attributed to the smallest average particle size of 4.60 mm
and highest specified surface area. However, the incorporation
of OPA produces the highest cure rate indices over
silica and carbon black. This is owed to the mineralogical
composition in OPA where various metal elements accelerate
the curing rate of rubber compound. The higher ts2
and tc90 of OPA filled vulcanizates compared to carbon
black-filled vulcanizates may be attributed to low OPA
loading (1 phr) compared to carbon black loading (50 phr).
3.2. Tensile properties
Fig. 1 illustrates the tensile strength and elongation at
break of the gum vulcanizates and the comparison between
various filled vulcanizates with respective optimum
loading. Ismail et al. [10] reported that a smaller particle
size normally gives better interaction with a natural rubber
matrix. The carbon black has the smallest particle size
(Table 1), thus the carbon black-filled NR vulcanizates
showed the highest tensile strength followed by OPA and
silica-filled vulcanizates. However, it is worth noting that
the optimum tensile strength of carbon black-filled natural
rubber vulcanizates was reached when filler loading was at
50 phr but this result is comparable with OPA-filled vulcanizates
where only 1 phr of OPA was used in this study,
even although the particle size of OPA was coarser than that
of silica and carbon black, as listed in Table 1. This may be
attributed to the different inherent properties of the filler
itself that correlate with the interaction between filler and
NR. The variations in strength with different fillers may be
due to morphology and functional groups of the filler as