Swelling characteristics
Knowledge of the swelling behavior of crosslinked
polymer can provide further insight into the crosslinks
microstructure, particularly for two-phase systems.
Figure 2 shows the variations of the swelling
coefficient against concentration of rubber curing
ingredients and PH for both SBR/PH and NBR/PH
blends. From Figure 2(a), it is observed that sulfur is
the most important parameter influencing the swelling
coefficient of the SBR/PH blends. As the swelling
coefficient of the rubber phase is dominated by
the crosslinking density,27–29 this result suggests that
increasing the sulfur content within the range of 2–
10 phr increases strongly the crosslinking density of
the SBR. Comparing Figure 2(a,b), it is found that
the extent of swelling and variation of swelling coefficient
of NBR/PH are less than those of SBR/PH,
but the sulfur has still higher influence with respect
to the other curing ingredients. This observation suggests
that the crosslinking density of the NBR does
not vary significantly as much as the SBR.
Figure 2(c) shows that the swelling coefficient
decreases by increasing the resin content. On the
other hand, the maximum extent of reduction in
swelling coefficient of both rubbers within the resin
content of 0–40 phr is approximately the same, i.e.,
extent of reduction is about 0.5 mL/g for both rubbers.
This behavior can be explained by the fact that
the rubber component is responsible for swelling of
the rubber/PH blends. Because, PH is a highly
crosslinked thermoset and can not be swelled anymore.
This explanation is confirmed by performing
separately swelling experiment for cured PH so as to
no swelling effect appears. In fact, the swelling of
PH is negligible and it acts as rigid filler. So incorporation
of PH decreases the content of rubber component
in the blend, leading to reduction of the swelling
coefficient of the blends, accordingly.
Table III presents the contribution of the components
of the blend (relative importance of the factors)
on the swelling coefficient. It is found that sulfur is
the only dominant parameters on the swelling coefficient
of the SBR/PH blends. However, for the NBR/
PH, sulfur and PH have almost the similar importance.
This could be attributed to lower extent of
swelling of the NBR vulcanizate due to its higher
crosslink density, as discussed earlier. On the other
hand, contribution of other curing ingredients,
namely MBTS, ZnO, and strearic acid in SBR/PH
blend is negligible, while these factors slightly affect
the NBR/PH blend. Slight influence of accelerator