morphology of the raw OPA changed after LENR coating.
The outer phase of the OPA (LENR) was able to behave as a
plasticizer imparting toughness to the OPA and, consequently,
reducing its rigidity. In addition, the LENR coating
may have changed the crosslink density of the natural
rubber composites, as torque variation is an indirect measure
of crosslink density [13]. This could be due to an increase
of rubber phase volume (with respect to the active
curing agent remaining constant); thus, the distance between
potential crosslinking sites became longer and
increased the mobility of the composite’s macromolecular
chains.
3.3. Mechanical properties
Table 2 presents the tensile properties and hardness of
the raw OPA and LENR-coated OPA filled NR composites.
From the results shown, the tensile strength and elongation
at break increased when a very low OPA loading was
added, but tended to reduce beyond 1 phr loading of the
OPA. The same trend was observed for the LENR-coated
OPA filled NR composites. Furthermore, the LENR-coated
OPA filled NR composites showed a higher value than
that of the raw OPA at a similar filler loading. It is deduced
that the coating layer of LENR had a positive contribution
to better adhesion by wetting the OPA more effectively
and reducing the micro-void within the raw OPA and the
NR matrix. As already noted, the LENR was able to penetrate
into the porous structure of the OPA, whilst the