Melt Spun Precursor

An isotropic pitch precursor is used to make a GP grade carbon fiber, whilst a mesophase pitch will produce a HP carbon fiber that can, if necessary, also be made from the same grade of isotropic pitch feedstock. Mesophase pitches can be divided into classes (Chapter 4) on the basis of their method of manufacture—preparation by means of pyrolysis, solvent extraction, hydrogenation or catalyst modification. A variant of the hydrogenation proeess, termed the Dormant Process, will produce a carbon fiber which is intermediate between GP and HP, with high elongation [1]. 

Property Grade of Dialead Single crystal graphite 

Source Reprinted from Mitsubishi technicai iiterature. 

Property KS352U K1352U K1392U K1382U K13C2U 

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Oxygen in Si-C-N-0 fibers is introduced in significant amounts when the melt spun precursor fibers are cured by oxidation, but oxygen is present only in trace amounts in Si-C-N fibers where curing of green fibers is achieved under anaerobic conditions. 

The types of pitch based melt spun precursor fibers that may be produced have already been discussed in Chapter 4 and that choice will obviously dictate the type of carbon fiber that can be made. A range of available pitch based carbon fibers is listed in Chapter 20. Table 7.1 lists the properties of Mitsubishi Chemical Corporation s Dialead and Table 7.2 lists the typical properties of the composite materials. An SEM micrograph of Dialead K13C2U at different magnifications is shown in Figure 7.1. 

Fibers. The principal type of phenoHc fiber is the novoloid fiber (98). The term novoloid designates a content of at least 85 wt % of a cross-linked novolak. Novoloid fibers are sold under the trademark Kynol, and Nippon Kynol and American Kynol are exclusive Hcensees. Novoloid fibers are made by acid-cataly2ed cross-linking of melt-spun novolak resin to form a fuUy cross-linked amorphous network. The fibers are infusible and insoluble, and possess physical and chemical properties that distinguish them from other fibers. AppHcations include a variety of flame- and chemical-resistant textiles and papers as weU as composites, gaskets, and friction materials. In addition, they are precursors for carbon fibers. 

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. 

Another process for silicon carbide fibers, developed by Verbeek and Winter of Bayer AG [45], also is based on polymeric precursors which contain [SiCH2] units, although linear polysilmethylenes are not involved. The pyrolysis of tetramethylsilane at 700°C, with provision for recycling of unconverted (CHg Si and lower boiling products, gave a polycarbosilane resin, yellow to red-brown in color, which was soluble in aromatic and in chlorinated hydrocarbons. Such resins could be melt-spun but required a cure-step to render them infusible before they were pyrolyzed to ceramic... 

Figure 1. Melt spun BN precursor fibers, magnification 500x.

Not only must precursor fibers be self-supporting as extruded, they must also remain intact (e.g. not melt or creep) during pyrolytic transformation to ceramic fibers. Thus, precursor fibers (especially melt spun fibers) must retain some chemical reactivity so that the fibers can be rendered infusible before or during pyrolysis. Infusibility is commonly obtained through reactions that provide extensive crosslinking. These include free radical, condensation, oxidatively or thermally induced molecular rearrangements. 

A boron nitride fiber can be very competitive commercially with carbon fiber. It has about the same density (2.2 g/cm ) as the carbon fiber, but has greater oxidation resistance and excellent dielectric properties. A method of converting boric oxide precursor fibers into boron nitride fibers has been developed (Economy and Anderson, 1967). Melt spun boric oxide precursor fiber is nitrided with ammonia according to the following reaction ... 

Besides the desired dehydrocoupling, Si-N bond cleavage of HPZ and formation of DEB-NHSiMej occurred. Nevertheless, the polymeric precursor had an ideal glass-transition temperature for melt spinning and stable melt viscosity. Melt spun Si-B-C-N-H green fibers were subsequently transformed into ceramic fibers by thermolysis at 1400°C. 

Two categories of pitch-based fiber exist isotropic carbon fiber produced from an isotropic pitch precursor, and an oriented, anisotropic fiber produced from a mesophase pitch precursor. Isotropic fibers were developed from low melting point isotropic pitches The precursor was melt-spun into fibers, which were oxidized to render them infusible, and then carbonized. Their low strengths and moduli make these fibers unsuitable for use in advanced composites. Orientation was accomplished by a hot-stretching process (>2200°C), but it is accompanied by the same processing difficulties encountered in the rayon precursor process. A different approach was suggested by the discovery of carbonaceous mesophase. ... 

Step 2. Melt spinning. The polymer is melt spun from a 500-hole nozzle at about 350°C under N2 to obtain the so-called preceramic continuous precursor fiber. ... 

One distinct advantage of oxide fibers in terms of processing is that several alumina precursors suitable for forming fibers are available. The aqueous chemistry of aluminum allows for the formation of viscous basic aluminum salt solutions that can be made into fibers by dry-spinning. Polymeric aluminoxanc precursors have also been used to produce alumina-based fibers. Recently, aluminoxanes that can be melt-spun into fibers have been produced. Details of both types of chemical precursors arc given below. 

Polymer Pyrolysis Derived SiC Fibers (PP-fibers) As shown 1976 by Yajima [54], pSiC fibers with a smaller diameter (8-30 pm) and without a central core can be manufactured by solid state pyrolysis of a polycarbosilane (PCS) precursor fiber. The melt-spun PCS fiber is first cured at 200°C in air to produce a thin layer to protect from melting later on, then heated up in inert atmosphere to 1500°C to convert the PCS in crystalline pSiC. The steps leading to the production of SiC can be summarized as follows ... 

In the case of thermosets, deliberate and extensive orientation is virtually unknown. This appears to be the result of the practical difficulties involved, rather than from any theoretical obstacle. For example, it is possible that the fibre Kynol produced by the Carborundum Corporation is oriented to some extent. This is produced from a melt-spun Novolak phenol-formaldehyde resin, which is later further cross-linked with formaldehyde. It is, of course, legitimate to consider carbon fibres as extreme examples of thermosets. Formed by the cyclisation and subsequent graphitisation of polyacrylonitrile (or other suitable precursors), they are highly oriented. 

Boron nitride fibers have been prepared in the laboratory by chemical vapor infiltration of boron oxide glass fibers with ammonia (Equation 8), a process that may alternatively be considered to be a nitridation of B2O3 precursor fibers [31]. The precursor fibers, in turn, are melt spun at a low temperature (480 C) from a viscous melt. Thus, the nitridation of a boron oxide fiber could alternatively be considered to be derived from a solid precursor fiber, a topic otherwise discussed in Chapters 8 to 12. In this process, the final step is the chemical conversion of a given precursor fiber at a high temperature in a highly reactive vapor phase environment. 

Despite its low molecular weight, the precursor can be melt spun at 100°C in air, and yields a continuous precursor fiber. This green fiber is heat treated in ammonia at lOOO C. Chemical vapor infiltration and the resulting reaction convert carbonaceous materials in the fiber into volatiles. A short heat treatment at 1800°C in nitrogen removes all residual volatiles and it stabilizes the boron nitride fiber and its microtexture. The final boron nitride fiber is white and has a density of 2.05 glom ... 

The fourth process which yieids high siiica giass fibers relies on acid leaching of borosilicate or aluminosiiicate precursor fibers in fabric form (Chapters 4 and 6). This is the oldest and least expensive process. Acid ieaching removes most of the compositionai oxides other than silica from a precursor fabric, individual fibers can be leached also, but they are not satisfactorily converted into sliver, braids of woven fabrics. Process details and properties of melt spun and acid leached silica substrate processes have been discussed in Chapter 4. Commercial applications of all four silica glass fibers will be discussed in Chapter 6. 

The ammonolysis of dimethyidichlorosilane offers another route to PSZ precursor fibers and ultimately silicon nitride fibers. In a first step, Me2SiCl2 and MeHSiCl2 are mixed in nearly a 1 1 molar ratio in benzene. The mixture is treated with NHs, NH4CI precipitates, and the solvent is removed. The ammonolysis product, a viscous polymer, is converted in a second step at 400°C into melt spinnable polymer, and is melt spun. The resulting PSZ precursor fiber has a (Si-N) backbone. Carbon is present in pendent methyl groups, and the empirical formula of the fiber is SiCi.7No.9oH57 [23]. 

Table 4.6 Strengths of melt spun PAN precursor based carbon fibers...

The aim of the second case study was to investigate the feasibility of creating a SPC of PA-6 using melt spun PA-6 micro fibers in the form of continuous yarn and anionic polymerization of the PA-6 precursor, -caprolactam. 

Low-cost carbon fibers are produced from an isotropic pitch with a low-softening point. The precursor is melt-spun, thermoset at relatively low temperature, and carbonized. The resulting fibers generally have low strength and modulus ( 35 - 70 GPa). They are suitable for insulation and filler applications. Their cost dropped to less than 20/kg in 1992. ... 

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