2.6 Some More Remarks on Rubber Reinforcement
37
They emphasized that the deformation necessarily produces cavity between filler
surface and rubber matrix except the case of the perfect adsorption. This cavitation is
to be considered the negative effect on reinforcement. After all, their theory formulated three factors for rubber reinforcement, hydrodynamic volume effect, adsorption
effect (bound rubber), and cavitation effect. Among the three, the last one is negative,
and the sum of the three is to give the reinforcement effect. Therefore, there can be
the minus reinforcement when the cavitation abounds.
Unhappily, three reasons prevent the theory from being applied to actual rubber–filler systems: First, the proposed equations contain parameters that cannot be
compared with experimental results. Second, proposed anisotropic behaviors of the
rubber–filler systems are not much common. Third, cavitation has not much observed
except calcium carbonate particle under limited conditions. Due to these reasons,
perhaps, few paper has checked this theory. On the significance of cavitation, A.
N. Gent recognized its significance [151], and recently, reviews have been published [152, 153]. Also, a report claimed the completion of cavitation and SIC in
self-reinforcement of NR [154], and cavitation was reported on nanosilica-loaded
cis-1,4-polybutadiene [155]. Particularly, on the fatigue failure and some other breakdown behaviors, cavitation has to be checked more. Focused on cavitation is one of
the credits of the Sato–Furukawa treatment. But, from the viewpoint of rubber reinforcement, they failed to recognize the structuring of nanofillers. Hence, their theory
may be regarded to be of use for explaining the behaviors of non-reinforcing fillers,
which are not forming agglomerates.
2.6.3 Rubber Mixing and Nanofiller Aggregate
The change in structuring of nanofiller during mechanical rubber mixing is to be
considered at this subsection. As explained at 2.5.3, commercially available furnace
CB after the manufacturing is fundamentally primary aggregate (see Fig. 2.4). During
the storage, it may be clustered into the higher aggregates. Figure 2.4 shows the TEM
image of them. This raw CB is subjected to rubber processing, and ‘how would be
the changes of CB aggregates during the mixing?’ is to be considered a little here.
Figure 2.1 shows a flowchart of the rubber processing. Actually, the rubber processing
is a kind of reactive processing since it contains vulcanization (chemical reaction)
step. However, this aspect is not taken into account in the present discussion.
There have been discussions on ‘whether these aggregates are decomposed to the
primary aggregate or even to a particulate level or not’ by the mechanical mixing.
Empirically, disaggregation has been clearly recognized together with further aggregation. The problem had been decomposing to the particulate level or not. Generally
accepted is the decomposition of the higher aggregates to the primary aggregates, not
to the independent particulate [156–158]. The long title of Ref. [156] is as follows:
Milling Black Reinforced Elastomers: Contrary to previous belief, the fused carbon chains
forming the persistent structure of high structure carbon blacks do not break down on milling
the filler elastomers.
37
They emphasized that the deformation necessarily produces cavity between filler
surface and rubber matrix except the case of the perfect adsorption. This cavitation is
to be considered the negative effect on reinforcement. After all, their theory formulated three factors for rubber reinforcement, hydrodynamic volume effect, adsorption
effect (bound rubber), and cavitation effect. Among the three, the last one is negative,
and the sum of the three is to give the reinforcement effect. Therefore, there can be
the minus reinforcement when the cavitation abounds.
Unhappily, three reasons prevent the theory from being applied to actual rubber–filler systems: First, the proposed equations contain parameters that cannot be
compared with experimental results. Second, proposed anisotropic behaviors of the
rubber–filler systems are not much common. Third, cavitation has not much observed
except calcium carbonate particle under limited conditions. Due to these reasons,
perhaps, few paper has checked this theory. On the significance of cavitation, A.
N. Gent recognized its significance [151], and recently, reviews have been published [152, 153]. Also, a report claimed the completion of cavitation and SIC in
self-reinforcement of NR [154], and cavitation was reported on nanosilica-loaded
cis-1,4-polybutadiene [155]. Particularly, on the fatigue failure and some other breakdown behaviors, cavitation has to be checked more. Focused on cavitation is one of
the credits of the Sato–Furukawa treatment. But, from the viewpoint of rubber reinforcement, they failed to recognize the structuring of nanofillers. Hence, their theory
may be regarded to be of use for explaining the behaviors of non-reinforcing fillers,
which are not forming agglomerates.
2.6.3 Rubber Mixing and Nanofiller Aggregate
The change in structuring of nanofiller during mechanical rubber mixing is to be
considered at this subsection. As explained at 2.5.3, commercially available furnace
CB after the manufacturing is fundamentally primary aggregate (see Fig. 2.4). During
the storage, it may be clustered into the higher aggregates. Figure 2.4 shows the TEM
image of them. This raw CB is subjected to rubber processing, and ‘how would be
the changes of CB aggregates during the mixing?’ is to be considered a little here.
Figure 2.1 shows a flowchart of the rubber processing. Actually, the rubber processing
is a kind of reactive processing since it contains vulcanization (chemical reaction)
step. However, this aspect is not taken into account in the present discussion.
There have been discussions on ‘whether these aggregates are decomposed to the
primary aggregate or even to a particulate level or not’ by the mechanical mixing.
Empirically, disaggregation has been clearly recognized together with further aggregation. The problem had been decomposing to the particulate level or not. Generally
accepted is the decomposition of the higher aggregates to the primary aggregates, not
to the independent particulate [156–158]. The long title of Ref. [156] is as follows:
Milling Black Reinforced Elastomers: Contrary to previous belief, the fused carbon chains
forming the persistent structure of high structure carbon blacks do not break down on milling
the filler elastomers.