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Treatment of pan fibres with SO2 and development of carbon fibres therefrom
Fibre Scienceand Technology 13 (1980) 155-162
T R E A T M E N T OF P A N FIBRES WITH SO 2 A N D D E V E L O P M E N T OF C A R B O N FIBRES T H E R E F R O M
O. P. BAHL, R. B. MATHUR and K. D. KUNDRA
Materials Division, National Physical Laboratory, New Dehli 110012 (India)
SUMMARY
High perJormance carbon fibres have been obtained by low temperature treatment of polyacrylonitrile (PA N) fibres in an atmosphere of SO 2, beJbre pyrolysis to 1000 ° C. The shrinkage behaviour, aromatisation index and the mechanical properties have been critically examined. These results are compared with those of the air-treated fibres. The process has beenJbund to be much quicker and the strength of the carbon fibres so obtained is about 25 % higher than that of those prepared by the conventional air-pretreatment method.
I.
INTRODUCTION
Polyacrylonitrile fibres possess an open chain structure and in order to make high performance carbon fibres from this raw material, one must convert it from the open chain to the closed chain form (ladder polymer structure). This is conventionally done by pretreating the precursor in the presence of oxygen-containing atmospheres. In this process not only is the chain structure cyclised but other functional groups are introduced into the structure 1-4 which help in cross-linking the molecular chains during high temperature pyrolysis. 5'6 Bahl and Manocha 7 and others a'9 have studied in detail the role of oxygen in the oxidation and subsequent carbonisation of PAN fibres. As the reaction of oxygen with PAN is diffusion controlled, it is very time consuming. Oxygen dehydrogenates the backbone in the beginning1° and also causes partial aromatisation of the structure. At the same time it reacts with the basic PAN chain and as a result of this, functional groups like C ~ O and C O O H are introduced into the molecules. It has been established that it is not useful to introduce more than 50 ~ cyclisation (in air-pretreatment) because of the reasons explained by Bahl and Manocha. 7 Continuous efforts are being made to 155 Fibre Science and Technology 0015-0568/80/0013-0155/$02.25 O Applied Science Publishers Ltd. England, 1980 Printed in Great Britain
] 56
O. P. BAHL, R. B. MATHUR, K. D. KUNDRA
obtain best quality carbon fibres with a minimum of energy and time consumption. Further progress towards this is due to a Japanese patent ~~ in which the precursor is impregnated with suitable amino siloxanes to enhance the cyclisation as well as the strength of the carbon fibres. Morita et al. ~2 and Raskovic et al. ~3 have treated PAN fibres with SO 2 and obtained good quality carbon fibres. There is however no report describing the detailed characterisation of SO 2 treated PAN fibres. The authors have carried out detailed studies of the effect of SO 2 on PAN fibres and have compared these results with those of the air-treated fibres. These are described here.
2.
EXPERIMENTAL
Beslon fibres were used throughout this study. The tow contained 6000 filaments of 1.5 denier each. Three per cent of methyl methacrylate was added to the parent monomer. Fibre tow of about 8 in length was tied between two hooks, one of which was fixed; the other carried a certain amount of load. The length of tow was then placed in a horizontal furnace in such a way that the fibre was in the constant temperature zone of the furnace. The furnace was switched on and the supply of SO 2 started. The temperature was gradually increased at the rate of about 4 °C/min up to the desired yolue. After the required soaking time the furnace supply was cut offand the SO 2 supply stopped. The SO2-pretreated fib(e tows were carbonised in a silica reaction vessel which was put in a suitable furgace the temperature of which could reach 1000°C, The carbonisation was carried out in the presence of ultra high pure N 2 gas. The set of experiments was repeated keeping the same time, temperature and tension but in place of SO 2, ordinary air was used. Mechanical properties were measured on an Instron tensile testing machine and the average of at least 25 readings is reported here. X-ray diffractograms were taken using a Philips diffractometer.
3.
RESULTS AND DISCUSSION
3.1. Changes in tensile strength and Young's modulus of SOz-treatedfibres Tows of PAN fibres were treated with SO 1 gas at 240°C for timings varying from 10min to 100rain. During the soaking period the furnace temperature was controlled within _+2 °C. Figure 1 shows the changes in the tensile strength with treatment time in the presence of SO z as well as air. A constant load of 700 g was applied in all the experiments reported here. The shape of the twocurves is the same in the beginning though the actual values differ. However, the fall in strength in the case of SO 2 is much steeper. It is surprising to note that the tensile strength increases in the case of SO2-treated fibres with increasing duration of treatment after having
TREATMENT
O F P A N FIBRES W I T H SO 2
157
o SOzTREATED FIBRES
90
• AIR TREATED FIBRES
80~
% c%6o ' ~- 70
x I
i-//~74/
N 5o 4O
~
30~)
o
~
~
]
I
I
I
I
I
I0
20
30
40
50
60
I 70
TIME IN MINUTES--~--
Fig. I.
Variation o f tensile strength with time.
fallen almost by 50 ~0 during the first few minutes of treatment. As described earlier by Bahl and Manocha 4 and Watt 14 the fall in the tensile strength is due to the conversion of C ~ N to C ~ N bonds or cyclisation. It therefore appears that SO 2 gas catalyses the cyclisation reaction at a rate which is much faster than that of oxygen. Figures 2 and 3 respectively show the variation with time of the primary and secondary Young's moduli of SO z as well as air-treated fibres. As is evident from the curves, the primary and secondary moduli of SO2-treated fibres drop down to about 50 ~o and 75 ~o respectively of the original value of the precursor within a few minutes of treatment, whereas in the case of air-treated fibres they register a gradual fall. The gradual increase in strength as well as modulus of the SO2-treated fibres after a steep fall can be attributed to the formation of 2-6
0 SO2 TREATED FIBRES
2.5,
• AIR TREATED FIBRES
cL 2 4
~23
× ~ 22 D o 20 >- 1 9 o~ ~< 1-8
16 10
20
30
40
50
60
TIME INMINUTES
Fig. 2.
Variation o f primary Young's m o d u l u s with time.
I 70
158
O. P. BAHL R. B. MATHUR~ K. D. KUNDRA 14~-
o S02TREATED FIBRES
13 \x
~2
• AIR TREATED FIBRES
x ~10 o
~o8 ~oz gz o 6
O5 O2
\,,.
0
/
o""
I
l
i
I
I
I
I
10
20
30
40
50
60
70
TIME IN MINUTES -a,,..-
Fig. 3.
Variation of secondary Young's modulus with time.
O --C--S--C groups as suggested by Raskovic et a/.~ 3 The sulphur atoms which go into the fibre during treatment cause intermolecular cross-linking by the formation of bridges between the molecular chains and therefore cause the increase in strength and modulus. It has been established now that the shrinkage is a criterion of cyclisation6.~ 5 in the case of PAN fibres. In other words, SO2-treated fibres should exhibit a fast rate of shrinkage in the beginning. In order to study this, shrinkage studies during sulphurisation were undertaken. 3.2. Shrinkage during treatment with S O 2 Figure 4 shows the percentage shrinkage versus temperature and time curves for different soaking times. The curves obtained are very much different than those obtained with air oxidation of PAN, and are peculiar too in appearance, resembling a saw tooth wave. Curve (a) is a typical curve for SO2-treatment whereas curve (b) represents typical data for air-treated fibres. The shrinkage in the first case (curve a) starts at a temperature as low as 90 °C and continues up to 140°C. The pattern is similar to the one followed in air oxidation. The percentage shrinkage, however, in the case of the SO2-treated fibre is comparatively higher. As described earlier by Bahl and Mathur ~6 and Rosenbaum ~~ this is supposed to be physical shrinkage and has nothing to do with later shrinkage which is due to various chemical reactions. However the amount of shrinkage in the case of SO 2 is about double the normal value obtained in air. This is the indication that besides physical shrinkage some chemical reaction also starts taking place with SOz-treatment at this low temperature, causing therefore the
TREATMENT
159
OF P A N FIBRES W I T H SO 2
TEMPERATURE'~ 90 140 165 190 215 240 il I I I I I I~ SOAKINGAT240°C
/c01 o/
o/o o 6
1
,..-=f~ I
0 10 Fig. 4.
o-o~o
I
50
I
I
50
I
•
I
I
I
I
I
70 90 110 TIME IN MINUTES-'~--
I
I
1:30
S h r i n k a g e b e h a v i o u r o f P A N . (a) S O 2 - t r e a t m e n t ;
I
I
150
(b) a i r - t r e a t m e n t .
cyclisation shrinkage as well. In other words the initial shrinkage is a mixture of physical as well as cyclisation shrinkage. From 140°C onwards the shrinkage is completely balanced by the elongation and the curve levels off similar to the air oxidation, and continues up thus to about 200 °C. At 200 °C the shrinkage behaviour of SO2-treated fibres departs markedly from the pattern followed by air-treated PAN. At this temperature the material (PAN) is known 6 to become plastic and therefore PAN fibres start elongating under tension. 16 In the case of air-treated fibres this elongation is completely balanced by the cyclisation as is evident from curve (b) in Fig. 4. However, in the case of SO2-treated PAN, elongation overtakes cyclisation in the temperature region of 200 to 215 °C. With a rise of temperature elongation and cyclisation are again completely balanced until at about 240 °C the cyclisation shrinkage again overtakes the elongation and continues with soaking time. In the case of air-treated fibres this shrinkage starts after a longer interval of time and at a slower rate, showing therefore that the cyclisation in the case of SO 2treated fibres is much faster. 3.3. X-ray diJJraction studies Using CuK radiations, PAN as such gives two peaks, one at 20 = 17 ° and the other at 20 = 29-5 °. When it is heated at a temperature higher than 180°C it has been observed Is that a new peak appears at 20 = 25.5 o corresponding to the sheet-like (002) structure of the ladder polymer. Figure 5 depicts three diffractograms, 'a' is for PAN as such, whereas 'b' and 'c' are for a 20 min treatment in SO2 and a 20 min treatment in air respectively. It is seen here that the original structure is almost completely transformed to the ladder polymer in a short duration of 20 min only in the presence of SO 2, whereas the effect is almost negligible in the case of air
160
O. P. BAHL, R. B. MATHUR, K. D. KUNDRA
31
29
27
25
23 21 -'*-20
19
17
15
13
Fig. 5. X-ray diffraction of different fibres. (a) PAN as such; (b) S O2-treated for 20 min; (c) air-treated for 20 rain.
oxidation. A rough calculation about the aromatisation index shows that it is as high as 65 ~o in the case of SO 2 and the value is less than 1 ~o in the case of the air-oxidised sample. This clearly demonstrates that cyclisation takes place at a very fast rate in the case of SO2-treated fibres and this in turn should result in a sudden fall in the tensile strength. This is exactly what has been observed by us in the tensile strength curves of Fig. 1.
3.4. Carbonisation of S02-pretreated fibres All PAN fibres with SO2-pretreatment were subjected to a similar carbonisation process. The shrinkage with temperature was monitored and it was observed that by and large, the shrinkage pattern was similar to that which had already been reported for air-oxidised fibres. 7'19 Table I summarises the mechanical properties of the carbon fibres at 1000 °C. The optimum tensile strength is obtained with 35rain only of SO 2 treatment. The optimum strength in air is obtained with 100 rain of soaking. Moreover the strength value is about 30 ~o higher in the case of SO2-treated fibres. It has been reported earlier 4 that in order to get optimum properties from PAN it is essential to attain an aromatisation index of about 50 ~ only. However in the case of SO2-treatment the aromatisation index with 20 min of soaking is 65 ~ and is 69 ~ and 75 ~ with 35 rain and 60 rain of soaking times respectively. Surprisingly the maximum in the tensile
TREATMENT OF PAN FIBRES WITH SO 2
161
TABLE 1 Soaking time rain
Fibres pretreated with SO 2 Strength Modulus x 103 lb in -2 × 106 lb in -2
Soaking time rain
20
245
20
20
35
270
22
35
60 i 00
184 Fibres very brittle
24 --
60 100
Fibres pretreated with air Strength Modulus x 10 a Ib in -2 x 106 Ib in -2
Fibres under stabilised Fibres under stabilised 180 215
Fibres under stabilised Fibres under stabilised 22 24
strength is observed at an aromatisation index of about 70 ~o when the PAN fibres are pretreated in the presence of SO2 instead of air. When PAN is pretreated in air or oxygen, oxygen not only cyclises the structure, but in addition oxidises the backbone of the chain. Cyclisation of the molecular chain helps in getting carbon fibres of good mechanical properties. In other words, the more cyclisation there is, the better should be the properties of the carbon fibres. However as seen above, there is another reaction that goes on side by side, i.e. the oxidation of the backbone. As has been explained by previous workers 14 certain amounts of C ~ O or COOH groups in the backbone help in cross-linking during carbonisation. It has been further reported by Bahl and Manocha 4 that the optimum level of the oxidation of the polymer backbone is reached when the degree of cyclisation is about 50 ~. Any further oxidation, however, although it increases the degree of cyclisation, at the same time increases the oxidation of the backbone. Proper cross-linking during carbonisation does not take place and the oxygen comes out during pyrolysis as CO or CO 2 thus introducing defects in the structure. This accounts for the lowering of the mechanical properties. With an aromatisation index of 65-70 ~ attained in the presence of oxygen, carbon fibres exhibited a much lower strength than the optimum value, as expected. Infra-red spectra of the SO2-treated fibres also show the presence of C ~ O groups, formed due to the comonomer initially present in the precursor. However, in this case the percentage of C ~ O groups is much less likely to cause any deterioration in the properties of the fibre due to oxidation of the backbone. On the other hand they will help in cross-linking the molecular chains. It is therefore possible to get carbon fibres of better mechanical properties with a very small duration of pretreatment. Table 1 depicts the strength and Young's modulus of carbon fibres pretreated with SO 2 and air. 3.5. Effect of load on the mechanical properties A set of experiments was also performed to see the effect of load, during treatment
162
O.P. BAHL, R.B. MATHUR, K.D. KUNDRA
with S O 2, on the m e c h a n i c a l p r o p e r t i e s o f the c a r b o n fibres so p r o d u c e d . These results have been c o m p a r e d with those o b t a i n e d f r o m the e x p e r i m e n t s p e r f o r m e d in the presence o f air, ~6 T h e results are c o m p i l e d in T a b l e 2. A s shown in T a b l e 2 irrespective o f the actual strength a n d m o d u l u s values, the trend is similar to the one o b s e r v e d d u r i n g p r e t r e a t m e n t o f P A N in the presence o f air. F u r t h e r , the o p t i m u m a m o u n t o f l o a d is also the same, i.e. 700 g. TABLE 2 Load in g
550 700 850
Fibres pretreated with SO 2 Strength Modulus x 103 Ib in -2 x 10e Ib in -2
200 270 185
22 22 25
Load in g
550 700 850
Fibres pretreated with air Strength Modulus x 103 Ib in -2 x 106 Ib #I -2
181"5 215 120.0
21.6 24 22
ACKNOWLEDGEMENTS The a u t h o r s are grateful to D r A. R. V e r m a , D i r e c t o r , N a t i o n a l Physical L a b o r a t o r y for his interest in the w o r k a n d for p e r m i t t i n g us to p u b l i s h this p a p e r . W e are i n d e b t e d to D r G . C . Jain a n d D r S. S. C h a r i for their c o n s t a n t e n c o u r a g e m e n t a n d interest in the w o r k . W e wish to t h a n k P r o f e s s o r E. F i t z e r for his very v a l u a b l e c o m m e n t s a n d suggestions. REFERENCES 1. 2. 3. 4. 5.
N. GRASS,Eand J. N. HAY, J. Po/ym. Sci., 56 (1962) p. 189. W. WATT, Carbon, 10 (1972) p. 121. A. J. CLARKEand J. E. BAIt~Y, J. Nature, 243 (1973) p. 146. O. P. BAXLand L. M. MANOCt~, Carbon, 12 (1974) p. 417. O. P. B~n., L. M. MANOCl~and G. C. JAIN,Die Angewandte Makromolekulare Chemic, 64 (1977) pp. 115-22. 6. D.J. MULLER,E. FITZERand A. K. FIEDI.ER,Proc. int. Conf. on Carbon Fibres, their Composites and Applications, The Plastic Institute, London, 1971, p. 10. 7. O. P. BAHLand L. M. MANOC~, Carbon, 13 (1975) p. 297. 8. A. SmNDO, Proc. Int. Conf. on Carbon Fibres, their Composites and Applications, The Plastic Institute, London, 1971. p. 18. 9. A.J. Cl.~RKJ~andJ. E. BAit~v, Proc. Int. Conf. on Carbon Fibres, their Composites and Applications, The Plastic Institute, London, 1971, p. 12. 10. E. FffZER and D. J. MULLS.R,Makromol. Chem., 44 (1971) p. 117. 11. Jap. Exlan. Co. Lid, British Patent 1499085, 25 Jan., 1978. 12. K. MORITA,H. Mnr&sm and Y. KINOSmTA.Carbon "72 Conf. Baden-Baden, 1972, p. 303. 13. V. RASKOVlC, 1. Dr~.Mtov and. S. MARINKOVIC, Private Communication. 14. W. WAr'r, Third Conf. on Ind. Carbon and Graphite, London, 1971, p. 431. 15. R. C. HouTz, Textile Res. J., 20 (1970) p. 786. 16. O. P. B,~IL and R. B. MATrlUR, Fib. Sci. & Tech., 12 (1979) p. 31. 17. S. J. ROS~N~UM, J. Appl. Polym. Sci., 9 (1965) p. 2071. 18. UCitlDATOSHIOet aL, Proc. lOth Biennial Carbon Conference, 1971, p. 131. 19. J. BROMLEY,Proc. Int. Conf. on Carbon Fibres their Composites and Applications, The Plastic Institute, London, 1971, p. 1.
T R E A T M E N T OF P A N FIBRES WITH SO 2 A N D D E V E L O P M E N T OF C A R B O N FIBRES T H E R E F R O M
O. P. BAHL, R. B. MATHUR and K. D. KUNDRA
Materials Division, National Physical Laboratory, New Dehli 110012 (India)
SUMMARY
High perJormance carbon fibres have been obtained by low temperature treatment of polyacrylonitrile (PA N) fibres in an atmosphere of SO 2, beJbre pyrolysis to 1000 ° C. The shrinkage behaviour, aromatisation index and the mechanical properties have been critically examined. These results are compared with those of the air-treated fibres. The process has beenJbund to be much quicker and the strength of the carbon fibres so obtained is about 25 % higher than that of those prepared by the conventional air-pretreatment method.
I.
INTRODUCTION
Polyacrylonitrile fibres possess an open chain structure and in order to make high performance carbon fibres from this raw material, one must convert it from the open chain to the closed chain form (ladder polymer structure). This is conventionally done by pretreating the precursor in the presence of oxygen-containing atmospheres. In this process not only is the chain structure cyclised but other functional groups are introduced into the structure 1-4 which help in cross-linking the molecular chains during high temperature pyrolysis. 5'6 Bahl and Manocha 7 and others a'9 have studied in detail the role of oxygen in the oxidation and subsequent carbonisation of PAN fibres. As the reaction of oxygen with PAN is diffusion controlled, it is very time consuming. Oxygen dehydrogenates the backbone in the beginning1° and also causes partial aromatisation of the structure. At the same time it reacts with the basic PAN chain and as a result of this, functional groups like C ~ O and C O O H are introduced into the molecules. It has been established that it is not useful to introduce more than 50 ~ cyclisation (in air-pretreatment) because of the reasons explained by Bahl and Manocha. 7 Continuous efforts are being made to 155 Fibre Science and Technology 0015-0568/80/0013-0155/$02.25 O Applied Science Publishers Ltd. England, 1980 Printed in Great Britain
] 56
O. P. BAHL, R. B. MATHUR, K. D. KUNDRA
obtain best quality carbon fibres with a minimum of energy and time consumption. Further progress towards this is due to a Japanese patent ~~ in which the precursor is impregnated with suitable amino siloxanes to enhance the cyclisation as well as the strength of the carbon fibres. Morita et al. ~2 and Raskovic et al. ~3 have treated PAN fibres with SO 2 and obtained good quality carbon fibres. There is however no report describing the detailed characterisation of SO 2 treated PAN fibres. The authors have carried out detailed studies of the effect of SO 2 on PAN fibres and have compared these results with those of the air-treated fibres. These are described here.
2.
EXPERIMENTAL
Beslon fibres were used throughout this study. The tow contained 6000 filaments of 1.5 denier each. Three per cent of methyl methacrylate was added to the parent monomer. Fibre tow of about 8 in length was tied between two hooks, one of which was fixed; the other carried a certain amount of load. The length of tow was then placed in a horizontal furnace in such a way that the fibre was in the constant temperature zone of the furnace. The furnace was switched on and the supply of SO 2 started. The temperature was gradually increased at the rate of about 4 °C/min up to the desired yolue. After the required soaking time the furnace supply was cut offand the SO 2 supply stopped. The SO2-pretreated fib(e tows were carbonised in a silica reaction vessel which was put in a suitable furgace the temperature of which could reach 1000°C, The carbonisation was carried out in the presence of ultra high pure N 2 gas. The set of experiments was repeated keeping the same time, temperature and tension but in place of SO 2, ordinary air was used. Mechanical properties were measured on an Instron tensile testing machine and the average of at least 25 readings is reported here. X-ray diffractograms were taken using a Philips diffractometer.
3.
RESULTS AND DISCUSSION
3.1. Changes in tensile strength and Young's modulus of SOz-treatedfibres Tows of PAN fibres were treated with SO 1 gas at 240°C for timings varying from 10min to 100rain. During the soaking period the furnace temperature was controlled within _+2 °C. Figure 1 shows the changes in the tensile strength with treatment time in the presence of SO z as well as air. A constant load of 700 g was applied in all the experiments reported here. The shape of the twocurves is the same in the beginning though the actual values differ. However, the fall in strength in the case of SO 2 is much steeper. It is surprising to note that the tensile strength increases in the case of SO2-treated fibres with increasing duration of treatment after having
TREATMENT
O F P A N FIBRES W I T H SO 2
157
o SOzTREATED FIBRES
90
• AIR TREATED FIBRES
80~
% c%6o ' ~- 70
x I
i-//~74/
N 5o 4O
~
30~)
o
~
~
]
I
I
I
I
I
I0
20
30
40
50
60
I 70
TIME IN MINUTES--~--
Fig. I.
Variation o f tensile strength with time.
fallen almost by 50 ~0 during the first few minutes of treatment. As described earlier by Bahl and Manocha 4 and Watt 14 the fall in the tensile strength is due to the conversion of C ~ N to C ~ N bonds or cyclisation. It therefore appears that SO 2 gas catalyses the cyclisation reaction at a rate which is much faster than that of oxygen. Figures 2 and 3 respectively show the variation with time of the primary and secondary Young's moduli of SO z as well as air-treated fibres. As is evident from the curves, the primary and secondary moduli of SO2-treated fibres drop down to about 50 ~o and 75 ~o respectively of the original value of the precursor within a few minutes of treatment, whereas in the case of air-treated fibres they register a gradual fall. The gradual increase in strength as well as modulus of the SO2-treated fibres after a steep fall can be attributed to the formation of 2-6
0 SO2 TREATED FIBRES
2.5,
• AIR TREATED FIBRES
cL 2 4
~23
× ~ 22 D o 20 >- 1 9 o~ ~< 1-8
16 10
20
30
40
50
60
TIME INMINUTES
Fig. 2.
Variation o f primary Young's m o d u l u s with time.
I 70
158
O. P. BAHL R. B. MATHUR~ K. D. KUNDRA 14~-
o S02TREATED FIBRES
13 \x
~2
• AIR TREATED FIBRES
x ~10 o
~o8 ~oz gz o 6
O5 O2
\,,.
0
/
o""
I
l
i
I
I
I
I
10
20
30
40
50
60
70
TIME IN MINUTES -a,,..-
Fig. 3.
Variation of secondary Young's modulus with time.
O --C--S--C groups as suggested by Raskovic et a/.~ 3 The sulphur atoms which go into the fibre during treatment cause intermolecular cross-linking by the formation of bridges between the molecular chains and therefore cause the increase in strength and modulus. It has been established now that the shrinkage is a criterion of cyclisation6.~ 5 in the case of PAN fibres. In other words, SO2-treated fibres should exhibit a fast rate of shrinkage in the beginning. In order to study this, shrinkage studies during sulphurisation were undertaken. 3.2. Shrinkage during treatment with S O 2 Figure 4 shows the percentage shrinkage versus temperature and time curves for different soaking times. The curves obtained are very much different than those obtained with air oxidation of PAN, and are peculiar too in appearance, resembling a saw tooth wave. Curve (a) is a typical curve for SO2-treatment whereas curve (b) represents typical data for air-treated fibres. The shrinkage in the first case (curve a) starts at a temperature as low as 90 °C and continues up to 140°C. The pattern is similar to the one followed in air oxidation. The percentage shrinkage, however, in the case of the SO2-treated fibre is comparatively higher. As described earlier by Bahl and Mathur ~6 and Rosenbaum ~~ this is supposed to be physical shrinkage and has nothing to do with later shrinkage which is due to various chemical reactions. However the amount of shrinkage in the case of SO 2 is about double the normal value obtained in air. This is the indication that besides physical shrinkage some chemical reaction also starts taking place with SOz-treatment at this low temperature, causing therefore the
TREATMENT
159
OF P A N FIBRES W I T H SO 2
TEMPERATURE'~ 90 140 165 190 215 240 il I I I I I I~ SOAKINGAT240°C
/c01 o/
o/o o 6
1
,..-=f~ I
0 10 Fig. 4.
o-o~o
I
50
I
I
50
I
•
I
I
I
I
I
70 90 110 TIME IN MINUTES-'~--
I
I
1:30
S h r i n k a g e b e h a v i o u r o f P A N . (a) S O 2 - t r e a t m e n t ;
I
I
150
(b) a i r - t r e a t m e n t .
cyclisation shrinkage as well. In other words the initial shrinkage is a mixture of physical as well as cyclisation shrinkage. From 140°C onwards the shrinkage is completely balanced by the elongation and the curve levels off similar to the air oxidation, and continues up thus to about 200 °C. At 200 °C the shrinkage behaviour of SO2-treated fibres departs markedly from the pattern followed by air-treated PAN. At this temperature the material (PAN) is known 6 to become plastic and therefore PAN fibres start elongating under tension. 16 In the case of air-treated fibres this elongation is completely balanced by the cyclisation as is evident from curve (b) in Fig. 4. However, in the case of SO2-treated PAN, elongation overtakes cyclisation in the temperature region of 200 to 215 °C. With a rise of temperature elongation and cyclisation are again completely balanced until at about 240 °C the cyclisation shrinkage again overtakes the elongation and continues with soaking time. In the case of air-treated fibres this shrinkage starts after a longer interval of time and at a slower rate, showing therefore that the cyclisation in the case of SO 2treated fibres is much faster. 3.3. X-ray diJJraction studies Using CuK radiations, PAN as such gives two peaks, one at 20 = 17 ° and the other at 20 = 29-5 °. When it is heated at a temperature higher than 180°C it has been observed Is that a new peak appears at 20 = 25.5 o corresponding to the sheet-like (002) structure of the ladder polymer. Figure 5 depicts three diffractograms, 'a' is for PAN as such, whereas 'b' and 'c' are for a 20 min treatment in SO2 and a 20 min treatment in air respectively. It is seen here that the original structure is almost completely transformed to the ladder polymer in a short duration of 20 min only in the presence of SO 2, whereas the effect is almost negligible in the case of air
160
O. P. BAHL, R. B. MATHUR, K. D. KUNDRA
31
29
27
25
23 21 -'*-20
19
17
15
13
Fig. 5. X-ray diffraction of different fibres. (a) PAN as such; (b) S O2-treated for 20 min; (c) air-treated for 20 rain.
oxidation. A rough calculation about the aromatisation index shows that it is as high as 65 ~o in the case of SO 2 and the value is less than 1 ~o in the case of the air-oxidised sample. This clearly demonstrates that cyclisation takes place at a very fast rate in the case of SO2-treated fibres and this in turn should result in a sudden fall in the tensile strength. This is exactly what has been observed by us in the tensile strength curves of Fig. 1.
3.4. Carbonisation of S02-pretreated fibres All PAN fibres with SO2-pretreatment were subjected to a similar carbonisation process. The shrinkage with temperature was monitored and it was observed that by and large, the shrinkage pattern was similar to that which had already been reported for air-oxidised fibres. 7'19 Table I summarises the mechanical properties of the carbon fibres at 1000 °C. The optimum tensile strength is obtained with 35rain only of SO 2 treatment. The optimum strength in air is obtained with 100 rain of soaking. Moreover the strength value is about 30 ~o higher in the case of SO2-treated fibres. It has been reported earlier 4 that in order to get optimum properties from PAN it is essential to attain an aromatisation index of about 50 ~ only. However in the case of SO2-treatment the aromatisation index with 20 min of soaking is 65 ~ and is 69 ~ and 75 ~ with 35 rain and 60 rain of soaking times respectively. Surprisingly the maximum in the tensile
TREATMENT OF PAN FIBRES WITH SO 2
161
TABLE 1 Soaking time rain
Fibres pretreated with SO 2 Strength Modulus x 103 lb in -2 × 106 lb in -2
Soaking time rain
20
245
20
20
35
270
22
35
60 i 00
184 Fibres very brittle
24 --
60 100
Fibres pretreated with air Strength Modulus x 10 a Ib in -2 x 106 Ib in -2
Fibres under stabilised Fibres under stabilised 180 215
Fibres under stabilised Fibres under stabilised 22 24
strength is observed at an aromatisation index of about 70 ~o when the PAN fibres are pretreated in the presence of SO2 instead of air. When PAN is pretreated in air or oxygen, oxygen not only cyclises the structure, but in addition oxidises the backbone of the chain. Cyclisation of the molecular chain helps in getting carbon fibres of good mechanical properties. In other words, the more cyclisation there is, the better should be the properties of the carbon fibres. However as seen above, there is another reaction that goes on side by side, i.e. the oxidation of the backbone. As has been explained by previous workers 14 certain amounts of C ~ O or COOH groups in the backbone help in cross-linking during carbonisation. It has been further reported by Bahl and Manocha 4 that the optimum level of the oxidation of the polymer backbone is reached when the degree of cyclisation is about 50 ~. Any further oxidation, however, although it increases the degree of cyclisation, at the same time increases the oxidation of the backbone. Proper cross-linking during carbonisation does not take place and the oxygen comes out during pyrolysis as CO or CO 2 thus introducing defects in the structure. This accounts for the lowering of the mechanical properties. With an aromatisation index of 65-70 ~ attained in the presence of oxygen, carbon fibres exhibited a much lower strength than the optimum value, as expected. Infra-red spectra of the SO2-treated fibres also show the presence of C ~ O groups, formed due to the comonomer initially present in the precursor. However, in this case the percentage of C ~ O groups is much less likely to cause any deterioration in the properties of the fibre due to oxidation of the backbone. On the other hand they will help in cross-linking the molecular chains. It is therefore possible to get carbon fibres of better mechanical properties with a very small duration of pretreatment. Table 1 depicts the strength and Young's modulus of carbon fibres pretreated with SO 2 and air. 3.5. Effect of load on the mechanical properties A set of experiments was also performed to see the effect of load, during treatment
162
O.P. BAHL, R.B. MATHUR, K.D. KUNDRA
with S O 2, on the m e c h a n i c a l p r o p e r t i e s o f the c a r b o n fibres so p r o d u c e d . These results have been c o m p a r e d with those o b t a i n e d f r o m the e x p e r i m e n t s p e r f o r m e d in the presence o f air, ~6 T h e results are c o m p i l e d in T a b l e 2. A s shown in T a b l e 2 irrespective o f the actual strength a n d m o d u l u s values, the trend is similar to the one o b s e r v e d d u r i n g p r e t r e a t m e n t o f P A N in the presence o f air. F u r t h e r , the o p t i m u m a m o u n t o f l o a d is also the same, i.e. 700 g. TABLE 2 Load in g
550 700 850
Fibres pretreated with SO 2 Strength Modulus x 103 Ib in -2 x 10e Ib in -2
200 270 185
22 22 25
Load in g
550 700 850
Fibres pretreated with air Strength Modulus x 103 Ib in -2 x 106 Ib #I -2
181"5 215 120.0
21.6 24 22
ACKNOWLEDGEMENTS The a u t h o r s are grateful to D r A. R. V e r m a , D i r e c t o r , N a t i o n a l Physical L a b o r a t o r y for his interest in the w o r k a n d for p e r m i t t i n g us to p u b l i s h this p a p e r . W e are i n d e b t e d to D r G . C . Jain a n d D r S. S. C h a r i for their c o n s t a n t e n c o u r a g e m e n t a n d interest in the w o r k . W e wish to t h a n k P r o f e s s o r E. F i t z e r for his very v a l u a b l e c o m m e n t s a n d suggestions. REFERENCES 1. 2. 3. 4. 5.
N. GRASS,Eand J. N. HAY, J. Po/ym. Sci., 56 (1962) p. 189. W. WATT, Carbon, 10 (1972) p. 121. A. J. CLARKEand J. E. BAIt~Y, J. Nature, 243 (1973) p. 146. O. P. BAXLand L. M. MANOCt~, Carbon, 12 (1974) p. 417. O. P. B~n., L. M. MANOCl~and G. C. JAIN,Die Angewandte Makromolekulare Chemic, 64 (1977) pp. 115-22. 6. D.J. MULLER,E. FITZERand A. K. FIEDI.ER,Proc. int. Conf. on Carbon Fibres, their Composites and Applications, The Plastic Institute, London, 1971, p. 10. 7. O. P. BAHLand L. M. MANOC~, Carbon, 13 (1975) p. 297. 8. A. SmNDO, Proc. Int. Conf. on Carbon Fibres, their Composites and Applications, The Plastic Institute, London, 1971. p. 18. 9. A.J. Cl.~RKJ~andJ. E. BAit~v, Proc. Int. Conf. on Carbon Fibres, their Composites and Applications, The Plastic Institute, London, 1971, p. 12. 10. E. FffZER and D. J. MULLS.R,Makromol. Chem., 44 (1971) p. 117. 11. Jap. Exlan. Co. Lid, British Patent 1499085, 25 Jan., 1978. 12. K. MORITA,H. Mnr&sm and Y. KINOSmTA.Carbon "72 Conf. Baden-Baden, 1972, p. 303. 13. V. RASKOVlC, 1. Dr~.Mtov and. S. MARINKOVIC, Private Communication. 14. W. WAr'r, Third Conf. on Ind. Carbon and Graphite, London, 1971, p. 431. 15. R. C. HouTz, Textile Res. J., 20 (1970) p. 786. 16. O. P. B,~IL and R. B. MATrlUR, Fib. Sci. & Tech., 12 (1979) p. 31. 17. S. J. ROS~N~UM, J. Appl. Polym. Sci., 9 (1965) p. 2071. 18. UCitlDATOSHIOet aL, Proc. lOth Biennial Carbon Conference, 1971, p. 131. 19. J. BROMLEY,Proc. Int. Conf. on Carbon Fibres their Composites and Applications, The Plastic Institute, London, 1971, p. 1.