NR composites are shown in Fig. 7. It can be clearly seen
that the LENR-coated OPA (Fig. 7b and d) was wellembedded
in the natural rubber matrix, compared to the
raw OPA filled NR composites (Fig. 7a and c). It was interesting
to note that the bridging rubber chain that penetrated
the OPA could be observed in the LENR-coated OPA
filled NR composites at 1 phr and even at 7 phr LENRcoated
OPA loading. This indicates why higher tensile
strength and elongation at breakwas observed when LENRcoated
OPA was added into the NR matrix.
3.5. Swelling assessment
Fig. 8 shows the swelling of the raw OPA and LENRcoated
OPA filled NR composites. As reported in our previous
study [7], the addition of OPA could restrict the
toluene solvent from penetrating into the NR composites.
However, in this study, the LENR-coated OPA filled NR
composites exhibited higher degrees of swelling than the
corresponding loading of raw OPA filled NR composites.
The higher absorption of toluene solvent was mainly
because the rubber phase volume was increased in the
LENR-coated OPA/NR system. The crosslink density was
evaluated according to the Flory Rehner equation [8] (as
shown in Fig. 8b). It was found that the crosslink density of
the OPA filled NR composites increased when the raw OPA
or LENR-coated OPA loading was increased. However, the
LENR-coated OPA filled NR composites indicated a lower
crosslink density than that of the raw OPA filled NR composites.
This observation was in line with the torque variation
results discussed earlier. The same reason may be
used to account for the molecular weight between crosslink
points in the rubber phase becoming longer; and,
consequently, inducing a more flexible chain in the rubber
network to allow a higher toluene uptake.
4. Conclusions
The following conclusions can be drawn for the effectiveness
of using LENR-coated OPA in natural rubber composites,
instead of raw OPA. The FTIR results show the
changes in the peaks of OPA spectra after LENR coating. The
scorch and cure times were reduced with the addition of
LENR-coated OPA, more than with the raw OPA. The torque
variation, tensile modulus, hardness and crosslink density
of LENR-coated OPA filled NR composites showed lower
values than those of raw OPA. Increases of tensile strength
and elongation at break were observed due to the formation
of a LENR interphase between OPA and NR matrixes,
which helped penetration into the porous-structured OPA
in order to make a stronger interfacial adhesion, whilst at
the same time imparting a more flexible chain. The better
physical interaction between LENR-coated OPA and the NR
matrix was demonstrated by SEM observation.