22
2 Filler and Rubber Reinforcement
While these analytical methods had been developed and applied to CB characterization for a long time, the interpretation of the results was often too complex and
ambiguous, or even suspicious enough to give a clear conclusion. Main reason of this
ambiguity is ascribable to the structuring of CB, i.e., primary aggregates of CB have
variously further associated to higher aggregates and/or agglomerates, depending on
the conditions of their handlings and storages. It is essential not only to improve
the analytical methods but also to control and standardize the handling and storage
methods if we reconsider the somewhat mixed-up results of past few decades. Surprisingly, when CB (the primary aggregate) is mechanically well mixed into rubber,
it exists as the aggregate with bound rubber, that is, the surface of primary CB aggregate is encircled by bound rubber (see 2.6.3). Here, the bound rubber formation is
apparently working as a compatibility enhancer of the primary CB aggregates in
rubber matrix (see 2.5.2 and 2.5.3).
At the time when the analyses from the surface science viewpoint were facing
difficulty, surface chemical modifications on CB did not make much progress, too
[51]. The surface modification had been thought to promote reinforcement since
the reinforcement of rubber by CB was once assumed due to the chemical reaction
between CB and rubber [52]. In those days, channel black was used, which was
chemically active due to the presence of, e.g., acidic phenolic hydroxyl group on
its surface. The chemical interaction with basic species was to be the reason of
reinforcement effect. From the 1950s on, use of CB has changed from channel and/or
thermal black to furnace black. The chemically functional groups on the furnace black
are much less than channel or thermal [48, 50], and reports on chemical modifications
of CB have rapidly decreased. In this connection, it is notable that the CB change from
acidic channel to basic furnace has brought about that of vulcanization accelerator,
too. Use of acidic channel CB was in harmony with the standard accelerators which
were basic. Newly appeared furnace black’s surface is basic, and it was found that the
delayed-action accelerators, e.g., sulfenamides were the preferred choice for scorchfree vulcanization of rubber. At present, sulfenamides are the standard accelerator
for the vulcanization of CB-loaded diene rubbers (NR, IR, SBR, and BR).
The effect of CB on the properties, especially the results of tensile measurements
of CB-loaded rubber vulcanizates have been reported in enormous number of publications. Even the number of those cited in Refs. [1–12, 14, 15, 37–39, 48] may be too
large to be scrutinized. In addition, many of the discussions on tensile properties or
the proposals of reinforcement mechanism have been based on the curves obtained at
a certain strain rate at a certain temperature (mostly at room temperature). Extensive
measurements at various strain rates and at various temperatures have been found
only in a few or several papers (additionally, most of them were published in 1950s
and 1960s, definitely not recently). Recent ones are usually the measurements based
on the ISO (or ASTM, BA, DIN, JIS, and any other equivalent) standard. They are
for the evaluation of commercial quality, not definitely for the scientific discussions.
In the previous book [12], we have cited a physicist’s advice given long time ago
[53]. It is worthy to cite again here:
2 Filler and Rubber Reinforcement
While these analytical methods had been developed and applied to CB characterization for a long time, the interpretation of the results was often too complex and
ambiguous, or even suspicious enough to give a clear conclusion. Main reason of this
ambiguity is ascribable to the structuring of CB, i.e., primary aggregates of CB have
variously further associated to higher aggregates and/or agglomerates, depending on
the conditions of their handlings and storages. It is essential not only to improve
the analytical methods but also to control and standardize the handling and storage
methods if we reconsider the somewhat mixed-up results of past few decades. Surprisingly, when CB (the primary aggregate) is mechanically well mixed into rubber,
it exists as the aggregate with bound rubber, that is, the surface of primary CB aggregate is encircled by bound rubber (see 2.6.3). Here, the bound rubber formation is
apparently working as a compatibility enhancer of the primary CB aggregates in
rubber matrix (see 2.5.2 and 2.5.3).
At the time when the analyses from the surface science viewpoint were facing
difficulty, surface chemical modifications on CB did not make much progress, too
[51]. The surface modification had been thought to promote reinforcement since
the reinforcement of rubber by CB was once assumed due to the chemical reaction
between CB and rubber [52]. In those days, channel black was used, which was
chemically active due to the presence of, e.g., acidic phenolic hydroxyl group on
its surface. The chemical interaction with basic species was to be the reason of
reinforcement effect. From the 1950s on, use of CB has changed from channel and/or
thermal black to furnace black. The chemically functional groups on the furnace black
are much less than channel or thermal [48, 50], and reports on chemical modifications
of CB have rapidly decreased. In this connection, it is notable that the CB change from
acidic channel to basic furnace has brought about that of vulcanization accelerator,
too. Use of acidic channel CB was in harmony with the standard accelerators which
were basic. Newly appeared furnace black’s surface is basic, and it was found that the
delayed-action accelerators, e.g., sulfenamides were the preferred choice for scorchfree vulcanization of rubber. At present, sulfenamides are the standard accelerator
for the vulcanization of CB-loaded diene rubbers (NR, IR, SBR, and BR).
The effect of CB on the properties, especially the results of tensile measurements
of CB-loaded rubber vulcanizates have been reported in enormous number of publications. Even the number of those cited in Refs. [1–12, 14, 15, 37–39, 48] may be too
large to be scrutinized. In addition, many of the discussions on tensile properties or
the proposals of reinforcement mechanism have been based on the curves obtained at
a certain strain rate at a certain temperature (mostly at room temperature). Extensive
measurements at various strain rates and at various temperatures have been found
only in a few or several papers (additionally, most of them were published in 1950s
and 1960s, definitely not recently). Recent ones are usually the measurements based
on the ISO (or ASTM, BA, DIN, JIS, and any other equivalent) standard. They are
for the evaluation of commercial quality, not definitely for the scientific discussions.
In the previous book [12], we have cited a physicist’s advice given long time ago
[53]. It is worthy to cite again here: