Effect of processing on carbon fibers 丁↓↓;↓↓↓↓↓↓ Fig. 12. Predicted transverse molecular orientation for mesophase pitch flowing through a circular capillary and transverse texture of carbonized mesophase fiber extruded from a circular capillar [571 Because its viscosity is highly temperature-depen- so the stabilization process involves simultaneous dent, mesophase pitch fibers draw down and cool diffusion and reaction. However, unlike PAN precur very quickly during fiber formaLion. In fact, at Typical sor fibers, the as-spun structure of inesopliase precur melt-spinning conditions, mesophase fibers are sor fibers is already highly oriented, so tension does already 100.c below their glass transition temper- not need to be applied during stabilization. Most ature by the time they are 2 cm from the spinneret. mesophase precursor fibers can be stabilized by As a result, they can break easily during spinning 230 to 280"C. Often, the tcmpcrature begins ncar the exposing them to air at temperatures ranging from and are extremely difficult to handle before they are carbonized. Although the rheology of mesophase softening temperature of the mesophase and makes control of the melt-spinning more difficult, its increased in a series of steps during the stabilization iquid crystalline nature gives this precursor advan- process, Numerous studies [43-45] have shown that tages compared to polymeric precursors such as he rate of oxidative stabilization is affected by the PAN. As Yoon et al.[42] showed, unlike polymeric temperature, the concentration of oxygen, and the fibers, the molecular orientation within a mesophase chemical structure of the mesophase molecules. Most precursor fiber can be improved by increasing spin researchers agree uring ling temperature oxidative stabilization, the mesophase fiber gains weight; ketones, aldehydes and carboxylic acids are 3.3 Stabilization of mesophase pitch precursor formed; and water is given off [43, 44]. At higher temperatures, the fiber begins to lose weight as CO2 Because mesophase pitch is a thermoplastic mate- is evolved. However, the exact nature of the reactions rial, the as-spun structure must be thermoset to that occur during the stabilization step is still the prevent relaxation during final heat treatment. Like subject of active research. As in the PAN carbon he PaN carbon fiber process, oxidative stabilization fiber process, the objective is to uniformly crosslink is normally employed to crosslink the as-spun fibers the precursor fiber as fast as possible with a minimumEffect of processing on carbon fibers 353 (4 vr = “8 = 0 n,=cosw(r) ne = sin o(r) n, = 0 Fig. 1 2. Predicted transverse molecular orientation for mesophase pitch flowing through a circular capillary a texture of carbonized mesophase fiber extruded from a circular capillar [57]. .nd transl Because its viscosity is highly temperature-depen￾dent, mesophase pitch fibers draw down and cool very quickly during fiber formation. In fact, at typical melt-spinning conditions, mesophase fibers are already 100°C below their glass transition temper￾ature by the time they are 2 cm from the spinneret. As a result, they can break easily during spinning and are extremely difficult to handle before they are carbonized. Although the rheology of mesophase makes control of the melt-spinning more difficult, its liquid crystalline nature gives this precursor advan￾tages compared to polymeric precursors such as PAN. As Yoon et al. [42] showed, unlike polymeric fibers, the molecular orientation within a mesophase precursor fiber can be improved by increasing spin￾ning temperature. 3.3 Stabilization of mesophasepitch precursor fibers Because mesophase pitch is a thermoplastic mate￾rial, the as-spun structure must be thermoset to prevent relaxation during final heat treatment. Like the PAN carbon fiber process, oxidative stabilization is normally employed to crosslink the as-spun fibers, so the stabilization process involves simultaneous diffusion and reaction. However, unlike PAN precur￾sor fibers, the as-spun structure of mesophase precur￾sor fibers is already highly oriented, so tension does not need to be applied during stabilization. Most mesophase precursor fibers can be stabilized by exposing them to air at temperatures ranging from 230 to 280°C. Often, the temperature begins near the softening temperature of the mesophase and is increased in a series of steps during the stabilization process. Numerous studies [43-451 have shown that the rate of oxidative stabilization is affected by the temperature, the concentration of oxygen, and the chemical structure of the mesophase molecules. Most researchers agree that, during the initial stages of oxidative stabilization, the mesophase fiber gains weight; ketones, aldehydes and carboxylic acids are formed; and water is given off [43,44]. At higher temperatures, the fiber begins to lose weight as CO, is evolved. However, the exact nature of the reactions that occur during the stabilization step is still the subject of active research. As in the PAN carbon fiber process, the objective is to uniformly crosslink the precursor fiber as fast as possible with a minimum