Page 297 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.89

For NR, it is necessary to first achieve a temperature above about 60 to 70°C to melt
out any crystallinity. Then, if the temperature becomes too high, the viscosity will drop too
much, preventing the development of sufficient stresses for the mechanochemical break-
down of the polymer.
Stage-One Mixing (Masterbatching). The additives must be thoroughly mixed with
the rubber polymer (or polymers) to achieve uniform dispersion of ingredients. Uncured
rubbers have high viscosity and, therefore, working of the rubber during mixing can in-
crease its temperature up to 150°C or more. If vulcanizing agents are present from the start
of mixing, premature vulcanization (scorch) might be the result. To avoid premature vul-
canization, a two-stage mixing process can be employed. In the first stage, nonvulcanizing
ingredients (filler, antidegradant, softeners or oils, wax, processing aids, and so forth) are
combined with the raw rubber in, for example, an internal mixer (although, for small
quantities, an open roll mill can be used). The cure activators can be added in this first
stage, but not sulfur and frequently not accelerator. The objectives of masterbatching are to
achieve good homogeneous blending of the polymer with chemical additives and good
dispersive mixing to achieve deagglomeration as well as distribution of the filler. The
product of the first stage is generally called a masterbatch. Because considerable heat is
generated the first stage of mixing, the masterbatch can be cooled by milling on a cooled
open two-roll mill. It may also be necessary to do some milling of the masterbatch to im-
prove the dispersive mixing and to make it homogeneous.
Stage-Two Mixing (Finish Mixing). After the first-stage mixing has been completed
and the masterbatch has been allowed to cool, stage two mixing is carried out, during
which the vulcanizing agents, such as sulfur, and accelerator are added. This second-stage
mixing, which finishes the mixing process, has been carried out on an open mill, but it is
more frequently done in a carefully temperature-controlled internal mixer. The maximum
temperature allowed is incorporated into mixing procedures as the controlled dump tem-
perature.

4.5.5.2 Shaping and Forming. Shaping processes for rubber products can be divided
into the basic categories of extrusion, calendering, and molding. Vulcanization usually
happens in a heated mold, but it can also occur in a steam autoclave, salt bath, or a hot air
oven after extrusion. Some products require assembly work as well as shaping or forming.
This is required for built-up products such as tires.
Extrusion. Screw extruders are generally used for extrusion of uncured rubber into
shaped rubber sections for later use (e.g., treads and side walls of tires) or for essentially
forming the shape of the final product (e.g., hoses, vehicle seals, and so on). The length/di-
ameter (L/D) ratio of the extruder barrel is less than for thermoplastics, typically in the
range 10 to 15, to reduce the risk of premature cross-linking due to heat build-up in the
barrel. The earlier “hot-feed” extruders were shorter, and the rubber was fed as a heated
strip. Die swell occurs in rubber extrudates, since the rubber polymer is highly elastic (due
to entanglements of its very long molecules) and exhibits die swell or “memory.”
Either hot or cooled finished mix is fed into the extruder (as a hot strip or crumb fed to
a hot-feed extruder, or a cooled strip or pellets into a cold-feed extruder). The extruder can
form extruded profiles suitable as components of built-up products, such as side walls or
treads of tires. The extruder can also form profiles that are vulcanized “on the run” by
passing the “endless” extrudates through a heated salt bath or hot oven, either of which is
long enough for vulcanization to sufficiently occur before the profile exits the oven or hot
bath. Profiles of unvulcanized products, such as hoses, can be vulcanized after extrusion in
a steam autoclave or hot oven.
Calendering. Rubber calenders consist of at least three rolls, which can be adjusted
for gap, speed, and temperature. Calendering can be used for forming sheets of uncured

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