Dynamic compression set results for various NR vulcanizates
are shown in Fig. 7. It is evident that compression
set is low for the gum. When a low loading of CB or silica
(6 phr) is added, the compression set is not significantly
changed. Nevertheless, clay-filled vulcanizate gives somewhat
higher compression set than that of CB- and silicafilled
vulcanizates when equal amounts of fillers are used.
At the same hardness level, silica-filled vulcanizate (S35) shows the highest compression set when compared to that
of C6 and CB14. This is attributed to the poor filler-rubber
interaction or poor filler dispersion when the silica loading
is high. Another possible explanation is the decrease in
crosslink density when silica loading is high. This result
also agrees well with the previous study showing that the
dynamic compression set tends to increase when the silica
loading is more than 30 phr [12].