Page 292 - Handbook of Plastics Technologies

ELASTOMERS

4.84 CHAPTER 4

Calcined clay (produced by heating a fine natural china clay to high temperatures in a
kiln) is used mostly in wire and cable coverings because of excellent water resistance and
electrical properties of its vulcanizates. Delaminated clays are also used in rubber com-
pounds. They are made by attrition milling the coarse clay fraction from the water-wash-
ing of soft clay. This breaks down the kaolinite stacks into thin, wide individual plates,
improving brightness, opacity, and barrier properties. Such clays impart very high stiffness
and low die swell because of their high shape factors.
4.5.4.5 Calcium Carbonate (Chalk, Whiting). Two general types of calcium carbonates
are used in the rubber industry: (1) wet or dry ground natural limestone, having particle
sizes between 700 and 5000 nm, and (2) precipitated calcium carbonate with fine and ul-
tra-fine products having average particle sizes as low as 40 nm. The ground products have
particles of low anisotropy (low structure or shape factor), low surface area, and low sur-
face activity. They are widely used only because of their low cost, and they can be used at
very high concentrations. Ground-calcium-carbonate vulcanizates have poor abrasion and
tear resistance. The dry-ground is the least expensive filler, and it can be used at the high-
est of levels. Precipitated calcium carbonates have much higher surface areas because of
their smaller particle size. Ultra-fine calcium carbonates, having particle sizes less than
100 nm, can have specific surface areas similar to those of hard clays.
Both the ground and precipitated calcium carbonates can by treated with stearic acid to
control water absorption, improve dispersability, and promote better wetting of the filler
by rubber. Silane treatment of these fillers is not effective. However, there is an ultra-fine
grade coated with carboxylated polybutadiene, which reactively links to the particle sur-
faces. Such treated ultra-fine products can give reinforcement of about the same level of
the semireinforcing thermal carbon blacks.

4.5.4.5.1 Other Fillers
Talc. Talc is little used in rubber applications. Platy talcs are hydrophobic, white, al-
kaline, and of high particulate asymmetry. They are readily treated with silanes and other
coupling agents. Unfortunately, particles of talc are generally too large for effective elas-
tomer reinforcement. Nevertheless, talcs can be micronized to reduce median particle
sizes to 1000 to 2000 nm. Such products are used but compete with less expensive clays.
Aluminum Oxyhydrate. This material is used for its ability to give off water at high
temperatures as a flame retardant.
Barite. Barite, ground barium sulfate, is used in acid-resistant vulcanizates, because it
is resistant to even strong acids that would attack other mineral fillers. It is also used where
high-density products are desired. It has little effect on cure, stiffness, or vulcanizate sta-
bility.
Mica. Because of its high aspect ratio and platyness, this material is sometimes used
as a semireinforcing filler. The platyness can also reduce swelling of compounds in oils,
solvents, and others.
Diatomite (Kieselguhr). Diatomaceous earth (as it is also called) is chemically inert,
but it has high adsorptive power. This can account for adsorption of curing ingredients that
interfere with accelerated-sulfur vulcanization. However, diatomite is used as a filler in sil-
icone rubber. Because of its high adsorptive capacity, it is used as a process aid in high-oil
rubber compounds.
4.5.4.6 Reinforcing Resins. The main types of reinforcing resins used in rubber com-
pounds are high-styrene resins and phenolic resins. The high-styrene resins are copoly-
mers of styrene and butadiene wherein 50 to 85 percent of the polymer is derived from
styrene. They are used to stiffen NR and SBR rubber compounds, for example, in shoe
soles. Phenolic resins are used for reinforcing NBR compounds. The phenolic resin is

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