Uses of virgin carbon fiber -Activated carbon fibers (ACF)-(1)

Rebouillat et al and Suzuke give good reviews of activated carbon fibers. Traditionally, activated carbon granules are made by the carbonization of a product such as coconut shells, which due to their physical granular form, tend to be difficult to handle and the development of an activated woven cloth by the British Chemical Defence establishment at Porton Down via the controlled heat treatment of a woven rayon cloth offers many advantages. The activated charcoal cloth (ACC) product was made under licence in 1977. One such process used a 1.8 m wide fabric, reducing to about 1.0 m at the end of the process. To aid carbonization, the cloth was treated with a solution of chemicals to confer a measure of flame retardancy. There are two forms of flame retardant- one where the flame retardant acts as a catalyst and promotes removal of the – OH groups and the other form, which actually reacts with the –OH groups.

After pre-treatment, the cloth is dried and carbonized up to about 850C in an atmosphere of N2, using a heating rate of about 20C /min and at this stage, the fabric is extremely brittle and is unable to withstand applied tension or rubbing. At 850-1000C, steam or CO2, is introduced to activate the fiber and sweep away the tars. The process of activation helps free the pores from occluded tars to give an apparent pore volume of about 0.5 cm3/g. One problem associated with the use of chemicals is that they can leave a residue, which may be unacceptable for certain specific end uses.

Toho Beslon developed a process to produce activated fiber from PAN using a Fe salt. The PAN fiber, usually in the form of a felt or fabric, is first pre-oxidized under tension in air, or O2, at 150-300C, using a lower temperature for a PAN fiber with comonomer content >6%, to prevent individual filaments sticking together. If, however, a Fe compound was initially incorporated in the PAN fiber, then sticking could be averted. A whole range of copolymers can be used and the polymer will contain 1000-2000 repeat units. Higher comonomer content favours easier PAN spinning, making stretching easier and increasing the yield and strength of the activated carbon fiber, but the tendency for filaments to stick will then increase. Either divalent or trivalent Fe compounds can be used, such as FeCl2 or FeCl3. The process can be further improved by undertaking a multistage pre-oxidation treatment. If the final Fe content is above 1%, the fiber is extracted with an organic acid to reduce the Fe level, preferably to below 0.3%, which prevents subsequent overactivation and loss of strength.

The oxidized fiber can then be treated with a chemical activation reagent such as ZnCl2, H3PO4 or HCl at 700-1000C or, alternatively, gaseous activation can be undertaken in CO2, NH3 or steam in the presence of N2 from 700-1000C. The product has a high N2 content with a specific surface area of some 300-2000 m2/g and a fiber strength of 0.25-0.35 Gpa.

Later, Toho introduced up to 0.3% P or B into the precursor to aid processing. Various studies on the preparation of PAN based activated carbon fiber have been undertaken, including work with a hollow fiber. The study of the activation stage has also been reported.



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