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Strain-induced density changes in PAN-based carbon fibres
PERGAMON
Carbon 38 (2000) 2007–2016
Strain-induced density changes in PAN-based carbon fibres S. Ozbek, D.H. Isaac* a
Department of Materials Engineering, University of Wales, Swansea, SA2 8 PP, UK Received 8 October 1999; accepted 19 February 2000
Abstract A series of density measurements has been carried out on PAN-based carbon fibres, which had been hot stretched under various processing regimes. It was found that all three process variables, namely temperature, stretching stress and dwell time, were important in determining the final density and that no simple factor related to the processing conditions could be used to characterise the density uniquely. Both Young’s modulus and tensile strength of the fibres were found to increase with increasing density but again the relationships depended on the specific combination of temperature, stretching stress and dwell time. On the other hand, it was found that both the preferred orientation and the apparent crystallite size, Lc , increased with increasing density, and both were directly related to density, independent of the precise combination of processing conditions. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibres; B. Heat treatment; D. Microstructure; Mechanical properties
1. Introduction Since the early 1960s considerable efforts have been made towards the production of high-strength highmodulus carbon fibres from a variety of polymeric precursors. Although a number of different starting materials [such as polyacrylonitrile (PAN), rayon, pitch, etc.] have been investigated, currently the most popular precursor material for carbon fibre production is (PAN)-based textile fibre. This precursor is conventionally subjected to lowtemperature stretching under oxidising conditions, which crosslinks the chains. Thus the fibre is stabilised with a degree of preferred orientation of the component chains impressed on the carbon precursor. The material is then carbonised by heat-treating to |13008C, free of stress, to produce high-strength carbon fibres, typically with a Young’s modulus of |230 GPa and a tensile strength of |4 GPa or more. Further heat treatment to |25008C results in high-modulus carbon fibres with modulus values typically as high as |400 GPa but a reduction in strength to less than |4 GPa. Various studies [1–5] in the 1960s and 1970s showed that further stretching of carbon fibres from different precursors can produce carbon fibre with a very high Young’s modulus. For example, Johnson [4] found that *Corresponding author. Tel.: 144-1792-295-288; fax: 1441792-295-244.
with the PAN-based fibre available at the time, an increase in axial modulus to |500 GPa could be achieved, combined with a strength of |2.5 GPa. This work also showed that many of the stress concentrations causing failure were not associated with impurities because these could not conceivably be removed by the hot-stretching process. Despite these results, the advantages of hot stretching were not exploited commercially, possibly due to limitations in the PAN-based precursors then available. More recently, developments in Japan by Toray [6] have shown that PAN-based carbon fibres can now be produced commercially (M60J) with a Young’s modulus of |590 GPa and a tensile strength of |3.9 GPa, although the production details for these fibres have not been published. However, the higher purity and lower defect population of fibres produced in recent years have led to a reassessment of hot-stretching experiments as a route for producing highmodulus and high-strength PAN-based carbon fibres in our laboratories [7,8]. In these experiments, commercial carbon fibres were hot stretched with varying stresses and dwell times at temperatures up to 30008C. The results indicated that all three of these process variables were important in determining the subsequent mechanical behaviour. Significant increases in modulus were recorded, although a loss in strength occurred following high-temperature processing without significant stretching. However, the strength was restored and in some cases improved when the high temperatures were accompanied by a
0008-6223 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00060-9
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reasonable degree of stretching. Microstructural changes in these hot-stretched fibres have also been investigated by X-ray diffraction [9,10] using synchrotron radiation. These studies showed substantial improvements in the preferred orientation parameter (Z) and increases in the apparent crystallite stack height (Lc ). Although many studies have been reported on the effects of processing on the structure and properties of carbon fibres [11,12], there is very little information available in the literature concerning the effects of process variables on the density of hot-stretched carbon fibres. However, one of the earliest studies on PAN-based carbon fibres by Shindo [13] showed that the density increased with heat-treatment temperature in the absence of stretching. Figures of |1950 kg m 23 at 25008C to |2030 kg m 23 at 30008C were reported by Shindo and he ascribed the increases to crystallite growth during heat-treatment. Later, Bacon [14] reported that the most important factor affecting the density of graphitised rayon-based fibres was the amount of stretch to which the fibres were subjected during graphitisation. Bacon collected his data from studies carried out on commercial fibres (Thornel) and experimental fibres prepared by similar processing techniques. The wide range of densities represented in his report showed a range of porosities from |40% (corresponding to a density of 1320 kg m 23 ) for low-modulus graphitised fibres, to only |12% (1936 kg m 23 ) for fibres with a Young’s modulus of |690 GPa. These porosities were calculated by comparing the filament bulk density with the X-ray density (approximately 2200 kg m 23 ). As outlined above, previous reports from this laboratory have considered the effects on mechanical properties [7,8] and microstructure [9,10] of hot-stretching carbon fibres. This report presents density data from some of these fibres, and relates these data to the process parameters and the final properties and microstructure of the fibres.
2. Experimental A graphite element resistance furnace from the ASTRO 1000A series, with a hot-zone length of 150 mm at 30008C, was adapted to allow the insertion of a carbon fibre tow subjected to a constant load. Further details of the equipment and the experimental arrangements have been published elsewhere [7,8]. Three sets of data are presented in this report, resulting from three series of experiments. Experiments for all series were performed on an experimental PAN-based 3000 filament batch, with a nominal diameter of 7 mm, supplied by Courtaulds that had been heat-treated to approximately 13008C during manufacture. The quoted tensile strength and modulus of this starting fibre were checked and confirmed to be 3.960.4 GPa and 180630 GPa, respectively, as measured by single fibre testing. The diameter and density of as-received fibres
were measured to be 7.060.2 mm and 173962 kg m 23 , respectively, figures which were consistent with those quoted by the manufacturers. For the first series, a set of twelve experimental fibre tows was produced by stretching 820-mm-long cut tows at four different temperatures of 27008C, 28008C, 29008C and 30008C with three loads, equivalent to stresses of 5 MPa, 43 MPa and 85 MPa, for a fixed dwell time of 5 min. For the second series, twelve different processing conditions were considered, using six stretching stresses of 26 MPa, 43 MPa, 51 MPa, 61 MPa, 79 MPa and 128 MPa at constant hot-zone temperatures of 26008C and 28008C for a fixed dwell time of 30 min. The third series of experiments produced a further set of 10 fibre tows using dwell times of 2, 5, 10, 20 and 30 min at constant hot-zone temperatures of 26008C and 2800 o C, with a fixed stretching stress of 79 MPa. The procedure for each processing run was to heat up a tow of fibres to the desired temperature at a rate of 27.38C min 21 , in a helium atmosphere at 50 kPa above ambient pressure, with minimal stretching. During this heating-up period, a small fixed extension of 1.5 mm was allowed in order to keep the fibres taut and to ensure that all the fibre tows had the same thermal–stretch history. When the final temperature was reached the load was applied by moving the platform away from the loading system. The tow was then allowed to stretch at a fixed temperature and constant load for the desired time. The extension was monitored as a function of time during each stretching experiment. The procedure for the first series was slightly different from the above in that the fibre tows were allowed to stretch during the heating-up period in addition to the 5 min dwell time. The as-received fibre samples were deresinated in ethyl methyl ketone (2-Butanone) before the density measurement, to remove the resin with which the fibres had been coated during manufacture. Densities of the fibre samples were measured using the flotation technique. Two types of liquid, tetrabromoethane (Br 2 CHCHBr 2 ), and carbon tetrachloride (CCl 4 ), with dissimilar densities 2950–2965 kg m 23 and 1594 kg m 23 at 208C, respectively, were mixed in the proportions required to ensure that the fibre samples floated in the middle of the mixture. The density of the mixture, and hence the sample density, was calculated from knowledge of the proportions of each component. For each processing condition, the densities of at least five small bundles were measured and the average and standard deviation determined. The Young’s modulus and tensile strength of individual filaments were measured on specially made single fibre testing equipment as described in a previous paper [8]. Modulus values were determined from the gradient of the central region of the load–extension curve, which was close to linear, ignoring the ends of the curve, which showed some non-linearity. Thus modulus values were not all measured at precisely the same fixed strain (a method
S. Ozbek, D.H. Isaac / Carbon 38 (2000) 2007 – 2016
suggested to account for variation with strain [15]) but were typically averaged over a range of between about 0.3 and 0.6%. Mechanical testing was carried out on a minimum of eight individual fibres, with a gauge length of 27 mm, for each processing condition, and an average and standard deviation determined. Individual fibre diameters were measured by optical microscopy using an image shearing microscope and confirmed by laser diffraction measurements. X-ray diffraction patterns of single fibres were taken using the synchrotron radiation source (SRS) at Daresbury Laboratory, UK. The preferred orientation parameter, Z (in 8), was taken to be the full width at half maximum height (FWHM) of an azimuthal scan through the (00.2) reflection. The measure of apparent crystallite stack height size, Lc , perpendicular to the basal planes was calculated from the FWHM of a radial scan through the (00.2) reflection. X-ray diffraction patterns from as-received monofilaments were too weak and diffuse for analysis, and so a bundle of as-received fibres was used for comparison purposes and a Z value of |388 was determined. The stacking height, Lc , of the as-received fibres was too small to estimate a meaningful value. Further details of the X-ray diffraction studies have been published elsewhere [9,10].
2009
3. Results and discussion Attempts have been made to establish the effects of each of the process variables on the final fibre density, to determine the relationships between density and the mechanical performance, and to elucidate any correlation between the fibre density and the structural parameters and these are discussed below.
3.1. Effects of process variables The effect of stretching temperature on the fibre density is evident from Fig. 1. For this graph, data were taken from the first series of experiments for three stretching stresses (5 MPa, 43 MPa and 85 MPa) at four processing temperatures (27008C, 28008C, 29008C and 30008C), for 5 min in each case. The best-fit curves to the data points show that the density increases with heat treatment temperature and that this density increase is more pronounced above 28508C, at which temperature larger plastic deformation starts to take place. These data in Fig. 1 also indicate that the fibre density is highly influenced by the stretching stress. This effect can be seen more graphically when density data are plotted against stretching stress for the
Fig. 1. The effects of processing temperature on fibre density for samples stretched for 5 min, with stresses of 5 MPa, 43 MPa or 85 MPa at temperatures between 27008C and 30008C. In all the figures, error bars for density values correspond to the standard deviation of measurements from at least five small bundles.
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data of the second series of experiments (Fig. 2). These data suggest that the density increases approximately linearly with stretching stress for the temperature range investigated. The work carried out by Shindo [13] on PAN-based carbon fibres showed that after heat-treatment to 27008C and 30008C, without stretching, the density rose to |2000 kg m 23 and |2030 kg m 23 , respectively. Although in the present work the absolute values of density are lower, consideration of the difference in densities between fibres treated to these two temperatures is illuminating. Thus for Shindo’s work, which involved no stretching, this density difference corresponds to |1.5%. The density results obtained in the present study, at the same heat-treatment temperatures but with a small stretching stress of 5 MPa, show a difference of |1.6% from |1857 kg m 23 to |1887 kg m 23 . With the intermediate load of 43 MPa, corresponding densities are |1859 kg m 23 and 1908 kg m 23 giving a difference of |2.6%. When the stretching stress is increased to 85 MPa the densities at these two temperatures are |1869 kg m 23 and |1929 kg m 23 corresponding to a difference of about 3%. Thus, although the absolute values of densities are somewhat lower that those reported by Shindo, probably due to differences in the precursors, the trends are remarkably consistent. Indeed, extrapolation of the present data to no stretching stress gives a density difference of |1.5%.
Bacon [14] collected data from various sources to establish a density–stretch relationship for stress-graphitised rayon-based carbon fibres and he found that the density was significantly affected by the degree of stretch. He reported that the density increased from |1340 kg m 23 to |1930 kg m 23 , when stretched at around 28008C, with effective strains of |2% and |110%, respectively. Bacon calculated the effective strain from the reduction in filament cross-sectional area compared with unstretched but graphitised fibres. When strains were determined in the same way for samples stretched at 28008C in the present study, it was found that the reduction in cross-sectional area increased from |8% for the lightly stretched fibre (5 MPa) to |34% for the highly stretched fibre (85 MPa), whilst the density only increased from |1860 kg m 23 to |1877 kg m 23 . It seems that the difference in the degree of stretch and density changes between these two studies is caused by the highly porous (about 40%) nature of the unstressed rayon-based fibres, as reported by Bacon [14]. Fig. 3 shows how the density of hot-stretched fibres varies with dwell time. It may be seen that the main density increase occurs in the first 5–10 min of the 30-min stretching period and then the rate of increase is significantly reduced. In this figure, best-fit logarithmic curves have been included in light of the observed logarithmic dependence of extension with time reported elsewhere [8]. It was not possible to measure the true strain experimen-
Fig. 2. The effects of stretching stress on the fibre density for samples stretched at 26008C or 28008C for 30 min with stretching stresses of between 26 and 128 MPa.
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Fig. 3. The effects of processing dwell time on the fibre density for samples stretched at 26008C or 28008C, with a stretching stress of 79 MPa for times between 2 and 30 min.
Fig. 4. The effects of induced extension of the fibres on the resulting density for samples stretched for 5 min, with stresses of 5 MPa, 43 MPa or 85 MPa at temperatures between 27008C and 30008C.
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tally due to the non-uniform temperature distribution along the furnace length. Hence, the total extension induced by each processing condition was measured to give an approximation to the relative strains. This extension is plotted against density in Fig. 4 for the first series of experiments, and it may be seen that there is an approximately linear relationship at each of the processing temperatures. Although the equivalent data for the second series of experiments also show an approximately linear relationship for each temperature, these data do not superimpose on the same temperature data in Fig. 4, which would have been expected if a simple combination of temperature and extension determined density. Thus, overall, these results indicate that the density of the hot-stretched fibres is highly influenced by the stretching stress, temperature and dwell time. The density is seen to be a function of a combination of these processing variables and by holding two of these variables constant, a reasonable relationship can be determined between the third variable and the density. However, there is no simple formula that can be used to relate all the process parameters to the density simultaneously, and further investigation is required to give any mathematical expression of these relationships. Furthermore, no simply measurable fibre parameter such as diameter, extension or reduction in
cross-sectional area can be related directly to density, independently of the process parameters.
3.2. Mechanical properties–density relationship Fig. 5 is a plot of density against Young’s modulus for the first series of experiments. It may be seen that the density increases approximately linearly with the modulus at each temperature. These results are similar to the density–Young’s modulus relationship of rayon-based fibres reported by Bacon [14]. He showed that the Young’s modulus increases almost linearly with increasing density, although his data were collected from measurements of commercial grades of fibres (Thornel) and experimental fibres prepared by similar processing techniques. These increases in density and modulus were found to be related to the substantial structural changes taking place during hot stretching, and this aspect is discussed further in the next section. Fig. 6 shows the relationship between density and tensile strength for the first series of experiments. Although these data indicate substantial scatter and large error bars, due to randomly occurring defects, there is evidence for increasing strength with density. The straight lines, which show linear regressions for each temperature,
Fig. 5. The relationship between density and Young’s modulus for samples stretched for 5 min, with stresses of 5 MPa, 43 MPa or 85 MPa at temperatures between 27008C and 30008C. At each temperature, higher stretching stresses produced higher modulus values. Error bars for modulus values correspond to the standard deviation of measurements from at least eight fibres.
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Fig. 6. The relationship between density and tensile strength (gauge length 27 mm) for samples stretched for 5 min, with stresses of 5 MPa, 43 MPa or 85 MPa at temperatures between 27008C and 30008C. The large error bars result from large variability in strength measurements due to a statistical distribution of defects. Nevertheless, there is a trend of increasing tensile strength with density at each temperature. Error bars for strength values correspond to the standard deviation of measurements from at least eight fibres.
are only meant to give an indication of the general trends, since there is not enough evidence to confirm a linear relationship. Fig. 6 also shows that, although the density increases substantially with high-temperature treatment and minimal stretching compared with the as-received value of 1739 kg m 23 , at the low-stretch end the strength is reduced to about half the original value of |3.9 GPa. When the extension is increased by using higher loads, the strength is improved substantially and the original strength is recovered, with a relatively small increase in density.
3.3. Microstructural parameters–density relationships Fig. 7 is a plot of density against the preferred orientation parameter, Z [FWHM of an azimuthal scan through the (00.2) reflection], for all three series of processing experiments. The straight line superimposed on this graph is the best fit to all the data points. It indicates that, for Z between |108 and |208, there is an approximately linear relationship between the density and preferred orientation of the fibres for all processing condition. It can be seen that this line passes through or very close to the error range of all points. Despite the apparent linear dependence between the two parameters there is some evidence to suggest that
there may be non-linear behaviour at high density values. Thus, whilst evidence for a linear relationship is not unequivocal, it is certainly apparent that the density bears a more general relationship with the structural parameter Z than with any of the processing parameters or mechanical properties discussed above. Since there is no literature published to date concerning the relationship between density and preferred orientation, it is not possible to make a direct comparison of the present results. However, the degree of preferred orientation of the carbon layer planes in carbon fibres has been strongly linked to the Young’s modulus. Several investigators have examined the relationship between preferred orientation and Young’s modulus in carbon fibres from various precursors: Bacon and Schalamon [1] and Ruland [16] for rayon-based fibres, Watt et al. [17], Johnson et al. [3], Diefendorf and Tokarsky [18], Isaac et al. [9] and Thitipoomdeja et al. [10] for PAN-based fibres, Hawthorne [2] for pitch-based fibres and Bright and Singer [19] for MP-based fibres. All of these studies found a very similar relationship between preferred orientation and Young’s modulus, namely, as the angular width (Z) falls, so the modulus rises. Although density and Young’s modulus are directly related to the preferred orientation, Fig. 5 indicates that
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Fig. 7. The relationship between density and the preferred orientation parameter (Z, in 8) for all three series of experimental fibres. Within the limits of experimental measurement, all points lie close to the general trend line produced by linear regression. Error bars for preferred orientation values correspond to the standard deviation of measurements, normally from three fibres.
Young’s modulus is not simply dependent on the density alone. The modulus is seen to be a function of a combination of processing conditions. The relationship between density and apparent crystallite stacking height (Lc ) is shown in Fig. 8 for all three series of processing experiments. This graph clearly shows that Lc increases as the density increases. Furthermore, there appears to be a general curve of density against Lc for all the various processing treatments used. Thus, this relationship appears to be independent of the precise heat-treatment temperature, stretching stress and dwell time employed. The trend line shown in Fig. 8 is a best-fit third-order polynomial curve, although this is purely to illustrate the general trend since there is no theoretical reason to expect such a polynomial relationship. In previous studies [9,10] it was demonstrated that Lc was significantly affected by the heat-treatment temperature and, to a somewhat lesser extent, by the stretching stress and the dwell time. These findings were in general agreement with the results of other workers. However, there is again a lack of information in the literature that relates density directly to Lc , for comparison with the results of the present study. The closest work derives from studies carried out during the late 1960s and early 1970s on the micro-porosity measurements of carbon fibres. Amongst these, Perret and Ruland [20,21] used low-angle
X-ray scattering to show that hot stretching affected the porosity (and hence the density) of rayon-based carbon fibres. For small (unspecified) degrees of stretch, the porosity was not affected by stretching and only slightly by the heat-treatment temperature. For larger degrees of stretch the porosity decreased to |1 / 3 of its original value (|30%) in the fibres. In addition, Fourdeux et al. [22] showed that the effect of temperature on porosity was less dramatic for PAN-based fibres than for rayon-based samples. They also found a general curve of increasing Lc with increase in the average distance between pores, which can be interpreted as a measure of density. Thus, overall, these low-angle X-ray studies appear to have some bearing on and are consistent with the density–Lc relationship found in this study.
4. Conclusions It has been demonstrated that hot stretching of PANbased carbon fibres has a profound effect on the density. The resulting density is dependent on all three processing variables, namely temperature, stretching stress and dwell time, and is not simply related to easily measured parameters such as extension. Of the three process parameters,
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Fig. 8. The relationship between density and the apparent crystallite stacking height (Lc ) for all three series of experimental fibres. The illustrated trend line is a best-fit third-order polynomial to indicate that density and Lc can be related, irrespective of the precise processing conditions. Error bars for Lc values correspond to the standard deviation of measurements, normally from three fibres.
temperature had the most significant effect within the ranges studied, and it was notable that above |28508C, when larger plastic deformation occurred, the density increased much more rapidly. On the other hand, density showed a linear relationship with stretching stress right up to the highest levels studied. Dwell time had the least significant effect, so that the density increase fell off approximately logarithmically with time leading to little measurable change after 5–10 min. Although Young’s modulus and the density are substantially increased by the hot-stretching procedures, there appears to be no simple relationship between these two variables independent of the process parameters. Similarly the tensile strength bears no direct relationship to the density. The substantial decrease in tensile strength that occurs with high temperature and low extension is accompanied by a large increase in density, whilst the increased strength induced by stretching is associated with a relatively small further increase in density. Large increases in the preferred orientation parameter (Z) have been achieved by hot stretching and an approximately linear dependence on density has been suggested, independent of the precise processing conditions. Similarly it has been found that the apparent crystallite stacking height of the basal planes (Lc ) increases with density and there again appears to be a general curve of Lc against
density independent of the precise combination of stretching stress, heat-treatment temperature and dwell time. Thus, in summary, it has been shown that the density of hot-stretched PAN-based carbon fibres can be closely related to the microstructural parameters such as preferred orientation and apparent crystallite size but has a far more complex relationship with the mechanical properties and the specific processing parameters that are used to induce the changes.
Acknowledgements We wish to thank EPSRC for financial support and the use of the SRS facility at Daresbury Laboratory. We also wish to express our gratitude to Mr. S. Smith of Courtaulds Research plc. for supply of carbon fibres and numerous discussions.
References [1] Bacon R, Schalamon WA. In: Preston J, editor, Applied Polymer Symposia, vol. 9, American Chemical Society, 1969, p. 285.
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[2] Hawthorne HM. In: Proc Intl Conf on Carbon Fibres and their Composites, London: Plastics Institute, 1971, p. 81. [3] Johnson JW, Marjoram JR, Rose PG. Nature 1969;221:357. [4] Johnson W. In: Proc 3rd Conf on Industrial Carbon and Graphite, London: Society of Chemical Industries, 1970, p. 447. [5] Hishiyama Y. In: 11th Biennial Conf on Carbon, Oak Ridge National Laboratory, Gatlinburg, TN, American Carbon Society, 1973:269. [6] Sumida A, Ono K, Kawazu Y. In: 34th SAMPE Symp, 1989:2579. [7] Ozbek S, Jenkins GM, Isaac DH. In: 20th Biennial Conf on Carbon, Santa Barbara, American Carbon Society, 1991:308. [8] Ozbek S, Isaac DH. Materials and Manufacturing Processes 1994;9(2):199. [9] Isaac DH, Ozbek S, Francis JG. Materials and Manufacturing Processes 1994;9(2):179. [10] Thitipoomdeja S, Ozbek S, Isaac DH. In: Processing, fabrication and manufacturing of composite materials, vol. 35, ASME, 1992, p. 87. [11] Edie DD. Carbon 1998;36:345.
[12] Mittal J, Konno H, Inagaki M, Bahl OP. Carbon 1998;36:1327. [13] Shindo A. Report No. 317, Government Industrial Research Institute, Osaka, Japan, 1961. [14] Bacon R. In: Walker Jr. PL, Thrower PA, editors, Chemistry and physics of carbon, vol. 9, New York: Marcel Dekker, 1973, p. 1. [15] Hughes JDH. Carbon 1986;24:551. [16] Ruland W. In: Preston J, editor, Applied Polymer Symposia, vol. 9, American Chemical Society, 1969, p. 293. [17] Watt W, Phillips LN, Johnson W. The Engineer 1966;221:815. [18] Diefendorf RJ, Tokarsky E. Polymer Eng Sci 1975;15(3):150. [19] Bright AA, Singer LS. Carbon 1979;17:59. [20] Perret R, Ruland W. In: Proc 9th Biennial Conf on Carbon, Boston College, MA, American Carbon Society, 1969:158. [21] Perret R, Ruland W. In: Proc Intl Carbon Conf, Carbon ’72, Baden-Baden, 1972:318. [22] Fourdeux A, Perret R, Ruland W. In: Proc 1st Intl Conf on Carbon Fibres, London: Plastics Institute, 1971, p. 57.
Carbon 38 (2000) 2007–2016
Strain-induced density changes in PAN-based carbon fibres S. Ozbek, D.H. Isaac* a
Department of Materials Engineering, University of Wales, Swansea, SA2 8 PP, UK Received 8 October 1999; accepted 19 February 2000
Abstract A series of density measurements has been carried out on PAN-based carbon fibres, which had been hot stretched under various processing regimes. It was found that all three process variables, namely temperature, stretching stress and dwell time, were important in determining the final density and that no simple factor related to the processing conditions could be used to characterise the density uniquely. Both Young’s modulus and tensile strength of the fibres were found to increase with increasing density but again the relationships depended on the specific combination of temperature, stretching stress and dwell time. On the other hand, it was found that both the preferred orientation and the apparent crystallite size, Lc , increased with increasing density, and both were directly related to density, independent of the precise combination of processing conditions. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibres; B. Heat treatment; D. Microstructure; Mechanical properties
1. Introduction Since the early 1960s considerable efforts have been made towards the production of high-strength highmodulus carbon fibres from a variety of polymeric precursors. Although a number of different starting materials [such as polyacrylonitrile (PAN), rayon, pitch, etc.] have been investigated, currently the most popular precursor material for carbon fibre production is (PAN)-based textile fibre. This precursor is conventionally subjected to lowtemperature stretching under oxidising conditions, which crosslinks the chains. Thus the fibre is stabilised with a degree of preferred orientation of the component chains impressed on the carbon precursor. The material is then carbonised by heat-treating to |13008C, free of stress, to produce high-strength carbon fibres, typically with a Young’s modulus of |230 GPa and a tensile strength of |4 GPa or more. Further heat treatment to |25008C results in high-modulus carbon fibres with modulus values typically as high as |400 GPa but a reduction in strength to less than |4 GPa. Various studies [1–5] in the 1960s and 1970s showed that further stretching of carbon fibres from different precursors can produce carbon fibre with a very high Young’s modulus. For example, Johnson [4] found that *Corresponding author. Tel.: 144-1792-295-288; fax: 1441792-295-244.
with the PAN-based fibre available at the time, an increase in axial modulus to |500 GPa could be achieved, combined with a strength of |2.5 GPa. This work also showed that many of the stress concentrations causing failure were not associated with impurities because these could not conceivably be removed by the hot-stretching process. Despite these results, the advantages of hot stretching were not exploited commercially, possibly due to limitations in the PAN-based precursors then available. More recently, developments in Japan by Toray [6] have shown that PAN-based carbon fibres can now be produced commercially (M60J) with a Young’s modulus of |590 GPa and a tensile strength of |3.9 GPa, although the production details for these fibres have not been published. However, the higher purity and lower defect population of fibres produced in recent years have led to a reassessment of hot-stretching experiments as a route for producing highmodulus and high-strength PAN-based carbon fibres in our laboratories [7,8]. In these experiments, commercial carbon fibres were hot stretched with varying stresses and dwell times at temperatures up to 30008C. The results indicated that all three of these process variables were important in determining the subsequent mechanical behaviour. Significant increases in modulus were recorded, although a loss in strength occurred following high-temperature processing without significant stretching. However, the strength was restored and in some cases improved when the high temperatures were accompanied by a
0008-6223 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00060-9
2008
S. Ozbek, D.H. Isaac / Carbon 38 (2000) 2007 – 2016
reasonable degree of stretching. Microstructural changes in these hot-stretched fibres have also been investigated by X-ray diffraction [9,10] using synchrotron radiation. These studies showed substantial improvements in the preferred orientation parameter (Z) and increases in the apparent crystallite stack height (Lc ). Although many studies have been reported on the effects of processing on the structure and properties of carbon fibres [11,12], there is very little information available in the literature concerning the effects of process variables on the density of hot-stretched carbon fibres. However, one of the earliest studies on PAN-based carbon fibres by Shindo [13] showed that the density increased with heat-treatment temperature in the absence of stretching. Figures of |1950 kg m 23 at 25008C to |2030 kg m 23 at 30008C were reported by Shindo and he ascribed the increases to crystallite growth during heat-treatment. Later, Bacon [14] reported that the most important factor affecting the density of graphitised rayon-based fibres was the amount of stretch to which the fibres were subjected during graphitisation. Bacon collected his data from studies carried out on commercial fibres (Thornel) and experimental fibres prepared by similar processing techniques. The wide range of densities represented in his report showed a range of porosities from |40% (corresponding to a density of 1320 kg m 23 ) for low-modulus graphitised fibres, to only |12% (1936 kg m 23 ) for fibres with a Young’s modulus of |690 GPa. These porosities were calculated by comparing the filament bulk density with the X-ray density (approximately 2200 kg m 23 ). As outlined above, previous reports from this laboratory have considered the effects on mechanical properties [7,8] and microstructure [9,10] of hot-stretching carbon fibres. This report presents density data from some of these fibres, and relates these data to the process parameters and the final properties and microstructure of the fibres.
2. Experimental A graphite element resistance furnace from the ASTRO 1000A series, with a hot-zone length of 150 mm at 30008C, was adapted to allow the insertion of a carbon fibre tow subjected to a constant load. Further details of the equipment and the experimental arrangements have been published elsewhere [7,8]. Three sets of data are presented in this report, resulting from three series of experiments. Experiments for all series were performed on an experimental PAN-based 3000 filament batch, with a nominal diameter of 7 mm, supplied by Courtaulds that had been heat-treated to approximately 13008C during manufacture. The quoted tensile strength and modulus of this starting fibre were checked and confirmed to be 3.960.4 GPa and 180630 GPa, respectively, as measured by single fibre testing. The diameter and density of as-received fibres
were measured to be 7.060.2 mm and 173962 kg m 23 , respectively, figures which were consistent with those quoted by the manufacturers. For the first series, a set of twelve experimental fibre tows was produced by stretching 820-mm-long cut tows at four different temperatures of 27008C, 28008C, 29008C and 30008C with three loads, equivalent to stresses of 5 MPa, 43 MPa and 85 MPa, for a fixed dwell time of 5 min. For the second series, twelve different processing conditions were considered, using six stretching stresses of 26 MPa, 43 MPa, 51 MPa, 61 MPa, 79 MPa and 128 MPa at constant hot-zone temperatures of 26008C and 28008C for a fixed dwell time of 30 min. The third series of experiments produced a further set of 10 fibre tows using dwell times of 2, 5, 10, 20 and 30 min at constant hot-zone temperatures of 26008C and 2800 o C, with a fixed stretching stress of 79 MPa. The procedure for each processing run was to heat up a tow of fibres to the desired temperature at a rate of 27.38C min 21 , in a helium atmosphere at 50 kPa above ambient pressure, with minimal stretching. During this heating-up period, a small fixed extension of 1.5 mm was allowed in order to keep the fibres taut and to ensure that all the fibre tows had the same thermal–stretch history. When the final temperature was reached the load was applied by moving the platform away from the loading system. The tow was then allowed to stretch at a fixed temperature and constant load for the desired time. The extension was monitored as a function of time during each stretching experiment. The procedure for the first series was slightly different from the above in that the fibre tows were allowed to stretch during the heating-up period in addition to the 5 min dwell time. The as-received fibre samples were deresinated in ethyl methyl ketone (2-Butanone) before the density measurement, to remove the resin with which the fibres had been coated during manufacture. Densities of the fibre samples were measured using the flotation technique. Two types of liquid, tetrabromoethane (Br 2 CHCHBr 2 ), and carbon tetrachloride (CCl 4 ), with dissimilar densities 2950–2965 kg m 23 and 1594 kg m 23 at 208C, respectively, were mixed in the proportions required to ensure that the fibre samples floated in the middle of the mixture. The density of the mixture, and hence the sample density, was calculated from knowledge of the proportions of each component. For each processing condition, the densities of at least five small bundles were measured and the average and standard deviation determined. The Young’s modulus and tensile strength of individual filaments were measured on specially made single fibre testing equipment as described in a previous paper [8]. Modulus values were determined from the gradient of the central region of the load–extension curve, which was close to linear, ignoring the ends of the curve, which showed some non-linearity. Thus modulus values were not all measured at precisely the same fixed strain (a method
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suggested to account for variation with strain [15]) but were typically averaged over a range of between about 0.3 and 0.6%. Mechanical testing was carried out on a minimum of eight individual fibres, with a gauge length of 27 mm, for each processing condition, and an average and standard deviation determined. Individual fibre diameters were measured by optical microscopy using an image shearing microscope and confirmed by laser diffraction measurements. X-ray diffraction patterns of single fibres were taken using the synchrotron radiation source (SRS) at Daresbury Laboratory, UK. The preferred orientation parameter, Z (in 8), was taken to be the full width at half maximum height (FWHM) of an azimuthal scan through the (00.2) reflection. The measure of apparent crystallite stack height size, Lc , perpendicular to the basal planes was calculated from the FWHM of a radial scan through the (00.2) reflection. X-ray diffraction patterns from as-received monofilaments were too weak and diffuse for analysis, and so a bundle of as-received fibres was used for comparison purposes and a Z value of |388 was determined. The stacking height, Lc , of the as-received fibres was too small to estimate a meaningful value. Further details of the X-ray diffraction studies have been published elsewhere [9,10].
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3. Results and discussion Attempts have been made to establish the effects of each of the process variables on the final fibre density, to determine the relationships between density and the mechanical performance, and to elucidate any correlation between the fibre density and the structural parameters and these are discussed below.
3.1. Effects of process variables The effect of stretching temperature on the fibre density is evident from Fig. 1. For this graph, data were taken from the first series of experiments for three stretching stresses (5 MPa, 43 MPa and 85 MPa) at four processing temperatures (27008C, 28008C, 29008C and 30008C), for 5 min in each case. The best-fit curves to the data points show that the density increases with heat treatment temperature and that this density increase is more pronounced above 28508C, at which temperature larger plastic deformation starts to take place. These data in Fig. 1 also indicate that the fibre density is highly influenced by the stretching stress. This effect can be seen more graphically when density data are plotted against stretching stress for the
Fig. 1. The effects of processing temperature on fibre density for samples stretched for 5 min, with stresses of 5 MPa, 43 MPa or 85 MPa at temperatures between 27008C and 30008C. In all the figures, error bars for density values correspond to the standard deviation of measurements from at least five small bundles.
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data of the second series of experiments (Fig. 2). These data suggest that the density increases approximately linearly with stretching stress for the temperature range investigated. The work carried out by Shindo [13] on PAN-based carbon fibres showed that after heat-treatment to 27008C and 30008C, without stretching, the density rose to |2000 kg m 23 and |2030 kg m 23 , respectively. Although in the present work the absolute values of density are lower, consideration of the difference in densities between fibres treated to these two temperatures is illuminating. Thus for Shindo’s work, which involved no stretching, this density difference corresponds to |1.5%. The density results obtained in the present study, at the same heat-treatment temperatures but with a small stretching stress of 5 MPa, show a difference of |1.6% from |1857 kg m 23 to |1887 kg m 23 . With the intermediate load of 43 MPa, corresponding densities are |1859 kg m 23 and 1908 kg m 23 giving a difference of |2.6%. When the stretching stress is increased to 85 MPa the densities at these two temperatures are |1869 kg m 23 and |1929 kg m 23 corresponding to a difference of about 3%. Thus, although the absolute values of densities are somewhat lower that those reported by Shindo, probably due to differences in the precursors, the trends are remarkably consistent. Indeed, extrapolation of the present data to no stretching stress gives a density difference of |1.5%.
Bacon [14] collected data from various sources to establish a density–stretch relationship for stress-graphitised rayon-based carbon fibres and he found that the density was significantly affected by the degree of stretch. He reported that the density increased from |1340 kg m 23 to |1930 kg m 23 , when stretched at around 28008C, with effective strains of |2% and |110%, respectively. Bacon calculated the effective strain from the reduction in filament cross-sectional area compared with unstretched but graphitised fibres. When strains were determined in the same way for samples stretched at 28008C in the present study, it was found that the reduction in cross-sectional area increased from |8% for the lightly stretched fibre (5 MPa) to |34% for the highly stretched fibre (85 MPa), whilst the density only increased from |1860 kg m 23 to |1877 kg m 23 . It seems that the difference in the degree of stretch and density changes between these two studies is caused by the highly porous (about 40%) nature of the unstressed rayon-based fibres, as reported by Bacon [14]. Fig. 3 shows how the density of hot-stretched fibres varies with dwell time. It may be seen that the main density increase occurs in the first 5–10 min of the 30-min stretching period and then the rate of increase is significantly reduced. In this figure, best-fit logarithmic curves have been included in light of the observed logarithmic dependence of extension with time reported elsewhere [8]. It was not possible to measure the true strain experimen-
Fig. 2. The effects of stretching stress on the fibre density for samples stretched at 26008C or 28008C for 30 min with stretching stresses of between 26 and 128 MPa.
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Fig. 3. The effects of processing dwell time on the fibre density for samples stretched at 26008C or 28008C, with a stretching stress of 79 MPa for times between 2 and 30 min.
Fig. 4. The effects of induced extension of the fibres on the resulting density for samples stretched for 5 min, with stresses of 5 MPa, 43 MPa or 85 MPa at temperatures between 27008C and 30008C.
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tally due to the non-uniform temperature distribution along the furnace length. Hence, the total extension induced by each processing condition was measured to give an approximation to the relative strains. This extension is plotted against density in Fig. 4 for the first series of experiments, and it may be seen that there is an approximately linear relationship at each of the processing temperatures. Although the equivalent data for the second series of experiments also show an approximately linear relationship for each temperature, these data do not superimpose on the same temperature data in Fig. 4, which would have been expected if a simple combination of temperature and extension determined density. Thus, overall, these results indicate that the density of the hot-stretched fibres is highly influenced by the stretching stress, temperature and dwell time. The density is seen to be a function of a combination of these processing variables and by holding two of these variables constant, a reasonable relationship can be determined between the third variable and the density. However, there is no simple formula that can be used to relate all the process parameters to the density simultaneously, and further investigation is required to give any mathematical expression of these relationships. Furthermore, no simply measurable fibre parameter such as diameter, extension or reduction in
cross-sectional area can be related directly to density, independently of the process parameters.
3.2. Mechanical properties–density relationship Fig. 5 is a plot of density against Young’s modulus for the first series of experiments. It may be seen that the density increases approximately linearly with the modulus at each temperature. These results are similar to the density–Young’s modulus relationship of rayon-based fibres reported by Bacon [14]. He showed that the Young’s modulus increases almost linearly with increasing density, although his data were collected from measurements of commercial grades of fibres (Thornel) and experimental fibres prepared by similar processing techniques. These increases in density and modulus were found to be related to the substantial structural changes taking place during hot stretching, and this aspect is discussed further in the next section. Fig. 6 shows the relationship between density and tensile strength for the first series of experiments. Although these data indicate substantial scatter and large error bars, due to randomly occurring defects, there is evidence for increasing strength with density. The straight lines, which show linear regressions for each temperature,
Fig. 5. The relationship between density and Young’s modulus for samples stretched for 5 min, with stresses of 5 MPa, 43 MPa or 85 MPa at temperatures between 27008C and 30008C. At each temperature, higher stretching stresses produced higher modulus values. Error bars for modulus values correspond to the standard deviation of measurements from at least eight fibres.
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Fig. 6. The relationship between density and tensile strength (gauge length 27 mm) for samples stretched for 5 min, with stresses of 5 MPa, 43 MPa or 85 MPa at temperatures between 27008C and 30008C. The large error bars result from large variability in strength measurements due to a statistical distribution of defects. Nevertheless, there is a trend of increasing tensile strength with density at each temperature. Error bars for strength values correspond to the standard deviation of measurements from at least eight fibres.
are only meant to give an indication of the general trends, since there is not enough evidence to confirm a linear relationship. Fig. 6 also shows that, although the density increases substantially with high-temperature treatment and minimal stretching compared with the as-received value of 1739 kg m 23 , at the low-stretch end the strength is reduced to about half the original value of |3.9 GPa. When the extension is increased by using higher loads, the strength is improved substantially and the original strength is recovered, with a relatively small increase in density.
3.3. Microstructural parameters–density relationships Fig. 7 is a plot of density against the preferred orientation parameter, Z [FWHM of an azimuthal scan through the (00.2) reflection], for all three series of processing experiments. The straight line superimposed on this graph is the best fit to all the data points. It indicates that, for Z between |108 and |208, there is an approximately linear relationship between the density and preferred orientation of the fibres for all processing condition. It can be seen that this line passes through or very close to the error range of all points. Despite the apparent linear dependence between the two parameters there is some evidence to suggest that
there may be non-linear behaviour at high density values. Thus, whilst evidence for a linear relationship is not unequivocal, it is certainly apparent that the density bears a more general relationship with the structural parameter Z than with any of the processing parameters or mechanical properties discussed above. Since there is no literature published to date concerning the relationship between density and preferred orientation, it is not possible to make a direct comparison of the present results. However, the degree of preferred orientation of the carbon layer planes in carbon fibres has been strongly linked to the Young’s modulus. Several investigators have examined the relationship between preferred orientation and Young’s modulus in carbon fibres from various precursors: Bacon and Schalamon [1] and Ruland [16] for rayon-based fibres, Watt et al. [17], Johnson et al. [3], Diefendorf and Tokarsky [18], Isaac et al. [9] and Thitipoomdeja et al. [10] for PAN-based fibres, Hawthorne [2] for pitch-based fibres and Bright and Singer [19] for MP-based fibres. All of these studies found a very similar relationship between preferred orientation and Young’s modulus, namely, as the angular width (Z) falls, so the modulus rises. Although density and Young’s modulus are directly related to the preferred orientation, Fig. 5 indicates that
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Fig. 7. The relationship between density and the preferred orientation parameter (Z, in 8) for all three series of experimental fibres. Within the limits of experimental measurement, all points lie close to the general trend line produced by linear regression. Error bars for preferred orientation values correspond to the standard deviation of measurements, normally from three fibres.
Young’s modulus is not simply dependent on the density alone. The modulus is seen to be a function of a combination of processing conditions. The relationship between density and apparent crystallite stacking height (Lc ) is shown in Fig. 8 for all three series of processing experiments. This graph clearly shows that Lc increases as the density increases. Furthermore, there appears to be a general curve of density against Lc for all the various processing treatments used. Thus, this relationship appears to be independent of the precise heat-treatment temperature, stretching stress and dwell time employed. The trend line shown in Fig. 8 is a best-fit third-order polynomial curve, although this is purely to illustrate the general trend since there is no theoretical reason to expect such a polynomial relationship. In previous studies [9,10] it was demonstrated that Lc was significantly affected by the heat-treatment temperature and, to a somewhat lesser extent, by the stretching stress and the dwell time. These findings were in general agreement with the results of other workers. However, there is again a lack of information in the literature that relates density directly to Lc , for comparison with the results of the present study. The closest work derives from studies carried out during the late 1960s and early 1970s on the micro-porosity measurements of carbon fibres. Amongst these, Perret and Ruland [20,21] used low-angle
X-ray scattering to show that hot stretching affected the porosity (and hence the density) of rayon-based carbon fibres. For small (unspecified) degrees of stretch, the porosity was not affected by stretching and only slightly by the heat-treatment temperature. For larger degrees of stretch the porosity decreased to |1 / 3 of its original value (|30%) in the fibres. In addition, Fourdeux et al. [22] showed that the effect of temperature on porosity was less dramatic for PAN-based fibres than for rayon-based samples. They also found a general curve of increasing Lc with increase in the average distance between pores, which can be interpreted as a measure of density. Thus, overall, these low-angle X-ray studies appear to have some bearing on and are consistent with the density–Lc relationship found in this study.
4. Conclusions It has been demonstrated that hot stretching of PANbased carbon fibres has a profound effect on the density. The resulting density is dependent on all three processing variables, namely temperature, stretching stress and dwell time, and is not simply related to easily measured parameters such as extension. Of the three process parameters,
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Fig. 8. The relationship between density and the apparent crystallite stacking height (Lc ) for all three series of experimental fibres. The illustrated trend line is a best-fit third-order polynomial to indicate that density and Lc can be related, irrespective of the precise processing conditions. Error bars for Lc values correspond to the standard deviation of measurements, normally from three fibres.
temperature had the most significant effect within the ranges studied, and it was notable that above |28508C, when larger plastic deformation occurred, the density increased much more rapidly. On the other hand, density showed a linear relationship with stretching stress right up to the highest levels studied. Dwell time had the least significant effect, so that the density increase fell off approximately logarithmically with time leading to little measurable change after 5–10 min. Although Young’s modulus and the density are substantially increased by the hot-stretching procedures, there appears to be no simple relationship between these two variables independent of the process parameters. Similarly the tensile strength bears no direct relationship to the density. The substantial decrease in tensile strength that occurs with high temperature and low extension is accompanied by a large increase in density, whilst the increased strength induced by stretching is associated with a relatively small further increase in density. Large increases in the preferred orientation parameter (Z) have been achieved by hot stretching and an approximately linear dependence on density has been suggested, independent of the precise processing conditions. Similarly it has been found that the apparent crystallite stacking height of the basal planes (Lc ) increases with density and there again appears to be a general curve of Lc against
density independent of the precise combination of stretching stress, heat-treatment temperature and dwell time. Thus, in summary, it has been shown that the density of hot-stretched PAN-based carbon fibres can be closely related to the microstructural parameters such as preferred orientation and apparent crystallite size but has a far more complex relationship with the mechanical properties and the specific processing parameters that are used to induce the changes.
Acknowledgements We wish to thank EPSRC for financial support and the use of the SRS facility at Daresbury Laboratory. We also wish to express our gratitude to Mr. S. Smith of Courtaulds Research plc. for supply of carbon fibres and numerous discussions.
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[2] Hawthorne HM. In: Proc Intl Conf on Carbon Fibres and their Composites, London: Plastics Institute, 1971, p. 81. [3] Johnson JW, Marjoram JR, Rose PG. Nature 1969;221:357. [4] Johnson W. In: Proc 3rd Conf on Industrial Carbon and Graphite, London: Society of Chemical Industries, 1970, p. 447. [5] Hishiyama Y. In: 11th Biennial Conf on Carbon, Oak Ridge National Laboratory, Gatlinburg, TN, American Carbon Society, 1973:269. [6] Sumida A, Ono K, Kawazu Y. In: 34th SAMPE Symp, 1989:2579. [7] Ozbek S, Jenkins GM, Isaac DH. In: 20th Biennial Conf on Carbon, Santa Barbara, American Carbon Society, 1991:308. [8] Ozbek S, Isaac DH. Materials and Manufacturing Processes 1994;9(2):199. [9] Isaac DH, Ozbek S, Francis JG. Materials and Manufacturing Processes 1994;9(2):179. [10] Thitipoomdeja S, Ozbek S, Isaac DH. In: Processing, fabrication and manufacturing of composite materials, vol. 35, ASME, 1992, p. 87. [11] Edie DD. Carbon 1998;36:345.
[12] Mittal J, Konno H, Inagaki M, Bahl OP. Carbon 1998;36:1327. [13] Shindo A. Report No. 317, Government Industrial Research Institute, Osaka, Japan, 1961. [14] Bacon R. In: Walker Jr. PL, Thrower PA, editors, Chemistry and physics of carbon, vol. 9, New York: Marcel Dekker, 1973, p. 1. [15] Hughes JDH. Carbon 1986;24:551. [16] Ruland W. In: Preston J, editor, Applied Polymer Symposia, vol. 9, American Chemical Society, 1969, p. 293. [17] Watt W, Phillips LN, Johnson W. The Engineer 1966;221:815. [18] Diefendorf RJ, Tokarsky E. Polymer Eng Sci 1975;15(3):150. [19] Bright AA, Singer LS. Carbon 1979;17:59. [20] Perret R, Ruland W. In: Proc 9th Biennial Conf on Carbon, Boston College, MA, American Carbon Society, 1969:158. [21] Perret R, Ruland W. In: Proc Intl Carbon Conf, Carbon ’72, Baden-Baden, 1972:318. [22] Fourdeux A, Perret R, Ruland W. In: Proc 1st Intl Conf on Carbon Fibres, London: Plastics Institute, 1971, p. 57.