Uses of virgin carbon fiber -Activated carbon fibers (ACF)-(2)

Courtaulds originally made a product from viscose rayon called Asgard, believed to be produced from a doped viscose fiber, or by treating the viscose fiber with a solution of diammonium dihydrogen phosphate and urea to give an uptake of 1-2% P on the fiber. This treated material was heated to give a black product, stable at temperatures up to 150C and able to withstand a 1300C Bunsen burner flame for 4 min. The product could be used alone, blended with aramids, or used as a precursor for the production of activated carbon fabrics.

Activated fibers can also be made from a phenolic resin fiber and pitch fiber.

Other work used hybrid of two fabrics, utilizing a PAN or phenolic fiber to supply strength and, yet, can be subsequently activated, whilst the second fiber, normally a cellulosic fiber, can be activated by a different process giving a surprisingly stronger product but, as expected, capable of adsorbing a wider range of molecules.

The activation process confer a high internal surface area to the carbon in a well defined pore structure, which can be determined by measuring the quantity of N2 which is adsorbed under controlled conditions, known as the Apparent BET N2 surface area, measured in m2/g. This internal surface area can be related to the pore volume. The higher the BET values, the greater will be the adsorption capacity.

As a chemical is adsorbed, its heat content changes and that heat is released as the heat of adsorption, which is known as the heat of wetting. To measure the efficiency of the activation process, it is usual to measure the heat of wetting with a solution calorimeter, which has been calibrated applying the precise and reproducible exothermic reaction of tris aminomethane and 0.1 N HCl. Typical adsorbates used for assessing the degree of activation are CCL4, n-hexane, benzene, toluene, di-n-butyl phthalate and silicone oil.

One reason that ACC is so effective is the shape of the pores, which are slit-shaped, long and narrow and able to accommodate quite large molecules, with the ability to permit the escape of pre-adsorbed water and allowing other molecules to be preferentially adsorbed. The time taken to fill the pore determines the effective filter life, whilst the total amount adsorbed determines the saturation vapor adsorption. In most cases, the ACC can be regenerated, without damage to its structure.

The measurement of HOW alone will not indicate the effectiveness of the ACC. It is the performance measured under dynamic conditions that is the crucial parameter. In general, the ACC can adsorb many organic molecules, especially in the form of vapors. Organic impurities in aqueous solution can be adsorbed as well as phosphates and nitrates. Most importantly, bacteria and viruses are also adsorbed.

Defense applications for ACC include NBC face masks, respirators, filters in closed air conditioning systems such as tanks, submarines and aircraft, protective clothing and combat gear.

Industrial applications include air conditioning, odor control, solvent recovery, liquid purification such as removal of Cl2 from water, enzyme and catalyst support with the ability to maintain uniform controllable heat over a large area, recovery of gold and other precious metals from waste liquors, control of premature ripening of products such as bananas, retardation of tarnishing of metal objects in display cabinets and protection of artifacts from external contamination.

In medical applications, it is used in bandages to remove offensive odors, for the control of odor with permanently artificial openings in the body, reduction of adsorption of anesthetics in face masks used by operating room personnel and for blood purification.



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