CONCLUSIONS
Thermal, mechanical, and microstructural characteristics
of NBR/PH and SBR/PH blends are investigated
in this study. It is shown that the glass-transition
of both rubbers is shifted slightly at various
compositions of rubber/PH mixtures, showing partial
solubility of the PH in the rubber phase. However,
these mixtures are incompatible blends and, as
evidenced by SEM micrographs, exhibit two-phase
microstructures in which rigid spherical particles of
PH are embedded in a soft rubber matrix. The
results obtained based on Taguchi’s analysis show
that sulfur has a dominant effect on the crosslinks
structure of the vulcananized rubber, although other
curing ingredients including accelerator and activator
influence the crosslinks network more or less.
Moreover, it is shown that crosslinking density of
the rubber phase in the blend is also affected by PH.
Soluble part of the resin phase, as supported by
dynamic mechanical analysis, is shown to be responsible
for alteration of crosslinking density of the rubber
phase due to curative role of PH. On the other
hand, presence of the rigid PH chains in the crosslink
network of the rubber restricts the molecular
motion and makes the rubber phase to be stiffer
than that of pure rubber vulcanizates. Deviation of
the blend modulus versus PH content from Halpin-
Tsai model is known as a further evidence for the
role of PH on the rubber phase.
Crosslinking density of the SBR is found to be
lower than that of NBR at a certain curing agent.
This is attributed to the hindrance effect of benzene
side group of SBR which prevents more crosslinks to
be formed in the polymeric chains. Additionally,
thermal stability of SBR blends decreases noticeably
by increasing the sulfur content when compared
with that of NBR/PH. This is probably due to the
formation of more polysulfidic crosslinks in SBR
vulcanizates.