Crack growth resistance, expressed as the length of
crack growth (lower length of crack growth indicates
higher crack growth resistance), is shown in Fig. 5. In this
experiment, the gum exhibits the highest crack growth
resistance. The explanation is given as its lower modulus
resulting in the lower stress concentration at its crack tip
[13,15]. For the vulcanizates having equal amounts of filler,
clay-filled vulcanizate shows the lowest crack growth
resistance followed by CB- and silica-filled vulcanizates,
respectively. It is apparent that these results correspond
well with their moduli. The highest modulus of C6 leads to
the highest stress concentration at its crack tip compared to
CB6 and S6. On the other hand, a decrease in crack growth
resistance is observed for S35 although its modulus is
similar to that of C6 and CB14. It is thought that the lower
crack growth resistance of S35 results from its poor silica
dispersion or lower filler-rubber interaction, which
overrides the effect of the modulus.
The fracture surfaces of NR vulcanizates filled with
various fillers are illustrated in Fig. 6. As can be seen, the