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Important Filler Properties
Filled elastomer properties are all influenced by different combinations of the following filler properties:
Filler shape, including a property known as structure
Filler to elastomer adhesion
Size and Specific Surface Area
While particle size is an easily understood concept for most particulate fillers, some explanation of its meaning is needed for the types of filler most widely used in elastomers, such as carbon black and synthetic silicas. Both of these are made up of very small primary particles, fused together into larger shapes (aggregates). The particle size referred to for these types of filler is normally the primary size, as determined by techniques such as electron microscopy, even though they will not be dispersed down to such particles in elastomer compounds.
Specific surface area is frequently quoted instead of, or as well as, particle size. For those fillers that do not form aggregates this is reasonable, as there is usually a good correspondence between particle size and specific surface area. With aggregating fillers caution is needed, as the level of aggregation will moderate the specific surface area. Nitrogen adsorption is the specific surface area test most commonly used for fine particulate fillers (usually designated as the BET surface area). While this is still used for fine fillers used in elastomers, one will also encounter another method using cetyl trimethylammonium bromide and known as the CTAB method. The logic behind the use of this much larger molecule is that the very fine fillers can contain surface accessible to the small nitrogen molecule, but not to CTAB and the latter gives a truer picture of the surface accessible to elastomer molecules. The CTAB area is usually less than the BET one, with the difference being regarded as an indication of the porosity of the particle.
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