surface of OPA fi lled natural rubber compound (Fig. 3b–d)
was rougher with deeper tearing lines and angular cracking
than unfilled natural rubber compound (Fig. 3a). This indicated that more force was required to break down the
samples. Moreover, it can be clearly seen that the OPA
particles (as shown in the white circle) were well dispersed
and embedded in natural rubber, which allowed efficient
stress transfer from the natural rubber matrix. These findings are consistent with the reinforced values of tensile
properties as reported before, which indicated that more
stress was needed to propagate the crack and break the
samples. It is interesting to note that the OPA particles were
embedded in the natural rubber matrix (as highlighted by
the white eclipse in Fig. 3c). This was due to the physical
interaction of OPA particles with natural rubber matrix in
which rubber chains penetrated into the pores of OPA and
become part of the filler (bound rubber); thus resulting in
higher values of tensile properties. At higher OPA loadings,
the filler particles were still in a well dispersed condition
but tended to agglomerate, as observed in Fig. 3d. Owing to
the filler agglomerates, the detachment of filler occurred
and this explained the deterioration of tensile strength and
elongation at break at higher OPA loading.