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 inE. 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 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 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 ex￾traction), 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 corre￾experimental 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 com￾monitoring 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