Chapter 6
Particulate Silica Reinforcement
of Rubber
Abstract Use of particulate silica for rubber is recently increasing, which may
continue during this century. So far, wet (or precipitated) silica has been employed
for rubber, especially for tire tread, in combination with silane coupling agent. Fillerto-filler interaction between silica particules is stronger than that of filler-to-rubber, in
contrast with carbon black (CB). Hence, silica has been considered inferior to CB in
terms of processability, in which good dispersion of filler is one of the key objectives.
Coupling agents are assumed to increase the filler-to-rubber interaction of wet silica.
On the other hand, in situ silica, which is generated in rubber matrix at place, suggests
a better processability than wet silica. This relatively novel technique of nanofiller
dispersion into rubber matrix is introduced and discussed in detail, together with the
advantages of new soft processing method of rubber.
Keywords Particulate silica · Coupling agent · Silica for tires · In situ silica · Soft
processing of rubber
6.1 Use of Particulate Silica for High-Performance Rubber
6.1.1 Utilization of Wet Silica in Rubber Compounding
The rubber vulcanizates are usually compounded with an appropriate nanofiller
to support and to improve their performance by reinforcement [1–6]. Among
nanofillers, carbon black (CB) was firstly used in rubber compounding for pneumatic tires early in the twentieth century (see Sects. 1.2 and 2.4), and since then
the consumption of CB for rubber has increased without interruption in accordance
with the development of the automobile and aircraft industries. On the other hand,
the rubber industry began to utilize particulate silica approximately 40 years later
[2]. As noticed in Sect. 2.4.3, here not ‘dry silica’ (also ‘fumed silica’ is used interchangeably) but ‘wet silica’ (also ‘precipitated silica’) is concerned. Since the radius
of silica particle is comparable to or even smaller than CB (see Fig. 2.2 at Sect. 2.1),
it was expected to make a better filler for rubber than CB. It is notable that at that
time most rubber engineers believed in a thesis ‘the smaller, the better’ as a rule of
thumb for filler reinforcement of rubber (see Sect. 5.2). However, the usage of silica
© Springer Nature Singapore Pte Ltd. 2020
S. Kohjiya et al., Reinforcement of Rubber, Springer Series on Polymer
and Composite Materials, https://doi.org/10.1007/978-981-15-3789-9_6
107
Particulate Silica Reinforcement
of Rubber
Abstract Use of particulate silica for rubber is recently increasing, which may
continue during this century. So far, wet (or precipitated) silica has been employed
for rubber, especially for tire tread, in combination with silane coupling agent. Fillerto-filler interaction between silica particules is stronger than that of filler-to-rubber, in
contrast with carbon black (CB). Hence, silica has been considered inferior to CB in
terms of processability, in which good dispersion of filler is one of the key objectives.
Coupling agents are assumed to increase the filler-to-rubber interaction of wet silica.
On the other hand, in situ silica, which is generated in rubber matrix at place, suggests
a better processability than wet silica. This relatively novel technique of nanofiller
dispersion into rubber matrix is introduced and discussed in detail, together with the
advantages of new soft processing method of rubber.
Keywords Particulate silica · Coupling agent · Silica for tires · In situ silica · Soft
processing of rubber
6.1 Use of Particulate Silica for High-Performance Rubber
6.1.1 Utilization of Wet Silica in Rubber Compounding
The rubber vulcanizates are usually compounded with an appropriate nanofiller
to support and to improve their performance by reinforcement [1–6]. Among
nanofillers, carbon black (CB) was firstly used in rubber compounding for pneumatic tires early in the twentieth century (see Sects. 1.2 and 2.4), and since then
the consumption of CB for rubber has increased without interruption in accordance
with the development of the automobile and aircraft industries. On the other hand,
the rubber industry began to utilize particulate silica approximately 40 years later
[2]. As noticed in Sect. 2.4.3, here not ‘dry silica’ (also ‘fumed silica’ is used interchangeably) but ‘wet silica’ (also ‘precipitated silica’) is concerned. Since the radius
of silica particle is comparable to or even smaller than CB (see Fig. 2.2 at Sect. 2.1),
it was expected to make a better filler for rubber than CB. It is notable that at that
time most rubber engineers believed in a thesis ‘the smaller, the better’ as a rule of
thumb for filler reinforcement of rubber (see Sect. 5.2). However, the usage of silica
© Springer Nature Singapore Pte Ltd. 2020
S. Kohjiya et al., Reinforcement of Rubber, Springer Series on Polymer
and Composite Materials, https://doi.org/10.1007/978-981-15-3789-9_6
107