Asbestos, surface area

Class D. Inorganic materials of high surface area or unusual surface properties which increase the water-retaining capacity of the mix. These include very fine clays (bentonites), pyrogenic silicas, condensed silica fume, milled asbestos and other fibrous materials. 

Now, let s imagine the gas is delivered to a patient through a gas scrubber filled with Ascarite to remove the carbon dioxide. Ascarite is sodium hydroxide on asbestos. Ascarite II uses silica instead of asbestos. As the gas flows over this high-surface-area medium, carbon dioxide in the sample reacts with the sodium hydroxide to give the nonvolatile product sodium bicarbonate. 

In heterogeneous catalysis, the solid catalyst is more effective when in a state of fine sub-division than when used in bulk. So, a lump of platinum will have much less catalytic activity than colloidal or platinised asbestos. Finely divided nickel is a better catalyst than lumps of solid nickel, because former occupies greater surface area than the latter. 

Polyolefin (Spectra from Honeywell-Allied Signal) and aramid (Kevlar from DuPont) fibers have much lower surface areas and polarities than asbestos fibers and do not bond... 

The carbon blacks consisted of Spheron 6 and Sterling FT (fine thermal) black, both non-heat-treated and 2700° C. heat-treated (obtained from the Godfrey L. Cabot Co., Boston, Mass.). Published surface areas for the four carbon blacks are 114, 84.1, 15.5, and 12.5 sq. meters per gram, respectively (31). The silica sand was supplied by the New Jersey Silica Sand Co., Millville, N. J., with the manufacturer s designation of No. 325 flour. Screen analysis by the supplier showed a considerable range in particle size about 60% was retained on a 90-mesh screen and 90% on a 200-mesh screen. Finally, a sample of asbestos was tested (obtained from the Canadian Johns-Manville Co., Port Credit, Toronto, with the designation 7R). Zettlemoyer (31) has studied this material. 

In general, it is recognized that there are multiple cellular and molecular responses to asbestos fibers, that no single mechanism is likely to account for all asbestos-related diseases, that the precise steps in pathogenic pathways leading to asbestos-related disease are not fiilly established, and that fiber structural and chemical properties (e.g., length, width, iron content, durability, surface areas) are important variables that play a role in the development of lung and pleural injury. 

The smaller the fiber diameter used in the prefilter, the greater the surface area for adsorption of particles and the better the retention of small particles. In the sixties, asbestos fibers were recognized as the best prefilter media. The individual fibrils were smaller than 0.01 ju and they had a positive zeta potential. However, when it was suspected that asbestos fibers presented a health hazard, fine diameter glass and synthetic polymer fibers were substituted. Unfortunately, neither media equals the performance of asbestos. Glass fibers are available in the finest diameters, but some users are fearful they may represent a similar health hazard. The trend has been to use polypropylene or polyester fiber prefilters. Melt blown or spun-bonded fibers are available in diameters near 1 ju. Multilayers of these media with appropriate calendering have resulted in surprisingly efficient prefilters. 

In the Steiner Tunnel of ASTM E 84-1981a (cf. Section 3.2.1, Fig. 3.93), flame spreading is measured on a specimen with surface area of 7320 mm x 508 mm. In the horizontal vent pipe of 408 mm dia. at the outlet of the tunnel, changes in the intensity of a vertical light beam are recorded during the test procedure. The area under the intensity vs, time curve for the specimen is divided by that for a red oak specimen and multiplied by 100, to establish a numerical index for comparison of the performance of the material to that of an asbestos-cement board and of red oak, taking these as limit points of an arbitrary centesimal scale (0 and 100, respectively). 

Fig. 20.15 Filter sheet performance. The relationship with Table 20.2 is apparent from the permeability data. Surface area refers to the gross area of asbestos fibres presented to liquid on filtration and is measured in cmVg. on a Rigden apparatus [11].

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