3. Results and discussion
Hardness and 300% modulus of all vulcanizates are
illustrated in Figs. 1 and 2, respectively. As expected, the
gum gives the lowest hardness and modulus while hardness and modulus increase noticeably when 6 phr of clay is
added to the NR. At equal amounts of filler, the clay-filled
vulcanizate exhibits the highest stiffness, followed by CB-filled and silica-filled, respectively. In addition, the
results show that 14 phr of CB or 35 phr of silica is required
for the vulcanizates to reach the same hardness level as the
vulcanizate containing 6 phr of clay. Compared to CB,
a higher amount of silica is needed in order to achieve the
same hardness level as the clay-filled vulcanizate. This is
thought to be due to the decrease in crosslink density when
high silica loading is used. In a previous study, crosslink
density of NR vulcanizates gradually decreases when silica
loading is more than 20 phr [12]. The explanation is given
as the adsorption of zinc complex on the silica surface, thus
lowering the sulfur vulcanization efficiency