2.1 Reinforcing Effect
15
Uniqueness of rubber processing is its involvement in a chemical reaction, i.e.,
the cross-linking reaction, which is the most notable difference from that of plastics
or fiber materials. In other words, rubber processing has been and is a reactive processing, which is an intrinsic difference from that of plastics. It is notable that the
vulcanization is to occur at the final step only. Hence, addition of a pre-vulcanization
inhibitor is a usual practice industrially. Since the vulcanization is the final step to
irreversibly determine the product shape, shaping is decisive, too. Among the three
steps, the mixing of filler onto rubber at the first step is supposedly the most influential
to the rubber reinforcement by nanofiller since the dispersion of nanofiller in rubber
matrix is assumed to be determined at the step of mixing. By the way, it is interesting
to note that rubber compounding is full of antimony: Mixing of filler makes rubber
hard, which necessitates the addition of softener, and pre-vulcanization inhibitor is
popular, while it is a must to mix accelerators in order to speed up the vulcanization
by sulfur! Apparently, they seem to be inconsistent, which are the rational results
of quite a lot of trials conducted for many years. However, the apparent contradictions are scientifically dissolved later. How to explain these contradictions may be
a good question to answer for improving your understanding of the practical rubber
technique.
The first step is to be considered here, where the filler mixing is almost simultaneously conducted together with the other reagents, in practice. Rubber mixing is a
mechanical process using some kind of mixer in order to mix fillers, curing (chemical
reactions toward a network structure) agents, and lots of other reagents into rubber.
Here, rubber is at a higher temperature than its glass-transition temperature (T g ) and
is technically a highly viscous liquid state. Practically, it means rubber is both liquid
and/or solid. At a time, it behaves like a solid and another time as a liquid, depending
on the stage or time of mixing [17, 18]. Therefore, rubber mixing is very sensitive to
the mechanical mixing conditions, and to find out the reasonable and rational mixing
conditions is a task of utmost importance for the skillful rubber engineers.
Additionally, it is very probable that some uninvited chemical reactions may be
involved during the mixing. Among them, unique feature in rubber mixing is mechanically induced chemical reactions, i.e., the mechanochemical reactions which are the
chemical reactions induced by mechanical energy. The possible mechanochemical
reactions of rubber in the mixer [19–22] include breakdown of a rubber molecule,
chemical bonding among rubbers, and that between rubber and filler or any other
mixed reagents. Sometimes, cavitation (formation of an empty space within the solid
body) may play an important role in the mechanochemical reactions during rubber
mixing under a high-speed rotation. Anyway, they are often much complicated, and
the cavitation is not amenable to an exact detection in general.
The other to be mentioned is chemical reactions due to curing reagents not at
the third step but at the first or second step. The chemical reaction at the steps (1)
and/or (2) is called ‘scorch,’ which is estimated mainly the cross-linking reaction
before the step (3). Scorch is absolutely to be avoided, and various means have
been worked out by controlling mixing conditions, in particular mixing temperature.
Occasionally, commercially available anti-scorching agents (the specific reagents
claiming to prohibit the scorch, also called pre-vulcanization inhibitor) are used.
15
Uniqueness of rubber processing is its involvement in a chemical reaction, i.e.,
the cross-linking reaction, which is the most notable difference from that of plastics
or fiber materials. In other words, rubber processing has been and is a reactive processing, which is an intrinsic difference from that of plastics. It is notable that the
vulcanization is to occur at the final step only. Hence, addition of a pre-vulcanization
inhibitor is a usual practice industrially. Since the vulcanization is the final step to
irreversibly determine the product shape, shaping is decisive, too. Among the three
steps, the mixing of filler onto rubber at the first step is supposedly the most influential
to the rubber reinforcement by nanofiller since the dispersion of nanofiller in rubber
matrix is assumed to be determined at the step of mixing. By the way, it is interesting
to note that rubber compounding is full of antimony: Mixing of filler makes rubber
hard, which necessitates the addition of softener, and pre-vulcanization inhibitor is
popular, while it is a must to mix accelerators in order to speed up the vulcanization
by sulfur! Apparently, they seem to be inconsistent, which are the rational results
of quite a lot of trials conducted for many years. However, the apparent contradictions are scientifically dissolved later. How to explain these contradictions may be
a good question to answer for improving your understanding of the practical rubber
technique.
The first step is to be considered here, where the filler mixing is almost simultaneously conducted together with the other reagents, in practice. Rubber mixing is a
mechanical process using some kind of mixer in order to mix fillers, curing (chemical
reactions toward a network structure) agents, and lots of other reagents into rubber.
Here, rubber is at a higher temperature than its glass-transition temperature (T g ) and
is technically a highly viscous liquid state. Practically, it means rubber is both liquid
and/or solid. At a time, it behaves like a solid and another time as a liquid, depending
on the stage or time of mixing [17, 18]. Therefore, rubber mixing is very sensitive to
the mechanical mixing conditions, and to find out the reasonable and rational mixing
conditions is a task of utmost importance for the skillful rubber engineers.
Additionally, it is very probable that some uninvited chemical reactions may be
involved during the mixing. Among them, unique feature in rubber mixing is mechanically induced chemical reactions, i.e., the mechanochemical reactions which are the
chemical reactions induced by mechanical energy. The possible mechanochemical
reactions of rubber in the mixer [19–22] include breakdown of a rubber molecule,
chemical bonding among rubbers, and that between rubber and filler or any other
mixed reagents. Sometimes, cavitation (formation of an empty space within the solid
body) may play an important role in the mechanochemical reactions during rubber
mixing under a high-speed rotation. Anyway, they are often much complicated, and
the cavitation is not amenable to an exact detection in general.
The other to be mentioned is chemical reactions due to curing reagents not at
the third step but at the first or second step. The chemical reaction at the steps (1)
and/or (2) is called ‘scorch,’ which is estimated mainly the cross-linking reaction
before the step (3). Scorch is absolutely to be avoided, and various means have
been worked out by controlling mixing conditions, in particular mixing temperature.
Occasionally, commercially available anti-scorching agents (the specific reagents
claiming to prohibit the scorch, also called pre-vulcanization inhibitor) are used.