Carbon Black Dispersion in Rubber. Effect of Fat Acids

Whether or not stearic acid assists in dispersing carbon black in rubber has not been settled. Blake was the first to claim that fat acids are dispersing agents for carbon black. He postulated a mechanism, based on Langmuir's work, with monomolecular films of fat acids on water and calculated that the fat acids naturally present in Hevea rubber, which he stated to be present to the extent of 2 per cent, are just sufficient to form a monomolecular film on each particle of 30 volumes of channel black in 100 volumes of rubber. He assumed that this monomolecular film serves to facilitate wetting the individual particles with rubber and prevents agglomeration or flocculation of the particles. Later developments, however, have invalidated Blake's calculations. Blake assumed that the diameter of the average channel-black particle is 200µ, whereas electron photomicrographs have shown that the actual average particle diameter is 28µ. Therefore, 30 volumes of channel black have about seven times the surface area calculated by Blake. The fat acids in Hevea rubber, even if present to the extent of 2 per cent and entirely concentrated on the surface of the particles, are insufficient to cover completely the surface of this quantity of channel black.