Oxidized PAN fiber opf

A particular advantage of using opf is that a preform can be more readily processed by conventional textile processes than by using the alternative carbon fiber. The opf route is favoured by BF Goodrich, Dunlop, Honeywell and Messier Bugatti. Chief suppliers of opf are SGL and Zoltek and, whilst Dunlop had their own inhouse facility to manufacture opf at one stage, this has now been discontinued and all supplies are purchased outside. 

The blank disks for carbon brakes are produced using modified conventional textile, but are strictly proprietary procedures. Due to the fact that the opf will be converted to a PAN based carbon fiber, the product will be non-graphitic, unsized and untreated. 

Carbon fibers are normally sized for improved handling and if this size is present, it would decompose during the first carbonization stage. Hence it is considered advisable, as a preliminary step, to burn off any size prior to further processing. 

Since the matrix and reinforcing fiber in carbon-carbon are both brittle materials, it is preferred to aim for a weak interface in the composite, permitting cracks to travel through the matrix in order to debond at the fiber/matrix interface. 

Fitzer and Burger [20] deduced that poor bonding was required, promoting longitudinal fissuring, into which carbon could be deposited to give a better bond and good load transfer. 

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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. 

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... 

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

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

Process. Any standard precursor material can be used, but the preferred material is wet spun Courtaulds special acrylic fiber (SAF), oxidized by RK Carbon Fibers Co. to form 6K Panox B oxidized polyacrylonitrile (PAN) fiber (OPF). This OPF is treated ia a nitrogen atmosphere at 450—750°C, preferably 525—595°C, to give fibers having between 69—70% C, 19% N density less than 2.5 g/mL and a specific resistivity under 10 ° ohm-cm. If crimp is desired, the fibers are first knit iato a sock before heat treating and then de-knit. Controlled carbonization of precursor filaments results ia a linear Dow fiber (LDF), whereas controlled carbonization of knit precursor fibers results ia a curly carbonaceous fiber (EDF). At higher carbonizing temperatures of 1000—1400°C the fibers become electrically conductive (22). 

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. 

Work undertaken at the NPL, New Delhi [99] showed that during the oxidation of PAN, a second small exotherm occurred with a maximum centered at about 350°C (Figure 5.39). PAN fiber oxidized at 350°C produced a more ordered stracture in the opf, which when carbonized gave a carbon fiber with improved mechanical properties (Table 5.10) and moreover, could be rapidly carbonized. 

All pass back rollers must rotate freely and tow tension in the first passes must be adjusted to ensure that the fiber band does not come into contact the floor of the oven. The fiber band must be of uniform thickness, with no gaps and must have a uniform tension. The inlet and outlet fiber speeds in oxidation control the mpul of the opf. The uniformity of colour across the first and second passes of the oxidized PAN tow band will be a good indication of uniform temperature distribution which must be within 2°C. The control of the level of oxidation can be ascertained by measuring the final oxidized density. 

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