A Study of Partial Carbonisation for the Developmentof Pitch Based Carbon Fibres

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Vol. 5, No. 1 March 2004 pp. 23-26

A Study of Partial Carbonisation for the Development of Pitch Based Carbon Fibres

R. K. Aggarwal

^♠

, G. Bhatia, V. Raman, M. Saha and A. Mishra

Carbon Technology Unit, Division of Engineering Materials, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi-110 012, India

♠e-mail: rkaggarwal@mail.nplindia.ernet.in (Received February 2, 2004; Accepted March 15, 2004)

Abstract

A study of partial carbonisation of green pitch fibres to temperatures in the range of 500-1000°C was carried out on three precursors − a neat pitch and two polymer modified pitches, with a view to find out a suitable temperature at which the fibres acquire considerably improved toughness or handleability (compared to that in the green stage) for their subsequent processing into carbon fibres. A partial carbonisation temperature of 500-600°C has been identified to result in a remarkable improvement in the toughness/handleability of the fibres in all the three cases. However, from techno-economical considerations, the neat pitch appears to provide the best precursor system for the production of pitch based carbon fibres.

Keywords : Carbon fibres, Polymer modified pitches, Handleability

1. Introduction

Pitch based general-purpose carbon fibres form an important variety of carbon fibres having high performance- to-cost ratio. These are used as a prominent reinforcement material in the production of carbon-carbon composites for applications like aircraft brake shoes, rocket nozzles, and rocket nose cones because of their outstanding mechanical properties at elevated temperatures above 2000°C, besides their other applications such as packing material, high temperature heat insulation, and additives for plastics to improve their wear and electrical conductivity properties [1- 4]. However, the technology of production of these carbon fibres involves the difficult problem of their handleability in the green stage because of their being extremely weak and brittle, the solution of which is obviously kept as a closely- guarded secret by the companies manufacturing this type of carbon fibres. Mochida et al. [5, 6] tried to solve this problem by blending polyvinylchloride (PVC) pitch and polyphenyleneoxide (PPO) with the precursor pitch. While the addition of PVC pitch was reported to cause a nominal improvement in the tensile strength of the pitch fibres upon carbonisation only, the addition of PPO did improve the strength of the pitch fibres by 29% in the green stage but at the cost of 19% reduction at the carbonised level. The present authors also had attempted to develop this variety of carbon fibres [7-10] and to solve the problem of their handleability [8] by modifying a suitable precursor pitch [7], with the addition of polymethylmethacrylate (PMMA), polystyrene (PS) and polycarbonate (PC) [9, 10]. Though the addition of PMMA (10% by wt. of pitch) caused a remark-

able improvement in the strength, strain-to-failure and handleability (toughness) of the resultant fibres in the green stage, it resulted in porous carbonized fibres exhibiting extraordi- narily poor mechanical properties. The addition of PS (30%

by wt. of pitch) and PC (10-20% by wt. of pitch), in turn, led to relatively lower improvements (compared to PMMA) in the strength and handleability of the fibres in the green stage, which however were very well retained upon carbonisation of the fibres [9, 10]. These improvements with PS or PC, thus, solved the problem of handleability only partially.

The other way to overcome the problem of handleability was indicated by Yamada et al. [11] and it consisted of thermal stabilization of the green pitch fibres on the very spool (heat-resistant) on which they are wound during spinning, followed by partial carbonisation to a temperature

~500°C at which they possess good tensile strength and shear ductility. However, no further details were provided by them. Based on this indication and the above-mentioned results involving significant improvements in the handleability of green pitch fibres by the incorporation of PS or PC, the present authors attempted to find out a suitable heat- treatment temperature (HTT), varying in the range of 500- 1000°C, at which the fibres acquire reasonably good strength (TS~250 MPa) and strain-to-failure (1-2%) while undergoing a small linear shrinkage (<5%) such that they exhibit considerably improved handleability for their subsequent processing into carbon fibres. Three precursor systems − a neat pitch and two other pitches modified by the addition of 30% PS or 20% PC to the neat pitch, were choosen for this purpose. Such a study was thought to be of great technological importance because knowing the shrinkage behaviour of the Carbon

Science

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24 R. K. Aggarwal et al. / Carbon Science Vol. 5, No. 1 (2004) 23-26

fibres with the HTT, one could design a special spool exhibiting a shrinkage behaviour commensurates with that of the fibres.

2. Experimental

Three series of experiments were conducted on the fibres based on neat and modified pitches. For modification of the neat pitch with the addition of PS or PC, both the pitch and the PS/PC were dissolved separately in a suitable solvent and the solutions obtained were mixed together briskly under reflux conditions, after which the solvent was distilled out to obtain the modified pitches. All the pitches, neat as well as modified, were then subjected to melt-spinning at suitable temperatures to obtain their fibres. Measured lengths of fibres of each precursor system were weighed and oxidised at temperatures up to 300°C and then carbonised to temperatures of 500-1000°C. At each HTT, the fibres were characterised with respect to weight loss, linear shrinkage and mechanical properties, and the results obtained are summarised in Table 1-3. The PS and PC were subjected to their pyrolysis behaviours up to 1000°C in an inert

atmosphere of nitrogen, which have been graphically shown in Fig. 1. The carbonised fibres (HTT = 1000°C) based on the neat pitch were also examined by a Scanning Electron Microscope (LEO-440) and the micrograph obtained is shown in Fig. 2.

Table 1. Characteristics of fibres based on neat precursor pitch at different heat-treatment temperatures (HTTs)

HTT (^oC)

Wt. loss (%)

Linear shrinkage

(%)

TS (MPa)

TM (GPa)

STF (%)

−* − − 2.9 1.3 0.2

500 4.0 4.3 109 4.9 2.2

600 7.3 4.9 281 12.7 2.2

700 10.3 6.1 447 25.2 1.8

800 12.3 6.7 647 39.9 1.6

1000 14.2 12.8 695 53.5 1.4

NOTE: TS=Tensile strength; TM=Tensile modulus; STF=Strain-to- failure

‘*’=refers to fibres in the green stage.

Table 2. Characteristics of fibres based on 30% PS-added pre- cursor pitch at different heat-treatment temperatures (HTTs)

HTT (^oC)

Wt. loss (%)

Linear shrinkage

(%)

TS (MPa)

TM (GPa)

STF (%)

−* − − 70 12.0 0.6

500 28.4 5.3 214 10.6 2.0

600 34.0 5.9 266 19.0 1.4

800 41.0 7.9 560 43.0 1.3

1000 42.8 11.2 800 90.2 0.9

Table 3. Characteristics of fibres based on 20% PC-added pre- cursor pitch at different heat-treatment temperatures (HTTs)

HTT (^oC)

Wt. loss (%)

Linear shrinkage

(%)

TS (MPa)

TM (GPa)

STF (%)

−* − − 43 13.6 0.3

500 14.8 9.8 134 4.8 2.8

600 15.9 14.3 425 29.6 1.4

700 17.7 17.9 542 33.0 1.6

800 18.4 19.6 620 44.0 1.4

1000 25.9 21.4 892 65.0 1.4

Fig. 1. Pyrolysis behaviour of polystyrene (PS) and polycarbon- ate (PC) with heat-treatment temperature (HTT).

Fig. 2. SEM photograph of carbonised pitch fibres based on neat pitch (HTT=1000°C).

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A Study of Partial Carbonisation for the Development of Pitch Based Carbon Fibres 25

3. Results and Discussion

It is seen from Table 1 that as the HTT increases from 500 to 1000°C, both the weight loss and the linear shrinkage of the fibres made from the neat pitch increase. The weight loss varies from 4.0% at 500°C to a value of 14.2% at 1000°C, while the linear shrinkage increases continuously from a value of 4.3% at 500°C to 12.8% at a HTT of 1000°C.

Along with the increase in the weight loss and the linear shrinkage, the tensile strength (TS) and tensile modulus (TM) of the resulting fibres also increase continuously in such a way that the strain-to-failure (STF) goes on decreasing from the highest value of 2.2% at 500°C to a value of 1.4% at a HTT of 1000°C. The continuous increase in the weight loss and linear shrinkage as well as TS and TM of the fibres with the increasing HTT is due to the increasing aromatisation of the pitch material caused by the removal of relatively low molecular weight components and other gaseous products resulting from the condensation, cyclisation and polymeri- sation reactions taking place among the various molecular species of the pitch.

It may be noted here that at a HTT of 600°C, the fibres undergo a linear shrinkage of 4.9% only and gain enough strength (TS = 281 MPa) attaining a strain-to-failure of 2.2%, compared to a strength of 2.9 MPa and a strain-to-failure of 0.2 in the green stage. This refers to a tremendous improvement in the toughness or handleability of the resultant fibres compared to the green fibres, and as mentioned already, this observation has a great technological implication, in the sense that it will be desirable to partially carbonise the green pitch fibres up to 600°C on the spool itself (using a suitably- designed spool capable of undergoing shrinkage commensurates with that of the fibres) and then process them further to the final HTT of 1000°C with much reduced handling problem. The resultant carbon fibres are found to have a reasonably good TS of 695 MPa, TM of 53.5 GPa and a strain-to-failure of 1.4%. The surface of the carbonised fibres is also found to be quite smooth as seen from the SEM photograph shown in Fig. 2.

Further, it is seen from Table 2, showing the results of the fibres based on the pitch modified with 30% PS, that here also the weight loss, linear shrinkage, TS and TM increase continuously with the HTT, while the strain-to-failure decreases from the maximum value of 2.0 at a HTT of 500°C to the minimum value of 0.9 at 1000°C. The same explanation as mentioned above in the case of the neat pitch holds good here too for these variations. It is interesting to note that though the linear shrinkage is of the same order (5.3-11.2%) as in the case of neat pitch (4.3-12.8%), the weight loss here is much higher (28.4-42.8%) compared to the case of neat pitch (4.0-14.2%). This is because of high content (30%) of PS in the precursor pitch, which undergoes fast degradation above a HTT of 400°C leaving zero coke yield (HTT = 1000

°C), as can be seen from Fig. 1. It is observed that at a HTT

of 500-600°C, there is a substantial improvement in the toughness/handleability of the fibres. However, it may be noted that the resultant carbon fibres here have a TS of 800 MPa with a STF of 0.9% only, compared to the correspond- ing values of 695 MPa and 1.4%, respectively, in the case of the neat pitch. As the ultimate carbon fibres, though having improved TS, are significantly more brittle in this case of PS addition, compared to the case of neat pitch, techno- economical considerations do not justify the modification of pitch with PS.

Furthermore, it is observed from Table 3, summarising the results of the fibres based on the pitch modified with 20%

PC that like in the case of the neat pitch or PS-modified pitch, as expected, here also the weight loss, linear shrinkage, TS and TM go on increasing continuously with the HTT, while the strain-to-failure goes on decreasing essentially from the maximum value of 2.8% at 500°C to the minimum value of 1.4% at 800-1000°C. The same explanation as mentioned in the other two cases for the different variations applies here also. It may be noted that in this case, the linear shrinkage is around double (9.8-21.4%) of what is observed in the other two cases, which may be attributed to the scission of the carbonate group from the backbone of the polymer during the heat-treatment. On the other hand, the weight loss (14.8-25.9%) lies in the midway of values observed in the other two cases of neat pitch (4.0-14.2%) and PS-added pitch (28.4-42.8%). This is because of relatively lower content (20%) of the polymer (PC) in the precursor pitch which gives a coke yield of 26.0%, compared to a value of 0.0% for the PS, as seen from Fig. 1.

It may also be noted from Table 3 that at a HTT of 600

°C, the fibres gain a substantially high strength of 425 MPa along with a STF of 1.4%, while undergoing a linear shrinkage of 14.3%. The ultimate carbon fibres (HTT = 1000

°C) are found to have a TS of 892 MPa, TM of 65 GPa and a STF of 1.4%.

It is interesting to note from the above results that at a HTT of 600°C, the neat pitch based fibres exhibit a TS of 281 MPa and a STF of 2.2%, while undergoing the lowest linear shrinkage of 4.9% and the lowest weight loss of 7.3%, as compared to a TS of 425 MPa and a STF of 1.4%, along with the highest linear shrinkage of 14.3% and a weight loss of 15.9%, obtained in the case of PC-modified pitch, while in both the cases, the resultant carbon fibres (HTT = 1000°C) possess the same STF of 1.4% but similar values of TS, namely, 695 MPa in the case of neat pitch and 892 MPa for the PC-modified pitch. Thus, for the similar quality of the end product (carbonised fibres), we have more tough or better handleable fibres at the partial carbonisation stage (HTT = 600°C) in the case of neat pitch, compared to the case of PC-modified pitch, which involves the additional step of modification of the precursor pitch. The yield of the final carbon fibres is also significantly higher in the former case (85.8%), compared to the latter case (74.1%). Hence,

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26 R. K. Aggarwal et al. / Carbon Science Vol. 5, No. 1 (2004) 23-26

from the techno-economical considerations, the neat pitch appears to provide the best precursor system. Further studies towards exploring other polymers as well as towards designing a suitable spool are in progress.

4. Conclusions

Partial carbonisation of green pitch fibres based on three precursors − a neat pitch and two polymer modified pitches, to a temperature of 500-600°C causes a remarkable improvement in the toughness/handleability of the fibres for their subsequent processing toward carbon fibers, in all the three cases. However, from techno-economical considerations, the neat pitch appears to provide the best precursor system for the production of pitch based carbon fibres.

Acknowledgements

The authors are grateful to Dr. Vikram Kumar, Director, National Physical Laboratory (NPL), New Delhi, for his keen interest in the work and the kind permission to publish the results, and to Dr. A. K. Gupta, Head, Division of Eng- ineering Materials, NPL, for his encouragement throughout this investigation. Thanks are due to Mr. P.R. Sengupta, Dr.

Ram Kishore and Mr. K.N. Sood for their valuable help in the present investigation. The authors are also thankful to ARDB, New Delhi, for sponsoring the project on pitch- based carbon monofilament, and to the Council of Scientific and Industrial Research, New Delhi, for the award of Research Associateship to Dr. M. Saha and Senior Research Fellowship to Mr. A. Mishra.

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