Oxidized PAN Fiber

Oxidized PAN fiber of density 1.37-1.40 gcm is treated with a water based proprietary antistat finish, dried and collected after plaiting, into boxes positioned on a plaiter table with longitudinal and transverse movements to plait the opf neatly into the boxes without entanglement. 

---

Fig. 1. The stmcture of Dow EDF fiber. EDF = elongatable Dow fiber OPF = oxidized PAN fiber.

Courtaulds developed many products [153] based on carbon fibers prepreg as warp sheet and tape, random fiber mats, preferred orientation mats, chopped fiber, thermoset and thermoplastic molding compounds and polytetrafluroethylene (PTFE) impregnated tow. Oxidized PAN fiber was available as continuous tow, tops, yarns, woven and knitted fabrics. 

In the oxidation stage, the PAN fiber will increase in density from 1.18 gcm to about 1.36-1.38 gcm for the oxidized PAN fiber (opf). The actual density will depend on whether the final product is to be used as opf or is required for onward processing to carbon fiber. The final density of an opf product will depend on the opf product specification, whereas for carbon fiber, the density must be at least 1.36 gcm , as otherwise, the fiber will tend to pull apart and break on entering the LT furnace. The upper opf density limit for the production of carbon fiber varies with the manufacturer and some manufacturers will use a value as high as 1.40 gcm , but others claim that this would produce inferior carbon fiber. The residual exothermicity of SAP heated in air at 230°C (Figure 5.8) after a 3 h treatment, has some 35% exothermic heat remaining in the oxidized fiber. 

Figure 5.11 Cyclic variation of the tensile strength of oxidized PAN fiber.

The degree of oxidation of an oxidized PAN fiber sample can be readily determined by preparing a suitable section with a microtome for examination under the microscope and this is detailed in Chapter 17, Section 17.3.8. Typical oxidized PAN microstructures are shown in Figure 3.51. 

X-ray diffraction can be used to monitor the production of PAN (Figure 12.24) and oxidized PAN fiber (Figure 12.25). Mean figures sampled over a four year period for the XRD d-spacings, crystal size and azimuthal half widths for TTP PAN precursor and Panox oxidized PAN fiber are given in Table 12.3. 

The precursor fiber type for reinforcing the carbon matrix can be an oxidized PAN fiber (opf), or either a PAN or pitch based carbon fiber. In some instances, for special applications, such as the Shuttle, a cellulose based carbon fiber is used. The reinforcements can be unidirectional have a random chopped fiber presentation as in a felt format a woven product from continuous fiber presented in a 2D, 3D, or in a Multi-D format (Section 21.1), or a non-woven carbon fiber. The chosen fiber architecture is most important for a given application and Lei et al [4] describe how, for example, 3-D braiding can be applied to carbon-carbon composites. One of the early forms of near net shape reinforcement used for carbon-carbon aircraft brakes was based on a weft knitted 3-D fabric made by the Pressure Foot process (Figure 14.1). 

TESTING OF OXIDIZED PAN FIBER (OPF) AND VIRGIN CARBON FIBER 17.3.1 Mass per unit length... 

It should be noted that oxidized PAN fiber will not burn in air when the density is above 1.37 g cm , but care must be taken when undertaking this test. 

Oxidized PAN fiber has an equilibrium moisture content and Figure 17.4 shows the effect of a moisture regain of 5% and 4% on the measured density, whilst Figure 17.5 shows a correction factor which can be applied to allow for the moisture regain. 

Table 17.4 Comparison of laser and Vickers image splitting eyepiece methods for determining the filament diameter of oxidized PAN fiber...

A method not used by the author for testing filaments is Sinclair s loop test (Supplement 1 at the end of this Chapter). The procedure has been replaced by some form of tow testing, except when testing oxidized PAN fiber, where filament testing is standard procedure. 

Oxidized PAN fiber is treated with a finish to act as a lubricant and this can be extracted with CH3OH by one of several routes ... 

To establish the degree of oxidation in an oxidized PAN fiber, it is advantageous to prepare a section for examination under the microscope. The following technique has been adapted from one used in hospitals to look at sections of bone. 

Figure 17.88 is an IR fingerprint of oxidized PAN fiber showing the presence of the correct fiber finish. 

Figure 17.88 Fingerprint of oxidized PAN fiber treated with a proprietary finish, (a) Finish as applied to oxidized PAN fiber, (b) Oxidized PAN fiber with finish.

Miscellaneous, covering items such as environmental factors (e.g. humidity level affecting moisture pick-up of oxidized PAN fiber), testing equipment (not correctly calibrated). 

Oxidized PAN fiber is made by a slightly modified first stage of the PAN based carbon fiber process and the properties are listed in Table 20.2. Oxidized PAN fiber, with a density above 1.37 gcm , is non-flammable. 

Lerner [189] found that oxidized PAN fibers subjected to heat treatment temperatures of 715-945 K are semiconductors. The room temperature conductivity is dominated by the contributions of impurity states, but these are not related to defects in the polymer. There is a decrease in conductivity on ageing in air caused by a decrease in the electron-phonon scattering time. The conductivity increases at temperatures above 473 K, as the samples aged in air outgas. 

The chemical resistance of oxidized PAN fiber in some common reagents is given in Table 20.25 and the outgassing of oxidized PAN fiber is given in Table 20.26. 

Since carbon fiber is electrically conductive, it is good practice for processors working with carbon fiber, to have designated work areas set aside, so that all electrics can be adequately protected from carbon liber fly and an efficient air filtration system built into the workplace to safeguard operatives. Oxidized PAN fiber is not electrically conductive, however, and does not have these problems. 

Property Typicai value for Panox oxidized PAN fiber (opf) Typical value for Asbestos... 

Textron Systems, Wilmington, Massachusetts, USA—formerly known as Avco and later as Textron Speciality Materials. Market Avcarb carbon fiber and Avox oxidized PAN fiber. The carbon-carbon and metal matrix composite parts are at Lowell, MA. 

---