8.3 Self-Reinforcement Behavior of NR
151
8.3 Self-Reinforcement Behavior of NR
8.3.1 Tensile Properties
Upon elongation over the onset stain, NR vulcanizates display a morphology of
dispersed shish-type crystallites in rubber matrix as explained so far. The detailed
morphology is still to be investigated further, but the assumed structure shown in
Fig. 8.5 at the right-hand side, may satisfy the three requirements for reinforcing
nanofillers:
(1) Diameter of the elementary particles or that of the primary aggregates is
definitely smaller than 1 μm,
(2) The elementary particle or the primary aggregate is accompanied by bound
rubber to be compatible with (wetted by) the rubber matrix,
(3) Finally, forming a more or less flexible network-like structure as an agglomerate
in the rubber matrix.
Hence, it is possible that the shish-type crystallites formed by SIC are estimated
to be functioning as nanofillers, i.e., they seem practically to be an oriented nanofiller
generated in situ [1, 39, 40, 62–73]. This parable has already been used in lots of
introductory textbooks on rubber technology. For example, a Japanese book entitled
‘The Basics of Rubber Technology’ [84] gives the following explanation at p. 54:
SIC introduces rigid crystallites in the rubber vulcanizate, which displays reinforcing effect.
In other words, due to the increase of melting temperature by elongation, which results in the
crystallization, the tensile strength of NR is as high as that of carbon black loaded SBR, NBR,
or any uncrystallizable rubbers, even without any fillers. (Translation from the Japanese text
by the present authors)
Such descriptions without presenting much evidence have been a commonplace
event, particularly among rubber engineers. As mentioned, the higher tensile strength
of NR seems to have been regarded to be at the center of self-reinforcement effect
of NR. In fact, Fig. 8.7 shows it very clearly, though the curves are not of simple
tensile but of hysteresis loss measurement [85]: The elongation was discontinued just
before the breakdown, and the contractile return curves are recorded in the figure.
The tensile strength is just a little higher than the top of these curves. In the first place,
tensile strength (T B ) of NR (pure gum stock, without any fillers) is near to 20 MPa,
which is more than 10 times of SBR. Secondly, there observed not much difference
between the two rubbers in the elongation at break (E B ) and in the tensile curves
begin to diverge only after the middle deformation region. Thirdly, it is notable that
the elongation ratio (λ), λ = 4.0, is approximately the onset strain of SIC (see Fig. 8.4
at the left-hand side). It is reasonably estimated that NR is self-reinforcing due to
SIC, while SBR is not because of the lack of SIC ability.
Compounding 50 phr of CB to SBR (SBR + 50 phr CB), however, has brought
about a high T B , comparable to that of NR. Comparison of NR and SBR + 50 phr CB
reveals that CB mixing has provided SBR with an equivalent tensile behavior to NR.
The observed hysteresis losses are also comparable (that of NR is a little smaller).
151
8.3 Self-Reinforcement Behavior of NR
8.3.1 Tensile Properties
Upon elongation over the onset stain, NR vulcanizates display a morphology of
dispersed shish-type crystallites in rubber matrix as explained so far. The detailed
morphology is still to be investigated further, but the assumed structure shown in
Fig. 8.5 at the right-hand side, may satisfy the three requirements for reinforcing
nanofillers:
(1) Diameter of the elementary particles or that of the primary aggregates is
definitely smaller than 1 μm,
(2) The elementary particle or the primary aggregate is accompanied by bound
rubber to be compatible with (wetted by) the rubber matrix,
(3) Finally, forming a more or less flexible network-like structure as an agglomerate
in the rubber matrix.
Hence, it is possible that the shish-type crystallites formed by SIC are estimated
to be functioning as nanofillers, i.e., they seem practically to be an oriented nanofiller
generated in situ [1, 39, 40, 62–73]. This parable has already been used in lots of
introductory textbooks on rubber technology. For example, a Japanese book entitled
‘The Basics of Rubber Technology’ [84] gives the following explanation at p. 54:
SIC introduces rigid crystallites in the rubber vulcanizate, which displays reinforcing effect.
In other words, due to the increase of melting temperature by elongation, which results in the
crystallization, the tensile strength of NR is as high as that of carbon black loaded SBR, NBR,
or any uncrystallizable rubbers, even without any fillers. (Translation from the Japanese text
by the present authors)
Such descriptions without presenting much evidence have been a commonplace
event, particularly among rubber engineers. As mentioned, the higher tensile strength
of NR seems to have been regarded to be at the center of self-reinforcement effect
of NR. In fact, Fig. 8.7 shows it very clearly, though the curves are not of simple
tensile but of hysteresis loss measurement [85]: The elongation was discontinued just
before the breakdown, and the contractile return curves are recorded in the figure.
The tensile strength is just a little higher than the top of these curves. In the first place,
tensile strength (T B ) of NR (pure gum stock, without any fillers) is near to 20 MPa,
which is more than 10 times of SBR. Secondly, there observed not much difference
between the two rubbers in the elongation at break (E B ) and in the tensile curves
begin to diverge only after the middle deformation region. Thirdly, it is notable that
the elongation ratio (λ), λ = 4.0, is approximately the onset strain of SIC (see Fig. 8.4
at the left-hand side). It is reasonably estimated that NR is self-reinforcing due to
SIC, while SBR is not because of the lack of SIC ability.
Compounding 50 phr of CB to SBR (SBR + 50 phr CB), however, has brought
about a high T B , comparable to that of NR. Comparison of NR and SBR + 50 phr CB
reveals that CB mixing has provided SBR with an equivalent tensile behavior to NR.
The observed hysteresis losses are also comparable (that of NR is a little smaller).