Figs. 2 and 3 individually present the impact of various
fillers on tensile modulus (M100 and M300) and hardness of
NR vulcanizates. The hardness approximately parallels the
modulus results (M100 and M300), both being improved
when the filler was incorporated. The carbon black-filled
NR vulcanizates showed the highest significant tensile
modulus and hardness compared to the silica and
OPA-filled vulcanizates. The same reason as for tensile
strength may be used to account for the trend of modulus
and hardness in which there is high carbon black loading
(50 phr) compared to silica (10 phr) and OPA (1 phr).