24
2 Filler and Rubber Reinforcement
CB in practice) into account, the macroscopic isotropy of rubber is not necessarily
inconsistent with anisotropic fillers. Even anisotropic fillers can be a good candidate
of reinforcing filler of rubber if we can make them homogeneously mixed into rubber
while keeping orientation of them random. Therefore, short fibers and some other
anisotropic materials are still worth to be studied for rubber reinforcement.
Also, not inorganic (CB and silica are inorganic) but organic fillers including
biofillers are to be studied more for rubber reinforcement. Lignin is surely one of
them [68, 70, 71], and in Chap. 7, possibility of it is to be discussed. In combination
with NR, lignin would provide us with unique example of soft and elastomeric ‘bionanocomposite,’ which would possibly be used in tires. If the tires are industrialized,
they would be fully in accord with the requirements of sustainable development
[12, 25, 72–75].
2.5 Reinforcing Factors of Particulate Nanofiller
2.5.1 Elucidation of Reinforcing Effects
As discussed in 2.2, the concept of rubber reinforcement was not established even
after the invention of vulcanization of rubber by Goodyear in 1839. The progress
had been slow, but the reagents relevant to vulcanization were classified separately
early in the twentieth century. For example, zinc oxide had been considered one
of the fillers for rubber reinforcement for long. However, the presence of it in the
recipes of rubber compounding now is not as reinforcing filler but as an activator of
vulcanization accelerators [2, 31, 76]. In the same way, lots of rubber reagents were at
first dichotomized [2]; A group of reagents on rubber vulcanization and processing,
and the other group on rubber reinforcement.
By the middle of twentieth century, it was found that almost all reinforcing fillers
were of smaller than 1 µm diameter. That is, being a submicron size particle of
the filler was recognized the first requirement for the rubber reinforcement and
accordingly, the studies on nanofillers have become the central issue together with
vulcanization, for rubber engineers in general.
Since then, the rubber reinforcement has been discussed on the basis of two
interactions, filler-to-filler and filler-to-rubber interactions as follows:
(1) The filler-to-rubber interaction has been considered to be more important. In
fact, once nanofiller was mixed into rubber, some rubber portion remained on
filler surface even when the compound was soaked in a good solvent of rubber
(e.g., toluene) repeatedly. The immobilized layer of rubber on the filler surface
is named ‘bound rubber.’ Experimentally, the bound rubber has been observed
in quite a number of the rubber/nanofiller combinations.
(2) When the filler-to-filler interaction is more important as in nanofillers (due to the
huge surface area per g of nanofiller), the nanofiller particles possibly associate
to form primary aggregate or even further higher aggregates and ultimately to
2 Filler and Rubber Reinforcement
CB in practice) into account, the macroscopic isotropy of rubber is not necessarily
inconsistent with anisotropic fillers. Even anisotropic fillers can be a good candidate
of reinforcing filler of rubber if we can make them homogeneously mixed into rubber
while keeping orientation of them random. Therefore, short fibers and some other
anisotropic materials are still worth to be studied for rubber reinforcement.
Also, not inorganic (CB and silica are inorganic) but organic fillers including
biofillers are to be studied more for rubber reinforcement. Lignin is surely one of
them [68, 70, 71], and in Chap. 7, possibility of it is to be discussed. In combination
with NR, lignin would provide us with unique example of soft and elastomeric ‘bionanocomposite,’ which would possibly be used in tires. If the tires are industrialized,
they would be fully in accord with the requirements of sustainable development
[12, 25, 72–75].
2.5 Reinforcing Factors of Particulate Nanofiller
2.5.1 Elucidation of Reinforcing Effects
As discussed in 2.2, the concept of rubber reinforcement was not established even
after the invention of vulcanization of rubber by Goodyear in 1839. The progress
had been slow, but the reagents relevant to vulcanization were classified separately
early in the twentieth century. For example, zinc oxide had been considered one
of the fillers for rubber reinforcement for long. However, the presence of it in the
recipes of rubber compounding now is not as reinforcing filler but as an activator of
vulcanization accelerators [2, 31, 76]. In the same way, lots of rubber reagents were at
first dichotomized [2]; A group of reagents on rubber vulcanization and processing,
and the other group on rubber reinforcement.
By the middle of twentieth century, it was found that almost all reinforcing fillers
were of smaller than 1 µm diameter. That is, being a submicron size particle of
the filler was recognized the first requirement for the rubber reinforcement and
accordingly, the studies on nanofillers have become the central issue together with
vulcanization, for rubber engineers in general.
Since then, the rubber reinforcement has been discussed on the basis of two
interactions, filler-to-filler and filler-to-rubber interactions as follows:
(1) The filler-to-rubber interaction has been considered to be more important. In
fact, once nanofiller was mixed into rubber, some rubber portion remained on
filler surface even when the compound was soaked in a good solvent of rubber
(e.g., toluene) repeatedly. The immobilized layer of rubber on the filler surface
is named ‘bound rubber.’ Experimentally, the bound rubber has been observed
in quite a number of the rubber/nanofiller combinations.
(2) When the filler-to-filler interaction is more important as in nanofillers (due to the
huge surface area per g of nanofiller), the nanofiller particles possibly associate
to form primary aggregate or even further higher aggregates and ultimately to