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Shrinkage behaviour of modified PAN precursors—Its influence on the properties of resulting carbon fibre
Polymer Degradation and Stability 14 (1986) 179-187
Shrinkage Behaviour of Modified PAN Precursors--Its Influence on the Properties of Resulting Carbon Fibre R. B. Mathur, T. L. Dhami & O. P. Bahl Carbon Technology Unit, National Physical Laboratory, New Delhi 110 012, India (Received: 19 July, 1985)
A BSTRA CT Carbon fibres have been developed using PA N precursors modified under different conditions. The shrinkage behaz'iour and weight loss of these modified precursors during proeessing into carbon fibres have been studied on a Mettler TA-3000 thermal analyzer. The implications offibre shrinkage during various stages of carbon .[ibre formation have been discussed.
INTRODUCTION For a variety of reasons 1-4 polyacrylonitrile (PAN) is one of the most commonly used precursors for making carbon fibres. The conversion of PAN to carbon fibres involves two major steps: 2'5 7 (i) thermal stabilization and (ii) carbonization. In the stabilization step, PAN is heated to 200-300 °C in an oxidizing atmosphere to convert it into non-plastic, thermally stable flame-proof material, which can then withstand temperatures up to 1000-3000°C in an inert atmosphere. The maximum temperature depends upon the type of carbon fibres ultimately required. 8 During thermal stabilization, PAN shows characteristic length changes, 9-,11 which depend upon the precursor as well as on the processing parameters during the stabilization step, such as stabilization temperature, time, rate of heating, load applied per filament, ambient 179
Polymer Degradation and Stabilio, 0141-3910/86/$03"50 ~') Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
180
R. B. Mathur, T. L. Dhami, O. P. Bahl
conditions, etc. According to Muller e t al. l o the shrinkage which occurs in PAN when it is heated to temperatures above the glass transition temperature is due to cyclization of PAN to form an imperfect ladder polymer structure. However, Bahl and Manocha 11 have reported two types of shrinkage--physical and chemical--which occur during the stabilization step. The physical shrinkage has been reported to be independent of temperature and oxidizing gas while the chemical shrinkage occurs as the result of chemical reactions taking place during the stabilization step. In this laboratory, we have developed carbon fibres from PAN which has been modified by employing various techniques, described elsewhere. 12 An attempt has been made to study the shrinkage behaviour of these modified precursors during oxidation and during carbonization and their causes and implications.
EXPERIMENTAL All the experiments reported here have been carried out with copolymer fibres supplied by Courtaulds Limited, Great Britain. A tow containing 6000 monofilaments, of 1.2 d-tex each, was used. Various methods which have been employed for the modification of the original precursor are described in detail elsewhere. 12 A TMA 40 cell of the Mettlar TA-3000 system was used for recording shrinkages. Two to three thousand monofilaments, approximately 20 mm long, were clamped in the holder with screws. The distance between the clamps was exactly 10 mm and the holder was attached to the TMA 40 cell as shown in Fig. I. A fixed load of 1 mg per filament was applied during the experiments. The fibres were first heated at a rate of 5 °C/min up to 230°C and kept at 230°C for 100 min in air. They were further heated from 230 °C to 900 °C at different heating rates in a nitrogen atmosphere containing a maximum of 10ppm of oxygen. Length changes of the order of 0.1/~m can be detected by the instrument. TGA experiments were carried out with the TGA 50 module of the same equipment. A I 0-mg sample was heated at 100 °C to constant weight and then further heated from 100 to 1000°C at a rate of 10°C/min in nitrogen containing a maximum of 10 ppm of oxygen. The DTA'20 unit of the TA-3000 system was used to study the heat flow behaviour of the samples. The values of activation energy were calculated using the
P A N precursors shrinkage--influence on carbon fibres
181
6
1 Fixed quortz hook
4. T.M.A. Measuring probe.
2. Clomp 3. Fibre
5. Somple eupport 6. Furnace
Fig. !.
Fibre attachment for measuring length changes.
Arrhenius equation 13 by the TC-10 Swiss Matrix 7775 microprocessor, attached to the unit. The modified precursors were converted into carbon fibres by optimizing various processing parameters as described earlier. ~2 The mechanical properties of the modified PAN fibres and those of the resulting carbon fibres were determined at 2 cm gauge length, using an lnstron 1122 tensile testing machine and single filament test methods. An average of twenty individual tests was reported for each sample.
RESULTS AND DISCUSSION
Shrinkage during thermal stabilization Table 1 shows the amount of shrinkage shown by the master sample (unmodified PAN) and by the modified precursors during an increase in temperature from room temperature to 230 °C and subsequent isothermal
A B C D E
Serial Samph' No. code
Precursor (untreated) C u C 1(stretched) N 2 (elongated) Air (elongated) Steam (stretched)
Treatment
8-87 15.34 7.17 12-06 13-86
40-180°C
(%)
Physwal shrmkage
9'22 9-52 15.60 8.91 8'80
180-230 °C
21.03 25'21 17.95 18-56 17.72
lOOmin ~othermal at 230°C 30'25 34.73 33.55 27-47 26.52
Total chemical shrinkage
Chemical shrinkage (%)
TABLE I Shrinkage Behaviour of Modified PAN Fibres
39.12 50"07 40.72 39.53 40.38
Total (%) (Chemical and physical) shrinkage
144.08 118.57 107-59 147-20
Activation energy, Ea (kJ/mol)
,.-.
Ix9
P A N precursors shrinkage--influence on carbon fibres
183
treatment at 230°C. The shrinkage can be subdivided into three temperature regions, namely: 40-180°C, 180-230°C and 230°C isothermal for 100 min. The CuC1 stretched precursor (B) shows the greatest physical shrinkage -153°//o compared with 7 . 2 ~ for the N 2 elongated sample (C) which was least. However, sample 'C' shows the greatest amount of chemical shrinkage (15.6)o), while the rest of the samples show a similar value of around 9 °/ o/ ' This therefore infers that sample 'C' undergoes the greatest amount of cyclization up to 230 °C compared with the other samples, or, in other words, the structure of sample 'C' is so modified that it is more conducive to the initiation of the cyclization reaction. On the other hand, sample C shows the least physical shrinkage of all the samples. It is well known 11 that greater amounts of physical shrinkage lead to the loss of preferred orientation. The aim should therefore be to avoid physical shrinkage to the greatest possible extent and, at the same time, have a maximum of chemical shrinkage which is due to the cyclization reaction. 1° Sample "C' seems to satisfy these conditions. In order to understand this in greater detail, the values of the activation energy for the cyclization of each of these samples were examined. These are presented in the last column in Table 1. The activation energy, E a, is least for sample 'C' which suggests that this sample possesses the best structure as far as the temperature of initiation of the cyclization reaction is concerned. This is the reason why it also shows the greatest cyclization shrinkage. It is also known that the cyclization of PAN is associated with a large exotherm. 14 Because of the lower value of E a, it is to be expected that the exotherm of sample 'C' will start at a lower temperature than those of other samples. This is confirmed by the DTA scans in Fig. 2 in which sample ~C' shows the lowest temperature of initiation of cyclization, which is -,- 160°C. The master sample and the steam stretched samples, which have the lowest values of cyclization shrinkage, show higher values of exotherm initiation temperature. It therefore seems that sample 'C' is the best stabilized sample. Examination of the shrinkage behaviour of these samples during isothermal treatment for 100 min at 230 °C, reveals that samples C, D and E shrink by about 18 ~o compared with 21 ~o for A and 25 !~ofor sample B and that overall chemical shrinkage during the isothermal stabilization is the same for samples B and C, but much less for samples A, D and E. On the basis of this shrinkage behaviour one may conclude that the stabilization
184
R. B. Mathur, T. L. Dhami, O. P. Bahl
T
(A) Precursor PAN (B) CuCI Stretching
"6 E
(C) 10% Elongation N 2 E
,4:
"'
(D) IO% Elongation Air
O
•
d
_o N b. o o 3:
W
b,i Z F-
'
I c~
II1
Fig. 2.
'
'
'
'
I
i
i
,
,
I
'
,
,
i
I
0
0
0
--
--
0 N
'
'
i
i
I 0
'
l
l
I
I 0
I
I
I
|
I
I
!
0
0
DTA curves of precursor PAN and its modified counterparts.
of the samples is in the decreasing order, BCAD and E. Of the carbon fibres prepared from such samples those from precursor C possess the highest values of mechanical properties, as shown in Table 2. One would have expected that sample B, which seems, from its shrinkage behaviour, to be best stabilized, must give the best carbon fibres. The study thus reveals that it is not only the control of chemical shrinkage which is important but that the amount of physical shrinkage also plays a dominant r61e so far as the development of carbon fibres from PAN is concerned. One should have the minimum possible amount of physical shrinkage, as in sample C. It seems possible that the amount of orientation in the molecular chains which occurs during modification in sample +B' is partially surrendered during this stage of shrinkage and this is the reason why resulting carbon fibres are not as good as anticipated. Experiments are in progress to limit this shrinkage in sample 'B' to only 7 ~0 and to examine the effect on the mechanical properties of resulting carbon fibres. This study also demonstrates the important point that one should not correlate the overall shrinkage with the stabilization of PAN. It may be misleading in the case of modified precursors. One must also take into account the physical shrinkage which occurs at various stages.
P A N precursors shrinkage--influence on carbon.fibres
185
TABLE 2 Mechanical Properties of Modified PAN Precursor and Resulting Carbon Fibres
Serial no.
Samph'
Mechanical properties
code Precursor P A N
I 2 3 4
Resulting carbon [ibres
Tensih" strength ( GPa)
Young's modulus ( GPa)
Tensile strength ( GPa)
Young's modulus ( GPa)
0.51 1.00 0.57 0.51
4.96 16.20 9.93 8-62
1.38 1-89 3.2 I-38
144-82 165-51 206-89 144.82
A B C D
Shrinkage during carbonization Table 3 shows the shrinkage behaviour and weight loss of stabilized PAN precursors A o to E o during carbonization. This shrinkage can be divided broadly into two temperature regions--230-500 °C and 500-900 °C. The rate of increase of temperature was 4 °C/min for all samples. Table 3 shows that the greatest shrinkage in the temperature region 230 500°C occurs in sample A o (4 i~',;), whereas the rest of the samples TABLE 3 Shrinkage Behaviour and Weight Loss During Carbonization of Modified Stabilized PAN Fibres
Serial No.
Temperature range 230 50O° C
Total shrinkage
50~900° C
('!~,)
......
( !'~,)
Shrinkage (",~,)
Weight loss
Shrinkage ( ",~)
(".) Ao Bo C~ Do E~I
Total weight loss
4"02 2' !9 2"30 1"25 1'31
6"53 5"57 5"94 5'51 5'57
Weight loss
(",,) 10"86 7"82 7" 19 4" 14 3"64
33"85 34' 10 32'27 34"87 34" 10
14'88 10"01 9'49 5"39 5"09
40"38 39"67 38"21 40"38 39'67
186
R. B. Mathur, T. L. Dhami, O. P. Bahl
show a smaller shrinkage of approximately 2 ~ . It is interesting to note also the weight losses of these fibres in this temperature region. It has already been mentioned that the weight loss, as well as the shrinkage, is the result of various chemical reactions and the evolution ofvolatiles. Sample A o, which shows the greatest shrinkage, also shows the greatest weight loss. It may be concluded that, in the absence of proper stabilization, uncyclized ring sequences in sample A o are driven off in the form of large molecular weight hydrocarbons (which condense to form tar), causing defect sites and the reorganization of molecular chains at these sites. This causes greater unwanted shrinkage as well as weight loss. Hence the mechanical properties of the resulting carbon fibres are also poor, as shown in Table 2. In the temperature region 500-900°C, the shrinkage and weight loss are mainly caused by the denitrogenation reaction and the evolution of HCN from the unreacted nitrile groups, is However, evolution of N 2 and HCN play conflicting r61es. Evolution of N 2 suggests the linking of two adjacent aromatic units to impart greater strength to the fibre structure. Evolution of HCN, on the other hand, implies the incorporation of defects in the structure, thereby lowering the strength of resulting carbon fibres. Since sample A o does not result in good carbon fibres, it may be inferred that the major part of its 11 ~o shrinkage in the region 500-900 °C is due to HCN evolution, whereas the major part of the 7 ~o shrinkage suffered by sample C Ois due to the denitrogenation reaction. This is the reason why C Obased carbon fibres show better mechanical properties. C o also shows the lowest weight loss during carbonization and a higher carbon yield. CONCLUSIONS This study reveals that PAN fibres modifed under different conditions result in carbon fibres with different mechanical properties. These results can be explained on the basis of their shrinkage behaviours during the various stages of processing. ACKNOWLEDGEMENTS The authors wish to thank Dr A. P. Mitra, Director, National Physical Laboratory, for his keen interest and permission to publish this work. The
PAN precursors shrinkage--influence on carbonfibres
187
encouragement received from Dr V. N. Bindal is also gratefully acknowledged,
REFERENCES 1. R. C. Houtz, Textih' Res. J., 20, 786 (1970). 2. A. K.!Shindo, Report 317, Govt. Ind. Res. Inst., Osaka (1961). 3. O. P. Bahl, L. M. Manocha, G. C. Jain, S. S. Chari and G. Bhatia, J, Sci. & Ind. Res., 38, 537 (1979). 4. R. M. Gill, Carbon fibres in composite materials. Published for The Plastics Institute, Butterworth, London (1972). 5. W. Johnson, L. N. Phillips and W. Watt, British Pat. 1110791 (1968). 6. J. Brandrup and L. H. Peebles, Macromolecules, 1, 64 (1968). 7. O. P. Bahl and L. M. Manocha, Carbon, 13, 297 (1975). 8. W. Watt, Proc. Roy. Soc. Lond., A319, 5 (1970). 9. J. W. Johnson and W. Watt, Polymer Reprints, Am. Chem. Soc. Atlantic City Meeting, 1968, Applied Polymer Symposia 9, 215 (1969). 10. D. J. Muller, E. Fitzer and A. K. Fiedler, Proc. Int. Carbon Fibres ConJl London, 10 (1971). 11. O. P. Bahl and L. M. Manocha, Angew. Makromol. Chem., 48, 145 (1975). 12. O. P. Bahl, R. B. Mathur and T. L. Dhami, J. Mat. Sci. andEng., 73, 105 (1985). 13. N. Poporske and I. Mladenov, Carbon, 21, 33 (1983). 14. O. P. Bahl and L. M. Manocha, Angew. Makromol. Chem., 64, 115 (1977). 15. L. M. Manocha, O. P. Bahl and G. C. Jain, Angew. Makromol. Chem., 67, 11 (1978).
Shrinkage Behaviour of Modified PAN Precursors--Its Influence on the Properties of Resulting Carbon Fibre R. B. Mathur, T. L. Dhami & O. P. Bahl Carbon Technology Unit, National Physical Laboratory, New Delhi 110 012, India (Received: 19 July, 1985)
A BSTRA CT Carbon fibres have been developed using PA N precursors modified under different conditions. The shrinkage behaz'iour and weight loss of these modified precursors during proeessing into carbon fibres have been studied on a Mettler TA-3000 thermal analyzer. The implications offibre shrinkage during various stages of carbon .[ibre formation have been discussed.
INTRODUCTION For a variety of reasons 1-4 polyacrylonitrile (PAN) is one of the most commonly used precursors for making carbon fibres. The conversion of PAN to carbon fibres involves two major steps: 2'5 7 (i) thermal stabilization and (ii) carbonization. In the stabilization step, PAN is heated to 200-300 °C in an oxidizing atmosphere to convert it into non-plastic, thermally stable flame-proof material, which can then withstand temperatures up to 1000-3000°C in an inert atmosphere. The maximum temperature depends upon the type of carbon fibres ultimately required. 8 During thermal stabilization, PAN shows characteristic length changes, 9-,11 which depend upon the precursor as well as on the processing parameters during the stabilization step, such as stabilization temperature, time, rate of heating, load applied per filament, ambient 179
Polymer Degradation and Stabilio, 0141-3910/86/$03"50 ~') Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
180
R. B. Mathur, T. L. Dhami, O. P. Bahl
conditions, etc. According to Muller e t al. l o the shrinkage which occurs in PAN when it is heated to temperatures above the glass transition temperature is due to cyclization of PAN to form an imperfect ladder polymer structure. However, Bahl and Manocha 11 have reported two types of shrinkage--physical and chemical--which occur during the stabilization step. The physical shrinkage has been reported to be independent of temperature and oxidizing gas while the chemical shrinkage occurs as the result of chemical reactions taking place during the stabilization step. In this laboratory, we have developed carbon fibres from PAN which has been modified by employing various techniques, described elsewhere. 12 An attempt has been made to study the shrinkage behaviour of these modified precursors during oxidation and during carbonization and their causes and implications.
EXPERIMENTAL All the experiments reported here have been carried out with copolymer fibres supplied by Courtaulds Limited, Great Britain. A tow containing 6000 monofilaments, of 1.2 d-tex each, was used. Various methods which have been employed for the modification of the original precursor are described in detail elsewhere. 12 A TMA 40 cell of the Mettlar TA-3000 system was used for recording shrinkages. Two to three thousand monofilaments, approximately 20 mm long, were clamped in the holder with screws. The distance between the clamps was exactly 10 mm and the holder was attached to the TMA 40 cell as shown in Fig. I. A fixed load of 1 mg per filament was applied during the experiments. The fibres were first heated at a rate of 5 °C/min up to 230°C and kept at 230°C for 100 min in air. They were further heated from 230 °C to 900 °C at different heating rates in a nitrogen atmosphere containing a maximum of 10ppm of oxygen. Length changes of the order of 0.1/~m can be detected by the instrument. TGA experiments were carried out with the TGA 50 module of the same equipment. A I 0-mg sample was heated at 100 °C to constant weight and then further heated from 100 to 1000°C at a rate of 10°C/min in nitrogen containing a maximum of 10 ppm of oxygen. The DTA'20 unit of the TA-3000 system was used to study the heat flow behaviour of the samples. The values of activation energy were calculated using the
P A N precursors shrinkage--influence on carbon fibres
181
6
1 Fixed quortz hook
4. T.M.A. Measuring probe.
2. Clomp 3. Fibre
5. Somple eupport 6. Furnace
Fig. !.
Fibre attachment for measuring length changes.
Arrhenius equation 13 by the TC-10 Swiss Matrix 7775 microprocessor, attached to the unit. The modified precursors were converted into carbon fibres by optimizing various processing parameters as described earlier. ~2 The mechanical properties of the modified PAN fibres and those of the resulting carbon fibres were determined at 2 cm gauge length, using an lnstron 1122 tensile testing machine and single filament test methods. An average of twenty individual tests was reported for each sample.
RESULTS AND DISCUSSION
Shrinkage during thermal stabilization Table 1 shows the amount of shrinkage shown by the master sample (unmodified PAN) and by the modified precursors during an increase in temperature from room temperature to 230 °C and subsequent isothermal
A B C D E
Serial Samph' No. code
Precursor (untreated) C u C 1(stretched) N 2 (elongated) Air (elongated) Steam (stretched)
Treatment
8-87 15.34 7.17 12-06 13-86
40-180°C
(%)
Physwal shrmkage
9'22 9-52 15.60 8.91 8'80
180-230 °C
21.03 25'21 17.95 18-56 17.72
lOOmin ~othermal at 230°C 30'25 34.73 33.55 27-47 26.52
Total chemical shrinkage
Chemical shrinkage (%)
TABLE I Shrinkage Behaviour of Modified PAN Fibres
39.12 50"07 40.72 39.53 40.38
Total (%) (Chemical and physical) shrinkage
144.08 118.57 107-59 147-20
Activation energy, Ea (kJ/mol)
,.-.
Ix9
P A N precursors shrinkage--influence on carbon fibres
183
treatment at 230°C. The shrinkage can be subdivided into three temperature regions, namely: 40-180°C, 180-230°C and 230°C isothermal for 100 min. The CuC1 stretched precursor (B) shows the greatest physical shrinkage -153°//o compared with 7 . 2 ~ for the N 2 elongated sample (C) which was least. However, sample 'C' shows the greatest amount of chemical shrinkage (15.6)o), while the rest of the samples show a similar value of around 9 °/ o/ ' This therefore infers that sample 'C' undergoes the greatest amount of cyclization up to 230 °C compared with the other samples, or, in other words, the structure of sample 'C' is so modified that it is more conducive to the initiation of the cyclization reaction. On the other hand, sample C shows the least physical shrinkage of all the samples. It is well known 11 that greater amounts of physical shrinkage lead to the loss of preferred orientation. The aim should therefore be to avoid physical shrinkage to the greatest possible extent and, at the same time, have a maximum of chemical shrinkage which is due to the cyclization reaction. 1° Sample "C' seems to satisfy these conditions. In order to understand this in greater detail, the values of the activation energy for the cyclization of each of these samples were examined. These are presented in the last column in Table 1. The activation energy, E a, is least for sample 'C' which suggests that this sample possesses the best structure as far as the temperature of initiation of the cyclization reaction is concerned. This is the reason why it also shows the greatest cyclization shrinkage. It is also known that the cyclization of PAN is associated with a large exotherm. 14 Because of the lower value of E a, it is to be expected that the exotherm of sample 'C' will start at a lower temperature than those of other samples. This is confirmed by the DTA scans in Fig. 2 in which sample ~C' shows the lowest temperature of initiation of cyclization, which is -,- 160°C. The master sample and the steam stretched samples, which have the lowest values of cyclization shrinkage, show higher values of exotherm initiation temperature. It therefore seems that sample 'C' is the best stabilized sample. Examination of the shrinkage behaviour of these samples during isothermal treatment for 100 min at 230 °C, reveals that samples C, D and E shrink by about 18 ~o compared with 21 ~o for A and 25 !~ofor sample B and that overall chemical shrinkage during the isothermal stabilization is the same for samples B and C, but much less for samples A, D and E. On the basis of this shrinkage behaviour one may conclude that the stabilization
184
R. B. Mathur, T. L. Dhami, O. P. Bahl
T
(A) Precursor PAN (B) CuCI Stretching
"6 E
(C) 10% Elongation N 2 E
,4:
"'
(D) IO% Elongation Air
O
•
d
_o N b. o o 3:
W
b,i Z F-
'
I c~
II1
Fig. 2.
'
'
'
'
I
i
i
,
,
I
'
,
,
i
I
0
0
0
--
--
0 N
'
'
i
i
I 0
'
l
l
I
I 0
I
I
I
|
I
I
!
0
0
DTA curves of precursor PAN and its modified counterparts.
of the samples is in the decreasing order, BCAD and E. Of the carbon fibres prepared from such samples those from precursor C possess the highest values of mechanical properties, as shown in Table 2. One would have expected that sample B, which seems, from its shrinkage behaviour, to be best stabilized, must give the best carbon fibres. The study thus reveals that it is not only the control of chemical shrinkage which is important but that the amount of physical shrinkage also plays a dominant r61e so far as the development of carbon fibres from PAN is concerned. One should have the minimum possible amount of physical shrinkage, as in sample C. It seems possible that the amount of orientation in the molecular chains which occurs during modification in sample +B' is partially surrendered during this stage of shrinkage and this is the reason why resulting carbon fibres are not as good as anticipated. Experiments are in progress to limit this shrinkage in sample 'B' to only 7 ~0 and to examine the effect on the mechanical properties of resulting carbon fibres. This study also demonstrates the important point that one should not correlate the overall shrinkage with the stabilization of PAN. It may be misleading in the case of modified precursors. One must also take into account the physical shrinkage which occurs at various stages.
P A N precursors shrinkage--influence on carbon.fibres
185
TABLE 2 Mechanical Properties of Modified PAN Precursor and Resulting Carbon Fibres
Serial no.
Samph'
Mechanical properties
code Precursor P A N
I 2 3 4
Resulting carbon [ibres
Tensih" strength ( GPa)
Young's modulus ( GPa)
Tensile strength ( GPa)
Young's modulus ( GPa)
0.51 1.00 0.57 0.51
4.96 16.20 9.93 8-62
1.38 1-89 3.2 I-38
144-82 165-51 206-89 144.82
A B C D
Shrinkage during carbonization Table 3 shows the shrinkage behaviour and weight loss of stabilized PAN precursors A o to E o during carbonization. This shrinkage can be divided broadly into two temperature regions--230-500 °C and 500-900 °C. The rate of increase of temperature was 4 °C/min for all samples. Table 3 shows that the greatest shrinkage in the temperature region 230 500°C occurs in sample A o (4 i~',;), whereas the rest of the samples TABLE 3 Shrinkage Behaviour and Weight Loss During Carbonization of Modified Stabilized PAN Fibres
Serial No.
Temperature range 230 50O° C
Total shrinkage
50~900° C
('!~,)
......
( !'~,)
Shrinkage (",~,)
Weight loss
Shrinkage ( ",~)
(".) Ao Bo C~ Do E~I
Total weight loss
4"02 2' !9 2"30 1"25 1'31
6"53 5"57 5"94 5'51 5'57
Weight loss
(",,) 10"86 7"82 7" 19 4" 14 3"64
33"85 34' 10 32'27 34"87 34" 10
14'88 10"01 9'49 5"39 5"09
40"38 39"67 38"21 40"38 39'67
186
R. B. Mathur, T. L. Dhami, O. P. Bahl
show a smaller shrinkage of approximately 2 ~ . It is interesting to note also the weight losses of these fibres in this temperature region. It has already been mentioned that the weight loss, as well as the shrinkage, is the result of various chemical reactions and the evolution ofvolatiles. Sample A o, which shows the greatest shrinkage, also shows the greatest weight loss. It may be concluded that, in the absence of proper stabilization, uncyclized ring sequences in sample A o are driven off in the form of large molecular weight hydrocarbons (which condense to form tar), causing defect sites and the reorganization of molecular chains at these sites. This causes greater unwanted shrinkage as well as weight loss. Hence the mechanical properties of the resulting carbon fibres are also poor, as shown in Table 2. In the temperature region 500-900°C, the shrinkage and weight loss are mainly caused by the denitrogenation reaction and the evolution of HCN from the unreacted nitrile groups, is However, evolution of N 2 and HCN play conflicting r61es. Evolution of N 2 suggests the linking of two adjacent aromatic units to impart greater strength to the fibre structure. Evolution of HCN, on the other hand, implies the incorporation of defects in the structure, thereby lowering the strength of resulting carbon fibres. Since sample A o does not result in good carbon fibres, it may be inferred that the major part of its 11 ~o shrinkage in the region 500-900 °C is due to HCN evolution, whereas the major part of the 7 ~o shrinkage suffered by sample C Ois due to the denitrogenation reaction. This is the reason why C Obased carbon fibres show better mechanical properties. C o also shows the lowest weight loss during carbonization and a higher carbon yield. CONCLUSIONS This study reveals that PAN fibres modifed under different conditions result in carbon fibres with different mechanical properties. These results can be explained on the basis of their shrinkage behaviours during the various stages of processing. ACKNOWLEDGEMENTS The authors wish to thank Dr A. P. Mitra, Director, National Physical Laboratory, for his keen interest and permission to publish this work. The
PAN precursors shrinkage--influence on carbonfibres
187
encouragement received from Dr V. N. Bindal is also gratefully acknowledged,
REFERENCES 1. R. C. Houtz, Textih' Res. J., 20, 786 (1970). 2. A. K.!Shindo, Report 317, Govt. Ind. Res. Inst., Osaka (1961). 3. O. P. Bahl, L. M. Manocha, G. C. Jain, S. S. Chari and G. Bhatia, J, Sci. & Ind. Res., 38, 537 (1979). 4. R. M. Gill, Carbon fibres in composite materials. Published for The Plastics Institute, Butterworth, London (1972). 5. W. Johnson, L. N. Phillips and W. Watt, British Pat. 1110791 (1968). 6. J. Brandrup and L. H. Peebles, Macromolecules, 1, 64 (1968). 7. O. P. Bahl and L. M. Manocha, Carbon, 13, 297 (1975). 8. W. Watt, Proc. Roy. Soc. Lond., A319, 5 (1970). 9. J. W. Johnson and W. Watt, Polymer Reprints, Am. Chem. Soc. Atlantic City Meeting, 1968, Applied Polymer Symposia 9, 215 (1969). 10. D. J. Muller, E. Fitzer and A. K. Fiedler, Proc. Int. Carbon Fibres ConJl London, 10 (1971). 11. O. P. Bahl and L. M. Manocha, Angew. Makromol. Chem., 48, 145 (1975). 12. O. P. Bahl, R. B. Mathur and T. L. Dhami, J. Mat. Sci. andEng., 73, 105 (1985). 13. N. Poporske and I. Mladenov, Carbon, 21, 33 (1983). 14. O. P. Bahl and L. M. Manocha, Angew. Makromol. Chem., 64, 115 (1977). 15. L. M. Manocha, O. P. Bahl and G. C. Jain, Angew. Makromol. Chem., 67, 11 (1978).