shows the main results of elemental and IGC measurements. The oxygen contents of the treated samples remain low, a result easily understandable knowing that at 500°C the thermal stability of the oxygenated surface

blacks

and specific interaction

parameters I,, ether (kJ mol-‘)

I,, acetone (kJ mol-‘)

%O 0.73 3.95 1.05 3.75

_ _ _ _

_ _ _ _

0.64 1.53 0.24 0.91 0.09 0.32

_ _ negligible 8.5 2 8.7

negligible 4.3 0.5 7.6 negl. 8.7

Table 7. Characteristics S (Jagiello) XE-2 XE-2 N115 N115 N326 N326 N772 N772

oxid. oxid. oxid.

oxid. N990 N990 oxid.

of air-oxidized (m2 g-‘)

carbon

yf (mJ m-‘)

blacks % 0

620 470 124 377 37 622 29

381 462 342 468 288 435 153

0.76 2.90 1.05 1.95 0.64 1.60 0.24

502 8 105

430 121 419

8.77 0.09 1.96

groups is limited. Table 7 also contains surface area evaluations according to the method developed by Jagiello and Papirer [23] and based on the results of IGC at infinite dilution conditions. This method is based on the exploitation of IGC results, using alkanes as solutes, at infinite dilution conditions. For statistically homogeneous surfaces (i.e. surfaces that appear homogeneous in comparison to the size of the alkanes used as probe molecules), the surface given by IGC is in agreement with that determined by application of the BET method.

1526

E.

PAPIRER et al.

For solids presenting highly active adsorption sites, the surface explored by the few injected solute molecules (corresponding to the very initial part of the isotherm) will not be representative of the total surface area of the solid. Hence IGC and BET results differ. However, when comparing similar solids, an increase of the surface will also be evidenced by the IGC method. Even though the absolute values of the specific surface areas may be criticized, their variation after air oxidation is real. It is observed after air oxidation that the areas of the blacks increases dramatically, in agreement with the literature [S], due to the generation of microporosity. It is then expected that all samples will show similar “surface” properties: the alkane probes essentially explore the pores where they will be strongly retained. Indeed, the 1~8values are exceedingly high and practically equivalent for all the samples examined, indicating once more that TGC at infinite dilution principally addresses the most active sites, in this instance the micropores. The higher yt values of the air-oxidized samples may be accounted for by the creation of micropores, i.e. of supplementary high-energy adsorption sites. This is the case for air oxidation, whereas the persulfate treatment concerns only the external surface of the carbon blacks. Attempts made to evaluate I,, did not meet with success, since pore-size exclusion effects are prevalent. 3.3 Surface properties of halogenated carbon blacks 3.3.1 Brominated samples Table 8 displays

the main results concerning carbon blacks brominated at 450°C. Elemental analysis of oxygen demonstrates that no significant oxidation has occurred during the treatment. The amount of Br introduced on carbon blacks was determined in the following manner: first, the carbon was burned in oxygen and the brominated derivatives that were formed were reacted with an aqueous solution of hydrazine sulfate. After neutralization, bromides were determined by potentiometry using silver nitrate, a silver electrode and a reference electrode. The accuracy of the method is less than 5%. Looking at the Br surface coverage, it can be seen that it diminishes as the surface area of the

carbon sample increases. formulated:

Several hypotheses

bromination concerns several atomic layers, the microstructure being “looser”; surface organisation is poorer for carbons having small surface areas; particles with larger dimensions possess aliphatic residues allowing polyhalogenation; the surface is degraded in some cases; and there is a strong physical retention of bromine. The first two hypotheses are not confirmed by scanning tunnelling microscopy since the carbon blacks preserve the aspect they had before bromination. The third hypothesis is very difficult to verify due to the lack of sensitivity of solid-state NMR. Finally, the last hypothesis is not supported by ESCA, which essentially identifies the formation of covalent bonds. Hence the question of the variation of the bromine surface coverage with particle diameter remains unanswered. However, our results suggest that the atomic surface structures of larger particles differ from those of small particles, in agreement with electron microscopy observations [ 11. Thermal analysis of brominated samples indicates that they are stable up to 400°C. For instance, for N990 the total weight loss (Aw) due possibly to the departure of water and adsorbed gases, between 100 and 4OO”C, amounts to less than 0.5%. In the temperature range 400-75O’C, Aw equals l%, and is 1.1% between 750 and 95O’C. From a comparison with the Aw of non-brominated blacks, one concludes that bromine derivatives leave the carbon black surface in the 450&75O”C temperature range. However, 15520% of the fixed Br resists the thermal treatment demonstrating that very stable carbonbromine bonds are created. Since the surface coverage with bromine and the ASA values both increase with the specific surface areas of the samples, a correlation does exist between these two quantities. However, the variation is monotonous rather than linear. d it can be seen that bromination Concerning ‘J,, increases the 7: values. At first sight, this result seems in agreement with the fact that bromination of ben-

Table 8. Characteristics of brominated carbon blacks

% occupied

2 (mJ mm’) XE-2 XE-2 N115 N115 N326 N326 N772 N772 N990 N990

brom. brom. brom. brom. brom.

1000 _ 145 _ 83 _ 29 _ 8

381 419 342 493 288 324 153 263 121 243

may be

% Br 0

6.64 0 5.85 0 6.69 0 3.86 0 1.1

surface 0

1.5 0 12 0 23 0 41 0 42

%O

0.73 0.80 1.05 1.05 0.64 0.64 0.24 0.36 0.09 0.17

Chemical modifications and surface properties of carbon blacks zene increases its surface tension from 29 to 36.5 mJ m-‘, since yp varies proportionally to the polarizability of the atoms or group of atoms that constitute the surface. However, so far, we have explained the pecularities of the carbon-black surface by supposing that the most active sites correspond to the peripheral surface of superimposed polyaromatic “scales”. However, one would guess that this surface is also the location of the most chemical reactive sites, where Br would be bonded. Hence, it is presently unclear what plays the major role: the peripheral surface or the carbon-bromine bonds. These bonds should definitely influence the surface acidity of the blacks, increasing the strength of existing carboxylic or phenolic groups. This can be shown using diethylether as a GC probe. Whereas no acidity was detected on N326, after bromination the I,, of ether amounts to 1.7 kJ mol-‘. For N772 and N990, the effect is even more pronounced, since after bromi5.2 and nation, the I,, values are, respectively, 7.4 kJ mol-i. 3.3.2 Chlorinated samples As described earlier, the samples were treated with a Cl&Cl, mix at 450°C. XPS detected various types of carbon-chloride bonds formed by an addition reaction and by hydrogen substitution [7]. Furthermore, it was shown that chlorination is much more complex than bromination, since the reaction is not limited to the outer part of the carbon particle but proceeds inside the particle. Also, with the particular treatment conditions, -Ccl, and Ccl, groups appear, but in minute amounts. Table 9 shows the results of the characterisation of the samples. More chlorine than bromine remains attached to the blacks. When calculating the surface coverage ratio, one observes values well beyond that of monolayer formation, confirming the ESCA observations for the coarser-particle carbon blacks. However, tunnelling electron microscopy does not point to major structure changes, but atomic resolution was not achieved due to the creation upon bromination of a non-electrical conductive surface layer. It is also seen that chlorination does not significantly change the oxygen content. The thermogravimetric analysis detects two domains of weight losses (Aw). Between 370 and 650°C in the case of

1527

N990, Aw is 2.5%, and between 650 and 950°C Aw equals 4.6%. Elemental analysis of N990, treated at 95O”C, reveals that 0.2-0.3% of carbon-chlorine bonds resist the heat treatment. Furthermore, the total weight loss lies beyond the Cl content, because Cl is eliminated in the form of chlorinated carbon derivatives. The interesting observation is that chlorinated samples are stable up to 350°C and may therefore safely be investigated by IGC. As for brominated samples, the yf of chlorinemodified carbon blacks increases, particularly for the coarser samples. The same explanation but also the same questions still apply as for brominated samples. Moreover, the I,, values of ether increase slightly, but to a lower extent. For instance, for N990, the I,, of the chlorinated sample is 2.8 kJ mol-i, whereas it is 7.4 kJ mol-’ for the brominated carbon.

3.4 Grafted samples For this study, only two carbon black samples were selected: N115 and N990. 3.4.1 Anionic grafting One of the major difficulties when analysing grafted carbon blacks is the determination of the amount of grafted hydrocarbon chains. This may be achieved by elemental analysis using samples of high surface area (N115) as shown in Table 10, but fails with low surface area samples on which very limited amounts of chains remain attached. The increase in hydrogen content of grafted N115 suggests chemical modification. However, there is also an increase in the oxygen content. This might be accounted for by a limited surface oxidation, despite the care taken to exclude 0, from the reaction Table 10. Characteristics of anionically grafted samples %H N115 N115 N115 N115 N990 N990 N990 N990 N990

(ethyl) (propyl) (pentyl) (initial) (extr.) ( BuLi)

(methyl) (butyl)

0.2 0.4 0.2 _ _ _ _

%0

(m;;-*)

(i$zz)

342 _ 148 200 79 125 76 63 60

60 _ 14 28 3 14 2 1 1

1.05 2.81 2.88 _ _ _ _ _

Table 9. Characteristics of chlorinated carbon blacks d

Ys

(mJ m-*) XE-2 XE-2 chlor. N115 N115 chlor. N326 N326 chlor. N772 N772 chlor. N990 N990 chlor.

1000 _ 145 83 29 _ 8 _

381 415 342 523 288 268 153 193 121 270

% Cl 0

7.37 0 5.42 0 6.92 0 7.55 0 4.15

% occupied surface 0

3.7 0 20 0 41 0 135 0 271

%O 0.73 1.49 1.05 0.85 0.64 0.67 0.24 0.44 0.09 0.21

1528

E. PAPIRERet al.

medium, and by Li salts (probably oxides and hydroxides) that are not water-extractable (about 0.2%). IGC, on the other hand, is much more sensitive to surface-modification treatments. This is particularly apparent for treated N115. A more detailed study was made with N990, trying to detect the influence of each step of the treatment: solvent purification, butyllithium reaction, and treatment of the latter with alkyliodides. Solvent purification induces an appreciable increase of yt. This is explainable by the elimination of hydrocarbon contaminants that are known to be particularly present on those carbon blacks. The butyllithium (BuLi) treatment itself, i.e. without further addition of alkyl iodides, decreases the yp quite significantly. BuLi will interact with oxygenated surface groups (carbonyl, hydroxyl) and with CH, giving rise to very stable ether or carbonalkyl bonds. The reaction of the BuLi modified blacks with alkyliodides leads to an additional decrease of yt, suggesting that a supplementary quantity of alkyl chains is linked to carbon blacks. Solid-state NMR experiments (Bruker 200 MHz) of 13C or proton at a high rate of spinning (16 kHz) cannot provide a quantitative description of the grafting procedure. Indeed, 13C NMR shows evidence, especially in the CPMAS mode, of the presence of oxygenated surface groups on the blacks (principally on N990) of the carbonyl type, suggesting that N990 is less organised than N115: a result in agreement with the higher reactivity of N990 toward oxygen and halogens. However, no evidence of the presence of methyl groups was recorded. However, proton NMR shows the introduction of such groups on the carbon surface. A peak having its maximum at 1.6 ppm is observed on N990. After contact with the BuLi/CH,I reagents, a fine peak at 0.65 ppm superrimposes the principal peak. The sample treated with BuLi/butyliodide exhibits a similar NMR peak with a maximum at 0.5 ppm. The differences in width and height of the peaks of N990 samples carrying either methyl or butyl grafts probably originate from the fact that a butyl chain may eventually be folded and readsorbed on the carbon surface [24]. Hence, the graft will lose part of its freedom of movement, generating a larger NMR peak. One may thus conclude that NMR gives evidence for the grafting of alkyl chains, but is unable to deliver quantitative information on the surface coverage, for instance by short alkyl chains. 3.4.2 Free-radical grafting Table 11 displays the main results obtained by grafting lauroyl chains onto the carbon blacks. The amount of grafted chains was calculated from the results of elemental analysis and also from the weight loss, under inert atmosphere, at 500°C. Upon heating, N115 loses 1.8% weight in the 40-140°C temperature range, 6.3% between 140-540°C and 2.6% between 540-920°C. The total

Table 11. Characteristics

%H N115 (lauroyl) N990 N990 (lauroyl)

Nll5

0.2 1.1 0.5 0.6

of free-radical

%0

grafted

samples

Number of grafts nm-’

Y: (mJ m-‘)

_

342 & 60 62+3 125+ 14 27kl

1.05 1.95 0.09 0.60

1.1 _ 18

weight loss (Aw) is 10.7%. For N990, Aw equals 4.5%. Taking into account the losses measured on nongrafted blacks, it is possible to calculate the number of grafted chains per unit of surface area (the maximum number, taking 0.06 nm’ as the cross-section of a CH2group, would be 16 grafts nm-“). The results shown in Table 10 indicate that the surface of N115 is only partially covered with lauroyl chains, whereas that of N990 is totally covered (one might even expect some lauroyl derivatives to be physically adsorbed and non-removable from the surface layer of N990). 13C MAS NMR gives clear evidence of the presence of -CH, and -CH,groups on the surfaces of both grafted samples. Methylene groups show up at 25-30 ppm, whereas methyl groups appear at 15-20 ppm. The resolution is even better when using CPMAS NMR, which enhances the methylene and methyl signals, allowing a mathematical deconvolution of the spectra and the approximate calculation of the number of lauroyl chains per nm’. For N115, this value is 2 grafts nm-‘, which is in satisfactory agreement with the value calculated from elemental or thermogravimetric measurements. However for N990, the value is much too high (45 instead of 18). However, the value of 18 is very approximate and is probably comparable to that of NMR. Proton NMR also confirms (peak at 1 ppm) the presence of aliphatic groups having sufficient mobility, i.e. non-restricted movements by interaction with the carbon surface. As expected, carbon blacks covered with dense layers of lauroyl chains do exhibit very different surface properties, as ascertained by IGC. The yt of the grafted N115 drops dramatically. One comes closer to the yt of graphite, suggesting that the “active” surface is not entirely shielded by alkyl groups. For N990, treated with lauroyl peroxide, 7: (at 100°C) reaches the very low value of 18 mJ m-‘, a value that is close to that of a solid entirely made of methylene groups, i.e. poly(ethylene) having a yf or (ys) of 35 mJ rn-’ at 20°C demonstrating that it is totally covered with alkyl chains.

4. CONCLUSION One of the main points to be stressed is that carbon blacks, under similar reaction conditions, have different behaviours depending on their mode of preparation. However, they have in common the dependence of their reactivity on their peculiar surface

Chemical modifications and surface properties of carbon blacks structure, being very heterogeneous as far as the distribution of the active surface or the chemically reactive surface group repartition are concerned. In general, towards a given modification procedure, they perform comparatively, but not to the same extent. Particles of larger dimensions are chemically more reactive (per unit surface area), leading to higher surface coverage by oxygen (moderate oxidation conditions), halogens and aliphatic grafts than are very fine particles. This difference originates from the different formation conditions (temperature, residence time in the combustion medium, etc.) causing different atomic surface structures. Acknowledgements-The authors thank Drs P. Ehrburger and J. Dentzer, who kindly facilitated the measurement of the active surface area of carbons in their laboratory.

REFERENCES 1.

2. 3.

4. 5. 6.

In Carbon Black Science and Technology (Edited by J. B. Donnet, R. C. Bansal and M. J. Wang). Marcel Dekker, New York (1993). J. B. Donnet and E. Custodtro, Carbon 30, 813 (1992). W. Hess and C. R. Herd, In Carbon Black Science and Technology (Edited by J. B. Donnet, R. C. Bansal and M. J. Wang). p. 89. Marcel Dekker, New York (1993). B. R. Puri, G. K. Sharma and S. K. Sharma, J. Indian Chem. Sot. 44, 6 (1967). J. B. Donnet and J. C. Bouland, Carbon 4,201 (1966). E. Papirer, R. Lacroix, J. B. Donnet, G. Nanst and P. Fioux, Carbon 32, 1341 (1994).

J. B. Donnet and E. Custodero,

7. 8. 9. 10. 11.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

E. Papirer,

1529

R. Lacroix, J. B. Donnet, G. Nan& and P. Fioux, Carbon 33, 63 (1995). G. Nan&, E. Papirer, P. Fioux, A. Tressaud and F. Moguet, Carbon, in press. R. Lacroix, Ph.D. Thesis, Mulhouse (1992). T. G. Gyul’Misayan, L. P. Gylyazetdinov, V. P. Zvev and V. F. Kareeva, Kauch. Rezina 26, 32 (1967). J. B. Donnet, E. Papirer and A. Vidal, In Chemistry and Physics of Carbon-(Edited by P. L. Walker and P. Thrower), no. 171-204. Marcel Dekker. New York (1975). ‘. -C. M. Lansinger and J. B. Donnet, Proc. IRCPO, Paris, p. 12 (1990). M. J. Wang, S. Wolff and J. B. Donnet, Rubber Chem. Technol. 64, 714 (1991). E. Papirer, S. Li, H. Balard and J. Jagiello, Carbon 29, 1135 (1991). H. Balard and E. Papirer, Progr. Org. Coatings 22, 1 (1993). G. M. Dorris and D. G. Gray, J. Colloid Interface Sci. 71, 93 (1979). N. R. Laine, F. J. Vastola and J. Walker, J. Phys. Chem. 67, 2030 (1963). P. Ehrburger, F. Louys and J. Lahaye, Carbon 27, 389 (1989). E. Papirer, J. Dentzer, S. Li and J. B. Donnet, Carbon 29, 69 (1991). P. Staszczuk, Mater. Chem. Phys 14, 279 (1986). C. Moreno-Castilla, M. Domingo-Garcia and F. J. Lopez-Garzon, J. Colloid Interface Sci. 112, 294 (1986). F. J. Lopez-Garzon and M. Domingo-Garcia, Chromatographia 21,447 (1986). J. Jagiello and E. Papirer, J. Colloid Interface Sci. 142, 232 (1991). E. Papirer and D. Y. Wu, Carbon 28, 393 (1990).