commmercially, the alteration of a rubber formulation is usually made in such a way as to
keep the hardness of the rubber product constant. This is because a specific hardness of the
rubber product sets the limit to its practical applications. Therefore, in this paper, natural
rubber (NR) vulcanizates containing various fillers were prepared to have the same
hardness level, and their mechanical properties were compared and related to the degree
of filler dispersion. The results show that higher amounts of carbon black (CB) and silica
are needed for CB- and silica-filled natural rubber vulcanizates to achieve the same
hardness value as a NR vulcanizate containing 6 phr of montmorillonite clay. At equal
loading of fillers, clay-filled vulcanizate exhibits higher modulus, hardness, tensile strength
and compression set, but lower heat build-up resistance and crack growth resistance than
those of the vulcanizates containing conventional fillers. For the vulcanizate having the
same hardness value, CB-filled vulcanizate gives the better overall mechanical properties
followed by the clay-filled and silica-filled vulcanizates, respectively. The explanation is
given as the better dispersion of carbon black, as can be seen in the SEM micrograph