2.2 Compounding Reagents for Rubber
19
preferred, organic peroxides (e.g., dicumyl peroxide) are employed often in combination with some coagents (e.g., triallyl isocyanurate). If the coloring is necessary,
appropriate inorganic pigments or organic dyes are mixed into rubber. NR is known
for its high tackiness (auto-adhesion tendency of rubber), and sometimes, anti-tack
agents are used to control it, while many synthetic rubbers need to be mixed with
tackifiers (various resins are used) to increase it. Tack is an important property of
rubber when shaping the product from a few or several rubber compounds just before
the cross-linking step. For rubber/cord composites (e.g., tires and conveyer belts), use
of good bonding agents is very useful which are supposed to chemically bond to both
rubber and cord. If the rubber product of some resistance to fire is very important,
compounding flame retardants are recommended.
So far mentioned reagents are all a kind of functionality materials. There is the
other kind of additives called extenders with not much functional capability. Mineral
oil is an example, and commercially available oil-extended rubber compounds are
popular in the market. If the extender is a powdery solid, it may be called filler. In
this book on rubber reinforcement, however, we do not classify it in filler due to the
lack of any reinforcing capability.
2.3 Non-reinforcing Filler
Filler is not much explained in the previous subsection, but it is one of the two
important ingredients in the rubber industry. Since cross-linking and reinforcing
are essential in using the rubber as a material of use, usages of ingredients and
making a suitable recipe for them are indispensable in rubber processing. Among
so many powdery solids investigated by rubber practitioners so far, rubber engineers
now recognize two classes of filler, reinforcing and non-reinforcing. The former is
mainly nanofillers, and they are focused in the following subsection. The latter is
usually of larger size than nanofillers.
A lot of minerals both natural and synthetic are now consumed as non-reinforcing
fillers in the rubber industry [2, 6, 10, 38, 39]. Their role may include that of extender, but at the same time, they display semi-reinforcing effect and/or some specific
functions beneficial to the final rubber products. Such non-reinforcing fillers include
particulate alumina (its trihydrate is of use for flame retardation), calcium carbonates
and silicates, clay (hard or soft), talc (platy talc is uniquely pure), thermal carbon,
wollastonite (very white and abrasive), zinc oxide (now mainly used as an activator
of the vulcanization accelerator, but it had been treated as a filler until recently), and
so on.
Among these, minerals whose diameter is submicron meters are semi-reinforcing
and may display structuring tendency (see 2.5.3 for structuring of filler) like
nanofillers. However, the formed aggregates may be decomposed under a rigorous mechanical mixing. It is needless to say that the smaller is the diameter of the
particles, the more difficult is the decomposition of the aggregates. Therefore, if nm
size particles, for example, calcium carbonate, are available in a large amount, it can
19
preferred, organic peroxides (e.g., dicumyl peroxide) are employed often in combination with some coagents (e.g., triallyl isocyanurate). If the coloring is necessary,
appropriate inorganic pigments or organic dyes are mixed into rubber. NR is known
for its high tackiness (auto-adhesion tendency of rubber), and sometimes, anti-tack
agents are used to control it, while many synthetic rubbers need to be mixed with
tackifiers (various resins are used) to increase it. Tack is an important property of
rubber when shaping the product from a few or several rubber compounds just before
the cross-linking step. For rubber/cord composites (e.g., tires and conveyer belts), use
of good bonding agents is very useful which are supposed to chemically bond to both
rubber and cord. If the rubber product of some resistance to fire is very important,
compounding flame retardants are recommended.
So far mentioned reagents are all a kind of functionality materials. There is the
other kind of additives called extenders with not much functional capability. Mineral
oil is an example, and commercially available oil-extended rubber compounds are
popular in the market. If the extender is a powdery solid, it may be called filler. In
this book on rubber reinforcement, however, we do not classify it in filler due to the
lack of any reinforcing capability.
2.3 Non-reinforcing Filler
Filler is not much explained in the previous subsection, but it is one of the two
important ingredients in the rubber industry. Since cross-linking and reinforcing
are essential in using the rubber as a material of use, usages of ingredients and
making a suitable recipe for them are indispensable in rubber processing. Among
so many powdery solids investigated by rubber practitioners so far, rubber engineers
now recognize two classes of filler, reinforcing and non-reinforcing. The former is
mainly nanofillers, and they are focused in the following subsection. The latter is
usually of larger size than nanofillers.
A lot of minerals both natural and synthetic are now consumed as non-reinforcing
fillers in the rubber industry [2, 6, 10, 38, 39]. Their role may include that of extender, but at the same time, they display semi-reinforcing effect and/or some specific
functions beneficial to the final rubber products. Such non-reinforcing fillers include
particulate alumina (its trihydrate is of use for flame retardation), calcium carbonates
and silicates, clay (hard or soft), talc (platy talc is uniquely pure), thermal carbon,
wollastonite (very white and abrasive), zinc oxide (now mainly used as an activator
of the vulcanization accelerator, but it had been treated as a filler until recently), and
so on.
Among these, minerals whose diameter is submicron meters are semi-reinforcing
and may display structuring tendency (see 2.5.3 for structuring of filler) like
nanofillers. However, the formed aggregates may be decomposed under a rigorous mechanical mixing. It is needless to say that the smaller is the diameter of the
particles, the more difficult is the decomposition of the aggregates. Therefore, if nm
size particles, for example, calcium carbonate, are available in a large amount, it can