M.. Prauchner et al. Carbon 43(2005)591-597 can be raised until the fibers become infusible [30, 31]. As ization and carbonization caused pronounced filament a higher initial SP permits stabilization to be started at a shrinkage, as is discussed later temperature, and crosslinking reactions ha In spite of the pre-treatment and consequent in- higher kinetics at higher temperatures, pitches with creases in SP, even the filaments produced from the higher SP can be stabilized using more elevated heating pitch pre-treated at 250C for 6h required very low rates without incurring problems of fiber deformation or heating rates to be stabilized without coalescence, fusing 0.08C/min, therefore implicating in a too time-consum y the other hand, the pitch spinnability is reduced ing process even though a final temperature of 180C with increasing pitch SP. This occurs because the higher was sufficient to make the fibers infusible. This heating the pitch SP, the higher the temperature necessary to rate is much lower than those usually employed to stabi spin it Since eucalyptus tar pitches have reactive oxygen lize fossil pitch filaments, 0.5-2C/min [30, 31]. There are containing functional groups and high volatile contents, two main reasons for this. The first one is that filaments high spinning temperatures give rise to bubble forma- with much higher SP can be used in the case of fossil tion, which damages the spinning process stability. In pitches, which is made possible because fossil pitches fact, if rheological studies are carried out for pitches present elevated thermal stability, and therefore, can with different SP, and the temperature is adjusted for be easily spun at temperatures sometimes even higher each sample in order to provide approximately the same than 350 C [32]. Having higher SP, the fossil pitch fila- viscosity(e. g, the spinning viscosity), viscosity instabil- ments can be stabilized at temperatures at which cross- ity increases with increasing pitch SP(Fig. 2)because linking reactions occur more rapidly. The another the corresponding temperature is higher. As pitch vis- main reason is the large amount of low molar mass mol- cosity is highly temperature-dependent [25], the pitch fil- ecules present even in the polymerized samples of euca- aments draw down and cool very quickly during lyptus tar pitches, which correspond to higher retention spinning. Since fiber tensile stress during spinning is time in the GPC curves of Fig 3. During oxidative ther nearly that required to break the filaments due to the mal treatment, these molecules can easily acquire energy brittle nature of the as-spun fibers, it is understandable sufficient to provoke the coalescence of the filaments hat oscillations during spinning can easily lead to fila- even at temperatures below the pitch SP. However,un ment failure der heating rates as low as 0.08 C/min, these molecules In practice, we had to find a compromise by increas- were slowly eliminated either by sublimation or ing pitch SP to a value as high as practicable without o "1 adm tton, Derbyshire et al. 4 reported that the running the risk of introducing insuperable problems for the fiber forming step. In this context, crude eucalyp- maximum effective heating rate during oxidative stabil- tus tar pitch(SP=76C)was submitted to a pre-treat- ization of pitch filaments decreases with increasing fiber ment aiming to increase its SP. Pre-treatment involved diameter due to the limited oxygen diffusion. Therefore, hermal polymerization at 250C [22, 23 the relatively large diameter of the as-spun filaments The pitch sample pre-treated at 250C for 6h was the produced in the present work is possibly contributing one with the highest SP(134 C)which presented good to make stabilization difficult spinnability. This pitch was spun at 175-180C, at a rate Despite oxygen incorporation(Table 1), eucalyptus of 48-50m/min and under a pressure of 2 bar. The fila- tar pitch filaments underwent pronounced weight loss ments had an average diameter of 46+ 2 um, which is and consequent shrinkage during stabilization table elatively large. However, the subsequent steps of stabil- 2), which can be attributed to the release of volatiles, 1E+06 1E+05 人思x 2h250° E+04 0.05.010015.020.025030.035040.0 Fig. 2. Viscosity as a function of shear time for crude pitch and pre-treated at 250C for 2 and 4h. Tests were carried out at 101, 118, and 156C, spectively, in order to provide similar viscosities for all samples.can be raised until the fibers become infusible [30,31]. As a higher initial SP permits stabilization to be started at a higher temperature, and crosslinking reactions have a higher kinetics at higher temperatures, pitches with higher SP can be stabilized using more elevated heating rates without incurring problems of fiber deformation or fusing. By the other hand, the pitch spinnability is reduced with increasing pitch SP. This occurs because the higher the pitch SP, the higher the temperature necessary to spin it. Since eucalyptus tar pitches have reactive oxygen containing functional groups and high volatile contents, high spinning temperatures give rise to bubble forma￾tion, which damages the spinning process stability. In fact, if rheological studies are carried out for pitches with different SP, and the temperature is adjusted for each sample in order to provide approximately the same viscosity (e.g., the spinning viscosity), viscosity instabil￾ity increases with increasing pitch SP (Fig. 2) because the corresponding temperature is higher. As pitch vis￾cosity is highly temperature-dependent [25], the pitch fil￾aments draw down and cool very quickly during spinning. Since fiber tensile stress during spinning is nearly that required to break the filaments due to the brittle nature of the as-spun fibers, it is understandable that oscillations during spinning can easily lead to fila￾ment failure. In practice, we had to find a compromise by increas￾ing pitch SP to a value as high as practicable without running the risk of introducing insuperable problems for the fiber forming step. In this context, crude eucalyp￾tus tar pitch (SP = 76C) was submitted to a pre-treat￾ment aiming to increase its SP. Pre-treatment involved thermal polymerization at 250C [22,23]. The pitch sample pre-treated at 250C for 6 h was the one with the highest SP (134C) which presented good spinnability. This pitch was spun at 175–180C, at a rate of 48–50m/min and under a pressure of 2 bar. The fila￾ments had an average diameter of 46 ± 2lm, which is relatively large. However, the subsequent steps of stabil￾ization and carbonization caused pronounced filament shrinkage, as is discussed later. In spite of the pre-treatment and consequent in￾creases in SP, even the filaments produced from the pitch pre-treated at 250C for 6 h required very low heating rates to be stabilized without coalescence, 0.08C/min, therefore implicating in a too time-consum￾ing process even though a final temperature of 180C was sufficient to make the fibers infusible. This heating rate is much lower than those usually employed to stabi￾lize fossil pitch filaments, 0.5–2 C/min [30,31]. There are two main reasons for this. The first one is that filaments with much higher SP can be used in the case of fossil pitches, which is made possible because fossil pitches present elevated thermal stability, and therefore, can be easily spun at temperatures sometimes even higher than 350C [32]. Having higher SP, the fossil pitch fila￾ments can be stabilized at temperatures at which cross￾linking reactions occur more rapidly. The another main reason is the large amount of low molar mass mol￾ecules present even in the polymerized samples of euca￾lyptus tar pitches, which correspond to higher retention time in the GPC curves of Fig. 3. During oxidative ther￾mal treatment, these molecules can easily acquire energy sufficient to provoke the coalescence of the filaments even at temperatures below the pitch SP. However, un￾der heating rates as low as 0.08C/min, these molecules were slowly eliminated either by sublimation or polymerization. In addition, Derbyshire et al. [4] reported that the maximum effective heating rate during oxidative stabil￾ization of pitch filaments decreases with increasing fiber diameter due to the limited oxygen diffusion. Therefore, the relatively large diameter of the as-spun filaments produced in the present work is possibly contributing to make stabilization difficult. Despite oxygen incorporation (Table 1), eucalyptus tar pitch filaments underwent pronounced weight loss and consequent shrinkage during stabilization (Table 2), which can be attributed to the release of volatiles, shear time (min) viscosity (cP) 1.E+04 1.E+05 1.E+06 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 crude pitch 2 h/250 ºC 4 h/250 ºC Fig. 2. Viscosity as a function of shear time for crude pitch and pre-treated at 250C for 2 and 4 h. Tests were carried out at 101, 118, and 156C, respectively, in order to provide similar viscosities for all samples. 594 M.J. Prauchner et al. / Carbon 43 (2005) 591–597