Carbon 1964, Vol. 1, pp. 315-320. Pergamon Press Ltd. Printed in Great Britain

FACTORS INFLUENCING THE GRAPHITIZATION

OF POLYMER CARBONS

J. J. KIPLING, J. N. SHERWOOD, P. V. SHOOTER and N. R. THOMPSON

Department of Chemistry, The University, Hull, England

(Received 23 _&ly 1963)

Abstract-Carbons have been prepared by carbonization of polymers to 7OO”C, and subsequent heating to 2700 or 3000°C. The helium density of the high-temperature carbons indicates whether or not they are graphitic ; this has been checked by X-ray examination. Whether or not a given carbon graphitizes at high temperature is determined by the behaviour in the carbonization stage, and in some cases in the very earliest stages of decompos%ion. A graphitizing carbon is not obtained unless fusion occurs during carbonization; substantial cross-linking therefore prevents graphitization. Such cross-linking includes that which occurs when the dehydrochlorination of polyvinyl chloride takes place at 200-250” in the presence of oxygen. For graphitization to be possible, it is probable that a fused state must be maintained over a range of temperature during carbonization. This suggests that the material decomposing at this stage should be of a heterogeneous character. This is discussed in relation to the chemical character of the polymers. Important evidence on the significance of the fusion zone is provided by examination of graphitizing and non-graphitizing carbons by polarized light.

1. INTRODUCTION

MORE specific requirements are being made as to the properties, such as density and pore structure, of synthetic graphite used in nuclear power instal-

lations. Attention is therefore being paid to

modifying existing commercial materials (e.g. by various impregnation processes) and to using new starting materials. In the choice of possible im- pregnants and other new materials, it is desirable to know whether the carbons formed from them

,vill undergo graphitization when heated to temperatures in the range 2700-3000”. The factors which influence graphitization are not yet well enough understood for prediction to be very satis- factory, and an investigation has therefore been made of a number of factors likely to be important.

In distinguishing between graphitizing and non- graphitizing carbons, FRANKLIN(~) used carbons derived from polyvinyl chloride and polyvinylidene chloride. The use of such polymer carbons is appropriate to such investigations, because more can generally be known of the genesis of the low- temperature carbon,(2) and even of its structure, than is the case with the raw materials used in the manufacture of commercial graphite. An examina-

tion of a further range of polymer carbons therefore seemed desirable. In view of the interest which exists in polynuclear aromatic substances which can be carbonized as single crystals,(3) we have included dibenzanthrone in these studies.

2. CRITERIA OF GRAPHITIZATION

The occurrence of graphitization must ulti- mately be assessed by X-ray methods, but it is useful to make an initial assessment in terms of a

property of the carbon which can be measured easily and quickly. Helium densities have been

found very satisfactory for this purpose, as is shown for a series of carbons in Table 1. The carbons were subjected to X-ray examination by Dr. H. E. BLAYDEN of the British Coke Research Association;* details of the method have been described previously. (4) The carbons showed either distinct three-dimensional graphitization or diffuse patterns characteristic of turbostratic structures. The latter gave crystallite dimensions (uncorrected for a number of interfering factors) in the range 20-40 A.

*And in one case, by Dr. H. P. STADLER of King's College, Newcastle.

315

316 J. J. KIPLING, J. N. SHERWOOD, P. V. SHOOTER and N. R. THOMPSON

The clear distinction between graphitic and ponding division between the graphitic and non- non-graphitic carbons made by X-ray examination graphitic carbons. This is valuable because the is paralleled by a clear separation of the helium mercury density is easier to measure than the densities. For particles of the size used in the helium density. experimental determinations, the helium density It is very valuable to find that a range of physical would not be expected to equal the value given by measurements all distinguish clearly between the X-ray measurement (2.26 g/cm3), as a small graphitizing and non-graphitizing carbons made volume of closed pores may be present in sub- from synthetic polymers. The properties now stantial pieces of the material. The results suggest, include X-ray diffraction, magnetic susceptibility,(4) however, that materials with helium densities of concentration of unpaired electrons,(4) densities in over 2 g/cm3 are likely to prove to be graphitic. various fluids, and the existence or otherwise of

The densities in mercury are also given in optical anisotropy (see below). Table 1. These were determined at little more than atmospheric pressure, so that the mercury would 3. FUSION DURING CARBONIZATION

penetrate between particles, but not into open Polyvinyl chloride and polyvinylidene chloride pores of diameter less than about lo5 A (10 p). differ in their modes of decomposition in a manner The values are therefore lower than those found which is probably significant for subsequent graph- with helium. They do, nevertheless, show a corres- itization. Polyvinyl chloride first loses hydrogen

TARLE~. HELIUM AND MERCURY DENSITIES OF CARBONS PREPARED AT 2700” AND 3000”

Starting material Density in helium

(g/cm? 2700” 3000”

Polyvinyl chloride Polyvinyl acetate Polyacenaphthylene

Dibenzanthrone

2.21 2.26 2.17 2.13 2.22 2.23

Density in mercury

(g/cm”) 2700” 3000”

1.81 1.70 1.97

1.94

1.97

graphitic graphitic

graphitic

graphitic

Cellulose 1.56 1.70 1.13 1.24 non-graphitic Coconut shell 1.36 1.46 1.23 1.35 non-graphitic Polyacrylonitrile 1.57 1.59 144 1.46 non-graphitic Phenol-formaldehyde (alkaline) 1.52 1.43 1.52 1.42 non-graphitic

TABLE 2. PROPERTIES ASSOCIATED I NI’ TH NON-GRAPHITIC CARBONS

Starting material Nature of

700” carbon

Density in helium Density in mercury (gl~“) (g/cm”)

2700” 3000” 2700” 3000”

Phenol-formaldehyde (alkaline) Char 1.52 1.52 Phenol-formaldehyde (acidic) Coke 1.49 1.43 Phenol-benzaldehyde (alkaline) Char 1.43 1.32 Phenol-benzaldehyde (acidic) Coke 1.42 144 1.42 1.41 Polyfurfuryl alcohol Coke 1.45 1.43 “Cascote” resin Coke 1.42 1.42 Cellulose triacetate Coke 144 1.46 1.38 Polymethyl a-chloroacrylate Char 1.34 1.16 Polyvinyl bromide Char 1.50 1.40 Polyvinylidene fluoride Coke 1.20 1.21

Styrene-50% divinylbenzene Coke 1.49 1.52 1.47 1.49 Styrene-25 y0 divinylbenzene Coke 1.52 1.62 1.43 1.51

X-ray result

FACTORS INFLUENCING THE GRAPHITIZATION OF POLYMER CARBONS 317

chloride, leaving a carbonaceous residue which then undergoes a carbonization process analogous to that of coal,(5*6f and during this stage passes through a fused state. By contrast, the entire decomposition of polyvinylidene chloride occurs with loss of hydrogen chloride, and no state of fusion occurs.(5)

A state of fusion occurs during the carbonization of all of the substances listed in Table 1 which give graphitic carbons, but for none of those which give non-graph& carbons.

Examination of a wider range of carbons (Table 2) shows that fusion sometimes occurs prior to the formation of a non-graphitic carbon, but a graphitic carbon has not been made without the occurrence of a fusion stage. Visual inspection of the 700” carbon is sufficient to detect the occur- rence or otherwise of fusion, the carbon having the appearance of a coke or a char, respectively, as these terms have been used for describing polymer carbons.f5, 6,

Further evidence is provided by examining the decomposition of polymers in a Gieseler plasto- meter. These tests were carried out by Mr. E. Bradburn of the National Coal Board, using a plastometer designed for the examination of coals. It showed that fusion of materials, e.g. ceIlulose triacetate, which give rise to non-graphitic carbons, occurred at a single temperature, whereas it occurred over a range of temperature when graphitic carbons were formed. This suggests that to produce graphitic carbons it may be desirable that the decomposing material should consist of material varying in moIecular complexity during the fusion stage, and not of identical molecules which would all undergo decomposition at the same temperature. This phenomenon is being examined in more detail.

4. THE EFFECT OF CROSS LINKING

4.1 By oxygen Carbonization of polymers is normally carried

out in an inert atmosphere. It has been observed, however, that if the dehydrochlorination of poly- vinyl chloride occurs in the presence of air (below ZSOO), a char is formed subsequently, instead of a coke, even when the subsequent carbonization (250-700”) is carried out in nitrogen.t7) The material which has been oxygenated in this way gives rise to a non-graphitic carbon. For materials which would, in any case, give non-graphitic carbons, the presence of oxygen in the first stage of heating (below 250”) is not important (Table 3).

The effect of oxygen on decomposing polyvinyl chloride shows that the property of graphitizing may be determined in the earliest stages of de- composition of the starting material. At this stage large percentages of oxygen are taken up by some substances,(‘) initially as hydroperoxide groups,(*) which then decompose to cross-link adjacent chains through carbonyl groups.(gf This extensive cross-linking would inhibit fusion during the later stage of carbonization.

4.2 In the original polymer It is almost certain that some degree of cross-

linking must occur if a decomposing linear polymer is to form a carbon. Extensive cross- linking, however, would give a three-dimensional structure which would probably not fuse and therefore could not give rise to a graphitic carbon. This is found with phenol-formaldehyde resin (poIymerized in the presence of alkali) which is believed to be cross-linked extensively.

4.3 During decomposition The degree to which cross-linking of the residue

occurs during the early stages of decomposition of

TABLE 3. EFFECT OF OXYGENATION BELOW 250" ON PROPERTIES OP CARBONS

Starting material

Polyvinyl chIcwide Polyac~Ioni~ile Cellulose Phenol-formaldehyde (akaline) Styrene-50% divinylbenzene

Nature of Density in helium Density in mercury

700” carbon (g/cm”) (gicms) 2700” 3000” 2700” 3000”

Char 1.49 1.52 1.34 1.37 Char 1.57 1.63 1.43 1.45 Char 1.48 169 1.15 1.25 Char 1.51 1.46 1.50 1.44 Char 1.41 l-46 1.34 1.34

318 J. J. KIPLING, J. N. SHERWOOD, P. V. SHOOTER and N. R. THOMPSON

the polymer is important because it should be possible to correlate this with the type of chemical grouping present in the polymer and to use the information in choosing substances likely to give either graphitizing or non-graphitizing carbons. This requires a study of the mechanisms of degra- dation which has not been undertaken in all cases. In two cases the position is clear. Inter-chain dehydration of cellulose (with formation of ether links) is the form of cross-linking most likely to be important. It has also been shown(6’10) that the cyanide group in polyacrylonitrile can function as an unsaturated group capable of undergoing both inter- and intra-chain condensation.

In addition to this, the structure of the polymer and its chemical character are important in de- termining whether or not fusion occurs before the onset of decomposition. Hydrogen-bonding is a factor preventing this in cellulose, but there are no polar groups to do so in polyacenaphthylene. On the other hand, the ease of breaking the C-Cl bond in polyvinyl chloride gives it an inherently low temperature of decomposition.

5. OPTICAL RESULTS

The importance of a zone of fusion is reinforced by results from optical studies made by Dr. H. E. BLAYDEN. Polished specimens of carbon were examined by polarized light microscopy.(4)

The non-graphitizing carbons were found to be optically isotropic whether prepared at low or at high temperatures. The graphitizing carbons, however, showed a distinct optical anisotropy. This was associated with striations in the carbon which appeared to be the result of flow of the decompos- ing material during carbonization. The effect was present in a 500” carbon (Fig. 1) made from polyvinyl chloride as well as in a carbon prepared at 2700” (Fig. 2).

Carbons made from polyvinyl chloride which had been decomposed with access to oxygen in the initial stages showed no such evidence of flow (Fig. 3) and no anistropy, even after being heated to 2700” (Fig. 4). The non-graphitizing carbons made from other polymers showed behaviour very similar to that illustrated in Figs. 3 and 4. Thus an orientation of the carbonaceous material appeared to occur at quite low temperatures in the precursor of a graphitizing carbon, but not in that of a non- graphitizing carbon. Such orientation, commenced

with the fragments of decomposing polymer, could be continued at temperatures above 2000”.

6. EXPERIMENTAL

(u) Polymers

The nature and source of most of the polymers used has been recorded previously.(’ .7) Polymethyl a-chloroacrylate was supplied in sheet form by Imperial Chemical Industries Limited, Plastics Division.

Dibenzanthrone (purified) was supplied by Imperial Chemical Industries Limited, Dyestuffs Division.

The polyvinylidene fluoride was a sample of “Kynar”, manufactured by Pennsalt Chemicals Corporation.

Polyacenaphthylene was prepared from the monomer essentially as described by SORENSON and CAMPBELL, except that boron trifluoride was used as initiator in the form of its complex with ether. The polymer can also be made by thermal polymerization of the monomer. A carbon, pre- pared at 1000” from the “thermal” polymer by Dr. BLAYDEN, was subsequently heated to 2700” and had substantially the same properties as the carbon prepared from the polymer made at a lower temperature.

Phenol-formaldehyde (acidic) was prepared by refluxing a mixture of 1.67 moles of formaldehyde (used as a 40 per cent aqueous solution) with 2 moles of phenol and 5 g of 37 per cent hydrochloric acid for about 3 weeks; the viscous liquid formed at this stage was then heated on a flat tray in an air-oven at 80” for several days.

Phenol-benzaldehyde (acidic) was prepared by RIESZ and SUSMAN’S method. In the alkali- catalysed preparation, the same proportions were used with the same numbers of equivalents of sodium carbonate as of acid in the acid-catalysed preparation.

Polyfurfuryl alcohol was made by heating 500 g of furfuryl alcohol and 0.4 per cent of 2 N hydro- chloric acid in a flat metal tray in stages to 180”, when polymerization occurred with violent effer- vescence. “Cascote” M.558 was supplied by Leicester, Love11 & Co. Ltd. It was polymerized similarly.

Styrene and divinyl benzene were co-polymer- ized in bulk by gentle heating in sealed tubes.

!m.;ny p 3

FIG. 3. Carbon produced at SOO”, carbonization with initial access to oxygen.

FIG. 4. Carbon produced at 2700”, carbonization with initial access to oxygen.

FACTORS INFLUENCING THE GRAPHITIZATION OF POLYMER CARBONS 319

Polyvinyl bromide was prepared by irradiating a solution of the purified monomer in ethylene dibromide with ultra-violet light.

Powdered polymers were compressed into pellets prior to carbonization.(5) In most cases carbonization was carried out at S”/min to 700”, in a stream of nitrogen. Polyfurfuryl alcohol and the “Cascote” resin, however, were heated in nitrogen at 6”/hr from 180 to 500” and then at a faster rate to 700”.

(c) Gruphitization

This was carried out in a stream of argon in a graphite-tube resistance furnace manufactured by Spembly Limited and similar in design to a iabora- tory furnace used at A.E.R.E., Harwell.

(d) Measurement of helium densities

Measurements of helium density were made at room temperature. The apparatus used is illus- trated in Fig. 5. It consists of a sample bulb surrounded by a thermostated jacket and con-

t Vacuum

_--Manometer arm

Calibrated

bulb

Mercury reservoir

FIG. 5 p Diagram of apparatus used for measuring helium density.

netted by capillary tubing and a cone-and-socket joint to a calibrated reference bulb, and hence to a mercury manometer. From pressure readings made at the three mercury levels shown (1, 2 and 31, it is possible to eliminate any variation in volume of the joint in the calculation of the volume of free space in the sample bulb. While this apparatus was being constructed, a description of an apparatus based on similar principles was published.(13) Our apparatus was designed, not to give results of the highest accuracy, but to provide data of diagnostic value rapidly for a considerable number of samples.

(e) Measurement of mercury densities

The apparatus was similar to that described by BEIRNE and HVTCXEOK;. f*4) The sample is de- gassed before the measurement is made. A pressure of mercury of about 40 cm above atmos- pheric is used; this is small enough to avoid penetration of mercury into any except the largest pores.

For measurement of helium and mercury densities, the carbon samples were sieved to 20-40 B.S.S.

Acknowledgements-m’e are indebted to Dr. H. E. BLAYDEN and to Dr. H. P. STADLER for permission to quote results of measurements, to Dr. BLA~DEN for permission to reproduce Figs. l-4, to Mr. E. BRADBUKN, of the National Coal Board, for the results of the plasto- meter test, and to Mrs. M. SCHOFIELD for assistance with the experimental work. We are grateful to the United Kingdom Atomic Energy Authority for financial support of this work and for the provision of the graphi- tizing furnace; to Imperial Chemical Industries Limited, to L&ester, Love11 & Co. Ltd., and to Pennsalt Chemicals Corporation for gifts of chemicals.

REFERENCES

1. FRANKLIN R., Proc. Roy. Sot. A7.09, 196 (1951).

2. WINSLOW F. H., MATREYEK ‘CV. and Y.%GER W. A., Industrial Carboll and Graphite, p. 190. Society of Chemical Industry, London (1958).

3. &ADLER H. P., Proceedings of the Fourth Carbon Cosferewce, p, 337. Pergamon Press, London (1960); STADLER H. P. and THOMSON C. M., Industrial Carbon and Graphite, p. 186. Society of Chemical Industry, London (1958).

4. BLAYDEN H. E. and WESTCOTT D. T., Proceedings of the Fifth Carbon Conference. Vol. II, p. 97. Pergamon Press, London (1963).

5. GILBERT J. B., KIPLING J. J., MCENANEY B. and SHERWOOD J. N., Polymer 3, 1 (1962).

6. GILBERT J. B. and KIPLING J. J., fjue141.493 (1962); Fuel 42, 5 (1963).

320 J. J. KIPLING, J. N. SHERWOOD, P. V. SHOOTER and N. R. THOMPSON

7. KIPLING J. J. and WILSON R. B., Proceedings of Residential Conference on Science in the Use of Coal (Shejield, April, 1958) p.C6. Institute of Fuel, London (1959).

8. WINKLER D. E., r. Polynt. Sci. 35, 3 (1959); CAMPBELL J. E. and RAUSC~ W. H., r. Polym. Sci. 18. 461 (1955): ACHHAMMER B. E.. Polvmer Degradhtion kIech&&ns. Circular No. i25 of the U.S. Bureau of Standards, p. 120. Washington D.C. (1953).

9. GILBERT J. B. and KIPLING J. J. Fuel 41, 249 (1962).

10. BURLANT W. J. and PARSONS J. r. Polym. Sci. 22, 249 (1956).

11. SORENSON W. and CAMPBELL T. W., Preparatiwe Methods of Polymer Chemistry. Interscience, New York (1961).

12. RIESZ C. H. and SUSMAN S., Proceedings of the Fourth Carbon Conference, p. 609. Pergamon Press, London (1960).

13. SMITH, J. W., Chem. &’ Ind. 885 (1958).

14. BEIRNE J. and HUTCHEON J. M., &it. J. Appl. Phys. Suppl. No. 3, p. 76 (1954).