3.2. M300
Figs. 4 and 5 show the effect of filler loading and type
on M300 (tensile stress at 300% elongation) of SMR
L/ENR 25 and SMR L/SBR blends. Both figures indicate
that M300 for carbon black- and silica-filled SMR
L/ENR 25 and SMR L/SBR blends increases with
increasing filler loading. In the case of calcium carbon-
ate, M300 is virtually independent of the filler loading,
indicating that calcium carbonate is non-reinforcing
filler. Again, for the range of filler loading studied, car-
bon black consistently exhibits the highest M300 followed by silica and calcium carbonate. M300 is affected
by several factors such as surface reactivity which deter-
mines the polymer–filler interaction, aggregates, size and
shape of particles, structure and filler particle dispersion
in rubber ([5], p. 342; [6]). For carbon black, the interac-
tion between filler and hydrocarbon rubber is stronger
than its attraction between aggregates. However, the
aggregate interaction is stronger in the case of silica and
calcium carbonate ([5], p. 342). Also, as mentioned earl-
ier, carbon black has larger surface area than silica and
calcium carbonate. This explains why carbon black-filled
SMR L/ENR 25 and SMR L/SBR blends have highest
M300 followed by silica and calcium carbonate. Fig. 6
shows the comparison of M300 of SMR L/ENR 25 and
SMR L/SBR blends at 40 phr of filler loading. As in the
case of Fig. 3 for tensile strength, SMR L/ENR 25 blend
exhibits higher M300 than SMR L/SBR blend. From this
observation, it is obvious that the ability of mutual rub-
ber reinforcement—as indicated by the former rubber
blend—and a better rubber–filler interphase interaction
are the two important factors in determining the strength
of filled rubber–rubber blends.
3.3. Elongation at break
The dependence of elongation at break of SMR
L/ENR 25 and SMR/SBR blends on the filler loading is
shown in Figs. 7 and 8, respectively. It is obvious that
for both blends, elongation at break increases steadily
for the calcium carbonate filled blends, whereas for the
carbon black- and silica-filled systems, it decreases
gradually with increase in the filler loading. This obser-
vation may be attributed to the non-reinforcement nature
of calcium carbonate as explained earlier for tensile
strength and M300. Calcium carbonate-filled blend
extends much more before rupture occurs and this extra
extension increases with increasing calcium carbonate
due to the increasing dilution effect imparted by calcium
carbonate as the filler loading is increased. On the con-