Page 291 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.83

• For low structure silica, 75 to 125 ml/g
• For very low structure silica, <75 ml/g

Silicas have strongly polar surface characteristics. This is because of the many hy-
droxyl groups occupying the silica surfaces. This causes the silica particles to bond to one
another, as apposed to bonding to or being wetted by the rubber molecules. This can cause
problems vis à vis the dispersion of silica into the rubber matrix during mixing. It can also
interfere with the general processability of the uncured rubber compound.
Because silicas are acidic, they retard the cure during accelerated-sulfur vulcanization.
Also, because of their polarity, they can adsorb such rubber chemicals as vulcanization ac-
celerators and reduce the efficiency of curing. It may be necessary to add additional
amounts of accelerator (e.g., DPG or DOTG) to compensate for the effects on the curing
system. Polyols and polyethers have been used as additives to compete for the silica polar
groups, reducing the amount of curing-system ingredients that are adsorbed by the silica.
To improve the bonding of silica to rubber molecules rather than to one another, silane
coupling agents are used. One of these is the commercially available bis-(triethoxysilyl-
propyl)tetrasulfide. An ethyoxy group of this molecule can react with a silica -OH group to
give ethanol and a linkage to a silica particle, whereas the tetrasulfide part of the coupling
molecule can interact with rubber, the overall result being a rubber-to-silica linkage: sil-
ica-O-Si([O-C H ] )CH -CH -CH -S -rubber. This coupling-agent additive also can be
2
2
x
2 5 2
2
used to reduce the reversion in natural vulcanizates. It is a slow curative that slowly cross-
links the natural rubber, compensating for the loss of cross-links during reversion. For
coupling silica particles to rubber molecules during high-temperature mixing, care must
be taken that the curing reaction is not so extensive so as to cause premature vulcanization
(scorch).
There is much interest in using silica fillers in tires, because it is possible to obtain
abrasion-resistant treads of lower hysteresis (thus better fuel economy) than that of car-
bon-black-filled treads. However, there have been problems with the processing of the sil-
ica-filled compounds. Also silica, being nonelectrically conductive, gives vulcanizates that
can hold static electrical charges due to rolling on the road. Efforts to get around these and
other problems have led to the introduction of hybrid silica-carbon fillers.
4.5.4.4 Clays. Kaolin clay fillers are generally used to reduce cost while improving cer-
tain physical and processing characteristics. There are two basic types of rubber filler
clays: (1) “hard clays,” having median particle sizes of 250 to 500 nm, and (2) “soft clays,”
having median particle sizes of 1000 to 2000 nm. The hard clays give vulcanizates of
higher tensile strength, stiffness, and abrasion resistance than do the soft clays. They are
semireinforcing. Soft clays can be used with higher loadings than can hard clays. Also,
faster extrusion rates are obtained with the soft clays.
More hard clays than soft clays are used in rubber compounds, because they are semi-
reinforcing fillers. Aminosilane and mercaptosilane treatment of hard clays enhances rein-
forcement. Sometimes, hard clay is used with other fillers, for example, to improve the
tensile strength and increase the modulus of calcium carbonate-filled vulcanizates. Clay is
sometimes used to replace a portion of the more expensive carbon black or silica, with lit-
tle loss of performance.
Airfloat clay, the type most used in rubber compounds, is dry-ground hydrous kaolin
that has been air-separated to reduce impurities and control particle-size distribution.
However, some water-washed clay (slurried in water and centrifuged or hydrocycloned to
remove impurities) is used, because it contains a lower level of impurities and gives com-
pounds that are more colorable. The water-washed clay also causes less die wear during
extrusion.

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