properties of the blends by performing a few number
of experiments. The trend of influence of a factor
can be determined by calculating the average effect
of a factor at a level. This latter one is obtained by
simple statistical calculation as follows25:
Ai ¼
P
n
j¼1 yj
i
n
(1)
where Ai is the average effect of factor A at level i, yj
i
represents the j-th observation (result) of factor A at
level i, and n stands for total number of observations
for factor A. By plotting the average factor effect
against the corresponding factor level, an average
plot is obtained from which the trend of influence of
a factor on the result is extracted. The relative influence
of factors (contribution) on the result can also
be obtained by somewhat more rigorous statistical
calculations known as analysis of variance
(ANOVA), see the details in Ref. 25. In this study,
calculations of both ANOVA and average factorlevel
effect are performed by using QT4 software.25
Preparation of compounds
The compounds are prepared by a laboratory scale
two-roll mill. The rubbers are first masticated at temperature
around 508C for 5 min. Then the masticated
rubbers are mixed further with PH and other ingredients
for around additional 25 min. The obtained
compounds are cured into a sheet using compression
molding at 1508C for 1 h under a pressure of 3.5
MPa in accordance to ASTM D-3182. The samples
required for mechanical and physical tests are cut
from the cured sheets.
Thermal analysis
Differential scanning calorimeter (DSC, Pyris 1, Perkin–
Elmer) is used to investigate the thermal events
occurred during the heating of the cured compounds.
To do this, DSC analysis is performed at a
heating rate of 208C/min under the nitrogen with
flow rate of 200 mL/min. Figure 1 shows the DSC
thermogram of typical compounds. As seen, there is
an endothermic peak in the graphs. This peak corresponds
to the maximum rate of mass loss of the
compounds, and in this study, it is taken as their
thermal stability.
Mechanical testing
The stress–strain (r–e) properties of the cured dumbbell-
shaped specimens are determined at room temperature
according to ASTM D412 by using an Instron
tensile testing machine, operated at a crosshead
speed of 60 mm/min. The tensile strength and elongation
at break are extracted directly from the r-e
curves, while the young’s modulus is calculated
from the initial slope of the curve, namely slope of
the curve within strain range of 0–2%. Dynamic mechanical
properties of the mixtures are determined
by a dynamic mechanical thermal analyzer (DMTATriton,
Tritic Technology, England) at a frequency of
1 Hz and temperature range from 2150 to 1808C
with a heating rate of 38C/min. The DMTA records
the variation of storage modulus (E0) and loss tangent
(tan d) with temperature.