CARBON PERGAMON Carbon40(2002)27192725 a study of pitch-based precursors for general purpose carbon fibres E. Mora. C. Blanco". V Prada. R. Santamaria. M. Granda. R. menendez Instituto Nacional del Carbon, La Corredoria s/n, Apdo 73, 33080 Oviedo, Spain Received 20 March 2002; accepted 24 May 2002 Abstract e The isotropic phase isolated from a thermally treated coal-tar pitch was studied as a possible precursor for carbon fibres Extraction with different solvents was performed in order to increase its softening point and so enable higher stabilisation emperatures to be used, with a significant reduction in time. The extraction conditions were selected studying the softer temperatures of the residues, the results of their thermogravimetric analysis and reactivity in air studied by means of differential scanning calorimetry. The residue obtained with a mixture of 40% acetone-60% acetonitrile was found to be the most suitable precursor for the fibres. The carbonised fibres showed a homogeneous surface and diameter, and had tensile properties comparable to other isotropic fibres described in the literature C 2002 Elsevier Science Ltd. All rights reserved Keywords: A. Coal tar pitch; Carbon fibre; B. Stabilisation, C. Thermal analysis; Differential scanning calorimetry 1. Introduction general purpose carbon fibres should meet are high carbon yield, low volatile content and narrow molecular weight General purpose carbon fibres(GPCF) are of increasing distribution. Additionally, the viscosity at the spinning interest to both scientists and industry GPCF are temperature must be appropriate. The softening point of isotropic and normally produced from coal-tar or petro- the pitch is also a critical parameter, as it determines the leum pitch through a melt spin process. Potential applica- temperature at which the stabilisation of the fibres should tions of these fibres are in construction materials [3, 4], as be carried out, and therefore, the rate of the stabilisation components of reinforced concrete with improved prop- reaction. In addition, any inorganic impurity should be erties, medical applications such as enteroadsorbents, avoided, as this would result in defects in the fibre electrodes of energy storage systems (battery anodes or tructure, weakening the mechanical properties of the fibre double layer capacitors)[5-8 or as activated carbons, Many precursors have been developed and studied to ideal for gas storage, removal of pollutants or catalyst produce GPCE, Pure compounds, coal-tar and petroleum support [1, 9-11]. The mechanical and transport properties pitches have been modified by thermal treatment, air- required for these applications are not as demanding as for blowing, chemical additives or catalytic reactions to in- the high performance carbon fibres [12, 13), and therefore, crease the softening point of the pitches [14-18]. Solvent general purpose carbon fibres produced at sig- extraction has also been widely used to remove the lighter nificantly lower costs. However, the cost of the general components of the pitch so as to achieve the desired purpose carbon fibres is still significantly higher than for properties [19,20] other established materials, this being the main drawback In the present work, a new precursor for general purpose to materialise their potential applications. carbon fibres is studied. This precursor was obtained from The main requirements that a suitable precursor for the isotropic phase isolated from thermally treated coal-tar the hot filtration Corresponding author. Tel +34-985-119-090, fax:+34. previous paper [21]. In this process, two different pre 985-297-66 cursors are obtained: a mesophase rich component that is E-mail address: clara @ incar csic es(C. Blanco) currently used to produce high density graphites on a 0008-6223/02/S-see front matter 2002 Elsevier Science Ltd. All rights reserved PII:S0008-6223(02)00185-9
Carbon 40 (2002) 2719–2725 A study of pitch-based precursors for general purpose carbon fibres E. Mora, C. Blanco , V. Prada, R. Santamarıa, M. Granda, R. Menendez * ´ ´ Instituto Nacional del Carbon´ , La Corredoria s/n, Apdo 73, 33080 Oviedo, Spain Received 20 March 2002; accepted 24 May 2002 Abstract The isotropic phase isolated from a thermally treated coal-tar pitch was studied as a possible precursor for carbon fibres. Extraction with different solvents was performed in order to increase its softening point and so enable higher stabilisation temperatures to be used, with a significant reduction in time. The extraction conditions were selected studying the softening temperatures of the residues, the results of their thermogravimetric analysis and reactivity in air studied by means of differential scanning calorimetry. The residue obtained with a mixture of 40% acetone–60% acetonitrile was found to be the most suitable precursor for the fibres. The carbonised fibres showed a homogeneous surface and diameter, and had tensile properties comparable to other isotropic fibres described in the literature. 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Coal tar pitch; Carbon fibre; B. Stabilisation; C. Thermal analysis; Differential scanning calorimetry 1. Introduction general purpose carbon fibres should meet are high carbon yield, low volatile content and narrow molecular weight General purpose carbon fibres (GPCF) are of increasing distribution. Additionally, the viscosity at the spinning interest to both scientists and industry [1,2]. GPCF are temperature must be appropriate. The softening point of isotropic and normally produced from coal-tar or petro- the pitch is also a critical parameter, as it determines the leum pitch through a melt spin process. Potential applica- temperature at which the stabilisation of the fibres should tions of these fibres are in construction materials [3,4], as be carried out, and therefore, the rate of the stabilisation components of reinforced concrete with improved prop- reaction. In addition, any inorganic impurity should be erties, medical applications such as enteroadsorbents, avoided, as this would result in defects in the fibre electrodes of energy storage systems (battery anodes or structure, weakening the mechanical properties of the fibre. double layer capacitors) [5–8] or as activated carbons, Many precursors have been developed and studied to ideal for gas storage, removal of pollutants or catalyst produce GPCF. Pure compounds, coal-tar and petroleum support [1,9–11]. The mechanical and transport properties pitches have been modified by thermal treatment, airrequired for these applications are not as demanding as for blowing, chemical additives or catalytic reactions to inthe high performance carbon fibres [12,13], and therefore, crease the softening point of the pitches [14–18]. Solvent general purpose carbon fibres can be produced at sig- extraction has also been widely used to remove the lighter nificantly lower costs. However, the cost of the general components of the pitch so as to achieve the desired purpose carbon fibres is still significantly higher than for properties [19,20]. other established materials, this being the main drawback In the present work, a new precursor for general purpose to materialise their potential applications. carbon fibres is studied. This precursor was obtained from The main requirements that a suitable precursor for the isotropic phase isolated from thermally treated coal-tar pitches, using the hot filtration process described in a *Corresponding author. Tel.: 134-985-119-090; fax: 134- previous paper [21]. In this process, two different pre- 985-297-662. cursors are obtained: a mesophase rich component that is E-mail address: clara@incar.csic.es (C. Blanco). currently used to produce high density graphites on a 0008-6223/02/$ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(02)00185-9
2720 E. Mora et al. Carbon 40(2002)2719-2725 ory scale [22, 23] and an isotropic pitch that is as-spun fibres was optimised according to the properties of ed in this study as a precursor of general purpose the E2 residue(softening point and reactivity in air). The fibres. Finding applications for both of the phases stabilisation conditions finally used were treatment in air at obtained by filtration is of great interest for the viability of 180C for I h, followed by treatment at 200C I h, 220C the whole process. This paper studies the properties and for 2 h, 240C for I h and finally 260C for 1 h. After optimisation of the carbon fibre precursor, as well as fibre each of these stages the samples were studied in the preparation and characterisation thermobalance(samples labelled st180, st200, st220, st240 and st260, according to the highest temperature used), sing a similar procedure to the one described above. The 2. Experimental stabilised fibres were carbonised in a horizontal furnace The temperature was raised to 1000C at a rate of 1C 2./. R materials min under a nitrogen atmosphere A commercial impregnating coal-tar pitch (B15) was 2.5. Characterisation of carbon fibres thermally treated at 430C under nitrogen for 4 h. The treatment was carried out in a 2 I stainless steel reactor in a Green and carbonised fibres were studied by optical similar way to that described in [24]. The pitch obtained microscopy, in order to determine the diameter of the was labelled C3. The resultant pitch was submitted to hot fibres. The fibres were individually stuck on a small piece filtration[21] in order to separate the isotropic phase from of paper, which was then embedded vertically in epoxy the mesophase. The resultant isotropic phase was labelled resin, Next the resin pellets were polished for optical microscopy examination so that the cross-sections of the fibres could be observed and measured the texture of the 22. Solvent extraction fibres was studied by scanning electron microscopy(SEM) and any possible defect in their structure was evaluated Pitch 13 was extracted with solvents of different ex- The tensile properties of the carbonised fibres were mea- traction capability in order to optimise its softening point sured and compared to commercial general purpose carbon for fibre spinning and stabilisation. Initially, hexane(H), fibres acetone(A), acetonitrile(AN) and toluene (t)were used Extractions were performed with 25 g of pitch and 100 ml of solvent placed in a 250 ml flask and maintained under 3. Results and discussion reflux for 30 min. The solution was then filtered on a number 4 ceramic plate. The residue obtained was labelled A commercial impregnating coal-tar pitch(B15) 3H, 13A, 113AN and 13T, respectively. Extractions were thermally treated at 430C for 4 h to obtain pitch C3, with also performed with mixtures of acetonitrile and acetone. a 37% mesophase content. After filtrating C3, a 100% The resultant residues were labelled El(75% acetonitrile isotropic pitch (13)was obtained. Some of the main and 25% acetone), E2(60% acetonitrile and 40% acetone) properties of these two pitches and the parent coal-tar pitch and E3(50% acetonitrile and 50% acetone) are summarised in Table 1. The relatively high carbon yield and insoluble content in the toluene and N-methylpir 23. Characterisation of the pitches rolidone of pitch 13 indicates that it has a high degree of polymerisation. As a result of the distillation of the lightest Pitch 13. and the different extraction residues ob compounds during the initial thermal treatment, and the were characterised by measuring its Mettler softening point removal of the mesophase( the most polymerised fractio and ALCan carbon yield, and also through elemental of the pitch) during filtration, pitch 13 has a narrower analysis, infrared spectroscopy, thermogravimetric analysis molecular weight distribution. These characteristics make and differential scanning calorimetry. The procedures followed have been described elsewhere [21, 24, 25 Table I 2. 4. Carbon fibres preparation Properties of parent, thermally treated and isotropic pitch NMP Carbon fibres were prepared using pitch E2 as precurso (wt%) The spinning of the fibres was performed in stainless steel B15 9549.0 6421.3 equipment, fitted with a graphite spinneret 300 Hm in C3 202 76.9 1.95 60.0 40.2 0.71 diameter. The temperature was 300C and the 3 183 69.8 1.85 49.7 18.8 0.71 itrogen pressure applied was 0.5 MPa. The fibres were SP, softening point in inert atmosphere; CY, carbon yield; C/H wound at different winding speeds(300-1000 rev /min)to atomic ratio; Tl, toluene insoluble content; NMPL, N-methylpir- obtain fibres of different diameters. Stabilisation of the rolidone insoluble content; lar, aromaticity index
2720 E. Mora et al. / Carbon 40 (2002) 2719–2725 laboratory scale [22,23]; and an isotropic pitch that is as-spun fibres was optimised according to the properties of evaluated in this study as a precursor of general purpose the E2 residue (softening point and reactivity in air). The carbon fibres. Finding applications for both of the phases stabilisation conditions finally used were treatment in air at obtained by filtration is of great interest for the viability of 180 8C for 1 h, followed by treatment at 200 8C 1 h, 220 8C the whole process. This paper studies the properties and for 2 h, 240 8C for 1 h and finally 260 8C for 1 h. After optimisation of the carbon fibre precursor, as well as fibre each of these stages the samples were studied in the preparation and characterisation. thermobalance (samples labelled st180, st200, st220, st240 and st260, according to the highest temperature used), using a similar procedure to the one described above. The 2. Experimental stabilised fibres were carbonised in a horizontal furnace. The temperature was raised to 1000 8C at a rate of 1 8C 21 2 .1. Raw materials min under a nitrogen atmosphere. A commercial impregnating coal-tar pitch (BI5) was 2 .5. Characterisation of carbon fibres thermally treated at 430 8C under nitrogen for 4 h. The treatment was carried out in a 2 l stainless steel reactor in a Green and carbonised fibres were studied by optical similar way to that described in [24]. The pitch obtained microscopy, in order to determine the diameter of the was labelled C3. The resultant pitch was submitted to hot fibres. The fibres were individually stuck on a small piece filtration [21] in order to separate the isotropic phase from of paper, which was then embedded vertically in epoxy the mesophase. The resultant isotropic phase was labelled resin. Next the resin pellets were polished for optical I3. microscopy examination so that the cross-sections of the fibres could be observed and measured. The texture of the 2 .2. Solvent extraction fibres was studied by scanning electron microscopy (SEM) and any possible defect in their structure was evaluated. Pitch I3 was extracted with solvents of different ex- The tensile properties of the carbonised fibres were meatraction capability in order to optimise its softening point sured and compared to commercial general purpose carbon for fibre spinning and stabilisation. Initially, hexane (H), fibres. acetone (A), acetonitrile (AN) and toluene (T) were used. Extractions were performed with 25 g of pitch and 100 ml of solvent placed in a 250 ml flask and maintained under 3. Results and discussion reflux for 30 min. The solution was then filtered on a number 4 ceramic plate. The residue obtained was labelled A commercial impregnating coal-tar pitch (BI5) was I3H, I3A, I13AN and I3T, respectively. Extractions were thermally treated at 430 8C for 4 h to obtain pitch C3, with also performed with mixtures of acetonitrile and acetone. a 37% mesophase content. After filtrating C3, a 100% The resultant residues were labelled E1 (75% acetonitrile isotropic pitch (I3) was obtained. Some of the main and 25% acetone), E2 (60% acetonitrile and 40% acetone) properties of these two pitches and the parent coal-tar pitch and E3 (50% acetonitrile and 50% acetone). are summarised in Table 1. The relatively high carbon yield and insoluble content in the toluene and N-methylpir- 2 .3. Characterisation of the pitches rolidone of pitch I3 indicates that it has a high degree of polymerisation. As a result of the distillation of the lightest Pitch I3, and the different extraction residues obtained compounds during the initial thermal treatment, and the were characterised by measuring its Mettler softening point removal of the mesophase (the most polymerised fraction and ALCAN carbon yield, and also through elemental of the pitch) during filtration, pitch I3 has a narrower analysis, infrared spectroscopy, thermogravimetric analysis molecular weight distribution. These characteristics make and differential scanning calorimetry. The procedures followed have been described elsewhere [21,24,25]. Table 1 Properties of parent, thermally treated and isotropic pitch 2 .4. Carbon fibres preparation Pitch SP CY C/H IT NMP Iar (wt.%) (wt.%) (wt.%) Carbon fibres were prepared using pitch E2 as precursor. The spinning of the fibres was performed in stainless steel BI5 95 49.0 1.64 21.3 4.9 0.63 equipment, fitted with a graphite spinneret 300 mm in C3 202 76.9 1.95 60.0 40.2 0.71 diameter. The spinning temperature was 300 8C and the I3 183 69.8 1.85 49.7 18.8 0.71 nitrogen pressure applied was 0.5 MPa. The fibres were SP, softening point in inert atmosphere; CY, carbon yield; C/H, wound at different winding speeds (300–1000 rev./min) to atomic ratio; TI, toluene insoluble content; NMPI, N-methylpirobtain fibres of different diameters. Stabilisation of the rolidone insoluble content; Iar, aromaticity index
E. Mora et al. Carbon 40(2002)2719-2725 2721 it a potential precursor for the production of isotropic carbon fibres. The main disadvantage of this precursor is its low intrinsic reactivity in air due its low content in alk groups, which is a characteristic of coal-tar pitches Therefore, high stabilisation temperatures would be re- quired in order to achieve stabilisation of the fibres within reasonable time. However, the softening point of the pitch is an upper limit to the increase in temperature, as stabilisation has to be necessarily carried out at tempera- tures well below the softening point. The approach then is e 2 to increase the softening point of the pitch, in order to be able higher stabilisation temperatures, at which the reactivity in air is enhanced. Among the possible methods 3 to increase the softening point (moderate temperature treatment, vacuum distillation, air-blowing, solvent ex- 01002003004005006007008009001000 traction), solvent extraction was preferred due to the easy -up require Fig. 1. DTG curves of pitch 13 and the residues obtained within 31. Solvent extraction different solvents Solvent extraction depends both on the solvent and the thermobalance. Fig. I shows the DtG curves corre- experimental conditions used. These determine not only sponding to pitch 13 and the extraction residues obtained the amount of material extracted from the pitch but also its with the different solvents. The distillation band, located characteristics. The extraction of the pitch removes the around 350C, gradually diminishes as the extraction ghtest components, increasing the softening point and capability of the solvent increases. This effect is especially arrowing the interval of molecular weight distribution noticeable for the residues obtained with acetone(13A)and Four different solvents (hexane H, acetonitrile an, toluene (13T)residues. The latter has a dtG curve in cetone A and toluene t) were used in this study. The which the band due to the distillation of light compounds main properties of the resultant residues(13H, 13an, 13A, has nearly disappeare of initial weight and 13T) are summarised in Table 2. Toluene, the most loss being around 400C. Of special interest is the aromatic of the solvents used, extracts nearly 50% of the behaviour of the residues at temperatures between 400 and pitch, yielding a residue with a softening temperature 50C, where two small bands are observed. At these higher than 350C, which is too high for the spinning of temperatures, the weight loss is due to the release of gases carbon fibres. Nevertheless, it is still worthwhile compar- produced during cracking and polymerisation reactions ing this residue with the ones obtained by using the other The intensity of these bands is similar for the 13H, 13AN olvents. There is a similarity in the effect that hexane and and 13A residues, while they are much smaller for the acetonitrile have, as can be seen from the extraction yield residue extracted with toluene(13T). This suggests that and the properties of the resultant residue(similar soften- although the amount of sample extracted by the hexane ing behaviour and carbon yield). The acetone has a acetonitrile and acetone is different in each case, the nature moderate effect in between toluene and acetonitrile of the compounds extracted is similar, whereas in the case a detailed study of the samples was carried out by of toluene a greater amount of the more reactive com- onitoring their evolution during thermal treatment in a pounds is extracted. It is also worthwhile noting that in the case of the residues extracted with toluene and acetone. a Table 2 part of the solvent remains in the residue and evaporates a temperatures between 100 and 200C Properties 13 and the residues obtained with differen The reactivity of the various residues with air was stud ied by dsC. in order to achieve a better understanding of their behaviour during stabilisation. Fig. 2 shows the (wt%) DSC curves obtained for the different residues at low temperature under air. The reaction with air becomes 3H 92.7 74.8 appreciable at around the softening temperature of the IaN Acetonitrile 85 sample, therefore at higher temperatures as the extraction Aceton 83.6 capability of the solvent increases. However, for 13, 13H and 13an the reactivity decreases thereafter, probably due Yext, yield of extraction, wt% of insoluble material; SP, to the decrease in the surface exposed to air if the material softening point in inert atmosphere, and CY, carbon yield. softens forming a film. This seems to be the case in
E. Mora et al. / Carbon 40 (2002) 2719–2725 2721 it a potential precursor for the production of isotropic carbon fibres. The main disadvantage of this precursor is its low intrinsic reactivity in air due its low content in alkyl groups, which is a characteristic of coal-tar pitches. Therefore, high stabilisation temperatures would be required in order to achieve stabilisation of the fibres within reasonable time. However, the softening point of the pitch is an upper limit to the increase in temperature, as stabilisation has to be necessarily carried out at temperatures well below the softening point. The approach then is to increase the softening point of the pitch, in order to be able to use higher stabilisation temperatures, at which the reactivity in air is enhanced. Among the possible methods to increase the softening point (moderate temperature treatment, vacuum distillation, air-blowing, solvent extraction), solvent extraction was preferred due to the easy experimental set-up required. Fig. 1. DTG curves of pitch I3 and the residues obtained within different solvents. 3 .1. Solvent extraction Solvent extraction depends both on the solvent and the thermobalance. Fig. 1 shows the DTG curves correexperimental conditions used. These determine not only sponding to pitch I3 and the extraction residues obtained the amount of material extracted from the pitch but also its with the different solvents. The distillation band, located characteristics. The extraction of the pitch removes the around 350 8C, gradually diminishes as the extraction lightest components, increasing the softening point and capability of the solvent increases. This effect is especially narrowing the interval of molecular weight distribution. noticeable for the residues obtained with acetone (I3A) and Four different solvents (hexane H, acetonitrile AN, toluene (I3T) residues. The latter has a DTG curve in acetone A and toluene T) were used in this study. The which the band due to the distillation of light compounds main properties of the resultant residues (I3H, I3AN, I3A, has nearly disappeared, the temperature of initial weight and I3T) are summarised in Table 2. Toluene, the most loss being around 400 8C. Of special interest is the aromatic of the solvents used, extracts nearly 50% of the behaviour of the residues at temperatures between 400 and pitch, yielding a residue with a softening temperature 550 8C, where two small bands are observed. At these higher than 350 8C, which is too high for the spinning of temperatures, the weight loss is due to the release of gases carbon fibres. Nevertheless, it is still worthwhile compar- produced during cracking and polymerisation reactions. ing this residue with the ones obtained by using the other The intensity of these bands is similar for the I3H, I3AN solvents. There is a similarity in the effect that hexane and and I3A residues, while they are much smaller for the acetonitrile have, as can be seen from the extraction yield residue extracted with toluene (I3T). This suggests that, and the properties of the resultant residue (similar soften- although the amount of sample extracted by the hexane, ing behaviour and carbon yield). The acetone has a acetonitrile and acetone is different in each case, the nature moderate effect in between toluene and acetonitrile. of the compounds extracted is similar, whereas in the case A detailed study of the samples was carried out by of toluene a greater amount of the more reactive commonitoring their evolution during thermal treatment in a pounds is extracted. It is also worthwhile noting that in the case of the residues extracted with toluene and acetone, a part of the solvent remains in the residue and evaporates at Table 2 temperatures between 100 and 200 8C. Properties of pitch I3 and the residues obtained with different The reactivity of the various residues with air was solvents studied by DSC, in order to achieve a better understanding Sample Solvent Yext SP CY of their behaviour during stabilisation. Fig. 2 shows the (wt.%) (8C) (wt.%) DSC curves obtained for the different residues at low I3 – – 183 69.8 temperature under air. The reaction with air becomes I3H Hexane 92.7 206 74.8 appreciable at around the softening temperature of the I3AN Acetonitrile 85.8 229 78.8 sample, therefore at higher temperatures as the extraction I3A Acetone 74.1 291 83.6 capability of the solvent increases. However, for I3, I3H I3T Toluene 49.7 .350 90.2 and I3AN the reactivity decreases thereafter, probably due Yext, yield of extraction, wt.% of insoluble material; SP, to the decrease in the surface exposed to air if the material softening point in inert atmosphere; and CY, carbon yield. softens forming a film. This seems to be the case in
2722 E. Mora et al. Carbon 40(2002)2719-2725 1.5 0.1 Fig. 2. DSC curves in air of residues obtained with different Fig. 3. DTG curves of residues obtained with solvent mixtures. 266° le mixtures with 25 and 50% acetone samples 13 and 13H, were the curve falls as far as the base tively. The carbon yield obtained for these samples is als line. The reactivity of residue 13An also decreases after similar, around 80 wt.%. The DTG curves of these extracts the softening temperature but in this case the curve does are shown in Fig 3, together with those corresponding to not reach the base line, indicating that the material does acetone and acetonitrile for purposes of comparison not soften completely. These results show that neither of Predictably the bands corresponding to the distillation of these samples (13H, 13AN) is a suitable fibre precursor, as pitch compounds(around 300C) diminish as the amount the oxidative stabilisation would have to be carried out of acetone in the solvent mixture increases. The bands temperatures below this maximum, so that very long observed at lower temperatures(100-200C), due to the stabilisation times would be needed. On the other hand the distillation of the solvent left behind in the residue reactivity of residues 13A and 1T progressively increases increase as the amount of acetone in the mixture increases above 150C, making these residues suitable for stabilise- The DSC results of the residues extracted with the tion. However, their softening temperature is higher than is mixtures of acetone and acetonitrile are shown in Fig 4 preferred for spinning All the curves, except the one corresponding to the residue In order to find a precursor with optimum properties for obtained with pure acetone, show a maximum close to the pinning and stabilisation, with a softening temperature softening temperature, as mentioned previously in relation lower than 13A and higher reactivity than 13AN, residues obtained with mixtures of both solvents were studied. the mixtures used were 25% acetone-75% acetonitrile (el), 40% acetone-60% acetonitrile (E2)and 50%acetone- 50% acetonitrile (E3). Table 3 shows the yield of each extraction and some properties of the resultant residues he extraction yield is similar for all the extractions, while the softening temperature increases as the acetone content in the solvent mixture increases, varying from 239 Table 3 Properties of residues obtained with mixtures of acetone and acetonitrile Acetone Next 45 03570105140175210245280315350 Yext, yield of extraction, wt. of insoluble material; SP, Fig. 4. DSC curves in air of residues obtained with solvent softening point in inert atmosphere, and CY, carbon yield
2722 E. Mora et al. / Carbon 40 (2002) 2719–2725 Fig. 2. DSC curves in air of residues obtained with different Fig. 3. DTG curves of residues obtained with solvent mixtures. solvents. 266 8C for the mixtures with 25 and 50% acetone, respecsamples I3 and I3H, were the curve falls as far as the base tively. The carbon yield obtained for these samples is also line. The reactivity of residue I3AN also decreases after similar, around 80 wt.%. The DTG curves of these extracts the softening temperature but in this case the curve does are shown in Fig. 3, together with those corresponding to not reach the base line, indicating that the material does acetone and acetonitrile for purposes of comparison. not soften completely. These results show that neither of Predictably the bands corresponding to the distillation of these samples (I3H, I3AN) is a suitable fibre precursor, as pitch compounds (around 300 8C) diminish as the amount the oxidative stabilisation would have to be carried out at of acetone in the solvent mixture increases. The bands temperatures below this maximum, so that very long observed at lower temperatures (100–200 8C), due to the stabilisation times would be needed. On the other hand, the distillation of the solvent left behind in the residue, reactivity of residues I3A and I3T progressively increases increase as the amount of acetone in the mixture increases. above 150 8C, making these residues suitable for stabilisa- The DSC results of the residues extracted with the tion. However, their softening temperature is higher than is mixtures of acetone and acetonitrile are shown in Fig. 4. preferred for spinning. All the curves, except the one corresponding to the residue In order to find a precursor with optimum properties for obtained with pure acetone, show a maximum close to the spinning and stabilisation, with a softening temperature softening temperature, as mentioned previously in relation lower than I3A and higher reactivity than I3AN, residues obtained with mixtures of both solvents were studied. The mixtures used were 25% acetone–75% acetonitrile (E1), 40% acetone–60% acetonitrile (E2) and 50% acetone– 50% acetonitrile (E3). Table 3 shows the yield of each extraction and some properties of the resultant residues. The extraction yield is similar for all the extractions, while the softening temperature increases as the acetone content in the solvent mixture increases, varying from 239 to Table 3 Properties of residues obtained with mixtures of acetone and acetonitrile Sample Acetone Yext SP CY (%) (wt.%) (8C) (wt.%) E1 25 84.9 239 79.1 E2 40 85.3 248 79.1 E3 50 86.2 266 79.5 Yext, yield of extraction, wt.% of insoluble material; SP, Fig. 4. DSC curves in air of residues obtained with solvent softening point in inert atmosphere; and CY, carbon yield. mixtures
E. Mora et al. Carbon 40(2002)2719-2725 2723 ith Fig. 2. This maximum becomes smaller as the acetone content in the solvent mixture increases The reaction with air starts at about the same temperature for all the extracts and continues after the maximum observed, which indi- cates that these samples do not soft completely Of the residues obtained with the mixtures of acetone and acetonitrile, residue E2 seems to be the most suitable F precursor of carbon fibres. It has a softening point要m|=m 248C, a high carbon yield and shows maximum reactivity with air at around 250C. Consequently stabilisation could 一9240 start at relatively high temperatures, i.e. 180C. Residue El would require a lower temperature of stabilisation and therefore much longer stabilisation times. Residue E2 was selected for the preparation of the carbon fibres 01002003004005006007009009001000 Temperature(C) 3.2. Carbon fibre preparation Residue e2 was easily spun into green carbon fibres at a temperature of 300C. The diameter of the as-spun fibres varied from 35 to 16 um as the winding speed increased The stabilisation process was then studied and optimised and the carbonised fibres were characterised The samples obtained at each stabilisation stage were studied by thermogravimetric analysis. The corresponding TG and DTG curves are shown in Fig. 5. The TG curves (Fig. 5a)show an increase in carbon yield as the degree of E&节 stabilisation increases, except for the fully stabilised fibre (st260)which has a slightly lower carbon yield than st220 and st240. This could be due to the greater amount of oxygen functional groups introduced, which decompose during carbonisation. The DtG curves(Fig. 5b)show the distillation band decreasing as the stabilisation degree 01002003004005006007008009001000 increases, this band disappearing completely for sample st260. In the case of samples st220, st240 and st260, there is a band situated at around 400C, not present in the parent sample(E2), which is probably due to two factors () reactions characteristic of pitch pyrolysis, around different des Fig. 5.(a) TG curves and (b) DTG curves of fibres stabilised to 450C; and (ii) decomposition of the functional grou introduced during stabilisation, around 350 and 400C Decomposition intensifies as the stabilisation degree in- the band situated at 2940 cm(aromatic C-H stretching creases, which is in agreement with the more intense band band)and the subsequent increase in the aromaticity index (Table 4). The oxygen was incorporated into the fibres The oxygen taken up during the stabilisation of the fibres was evaluated by elemental analysis, in order corroborate the idea of more functional groups being Table 4 ntroduced into the sample as stabilisation takes place. The Elemental analysis and infrared data of samples at different stabilisation stage were studied and the results are summarised in Table 4. The oxygen C/H content increased progressively as the stabilisation degree E2 185 increased, from 1.04% for the green fibres to 3. 19% for the 1.30 ully stabilised fibres. The samples were also studied by means of infrared spectroscopy in order to determine the increase in oxygen-containing functional groups. The 2.19 results showed that the reaction with air mainly affected t260 the aliphatic hydrogens, which were almost entirely re- C/H, atomic ratio, lar, aromaticity index, O, oxygen content, moved during stabilisation, as indicated by the reduction in wt % and Ico, carbonyl index
E. Mora et al. / Carbon 40 (2002) 2719–2725 2723 with Fig. 2. This maximum becomes smaller as the acetone content in the solvent mixture increases. The reaction with air starts at about the same temperature for all the extracts and continues after the maximum observed, which indicates that these samples do not soft completely. Of the residues obtained with the mixtures of acetone and acetonitrile, residue E2 seems to be the most suitable precursor of carbon fibres. It has a softening point of 248 8C, a high carbon yield and shows maximum reactivity with air at around 250 8C. Consequently stabilisation could start at relatively high temperatures, i.e. 180 8C. Residue E1 would require a lower temperature of stabilisation and therefore much longer stabilisation times. Residue E2 was selected for the preparation of the carbon fibres. 3 .2. Carbon fibre preparation Residue E2 was easily spun into green carbon fibres at a temperature of 300 8C. The diameter of the as-spun fibres varied from 35 to 16 mm as the winding speed increased. The stabilisation process was then studied and optimised and the carbonised fibres were characterised. The samples obtained at each stabilisation stage were studied by thermogravimetric analysis. The corresponding TG and DTG curves are shown in Fig. 5. The TG curves (Fig. 5a) show an increase in carbon yield as the degree of stabilisation increases, except for the fully stabilised fibre (st260) which has a slightly lower carbon yield than st220 and st240. This could be due to the greater amount of oxygen functional groups introduced, which decompose during carbonisation. The DTG curves (Fig. 5b) show the distillation band decreasing as the stabilisation degree increases, this band disappearing completely for sample st260. In the case of samples st220, st240 and st260, there is a band situated at around 400 8C, not present in the parent sample (E2), which is probably due to two factors: Fig. 5. (a) TG curves and (b) DTG curves of fibres stabilised to (i) reactions characteristic of pitch pyrolysis, around different degrees. 450 8C; and (ii) decomposition of the functional groups introduced during stabilisation, around 350 and 400 8C. 21 Decomposition intensifies as the stabilisation degree in- the band situated at 2940 cm (aromatic C–H stretching band) and the subsequent increase in the aromaticity index creases, which is in agreement with the more intense band (Table 4). The oxygen was incorporated into the fibres obtained for st260. The oxygen taken up during the stabilisation of the fibres was evaluated by elemental analysis, in order to Table 4 corroborate the idea of more functional groups being Elemental analysis and infrared data of samples at different introduced into the sample as stabilisation takes place. The stabilisation degrees samples obtained at each stabilisation stage were studied and the results are summarised in Table 4. The oxygen Sample C/H O Iar Ico content increased progressively as the stabilisation degree E2 1.85 1.04 0.68 0.12 increased, from 1.04% for the green fibres to 3.19% for the st180 2.05 1.30 0.69 0.25 fully stabilised fibres. The samples were also studied by st200 2.13 1.64 0.70 0.40 means of infrared spectroscopy in order to determine the st220 2.13 2.08 0.72 0.48 increase in oxygen-containing functional groups. The st240 2.19 2.68 0.74 0.54 st260 2.23 3.19 0.74 0.62 results showed that the reaction with air mainly affected the aliphatic hydrogens, which were almost entirely re- C/H, atomic ratio; Iar, aromaticity index; O, oxygen content, moved during stabilisation, as indicated by the reduction in wt.%; and Ico, carbonyl index
2724 E. Mora et al./ Carbon 40(2002)2719-2725 f carbonyl I groups (band at 1700 cm ) The SEM. The fibres exhibit a smooth and homogeneous I groups increased with increasing surface, an apparent lack any defects as a result of a good an increase in the carbonyl index, a stabilisation process. The thickness of the fibres was also can be seen from Table 4 homogeneous, the diameter being very similar for each set 1000C. Fig. 6 shows the appearance of the fibres under Although isotropic carbon fibres are not generally used for structural applications, the mechanical properties were measured and evaluated. As might be expected, the tensile strength of the fibres is higher for the fibres with a smaller diameter, this being 143 MPa for the 35 um diameter fibres and 414 MPa for the 16 um ones. The strength found for the thinnest fibres is comparable to other general purpose carbon fibres reported in the literature [2]. The modulus increases slightly from 28.9 to 36.3 GPa as the fibre diameter decreases The new precursor studied in this paper has been proven to be suitable for the production of general purpose carbon fibres. This precursor is obtained through hot filtration of thermally treated coal-tar pitches, a relatively simple and new procedure studied by the authors in previous pape [21]. The isotropic phase that is obtained in this process has been, for the first time, evaluated as precursor for carbon fibres. The results obtained seem promising, as the quality of the fibres obtained is reasonable for general purpose carbon fibres. Despite an additional step of solvent extraction being required to raise the softening point of the in order to be able to stabilise at higher tures, the overall process is still of great interest, as in the BRAS INCAR filtration process a mesophase-rich comp is also obtained alongside the fibre precursor. The mesophase produced has been recently used to added-value products are obtained through the same process, which is interesting from the economic point of The isotropic phase obtained by hot filtration of a thermally treated coal-tar pitch can be used as a precursor for carbon fibres. In order to reduce the duration of the stabilisation process, the softening point of the pitch was The residue obtained with a mixture of 40% acetone-60% acetonitrile was found to be the most suitable precursor carbon fibres. The fibres obtained after carbonisation did not present any appreciable defects and were homogeneous ies comparable other isotropic fibres described in the literature. The advantage of the technology proposed in this paper is that 乙唾圈围 it allows the use of the whole thermally treated coal-tar pitch for the preparation of two types of high added-value
2724 E. Mora et al. / Carbon 40 (2002) 2719–2725 21 mainly as carbonyl groups (band at 1700 cm ). The SEM. The fibres exhibit a smooth and homogeneous amount of carbonyl groups increased with increasing surface, an apparent lack any defects as a result of a good stabilisation, causing an increase in the carbonyl index, as stabilisation process. The thickness of the fibres was also can be seen from Table 4. homogeneous, the diameter being very similar for each set After stabilisation, the fibres were carbonised to of fibres. 1000 8C. Fig. 6 shows the appearance of the fibres under Although isotropic carbon fibres are not generally used for structural applications, the mechanical properties were measured and evaluated. As might be expected, the tensile strength of the fibres is higher for the fibres with a smaller diameter, this being 143 MPa for the 35 mm diameter fibres and 414 MPa for the 16 mm ones. The strength found for the thinnest fibres is comparable to other general purpose carbon fibres reported in the literature [2]. The modulus increases slightly from 28.9 to 36.3 GPa as the fibre diameter decreases. The new precursor studied in this paper has been proven to be suitable for the production of general purpose carbon fibres. This precursor is obtained through hot filtration of thermally treated coal-tar pitches, a relatively simple and new procedure studied by the authors in previous papers [21]. The isotropic phase that is obtained in this process has been, for the first time, evaluated as precursor for carbon fibres. The results obtained seem promising, as the quality of the fibres obtained is reasonable for general purpose carbon fibres. Despite an additional step of solvent extraction being required to raise the softening point of the precursor in order to be able to stabilise at higher temperatures, the overall process is still of great interest, as in the filtration process a mesophase-rich component is also obtained alongside the fibre precursor. The mesophase produced has been recently used to prepare high density graphites [22,23]. The advantage offered by the hot filtration process is that two different precursors of high added-value products are obtained through the same process, which is interesting from the economic point of view. 4. Conclusions The isotropic phase obtained by hot filtration of a thermally treated coal-tar pitch can be used as a precursor for carbon fibres. In order to reduce the duration of the stabilisation process, the softening point of the pitch was increased to the desired temperature by solvent extraction. The residue obtained with a mixture of 40% acetone–60% acetonitrile was found to be the most suitable precursor for carbon fibres. The fibres obtained after carbonisation did not present any appreciable defects and were homogeneous in diameter, having mechanical properties comparable to other isotropic fibres described in the literature. The advantage of the technology proposed in this paper is that it allows the use of the whole thermally treated coal-tar pitch for the preparation of two types of high added-value Fig. 6. SEM images of carbon fibres. carbon materials
E. Mora et al. Carbon 40(2002)2719-2725 2725 Ack ts [13] Murdie N. Carbon fibers/carbon composites properties and applications. In: Marsh H, Heintz EA, Rod- This work was supported by CICYT-FEDER(Project guezReinoso F, editors, Introduction to carbon tech- 1FDI997-1657MAT)C. Blanco and V. Prada are grateful slogies, Alicante: University of Alicante, 1997, pp. 597- to mec for a research grant. [14] Mochida I, An KH, Korai Y, Kojima T, Komatsu M Yoshikawa M. Activated carbon fibres prepared from References 1998:41:399-405 [15] Yang Ks, Lee DJ, Ryu Sk, Korai Y, Kim YJ, Mochida I [1] Mays TJ. Active carbon fibres. In: Burchell TD, editor, orean. Isotropic carbon fibres and graphite fibres from Carbon materials for advanced technologies, New York: chemically modify pitches. J Chem Eng 1999: 16: 518-24 Pergamon, 1999, pp. 95-118 [16] Alcaniz- Monge J, Cazorla Amoros D, Linares-Solano A 2] Alcaniz-Monge J, Cazorla-Amoros D, Linares-Solano A Dya A, Sakamoto A, Hoshi K. Preparation of general Fibras de carbon. Prepara purpose carbon fibres from coal tar pitches with low soften- Publicaciones de la universidad de ing point. Carbon 1997: 35: 1079-8 3] Fu X, Lu w, Chung DDL Ozone treatment of carbon fibres [17 Korai Y, Ishida S, Watanabe F, Yoon SH, Wang YG, Mochida for reinforcing cement. Carbon 1998: 36: 1337-45 L, Kato I, Nakamura T, Sakai Y, Komatsu M. Preparation of 4 Xu Y, Chung DDL Silane-treated carbon fiber for reinforc- arbon fibres from isotro ing cement. Carbon 2001: 39: 1995-2001 Carbon1997;35:1733-7 5] Roh YB, Jeong KM, Cho HG, Kang HY, Lee YS [18 Watanabe F, Ishida S, Korai Y, Mochida I, Kato l, Sakai Y, Lee Bs. Unique charge-discharge properties of Komatsu M. Pitch-based carbon fibres of high compressive aterials with different structures. J Power strength prepared from synthetic isotropic pitch containing 199768:271-6. cophase spheres. Carbon 1999: 37: 961-7 6 Momma T, Liu X, Osaka T, Sawada T. Electrochemical [ White Kimber G. Extraction using a modification of activated carbon fibre electrode and its ixture of compressed CO, and toluene. Ind Eng Chem Res application to double-layer capacitor. J Power Sources 1996;60:249-53 20 Dauche FM, Barnes AB, Gallego NC, Edie DD, Thies MC H, Shudo A, Miura K. High capacity electric cally extracted itor with high density activated carbon mesophase pitches. Carbon 1998, 36: 1238-40. fibre electrode. J Electrochem Soc 2000: 147: 38-42. 21 Blanco C, Santamaria R, Bermejo J, Menende 8]Egashira M, Takatsuji H, Olada S, Yamaki J. Pro ration and characterisation of the isotropic phase Sn containing nanoparticles activated carbon fibres for a kisting mesophase in thermally treated coal-tar negative electrode in lithium batteries, J Power Sources Carbon2000;38:1169-76 2002;107:5 22 Fanjul ation of preparation conditions for polygranular carbons Oya A Methane storage in activated carbon fibres. Extended Proceedings of the American Carbon Society of Carbon 01 abstracts. 22nd Biennial conference on carbon uc califor- onference CD-ROM Poster Session 2.23 ia San Diego, USA: American Carbon Society, 1995, pp. [23] Fanjul F, Granda M, Santamaria R, Bermejo J, ssessment of the oxidative stabilisation of [10] Li K, Licheng L, Lu C, Qiao W, Liu Z, Liu L, Mochida I mesophase by thermal analysis techniques. J Anal Appl Catalytic removal of so, over ammonia-activated carbon Pyrolysis2001;58-59:91l-26 fibres. Carbon 2001: 39: 1803-8 24 Blanco C, Santamaria R, Bermejo J, Menendez R. A [11 Mochida I, Kawano S, Shirahama N, Enjoji T, Moon SH, omparative study of air-blown and thermally treated coal-ta akanishi K, Korai Y, Yasutake A, Yoshikawa M. Catalytic pitches. Carbon 2000, 38: 517-23 ctivity of pitch-based activated carbon fiber of large surface [25] Blanco C, Prada V, Santamaria R, Bermejo J, Menendez r. area heat-treated at high temperature and its regeneration for Pyrolysis behaviour of mesophase and isotropic pha NO-NH, reaction at lated from the same pitch. J Anal Appl Pyrolysis 2002;63:251-6 [12 Eddie DD, McHugh JJ. High performance carbon fibres In Burchell TD. editor. Carbon materials for advanced tech- ologies, New York: Pergamon, 1999, pp. 119-38
E. Mora et al. / Carbon 40 (2002) 2719–2725 2725 Acknowledgements [13] Murdie N. Carbon fibers/carbon composites: production, properties and applications. In: Marsh H, Heintz EA, Rodrıguez-Reinoso F, editors, Introduction to carbon tech- ´ This work was supported by CICYT-FEDER (Project nologies, Alicante: University of Alicante, 1997, pp. 597– 1FD1997-1657MAT). C. Blanco and V. Prada are grateful 633. to MEC for a research grant. [14] Mochida I, An KH, Korai Y, Kojima T, Komatsu M, Yoshikawa M. Activated carbon fibres prepared from quinoline and isoquinoline pitches. Sekiyu Gakkaishi References 1998;41:399–405. [15] Yang KS, Lee DJ, Ryu SK, Korai Y, Kim YJ, Mochida I. [1] Mays TJ. Active carbon fibres. In: Burchell TD, editor, Korean. Isotropic carbon fibres and graphite fibres from Carbon materials for advanced technologies, New York: chemically modify pitches. J Chem Eng 1999;16:518–24. Pergamon, 1999, pp. 95–118. [16] Alcaniz-Monge J, Cazorla Amoros D, Linares-Solano A, ˜ [2] Alcaniz-Monge J, Cazorla-Amoros D, Linares-Solano A. Oya A, Sakamoto A, Hoshi K. Preparation of general ˜ Fibras de carbon. Preparacion y aplicaciones. Alicante: purpose carbon fibres from coal tar pitches with low soften- ´ ´ Publicaciones de la Universidad de Alicante, 1998. ing point. Carbon 1997;35:1079–87. [3] Fu X, Lu W, Chung DDL. Ozone treatment of carbon fibres [17] Korai Y, Ishida S, Watanabe F, Yoon SH, Wang YG, Mochida for reinforcing cement. Carbon 1998;36:1337–45. I, Kato I, Nakamura T, Sakai Y, Komatsu M. Preparation of [4] Xu Y, Chung DDL. Silane-treated carbon fiber for reinforc- carbon fibres from isotropic pitch containing mesophase ing cement. Carbon 2001;39:1995–2001. spheres. Carbon 1997;35:1733–7. [5] Roh YB, Jeong KM, Cho HG, Kang HY, Lee YS, Ryu SK, [18] Watanabe F, Ishida S, Korai Y, Mochida I, Kato I, Sakai Y, Lee BS. Unique charge–discharge properties of carbon Komatsu M. Pitch-based carbon fibres of high compressive materials with different structures. J Power Sources strength prepared from synthetic isotropic pitch containing 1997;68:271–6. mesophase spheres. Carbon 1999;37:961–7. [6] Momma T, Liu XJ, Osaka T, Sawada T. Electrochemical [19] White KL, Knutson BL, Kimber G. Extraction using a modification of activated carbon fibre electrode and its mixture of compressed CO and toluene. Ind Eng Chem Res 2 application to double-layer capacitor. J Power Sources 1999;38:3360–6. 1996;60:249–53. [20] Dauche FM, Barnes AB, Gallego NC, Edie DD, Thies MC. [7] Nakagawa H, Shudo A, Miura K. High capacity electric Ribbon-shaped carbon fibres from supercritically extracted double-layer capacitor with high density activated carbon mesophase pitches. Carbon 1998;36:1238–40. fibre electrode. J Electrochem Soc 2000;147:38–42. [21] Blanco C, Santamarıa R, Bermejo J, Menendez R. Sepa- ´ ´ [8] Egashira M, Takatsuji H, Olada S, Yamaki J. Properties of ration and characterisation of the isotropic phase and coSn containing nanoparticles activated carbon fibres for a existing mesophase in thermally treated coal-tar pitches. negative electrode in lithium batteries. J Power Sources Carbon 2000;38:1169–76. 2002;107:56–60. [22] Fanjul F, Granda M, Santamarıa R, Menendez R. Optimi- ´ ´ [9] Alcaniz-Monge J, Cazorla-Amoros D, Linares-Solano A, sation of preparation conditions for polygranular carbons. ˜ Oya A. Methane storage in activated carbon fibres. Extended Proceedings of the American Carbon Society of Carbon ’01 abstracts, 22nd Biennial Conference on carbon UC Califor- Conference. CD-ROM Poster Session 2.23. nia San Diego, USA: American Carbon Society, 1995, pp. [23] Fanjul F, Granda M, Santamarıa R, Bermejo J, Menendez R. ´ ´ 516–517. Assessment of the oxidative stabilisation of carbonaceous [10] Li K, Licheng L, Lu C, Qiao W, Liu Z, Liu L, Mochida I. mesophase by thermal analysis techniques. J Anal Appl Catalytic removal of SO over ammonia-activated carbon Pyrolysis 2001;58–59:911–26. 2 fibres. Carbon 2001;39:1803–8. [24] Blanco C, Santamarıa R, Bermejo J, Menendez R. A ´ ´ [11] Mochida I, Kawano S, Shirahama N, Enjoji T, Moon SH, comparative study of air-blown and thermally treated coal-tar Sakanishi K, Korai Y, Yasutake A, Yoshikawa M. Catalytic pitches. Carbon 2000;38:517–23. activity of pitch-based activated carbon fiber of large surface [25] Blanco C, Prada V, Santamarıa R, Bermejo J, Menendez R. ´ ´ area heat-treated at high temperature and its regeneration for Pyrolysis behaviour of mesophase and isotropic phase isoNO–NH reaction at ambient temperatures. Fuel lated from the same pitch. J Anal Appl Pyrolysis 3 2001;80:2227–33. 2002;63:251–65. [12] Eddie DD, McHugh JJ. High performance carbon fibres. In: Burchell TD, editor, Carbon materials for advanced technologies, New York: Pergamon, 1999, pp. 119–38
