Modification of coal-tar pitch by air-blowing — I. Variation of pitch composition and properties

Carbon, Vol. 33, No. 3, pp. 295-307, 1995 Copyright 0 1995 Elsevier ScienceLtd Printedin Great Britain. All rightsreserved

Pergamon

000%6223195 $9.50 + .OO

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MODIFICATION OF COAL-TAR PITCH BY AIR-BLOWING-I. VARIATION OF PITCH COMPOSITION AND PROPERTIES J. J. FERN~~NDEZ,

Institute

FIGUEIRAS,

A.

National

M.

de1 Carbon,

GRANDA,

CSIC,

J. BERMEJO,

Apartado

and

73, 33080 Oviedo,

R.

MEN~NDEZ

Spain

(Received 29 July 1994; accepted 20 September 1994) Abstract-Pitch oxidation by air-blowing has been studied as a possible method to modify pitch properties for composite preparation. Two commercial coal-tar pitches, an impregnating pitch and a binder pitch, were air-blown at temperatures between 250-3OO”C, for 18 and 14 h, respectively, to produce pitches of similar softening point ranging from 148 to 223°C. Parent and treated pitches were characterized by elemental analvsis. FTIR, thermal analysis (TMA, TGA. and DTA), solubility in toluene and quinoline, XPS, X-ray diffraction, and optical microscopy. Additionally, parent pitches were also characterized by extrography. Pyrolysis products of parent and treated pitches, obtained at 45O”C, were characterized by optical microscopy to monitor the influence of air-blowing on mesophase formation. Results show that pitch modification by air-blowing produces a significant increase in carbon yield, without restricting the fluidity necessary to pitch for impregnation. During air-blowing, the reactions of dehydrogenative polymerization and crossing-linking of oligomers occur. Key Words-Coal-tar composites.

pitch,

air blowing,

chemical

1. INTRODUCTION

Because

of their

are of worldwide tedious

manufacture

unique interest.

C-C composites

properties, However,

processing,

high

cost due to

underdeveloped

de-

poor fibre properties translation, and susceptibility to oxidation at high temperatures have restricted their use[ 1,2]. Therefore, C-C composites are used only in those critical applications where no other materials are suitable[3]. Despite these limitations, a strong demand for high-performance C-C composites at reasonable cost exists for several applications[4]. For such purposes the development of adequate matrix precursors contributes to increased composite performance, lowering cost processing. Moreover, C-C composites also can offer a modernization of conventional technology, which justifies their use for non high-technology applications[2,5]. Matrix precursors for C-C composites should meet the following requirements[2]: (a) high carbon yieldeven in carbonization at atmospheric pressure, (b) acceptable fluidity at temperatures suitable for filler impregnation and efficient wetting to the carbon substrate, and (c) ability to produce carbons with low porosity, high strength, and resistance to oxygen at high temperatures. Pitches have an important role as raw materials in the carbon manufacturing industry. Properties of derived carbons are controlled by chemical and physical properties of pitch and its behaviour during carbonization[6,7]. Moreover, operating conditions also make a contribution. The main disadvantages of commercially available pitches for high-density materials preparation are low carbon yields and a significant pore volume after carbonization, which produces undesirable effects on physical and mechanical propersign

methodology,

composition,

pitch properties,

pitch pyrolysis,

C-C

ties. Several impregnation and carbonization cycles are necessary to produce a dense-matrix carbon. Despite these limitations, pitches have potential as promising precursors of composite matrices because of their low price and flexibility to produce carbons of different characteristics depending on the processing conditions. In order to optimize pitch utilization, several procedures have been developed to modify pitch properties to achieve the preceding requirements (i.e., extensive removal of volatile fractions[8], controlled condensation[9], and addition of coking accelerators[ 10,l I]. Pitch oxidation by air-blowing promises to be very effective. Molecular cross-linking induced by oxygen functionality[l2-141 at low temperatures increases the molecular weight of some molecules, preventing their distillation and removal during the carbonization stage. Such treatments result in a viscosity increase and, simultaneously, the lamellar orientation of the aromatic molecules becomes more difficult to achieve. Both factors limit the growth and coalescence of mesophase formed during pitch heat treatment. Current studies are mainly concerned with the air-blowing of coal-tar pitches used as precursors of isotropic carbon fibres[l5-171, and relate to pitch ability for spinning. Fabrication of pitch-based C-C composites can be substantially improved from a better understanding of the chemical and physical properties of precursors and the chemistry involved in the whole process of preparation, which in turn controls the structure and properties of the final material. This will cover an area of limited understanding of C-C composites, associated with the “disconnection” between the fundamentals of characterization/processing and materials engineering. This paper is the first of a series of three dealing with the use of air-blown coal-tar pitches for the preparation of unidirectional C-C composites, and is con-

J. J. FERN~NDEZ

296

cerned with the effects of air-blowing, at different temperatures, on the structure and pyrolysis behaviour of two commercial pitches, discussing their potential use as matrix precursors for C-C composites preparation. Air-blown pitch properties are initially controlled by their softening point (SP), carbon yield (CY) and solvent solubilities. Spectroscopic and chromatographic techniques have been used to monitor the chemical changes involved during the oxidative treatment and these are related to pitch pyrolysis behaviour as determined by thermal analysis of pitches (TG, DTG and DTA) and the evaluation of mesophase by optical microscopy of pyrolyzed pitches.

et ul.

conditions were used at 275 and 300°C. Resultant pitches were ground and sieved to <0.4 mm particle size. CTPA and CTPB were also heated to 25O”C, under the same conditions, in an inert atmosphere to control the effect of the thermal treatment itself. Consequently, two related series of five pitches were prepared: one untreated pitch, one thermally treated pitch, and three air-blown pitches obtained at three different temperatures. Yields of air-blown pitches were determined from the weight of the pitch remaining in the reactor.

2.2 Characterization of pitches 2.2.1 Softening point. The softening

2. EXPERIMENTAL

2.1 Oxidation of pitches Two industrial coal-tar pitches, a binder pitch (CTPA) and an impregnating pitch (CTPB), were used as raw materials. Analysis and specifications are given in Table 1. The oxidation treatment used is similar to that described by Barr and Lewis[ 121. Pitches were air-blown in a 2-liter Pyrex reactor fitted with a stirrer. The reactor was heated with an electric mantle. The temperature was regulated with a Honeywell controller with a thermocouple immersed into the pitch. Variations in temperature were always within 1°C. One thousand g of pitch, <6 mm particle diameter, was preheated to 175’C for 30 min in nitrogen. At these conditions, the studied pitches soften to an adequate viscosity for the stirring operation. Heating was continued to 250°C at 3°C min-‘, under an air flow of 80 1 hh’ , and maintained until the viscosity of the pitch did not allow further stirring. CTPA and CTPB required 14 h and 18 h, respectively. The pitch was then quench-cooled to room temperature. The same

point of pitches (Table 1) was determined with thermomechanic Seiko Instruments equipment (TMA). Fifteen mg of sample was initially softened in an aluminium crucible, and then placed in the TMA with a section probe of 1 mm2 and a penetrating load of 10 g. Heating was carried out at 5°C min-’ under a nitrogen flow of 60 ml min.-‘. The softening point was determined from the intersection of tangents before and after the penetration. 2.2.2 Carbon yield. Four g of pitch was carbonized in a horizontal tube furnace to 9OO”C, at 1°C min-’ for 30 min in a nitrogen flow of 30 1 hh’. Pitch was placed in a ceramic pot 42 mm internal diameter and 11 mm height. Carbon yield was determined from the weight of the carbonaceous residue. 2.2.3 Solubility of pitches. The solubility of pitch in toluene was determined according to a standard procedure[ 181. Two g of pitch, <0.4 mm particle diameter, and 100 ml of toluene were placed in a 500 ml flask, heated to the boiling point, and maintained under reflux for 30 min. Filtering was carried out with a porous ceramic plate no. 4 and the residue

Table 1. Selected properties of studied pitches Elemental analysis (daf, %) Pitch CTPA CTPAO CTPA 1 CTPA2 CTPA3 CTPB CTPBO CTPBl CTPBZ CTPB3

CY (@IO)

TI (070)

Treatment

C

H

N

S

0*

C/H

SP (“C)

_

93.7 94.0 93.8 94.0 93.2

4.1 3.9 3.5 3.5 3.4

1.1 1.1 1.1 1.1 1 .o

l.li 0.5 0.3 0.4 0.5

0.5 1.3 1.0 1.9

1.9 2.0 2.2 2.2 2.3

72 92 151 171 223

48.4 54.3 70.8 72.1 19.4

35.1 42.9 59.8 62.6 70.0

13.0 15.8 39.3 49.2 53.5

92.0 92.3 92.7 93.1 92.1

4.5 4.5 4.0 4.1 4.0

1.1 1.2 1.1 I .2 1.1

0.5 0.4 0.4 0.4 0.6

1.9 1.6 1.8 1.2 1.6

1.7 1.7 1.9 1.9 1.9

54 70 148 164 219

35.2 37.8 62.4 64.4 61.9

20.8 23.7 52.7 53.8 60.5

3.0 4.2 39.2 42.9 45.9

250”C/N, 250”CIAir 2lS”C/Air 300”C/Air _ 250”C/N, 250”CIAir 275”C/Air 300”C/Air

*, by difference. t, sulfur and oxygen. C/H, atomic ratio C/H. SP, softening point TMA.

CY, carbon yield (1OOO’C). TI, toluene insolubles. QI, quinoline insolubles.

QJ (70)

Modification

of coal-tar

washed with hot toluene for complete removal of toluene solubles, and 10 ml of acetone. Solubility of pitch in quinoline was determined according to ASTM standards[l9]. Two g of pitch, <0.4 mm particle diameter, was placed in a lOO-ml flask with 25 ml of quinoline, and then heated in a water bath to 75°C for 20 min. The solution was filtered with a porous ceramic plate no. 5, and the residue washed with toluene and acetone for complete quinoline removal. 2.2.4 Extrography. Fractionation of parent pitches into classes of compounds of different functionality and molecular size was carried out by extrography, with a sequence of six solvents of increasing polarity (n-hexane, 64% n-hexane/36% benzene, chloroform, 95% chloroform/5% diethyl ether, 93% chloroform/7% ethanol, and pyridine), as described by Granda et al.[20]. 2.2.5 XPS. Oxygen contents of pitches were obtained both by elemental analysis and by XPS analysis of CTPB series, using a Leybold Heraeus equipment with MgKol X-rays of energy 1253.6 eV at a power of 130 W and in a residual vacuum lower than 5. 10m2 mPa. All peaks have been charge referenced to the major C-C/C-H 1s peak at 284.6 eV. An analyzer pass energy of 150 eV has been used to record broad scan spectra from which surface composition data have been calculated. High-resolution spectra on Cls and 01s zones have been obtained for each sample with a pass energy of 50 eV. 2.2.6 Infrared spectroscopy. FTIR spectra of parent and air-blown pitches were obtained with KBr pellets in a Perkin Elmer 1750 spectrometer. Two pellets of each sample (sample/KBr = 1:lOO) were used to minimize experimental errors. Spectra were corrected for scattering using two baselines (3800-1800 and then calibrated to cm-’ and 1800-660 cm-‘), 1 mg.cm-*[21]. 2.2.7 X-ray diffraction. X-ray diffractograms of powdered solids were obtained according to the Japan Society for the Promotion of Science procedure[22], with a wide-angle X-ray RIGAKU GeigerFlex diffractometer using Ni-filtered CuKoc radiation.

2.3 Pyrolysis of pitches TG/DTG analyses of pitches were carried out in a SETARAM thermal analyser. Thirty mg of pitch was heated to lOOO”C, at 10°C min-‘, under a nitrogen flow of 50 ml mini’ in a lOO-mm3 PtRhlO% crucible (5 mm diameter and 6.5 mm height). Pitch evolution during pyrolysis was monitored by measuring the mesophase in parent and air-blown pitches. About 4 g of each of the 10 pitches was placed in a ceramic pot, which was flushed with nitrogen and positioned in the centre of a horizontal tube furnace. The pitches were heated under nitrogen from 25 to and maintained at the maxi450°C at S”C.min-‘, mum temperature for 30 min. The sample was then removed from the furnace and rapidly cooled under nitrogen.

pitch by air-blowing-I

297

2.4 Optical microscopy of pyrolyzed pitches Pyrolysis products of each pitch were mounted in epoxy resin, polished, and examined using a Leitz microscope fitted with cross-polars and a one-wave retarder plate to record the optical texture of the pyrolyzed pitches. The different structural features were evaluated by a point-counting procedure on 500 points, statistically selected. These include: aggregates of QI and mesophase (similar to coke mosaics), mesophase spheres, coalesced mesophase (similar to coke domains), and isotropic material (remaining pitch). A series of coloured micrographs was taken for each heated pitch at 25x and 60x magnifications.

3. RESULTS AND DISCUSSION

3. I Characterization of parent pitches CTPA contains a high percentage of primary Ql (13 wt%), as shown in Table 1. Primary QI is a component of commercial coal-tar pitches, and is difficult to remove and consequently expensive. Furthermore, the influence of QI on carbon-carbon composite matrix properties (i.e., porosity, strength, and reactivity) is not clearly established. CTPA is more aromatic and less soluble than CTPB (Table 1). To gain a better knowledge of the chemical composition of the selected pitches and its relation to their behaviour during air-blowing, pitches were characterized by extrography, in terms of classes of compounds of different functionality and molecular size, and thermal analysis. Figure 1 shows the distribution of the extrography fractions of the two pitches. CTPB contains higher amounts of light neutral compounds (fraction II), and phenolic and nitrogen basic compounds (fraction IV) than CTPA[23]. Unrecovered material of CTPA is as high as 20 wt%, and is mainly composed of primary QI. Considering that CTPB is an impregnating pitch and considering the analytical data in Table 1, which shows a higher content of oxygen for CTPB than for CTPA, it is supposed that fraction IV of CTPB is richer in phenolic and other oxygen containing compounds. TG and DTG curves of pitches show interesting differences, as shown in Figure 2. As expected from the former analyses, the weight loss of CTPB is higher than that of CTPA at any temperature, and the differences become more important above 500°C. The maximum weight loss rate of CTPB at 395°C is due to its higher content of light compounds. Besides the principal maximum weight loss rate at 42O”C, the DTG curve of CTPA shows significant weight losses at 480°C and 535°C. As will be further discussed, the compounds lost at these temperatures are not affected by air-bIowing.

3.2 Properties of air-blown pitches Experiments were addressed to the preparation of air-blown pitches of a high carbon yield, without destroying their capacity for wetting and their ability to

J. 1. FERNANDEZ

298

et al.

lmCTPA

Fr.V

Fr.IV

Fr.1

Exttoowhy

Fr.VI

Fr.VII

Ql

UnncoMred

Fractions

Fig. 1. Relative distribution of fractions obtained by extrography, primary Ql content, and extrography unrecovered material of pitches CTPA and CTPB.

form well structured cokes, Therefore, air-blowing was carried out at temperatures of 250, 275, and 300°C and times long enough to achieve the maximum softening point (SP) compatible with an efficient stirring of pitches at 250°C. Required times were 14 h for CTPA and 18 h for CTPB. Original pitches were also thermally treated at 250°C and 14 h or 18 h, under nitrogen, to discriminate between the effects of airblowing and temperature. Neither air-blowing nor temperature (between 250 and 300°C) had a significant influence on pitch yields,

0.1

0.0

-0.1

70 60

-0.2

-0.3 40 -o-4oo

200

300

400

500

600

700

900

sod0 I

Temperature (’ C)

Fig. 2. TG and DTG curves of CTPA and CTPB.

5

which were similar for all pitches. Yields of 88 wt% 86-87 wt% were obtained for pitches from CTPA and CTPB, respectively. Air-blowing produced a significant increase in the SP of both pitches, whereas effects of temperature were substantially less (Table 1). The similar SP of both series of air-blown pitches is a consequence of the experimental conditions used (different times). The SP of air-blown pitches ranges from 151 to 223°C for CTPA series and from 148 to 219°C for those of CTPB. Coking capacity of pitches, as measured by the carbon yield (CY) obtained on pyrolysis at 9OO”C, increases in the same sense as the SP, varying at the initial stage of the air-blowing treatment from 48.4 to 79.4 wt%, and from 35.2 to 67.9 wt%, for CTPA and CTPB, respectively (Table 1). However, as the temperature increased, CY variations became smaller, showing in the range of 250-300°C only an increase of about 8 and 6 wt%, for CTPA and CTPB series, respectively. The major influence of air-blowing on CY is revealed by the big difference between the values obtained at 250°C under air and nitrogen. Under nitrogen, the increases of CY, TI, and QI are mainly due to volatile release during treatment. Solubility tests in toluene and quinoline (Table 1) show that the toluene-insoluble content of parent pitches is smaller than that of the lowest-temperature air-blown pitches by a factor of 2.0 and 2.5 for CTPA and CTPB, respectively. The increase with temperature is more moderate. Variations of the QI fraction and

Modification of coal-tar pitch by air-blowingwith air blowing are even more spectacular, especially for the case of the impregnating pitch, where the increase is by a factor of 13. Insoluble fractions of airblown pitches are much larger than those of the thermally treated pitches, which means that chemical reactions caused by air-blowing transform a considerable amount of TS and TI-QS into QI. However, in spite of the strong increase of QI content, the airblown pitches are totally isotropic, as observed by optical microscopy. The increase of SP, CY, TI, and QI produced by air-blowing is always higher for CTPB than for CTPA, probably because of the longer time of treatment. It is to be noted that an increase of temperature from 250 to 275°C produces increases of CY and SP much lower than those reached when temperature rises from 275 to 300°C. Despite the fact that SP is not the only parameter that controls pitch fluidity, in the case of CTPs with similar characteristics it can be taken as an approximate reference. Therefore, the variation of ACY/ASP ratio with the air-blowing temperature can be considered for the selection of adequate operating conditions for the preparation of C-C composites. From this point of view, pitches air-blown at 275°C can be considered as better matrix precursors.

3.3 Chemical composition air-blown pitches

of

Nitrogen and sulphur contents are not significantly affected by any treatment (Table 1). For oxygen, small changes are observed. CTPA and CTPB seem to have different behaviours during treatment, as deduced from variations of oxygen content. While CTPA exhibits a continuous increase of oxygen with the increase of air-blowing temperature, except for 275°C CTPB shows an irregular variation. Variations for CTPB can be additionally influenced by the distillation of light compounds bearing oxygen functionality. The two pitches show a minimum oxygen content after air-blowing at 275°C. As oxygen content calculated by difference could be considered unreliable,

I

299

XPS was also used to determine C/O ratios for the CTPB series. Results vary in the same way, showing a maximum of C/O ratio for the pitch air-blown at 275”C, as observed in Fig. 3. This variation could be indicative of a new pathway of oxidation-polymerization, which starts at a temperature near 275”C[24]. The strong increase of the SP between 275 and 300°C supports this hypothesis. FTIR spectra of air-blown pitches are similar to those of parent pitches, showing only a decrease of the aliphatic stretching absorbances. Despite the limitations of FTIR techniques for quantitative studies, relevant information can be obtained when applied to samples of similar origin, in a comparative way. To gain an insight into the mechanism of polymerization, ortho-substitution indices, Abs750/(Abs750 + Abss4e + Abss6& and aromaticity, Abs,Oso/(Abs,OsO + AbsZ9zo) were calculated from the out-of-plane bending aromatic carbonhydrogen bonds (700-900 cm-‘) and the stretching region of aromatic (3050 cm-‘) and aliphatic (2920 cm-‘) C-H bonds, according to Guillen et al. [21]. The variations of ortho-substitution and aromaticity indices with the different treatments of pitches are given in Figs, 4 and 5. Thermal treatment causes a decrease of the two indices because of the distillation of the smallest components. On the contrary, with airblowing there is a decrease of the ortho-substitution index and an increase of the aromaticity index. Values of the ortho-substitution index of CTPA and CTPB corroborate that the condensation degree of components of CTPA is higher than that of CTPB, and this is maintained throughout the air-blown series. However, the aromaticity index is higher for CTPB, and the evolution observed with air-blowing follows a clear increasing trend for the two series. Pitch polymerization, as indicated by the decrease of the ortho-substitution index, increases with airblowing temperature, especially for CTPA at 300°C. Moreover, the increase of the aromaticity index and C/H atomic ratio (Table 1) suggests that polymerization involves a significant loss of aliphatic hydrogen.

Ortho-6ubstituUonindex (xl 06)

70 76 60

Original 250

276

300

260%. N2 250C.Air

276X.Air

3W%,Air

Heat Treatment

Temperature (” C) Fig. 3. XPS analysis of CTPB series.

Fig. 4. Variation of the ortho-substitution with treatment.

index of pitches

.l. J

300

FERNANDEZ et al.

Aromatlclty index (x100) w)

Y =CTPA Swles

q CTPB Berlu,

B!-

80

Original

2M)C, N2 250X,Air

275C.Air

3WC,

/

Air

HeatTreatment Fig. 5. Variation

of the aromaticity treatment.

index of pitches

with

Hein[l3] and Barr and Lewis[l2] proposed, as the principal mechanism for molecular growth produced by air-blowing of pitches, the formation of intermolecular linkages by dehydration of oxidized molecules, according to the general equation:

k O2 + 2Ar + Ar-Ar

+ H,O.

Oxygen groups formed by oxidation of alkyl sidechains and naphthenic structures decompose to give Ar-Ar. Nevertheless, coal-tar pitches are highly aromatic mixtures, having only a limited amount of aliphatic hydrogen to ease the polymerization. The strong polymerization showed by increases of SP, TI, and QI of studied pitches must follow, at least in part, other pathways. Zent et al. [ 161, on the basis of an exhaustive chemical and structural analysis of a coal-tar pitch (80°C of SP, 0 wt% of QI, 87% of H,,) airblown at 3OO”C, concluded that after elimination of alkyl side-chains and aromatization of naphthenic structures, air-blowing created oxy-radicals, which promoted aromatic ring condensation by dehydrogenation and aromatization, giving rise to large and planar aromatic molecules similar to those obtained by thermal polymerization. As usual when dealing with such complex mixtures as pitches, it is difficult to distinguish between possible mechanisms involved in any kind of treatment, es-

Table 2. Varratron of I,,,,, air-blowing

pecially because several reactions can take place in concurrence. It is better to gain an insight into the prevailing mechanism through changes in the molecular structure. The first of the mechanisms above-mentioned leads to oligomeric cross-linked structures, whereas the second gives rise to highly condensed aromatic molecules. Changes in molecular structure promoted by air-blowing could be shown by the variation of Iortho and IAr of the TS and TI of the original and airblown pitches, as well as by the interlayer spacing, d 002 * of 10021 basal planes determined by X-ray diffraction. The changes in Iorrho and I,,.,, of CTPB series are shown in Table 2. The Iortho of the TS fraction slightly increases with air-blowing and air-blowing temperature, whereas in the TI fraction increases with air-blowing at 250 and 275”C, and slightly changes at 300°C. The Iortho increase means that the Tl compounds formed by polymerization at 250 and 275°C have a lower average condensation extent than those previously existing in CTPB, whereas those formed at 300°C have similar condensation extent. The variation of IAr values of TS fraction reveals that air-blowing at 250°C mainly promotes polymerization, which occurs with significant removal of H,, TS components with alkyl side-chains and naphthenic structures polymerize, giving rise to TI compounds. In this way, a significant amount of H,, is removed from TS fraction. At 3OO”C, however, not only H,, is removed, but also H,,, suggesting that at this temperature, polymerization also takes place by a mechanism that involves removal of H,,. The variation of I,, of TI fraction follows a similar trend. For similar H,,, the TI components formed at 250 and 275°C have higher existing in amounts of HAr than those previously CTPB, that is to say, the components formed by polymerization are less condensed than those existing in the original pitch. The contrary situation occurs at 300°C which suggests that at this temperature polymerization takes place by mechanisms that lead to very condensed aromatic structures. In summary, for CTPB pitch it seems that polymerization can occur by two different mechanisms, and air-blowing temperature has a marked influence on the proportion of one to the other. The increase in temperature probably promotes preferentially the mechanism proposed by Zent et al. [16], whereas at lower temperatures the main mechanism for CTPB pitch is that reported by Hein[l3] and Barr and Lewis[l2]. It is logic to suppose that the ratio of one

and I,,, calculated from FTIR spectra, with for the CTPB series

temperature

TI fraction

TS fraction CTPB

CTPBl

CTPB3

CTPB

CTPBl

CTPBZ

CTPB3

Io,ttlo

0.50

1Al

0.47

0.51 0.55

0.52 0.49

0.40 0.68

0.44 0.73

0.44 0.70

0.41 0.64

Modification

of coal-tar

Table 3. X-ray interlayer spacing [002] basal planes Pitch CTPA CTPAO CTPAl CTPA2 CTPA3

doe, (A)

Pitch

dooz (A)

3.60 3.58 3.51 3.56 3.54

CTPB CTPBO CTPBl CTPBZ CTPB3

3.68 3.61 3.63 3.63 3.63

to the other mechanism is also influenced by the composition and molecular structure of the parent pitch. Results of X-ray diffraction in Table 3 show that air-blowing causes a significant decrease of d,,, similar for the two series, although the influence of airblowing temperature is only evident for CTPA. On the other hand, the high value of dW2 for CTPB series reduces their relevance in relation with the planarity of the constituents macromolecules. On the other hand, values of CTPA series approach those of some structured materials. Therefore, one can conclude that the decrease of dooz points to the prevalence in the two pitches of the mechanism of polymerization proposed by Zent et al. [ 161, especially for CTPA. The difference between doo2 values of both series of pitches highlights the strong influence of parent pitch characteristics on resulting molecular structures, as well as the minor relevance of the air-blowing temperature for CTPB pitch. The low value of doo2 exhibited by CTPA3 agrees with the significant fall in the orthosubstitution index of this pitch (Fig. 5).

3.4 Effects of air-blowing on pitch pyrolysis behaviour determined by thermal analysis Table 4 summarizes the main results obtained from the TGA and DTG curves of the two series of pitches. Air-blown pitches from CTPA show lower weight loss than those from CTPB, this difference being higher than 10% at 6OO”C, and closely related to the weight

Table 4. Results of pyrolysis of pitches obtained analysis

by thermal

Weight loss at 600°C

Ts

Tf

Tm

CTPA CTPAO CTPA 1 CTPAZ CTPA3

51.2 44.1 28.8 26.4 20.5

230 260 330 350 360

560 560 580 600 600

400 420 440 445 450

CTPB CTPBO CTPBl CTPB2 CTPB3

64.0 57.4 39.3 38.0 34.0

200 200 320 330 330

560 560 565 575 580

395 410 440 440 440

Pitch

Ts: Weight loss starting temperature (“C). Tf: Weight loss final temperature (“C). Tm: Temperature of maximum weight loss rate (“C).

pitch by air-blowing--I

301

loss of parent pitches (Table 1 and Fig. 2). It is to be noted that weight losses of CTPAl and CTPA2 are similar, whereas CTPA3 losses are less. Similar trends are observed in the CTPB series suggesting, again, the occurrence of some changes in the polymerization mechanism at temperatures over 275°C. Concerning the weight loss and the composition of parent pitches above mentioned, the temperatures at which the weight loss starts (T,) and finishes (rJ) are lower for the CTPB Series than for the CTPA series. On the other hand, the temperature of maximum weight loss rate (7”) is similar, although DTG curves of the airblown CTPA pitches show two other small maxima at higher temperatures, exactly the same as that for CTPA (Fig. 2). This fact suggests that the pitch components that distill at these temperatures are not affected by air-blowing. The shift of the maximum weight loss rate to higher temperatures as a consequence of air-blowing is clearly revealed by values shown in Table 4. The amount of volatile components evolved in airblowing polymerization can be estimated from the weight loss of parent and treated pitches. For the CTPA series, the amounts are 8, 9, and 13 wt%, for CTPAl, CTPA2, and CTPA3, respectively. DTA curves (Fig. 6) are of particular interest. They show an interval between 420 and 450°C where “endo” effects of the distillation are predominant. These effects are more marked for the pitches of the CTPB series and decrease in both series with air-blowing temperature. Between 450 and 520°C the “exo” effect of the polymerization prevails, overlapping small “endo” effects attributable to cracking reactions. CTPB and CTPBO show a maximum “exo” peak at 52O”C, which shifts to slightly higher temperature with air-blowing severity. This peak has been connected with the hardening of pitch[25]. Above 520°C CTPB and CTPBO do not show relevant peculiarities. However, air-blown pitches show a notable “endo” effect at 600°C of decreasing intensity with air-blowing temperature. This effect, due to cracking reactions, reveals the instability of the large macromolecules formed by air-blowing of CTPB at this temperature, and it is particularly strong for CTPBl. Pitches of CTPA series do not show any notable effect over 550°C. On the other hand, CTPA and CTPAO show an important “endo” effect at 53554O”C, which decreases with air-blowing temperature and disappears for CTPAS. Furthermore, CTPA3 shows a sequence of sharp “endo” and “exo” peaks between 460 and 52O”C, which could be understood as a more extensive participation of cracking and repolymerization reactions in the mechanism of molecular growth of this pitch. Understandings of cause-effect relationships in DTA do not give reliable information from DTA curves about differences in pitch structure and composition originated by air-blowing and their relation with the composition of parent pitches and air-blowing temperature. However, some speculation is possible.

J. J. FERNANDEZ ef al.

302

CTPA

CTPB

SERIES

SERIES

CTIA2

4

CTPB

CTPA

)

Tomperoturr

200

400

Temperature

(“C)

Fig. 6. DTA curves of CTPA

300

and CTPB

series.

500

(“C)

600

Modification

of coal-tar

First of all, the shift to a higher temperature of the “exe” peaks placed at 520°C for CTPB could be indicative of a delay in the solidification stage because of decreased reactivity of the constituents macromolecules. More significant seems to be the strong “endo” peak for CTPBl at 600 and 7OO”C, which shows the peculiar composition of this pitch and the influence of air-blowing temperature on the overall results of the polymerization. Regarding pitches of CTPA series, the decrease of the intensity of the notable “endo” effect of CTPA at 540°C as the severity of air-blowing increases is indicative of the influence of air-blowing temperature on the transformation of specific compounds of CTPA, and probably of changes in the mechanism of airblowing polymerization; this is also supported by the peculiar sequence of “endo” and “exe” peaks of CTPA3. Except for the differences mentioned, DTA curves of air-blown pitches resemble those of the parent pitches, showing that some of the properties of the parent pitches remain after air-blowing.

3.5 Effects of air-blowing of pitch on mesophase development It is expected that changes in size and chemical structure of pitch components produced by airblowing will affect the development of mesophase during pitch pyrolysis. The ability of air-blown pitches to form mesophase was monitored by optical microscopy of their pyrolysis products. Optical micrographs of Figs. 7 and 8 show the main structural features of some of the pyrolyzed pitches. In the pyrolysis products of the CTPA series, primary QI are not uniformly distributed. They are generally grouped mainly in the lower part of the ceramic pot, associated with mesophase, forming aggregates with a structure similar to that of cokemosaics (Fig. 7d, position D). CTPA air-blown pitches also show large and clean mesophase spheres, QI free, which are reminiscent of those typical from low-viscosity systems (Fig. 7~). On the other hand, in the parent CTPA and that thermally treated, QI particles always appear surrounding the mesophase spheres, and consequently hinder their growth and produce deformations (Figs. 7a,7b). Besides mesophase spheres and aggregates, pitch

Table 5. Main structure

Pitch

QI

features

Isotropic

pitch by air-blowing-

303

pyrolysis products also contain areas of coalesced mesophase, similar to coke domains (Fig. 7d, position C), and isotropic pitch (Fig. 7, position B). The negative effect of the primary QI on the mesophase development is more evident in the parent and thermally treated pitches (Figs. 7a,7b). Pyrolysis products of CTPB series are different from those of CTPA. CTPB and that thermally treated mainly consist of small and well shaped spheres of mesophase in an isotropic pitch matrix (Fig. 8a). The air-blown pitches, however, show areas of coalesced mesophase, which decrease in size with increasing severity of air-blowing treatment. These areas resemble clusters of grapes of very small size, difficult to classify as mesophase spheres or coalesced mesophase (Fig. 8b). The effect of primary QI is irrelevant in this series. Table 5 shows the participation of the different mesophase forms and the remaining isotropic pitch present in the pyrolysis products of the two series of pitches. Total anisotropic material content is given in Table 5. Measurement of the anisotropic part of the aggregates is extremely difficult because of the small size and irregular shape of the mesophase. Therefore, the anisotropic material in aggregates was calculated by subtracting from the percentage of aggregates, the primary QI content. Data in Table 5 show that airblowing of CTPA increases the formation and development of mesophase. The amount of anisotropic material increases from 44.1 ~01% for CTPA to 55.4 ~01% for CTPA3; coalesced mesophase goes from 0.0 ~01% to about 25.0 ~01%. On the other hand, in the CTPB series, a slight negative effect of air-blowing on the generation of mesophase is observed, in addition to a decrease in size of the crystalline microstructures with the increase of the air-blowing temperature. This different behaviour of air-blown pitches between series confirms the existence of two mechanisms of polymerization that take place simultaneously during air-blowing, and contribute to differing extents, depending on the nature and structure of the components of pitch and the air-blowing temperature. The delay in the mesophase formation and the decrease in size of the crystalline microstructure for CTPB airblown series could be a consequence of the higher con-

of pyrolyzed Aggregates minus QI

I

pitches, Mesophase spheres

obtained

by optical

Coalesced mesophase

microscopy

Anisotropic

CTPA CTPAO CTPAl CTPAZ CTPA3

19.9 21.0 18.2 17.4 16.0

36.0 36.0 38.6 30.4 28.5

21.6 25.0 26.2 18.8 25.6

22.5 12.7 5.6 5.6 6.4

0.0 5.2 11.3 27.8 23.4

44.1 43.0 43.2 52.2 55.4

CTPB CTPBO CTPBI CTPB2 CTPB3

6.0 6.2 5.4 5.3 4.7

46.2 43.5 47.7 52.5 50.3

0.0 0.0 0.7 1.7 0.0

47.8 50.3

0.0 0.0 -

47.8 50.3 48.0 41.2 45.0

-

304

J. J

FERNANDEZ

et ul.

Fig. 7. Optical micrographs of pyrolyzed pitch/mesophase systems, obtained at 45O”C, with a residence time of 30 min, from (a) CTPA, (b) CTPAO, (c) CTPAl, and (d) CTPAZ. A: mesophase sphere; B: isotropic pitch; C: coalesced mesophase; and D: aggregates.

Modification

of coal-tar

pitch by air-blowing-I

Fig. 7 continued.

305

306

J. J. FERNANDEZ

et al.

Fig. 8. Optical micrographs of pyrolyzed pitch/mesophase systems, obtained at 45O”C, with a residence time of 30 min, from (a) CTPB, (b) CTPBl. A: mesophase sphere; and B: isotropic pitch.

tribution of the polymerization mechanism proposed in refs. [12] and [13], which leads to the formation of cross-linked oligoaryl macromolecules. Cross-linkages have to be cleaved and restructured before forming

mesophase. This is in agreement with the results obtained by IR-spectroscopy, X-ray diffraction profiles, and thermal analysis. On the other hand, in the polymerization by air-blowing of CTPA, the prevailing

Modification

of coal-tar

mechanism should be that proposed by Zent et al. [161, and its contribution increases with the temperature of treatment. The variation of C/H ratio with air-blowing (Table 1) also supports this conclusion. C/H ratio of CTPA series changes from 1.9 to 2.3; the variation is mainly due to air-blowing. The values of CTPB series go from 1.7 to 1.9, although CTPA3 and CTPB3 have similar softening points, indicative of similar amounts and size of macromolecules. Therefore, compounds formed from CTPA are much more condensed than those obtained from CTPB at similar leads of polymerization.

4. CONCLUSIONS

Coal-tar pitch modification by air-blowing, at temperatures between 250-3OO”C, for periods of 14-18 h, results in a significant increase in carbon yield without destroying the ability of the pitch to form anisotropic carbons. Variations of ACY/ASP ratio with air-blowing temperature suggest that pitches air-blown at 275°C may be the most suitable of matrix precursors. Variations in chemical and physical properties of pitches with air-blowing, as well as their thermal behaviour, suggest the concurrence of two main mechanisms in air-blowing polymerizations. One leads to the formation of cross-linked olygomers and the other to the formation of large planar macromolecules through extensive ring condensation. The relative participation of these reactions depends on the chemical composition of the parent pitch and the temperature of air-blowing. The second mechanism leads to better structured aromatic units, which accelerate mesophase formation and improve the microcrystalline structure of cokes. The formation of cross-linked molecules can delay mesophase formation and impair its structure, because they need bond cleavage associated with a high-viscosity system. Primary QI had a more negative effect on the mesophase development in the parent and thermally treated pitches, being more relevant for the binder pitch series. Acknowledgements-This research was supported by the CICYT (Project MAT91-0877) and FICYT. Pitches have been

pitch by air-blowing-

I

307

provided by Industrial Quimica de1 Nal6n. The authors would like to thank Angeles Martin for XPS analysis.

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