108
6 Particulate Silica Reinforcement of Rubber
did not spread much due to the difficulty of homogeneous dispersion (then, a simple
optical observation of the surface was employed for judging good or bad of the filler
dispersion) of silica compared with that of CB under the same mixing conditions.
Historically and technically, they were working on nanoparticles but knew little of the
nanolevel science which was still to be established then. However, it is not essential
in terms of structuring the tendency of the nanofiller, to differentiate between CB and
silica. Relative comparison between rubber-to-filler and filler-to-filler interactions is
more important: Rationalization of this statement has been given in several relevant
descriptions in Parts 1 and 2 in this book.
One more drawback of the silica compound was the delay of vulcanization, which
was correctly estimated due to deactivation of a part of the accelerator by its adsorption onto the silica surface. This was later got over by adding a few reagents such as
diethylene glycol (e.g., see Table 4.1 at Sect. 4.2). They are assumed to be adsorbed
prior to the accelerator to result in distancing it from the surface. However, it has
taken much more time to overcome the first difficulty of the dispersion in the rubber.
Through lots of practical experiences, rubber engineers have assumed that the aggregation of silica particles is the reason of their inhomogeneous dispersion in rubber.
In other words, it is correctly estimated that the filler-to-filler interaction of silica is
much stronger than the filler-to-rubber interaction, when compared with CB. As one
of the probable reasons, the presence of a silanol group on the surface is mentioned,
which may afford, e.g., hydrogen bonding between silica particles.
Under these situations, the utilization of particulate silica in rubber has gradually
increased, and now the use of silica compounds is one of the fashionable current
topics in rubber industry from the viewpoint of sustainable development (SD) [6–
10]. One technical factor supporting this trend for long is the use of silane coupling
agents, taking advantage of the presence of functional groups on the silica surface,
the silanol group, in particular [4, 6, 9]. These are the background for wet silica
usages, which are to be explained more in the next subsection.
6.1.2 Wet Silica for Higher Performances
The recent trend of expanding utilization of particulate silica grounds on two issues,
ecological and energy-saving considerations, both of which are related to SD as the
fundamental principle [6–13]. While CB is a petrochemical product, the ingredient of
which is petroleum (see Sects. 2.4.2 and 2.5.3), silicon is abundant on the earth. From
the social and particularly SD prospects, silica is preferred to CB. In fact, CB is to be
avoided under the most recent trend of decarbonization [6, 14–17]. From the technical
side, the use of silica in combination with silane coupling agents, in particular, has
been playing the most important role in practice [6, 18–23]. The first breakthrough
on this line is the commercialization of bis(triethoxysilylpropyl) tetrasulfide (TESPT
or Si 69) as a new coupling agent [19], followed by its applications [20–23]. The
polysulfide group in TESPT enables it to be involved in sulfur vulcanization reactions,
6 Particulate Silica Reinforcement of Rubber
did not spread much due to the difficulty of homogeneous dispersion (then, a simple
optical observation of the surface was employed for judging good or bad of the filler
dispersion) of silica compared with that of CB under the same mixing conditions.
Historically and technically, they were working on nanoparticles but knew little of the
nanolevel science which was still to be established then. However, it is not essential
in terms of structuring the tendency of the nanofiller, to differentiate between CB and
silica. Relative comparison between rubber-to-filler and filler-to-filler interactions is
more important: Rationalization of this statement has been given in several relevant
descriptions in Parts 1 and 2 in this book.
One more drawback of the silica compound was the delay of vulcanization, which
was correctly estimated due to deactivation of a part of the accelerator by its adsorption onto the silica surface. This was later got over by adding a few reagents such as
diethylene glycol (e.g., see Table 4.1 at Sect. 4.2). They are assumed to be adsorbed
prior to the accelerator to result in distancing it from the surface. However, it has
taken much more time to overcome the first difficulty of the dispersion in the rubber.
Through lots of practical experiences, rubber engineers have assumed that the aggregation of silica particles is the reason of their inhomogeneous dispersion in rubber.
In other words, it is correctly estimated that the filler-to-filler interaction of silica is
much stronger than the filler-to-rubber interaction, when compared with CB. As one
of the probable reasons, the presence of a silanol group on the surface is mentioned,
which may afford, e.g., hydrogen bonding between silica particles.
Under these situations, the utilization of particulate silica in rubber has gradually
increased, and now the use of silica compounds is one of the fashionable current
topics in rubber industry from the viewpoint of sustainable development (SD) [6–
10]. One technical factor supporting this trend for long is the use of silane coupling
agents, taking advantage of the presence of functional groups on the silica surface,
the silanol group, in particular [4, 6, 9]. These are the background for wet silica
usages, which are to be explained more in the next subsection.
6.1.2 Wet Silica for Higher Performances
The recent trend of expanding utilization of particulate silica grounds on two issues,
ecological and energy-saving considerations, both of which are related to SD as the
fundamental principle [6–13]. While CB is a petrochemical product, the ingredient of
which is petroleum (see Sects. 2.4.2 and 2.5.3), silicon is abundant on the earth. From
the social and particularly SD prospects, silica is preferred to CB. In fact, CB is to be
avoided under the most recent trend of decarbonization [6, 14–17]. From the technical
side, the use of silica in combination with silane coupling agents, in particular, has
been playing the most important role in practice [6, 18–23]. The first breakthrough
on this line is the commercialization of bis(triethoxysilylpropyl) tetrasulfide (TESPT
or Si 69) as a new coupling agent [19], followed by its applications [20–23]. The
polysulfide group in TESPT enables it to be involved in sulfur vulcanization reactions,