Carbon fiber stabilization

With regard to carbon fiber stabilization. Table 1 and Figure 3 clearly show that the vinyl halides depress the melting point considerably less than more voluminous monomers such as methyl acrylate, methyl methacrylate or vinyl acetate, at comparable molar level. It can be concluded that fiber fusion will be less of a problem with the former, as compared with the latter. Figure 4 illustrates this point for copolymers AN/VBr as compared to AN/VA. The melting points are calculated using Eq. (2),... 

Table 3. Elementary analyses of carbon fibers. Stabilization 3 h, 250 °C time of carbonization 5 min...

XPS was used by Brewis et al [58] to determine the levels and nature of oxidation in carbon fiber. ESCA revealed the presence on the carbon fiber surface of =C==0, =C—OH, Na, —S04, =C=C=, unoxidized nitrogen and silicone type Si. A direct relationship was found to exist between carbon fiber stability and the sodium present as Na2S04 (Figure 12.31). Figure 12.32 shows selected ESCA spectra [59]. 

Figure 20.34 Relationship between total sodium content and bare carbon fiber stability. Source Reprinted with permission from Gibbs HH, Wendt RC, Wilson FC, 33rd Ann Tech Conf SPI, 1978. Copyright 1978, The Society of Plastics Engineers.

See carbon fibers, carbonization, pitch-based carbon fibers, stabilization treatment... 

More than 95% of current carbon fiber production for advanced composite appHcations is based on the thermal conversion of polyacrylonitrile (PAN) or pitch precursors to carbon or graphite fibers. Generally, the conversion of PAN or pitch precursor to carbon fiber involves similar process steps fiber formation, ie, spinning, stabilization to thermoset the fiber, carbonization—graphitization, surface treatment, and sizing. Schematic process flow diagrams are shown in Eigure 4. However, specific process details differ. 

Mechanical Properties and Stability at Elevated Temperature. One increasingly important characteristic of carbon fibers is their excellent performance at elevated temperatures. Strength tested in an inert environment remains constant or slightly increases to temperatures exceeding 2500°C. Amoco s high modulus pitch carbon fiber P-50 maintains approximately 80% of room temperature modulus at temperatures up to 1500°C, then decreases more rapidly to 30% at 2800°C (64). The rapid decrease in modulus is a result of increased atomic mobiHty, increa sing fiber plasticity. 

Fibers produced from pitch precursors can be manufactured by heat treating isotropic pitch at 400 to 450°C in an inert environment to transform it into a hquid crystalline state. The pitch is then spun into fibers and allowed to thermoset at 300°C for short periods of time. The fibers are subsequendy carbonized and graphitized at temperatures similar to those used in the manufacture of PAN-based fibers. The isotropic pitch precursor has not proved attractive to industry. However, a process based on anisotropic mesophase pitch (30), in which commercial pitch is spun and polymerized to form the mesophase, which is then melt spun, stabilized in air at about 300°C, carbonized at 1300°C, and graphitized at 3000°C, produces ultrahigh modulus (UHM) carbon fibers. In this process tension is not requited in the stabilization and graphitization stages. 

Further improvements in the properties of PAN-based carbon fibers are likely to emerge through improved stabilization, that is, by creating the ideally cross-linked fiber. On the other hand, as purer pitch precursors become available, further improvements in mesophase pitch-based carbon fibers are likely to arise from optimized spinnerette designs and enhanced understanding of the relationship between pitch chemistry and its flow/orientation behavior. Of course, the development of new precursors offers the potential to form carbon fibers with a balance of properties ideal for a given application. 

Dunham, M. G., Stabilization of polyacrylonitrile carbon fiber precursors. Ph.D. dissertation, Clemson University, Clemson, SC, 1990. 

There is recent evidence that stabilization to elevated temperatures (over 350°C) yields a structure with additional intermolecular cross-linking that results in improved mechanical properties in carbonized fibers [10,11], In addition, it has been noted that the addition of ammonia to the stabilizing environment accelerates stabilization [12],... 

In the energy domain, new and efficient uses in gas lines, electric cable ducts and the like, will promote surface stabilization and endurance as well as complex stress capability of various extruded or cast systems. Such reactants as acetylene terminated polymers have yielded cross-linked cured, networks of exceptional density and durability. A diimide dianhydride combined with (3) ethynylaniline yields an acetylene terminated tetraimide. On further polymerization at 250°C, the cross-linked structure derived can be used continuously at about 230°C. When this is combined with polymer carbon fibers or filaments, an exceedingly refractory and tough binder is produced. 

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