158
8 Self-Reinforcement in Natural Rubber (NR): Template Crystallization
The details of this cavitation (for cavitation, see Sect. 2.6.2) are not much clear,
but the paper claims its relation with zinc oxide, namely de-cohesion from rubber
matrix, and the competition between cavitation and SIC, i.e., template crystallization.
This estimation may be assumed to be related with the general competition between
cavitation and nanofiller in rubber reinforcement. Further progress in mechanism
elucidation of template crystallization would contribute to more clarifying these
issues.
From these results, the following two facts are estimated:
(1) The crystalline ligaments, which are present at the crack tip under the tear mode
deformation, reinforce relatively weak elliptical areas.
(2) Cavitation due to poor adhesion between zinc oxide and rubber matrix induces
rupture of elliptical area at the crack tip.
Taking the two into account, a possible mechanism of crack growth is proposed
[88, 89]. This study can be a first step toward elucidating the role of template crystallization against the fatigue failure of NR vulcanizates. Separate from those experimental approaches, a large-scale numerical simulation study is reported [112]. These
types of researches are to be recommended more, not as an assistance to the empirical approach, but as an independent theoretical study. The more complicated is the
targeted phenomenon, the more important is the audacious simplification, would be
the case in coping with a technical complexity.
8.3.4 Novel Functional Properties of Cross-Linked NR
It does not need to mention that reinforcement is fundamentally a concept in mechanical properties. It has been emphasized, however, that lots of other properties are
also concerned with rubber reinforcement, self-reinforcement of NR in particular.
Functional consideration of template crystallization in dynamic friction and wear
performance of NR vulcanizates (see Sect. 8.2.3) is still one of the future issues to be
resolved in relation with a higher tire performance. Here, the unique reversibility of
template crystallization of NR vulcanizates, that is, instantaneous template crystallization followed by spontaneous melting on its contractile step is to be reconsidered.
This was displayed as a specific hysteresis loss in NR vulcanizate in Fig. 8.1. Note
that the loss is reported on NRs collected from guayule (Parthenium argentatum)
and rubber dandelion (Taraxacum kok-saghyz), too [113, 114].
As already stated, one cause of reversibility is the maximal degree of crystallization, which does not exceed 20% or so. Hence, the crystal region remains to be a
dispersed phase, definitely not a continuous phase to the rupture elongation. The hysteresis loss by the crystal melting on the contraction of stretched NRs seems to suggest
its possible applications, and one of the notable functions so far proposed is shapememory effect [115–118]. NR vulcanizates of relatively low network chain density
is recommended for the clear memory [118]. A research group in Lyon has reported
8 Self-Reinforcement in Natural Rubber (NR): Template Crystallization
The details of this cavitation (for cavitation, see Sect. 2.6.2) are not much clear,
but the paper claims its relation with zinc oxide, namely de-cohesion from rubber
matrix, and the competition between cavitation and SIC, i.e., template crystallization.
This estimation may be assumed to be related with the general competition between
cavitation and nanofiller in rubber reinforcement. Further progress in mechanism
elucidation of template crystallization would contribute to more clarifying these
issues.
From these results, the following two facts are estimated:
(1) The crystalline ligaments, which are present at the crack tip under the tear mode
deformation, reinforce relatively weak elliptical areas.
(2) Cavitation due to poor adhesion between zinc oxide and rubber matrix induces
rupture of elliptical area at the crack tip.
Taking the two into account, a possible mechanism of crack growth is proposed
[88, 89]. This study can be a first step toward elucidating the role of template crystallization against the fatigue failure of NR vulcanizates. Separate from those experimental approaches, a large-scale numerical simulation study is reported [112]. These
types of researches are to be recommended more, not as an assistance to the empirical approach, but as an independent theoretical study. The more complicated is the
targeted phenomenon, the more important is the audacious simplification, would be
the case in coping with a technical complexity.
8.3.4 Novel Functional Properties of Cross-Linked NR
It does not need to mention that reinforcement is fundamentally a concept in mechanical properties. It has been emphasized, however, that lots of other properties are
also concerned with rubber reinforcement, self-reinforcement of NR in particular.
Functional consideration of template crystallization in dynamic friction and wear
performance of NR vulcanizates (see Sect. 8.2.3) is still one of the future issues to be
resolved in relation with a higher tire performance. Here, the unique reversibility of
template crystallization of NR vulcanizates, that is, instantaneous template crystallization followed by spontaneous melting on its contractile step is to be reconsidered.
This was displayed as a specific hysteresis loss in NR vulcanizate in Fig. 8.1. Note
that the loss is reported on NRs collected from guayule (Parthenium argentatum)
and rubber dandelion (Taraxacum kok-saghyz), too [113, 114].
As already stated, one cause of reversibility is the maximal degree of crystallization, which does not exceed 20% or so. Hence, the crystal region remains to be a
dispersed phase, definitely not a continuous phase to the rupture elongation. The hysteresis loss by the crystal melting on the contraction of stretched NRs seems to suggest
its possible applications, and one of the notable functions so far proposed is shapememory effect [115–118]. NR vulcanizates of relatively low network chain density
is recommended for the clear memory [118]. A research group in Lyon has reported