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Specific Surface Area (SSA)
An alternative measure of particle size is provided by the SSA, which is the area of surface present per unit weight of the filler and usually given the units m2g~
The SSA is obviously related to the particle size distribution of the filler and with materials of the same density and shape, a higher specific surface area means a smaller particle size.
An indirect method for expressing particle size, SSA, has some advantages, especially for very small particles. Fillers in common use have SSAs ranging from under 1 to more than 200 m2g~ In some cases, the SSA is calculated from the ESD particle size information, but it is more usually measured by adsorption methods, such as nitrogen or dye adsorption. The most rigorous method is nitrogen adsorption using the BET (Brunauer, Emmett, Teller) procedure. This is a very reproducible method, but requires relatively expensive equipment not always possessed by filler producers. As nitrogen is a very small molecule, it can access surface features that may not be accessible to polymer molecules and thus give a falsely high result for practical purposes. Dye adsorption uses much larger molecules adsorbed from solution and is less expensive to carry out and can come closer to the value relevant in polymer composites. The carbon black industry uses a surfactant (CTAB, cetyltri- methylammonium bromide) for this purpose.
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