Page 293 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.85

cross-linked during vulcanization, and its presence can give rise to increased hardness,
tensile strength, tear strength, and abrasion resistance. Before curing, the phenolics act as
processing aids.
4.5.4.7 Pigments. Both organic and inorganic pigments are used in colored rubber com-
pounds. Pigments are insoluble in rubber and rubber solvents. They must be easily dis-
persed in rubber compounds and insensitive to vulcanization conditions, vulcanizing
agents, and other additives. They must be light fast and insensitive to conditions encoun-
tered in product use (e.g. acid or base). They are generally free of strong pro-oxidants such
as copper and manganese compounds.
White Pigments. Various types of titanium dioxide are probably the most important
white pigments for rubber. Although they are fairly expensive, they are economically used
because of their great whitening power, and only small amounts are required. They also
have minimum effects on vulcanizate properties unless concentrations of about 20 phr or
more are used. Lithopone (a white pigment consisting of a mixture of zinc sulfide, zinc ox-
ide, and barium sulfate) has relatively low whitening power; thus, large amounts must be
used. This can degrade the vulcanizate properties. For this reason, titanium dioxide is pre-
ferred.
There are two forms of titanium dioxide used in rubber: anatas and rutile types that dif-
fer in crystalline structure. An anatase-type titanium oxide pigmented vulcanizate can
have an outstanding (bluish) white color, while most rutile titanium dioxides give a cream-
colored white rubber vulcanizate. However, rutile types have 20 percent more covering
power than do anatas types. Also, rutile types give the more light- and weather-resistant
vulcanizates. Nevertheless, anatas types are used where a more nearly pure white material
is required.
Inorganic Colored Pigments. Inorganic pigments do not have the brilliance of some
of the organic ones, but they have the better weathering properties and good chemical re-
sistance. Also, they can be low in cost. They are used in low concentrations lest they unfa-
vorably influence the performance properties of the vulcanizates.
Iron oxide pigments are used to obtain reddish, brown, beige, and yellow hues. Iron ox-
ide pigments should be free of such pro-oxidants as manganese impurities. Chromium ox-
ide pigments are used for greenish and yellowish green hues. Cadmium-containing
pigments are used for brilliant yellow, orange, and red colors. However, cadmium com-
pounds are restricted in some countries for toxicological reasons. Ultramarines are used
for blue colors.
Organic Colored Pigments. The organic pigments are more efficient than the inor-
ganic ones. They give brilliant colors but are not as resistant to light and weather, and they
have less covering ability. They are also generally more expensive. Suitable materials in-
clude azo dyes, for example, from the diazo coupling of o-chloroaniline with p-nitrophe-
nyl-3-methyl-5-pyrazole to produce an orange pigment. Other examples are alizarine
dyes, and for blues and greens, the phthalocyanine dies. These pigments are available as
pure powders or in paste form.

4.5.4.8 Other Compounding Ingredients
Softeners, Tackifiers, and Processing Aids. Softeners (e.g., extender oils, process
aids, and tackifiers) are added to (1) improve processing characteristics of the compound,
(2) to modify the final compound properties (e.g., hardness), (3) to reduce the cost of the
compound (i.e., an extender oil, being inexpensive and enabling greater levels of inexpen-
sive filler), and (4) to reduce the power consumption during processing.
Differences among softeners, tackifier resins, and softeners are blurred, and many are
dual-purpose ingredients of rubber compounds. Plasticizers also act as softeners and pro-

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