2.4. Composite preparation
The OPA loading levels used in this study were 0.5,1, 3, 7,
and 9 phr. The detailed formulations are shown in Table 1.
Mixing of raw materials was done using a conventional
laboratory sized two-roll mill (model XK 160). A total
mixing time of about 20 minutes avoided premature
vulcanisation due to excessive heat generated during
compounding. The mixing sequences were kept constant
for all mixes. Curing characteristics, such as scorch time
(ts2), cure time (tc90) and maximum torque (MH), of OPA
filled NR composites were determined using a Monsanto
Moving Die Rheometer (model MDR 2000); according to
ISO 3417, at 150 C, followed by compression moulding at
150 C according to their respective tc90 values. The moulded
sheets were conditioned in desiccators for 24 hours
prior to further testing. The LENR-coated OPA was designated
as LOPA and compounded in the same manner as raw
OPA filled NR composites (control).
2.5. Mechanical tests
Tensile testing of five dumbbell shaped specimens cut
from moulded sheet with a thickness of about 2 mm was
conducted according to ISO 37, using a universal tensile
testing machine (model Instron 3366) at a crosshead speed
of 500 mm/min. The tensile strength, elongation at break,
and tensile modulus (M100 and M300) were evaluated from
stress-strain determinations and the averages of the five
repeated tests for each compound were recorded. Hardness
test measurement was conducted using a Shore A type
durometer under conditions according to ISO 7619.