3. Results and discussion
3.1. LOPA characterization
The FTIR spectra of raw OPA particles and LENR-coated
OPA particles are shown in Fig. 1. The characteristic vibration
bands for the raw OPA have already been noted in a
previous report [10] where the existence of Si–O, Al–O or
Si–O–Al and also calcite was confirmed. However, with the
LENR coating, the FTIR analysis in the 450–4000 cm1
indicated that the OPA functional group was changing. The
new peak observed within 2800–3000 cm1 was assigned
to the stretching vibration of CH2 and CH3 modes
from the isoprene unit of ENR itself. Also, the bands from
850–1300 cm1 tended to broaden compared to the FTIR
spectra of raw OPA. It is suggested that there was a combination
or physical interactions between the functional
group (Si–O) of OPA with the oxirane groups from the LENR
during the coating process.
The surface morphologies of raw OPA and LENR-coated
OPA were studied and are compared in Fig. 2. The raw OPA
typically showed irregular shapes with a rough surface and
a porous structure. The surface of the ash is capable of
absorption, and thus the LENR would penetrate into the
porous structure of the OPA during the coating process and
form a coating layer. This LENR layer was believed to form
an interphase within OPA and NR, thereby improving the
compatibility between OPA and NR matrices. Unfortunately,
the LENR coating method used may lead to an
agglomeration of OPA (as shown in Fig. 2b), which may
cause difficulties in the compounding process.
3.2. Curing characteristics
Figs. 3–5 show the curing characteristics i.e., scorch time
(ts2), cure time (tc90) and torque variation (MH – ML) of
LENR-coated OPA filled NR composites, and compares them
to the raw OPA filled NR composites (control). The ts2 and
tc90 of the OPA filled NR composites showed a decrease
whilst the torque variation increased when the OPA loading
was increased for both raw OPA and LENR-coated OPAs. At
the same filler loading, the curing characteristics of LENRcoated
OPA filled NR composites were found to be lower
than that of raw OPA filled NR composites. This may be
attributed to the adjacent double bond by the oxirane
group from ENR, which was accelerated in the vulcanization
reaction [11,12]. Even although the addition of a
compatibilizer was expected to improve the torque variation
of composites; the LENR coating showed a contrasting
effect by lowering the torque variation of the OPA filled NR
composites. This was mainly caused by softening of the
composites due to the ratio of the rubber phase volume to
raw filler being increased when LENR-coated OPA was
added. This is clearly shown in Fig. 2, where the surface