suggests that the BFA filler did not reinforce the PSi or CB filled NR
vulcanizates. However, when taking a closer look at the total filler
of 15 phr in Figs. 12 and 13, the addition of BFA was of great benefit
in reducing the primary filler content. In other words, BFA filler
could be added into the NR vulcanizates to up to 75% of the total
filler volume (up to a ratio of 75:25 of BFA:PSi and BFA:CB) without
worsening the tensile strength and elongation at break. Excess BFA
content in the hybrid filler probably caused a worsening effect on
the strength of the rubber as a result of the non-silica content or
metal oxides in the BFA filler, the metal oxide content in the BFA
filler being about 24.6 wt.% (see Table 2). The decreased mechanical
strength of the vulcanizates was also caused by the porosities of
the BFA particles [39] as shown in Fig. 8d. Having compared the
changes in tensile strength and elongation at break for BFA:PSi
and BFA:CB vulcanizates, it was found that the addition and increase
of BFA content in CB was more sensitive to the decreases