2.3. Determination of tensile and tear properties
All the test specimens were compression molded at
150 °C using the respective cure times determined from
the MDR 2000. For the tensile experiment, dumbbell
samples were cut from a 2 mm thick molded rubber
sheet. The gauge length and width of the dumbbell was
33 ± 2 and 6.3 ± 0.1mm respectively. ASTM D 412-92
(Test Method A) was adopted for the tensile testing pro-
cedure of the rubber samples. In the case of tear pro-
perty, the testing was conducted according to ASTM D
624-91. Crescent test sample was prepared by using Die
B cutter. A nick of 0.50 0.05 mm depth was cut by
using a sharp razor blade. A Monsanto Tensometer
(Model T10) operating at 50 cm/min was used through-
out the tensile and tear measurements.
3. Results and discussion
3.1. Tensile strength
The effect of filler loading and type on tensile strength
of SMR L/ENR 25 and SMR L/SBR blends is shown in
Figs. 1 and 2, respectively. It can be seen in both figures
that the tensile strength for carbon black- and silica-filled
SMR L/ENR 25 and SMR L/SBR blends increases with
increasing filler loading whereas for calcium carbonate
it shows a decreasing trend. At a fixed filler loading, car-
bon black exhibits the highest value followed by silica
and calcium carbonate. As indicated in Table 1, carbon
black (N330) has larger surface area than silica (Vulcasil
C ) and calcium carbonate. It is well known that the
larger the surface area of particulate filler, the greater the
interaction between the filler and rubber matrix. For sil-
ica-filled SMR L/ENR 25 and SMR L/SBR blends, the lower tensile strength than carbon black-filled blends was
due to smaller surface area, weak silica–hydrocarbon
phase interaction and poor filler dispersion compared to
carbon black ([3], p. 229; [4]). The decreasing trend and
lowest value of calcium carbonate-filled SMR L/ENR 25
and SMR L/SBR blends was due to the smaller surface
area of calcium carbonate which gives rise to a weaker
interaction between the filler and rubber matrix and thus
acts as a non-reinforcing filler. The comparison of tensile
strength of SMR L/ENR 25 and SMR L/SBR blends at
40 phr of filler loading is shown in Fig. 3. It can be seen
that for all fillers, the tensile strength for SMR L/ENR
25 blend is higher than for SMR L/SBR blend. This
observation might be due to the combined effect of SMR
L and ENR 25, both of which undergo strain-induced
crystallization upon stretching. The two rubbers
reinforce each other when subjected to a tensile test. The other possibility might be better filler–rubber interphase
interaction in SMR L/ENR 25 blends compared with
SMR L/SBR blends, particularly between silica and
ENR 25.