Page 311 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.103

The characteristics of these TPEs depend on the relative proportions of the polymer-
ized styrene and diene units as well as the chemical nature of the monomers. At low sty-
rene levels, the TPEs are soft and rubbery. With increasing styrene content, the TPE
progressively becomes stiffer at room temperature, and it becomes a glossy, hard material
similar to an impact-modified polystyrene (HIPS). Removal of the rubbery-block unsatur-
ation by hydrogenation (to give S-EB-S materials) makes the styrenic TPEs much more
resistant to oxidation and ozone attack. Commercially available styrenic TPEs range in
hardness from 20 Shore A to 60 Shore D.
Styrenics have a broad service temperature range, from about –70 to about 70 to
100°C, but the upper limit is sufficiently low to restrict their use to lower-temperature ap-
plications. They are very resistant to water and other polar fluids but lose much of their ef-
fectiveness if exposed to oils, fuels, and other nonpolar organic solvents. Their
compression and tension set (resistance to plastic deformation under stress) is good (that
is, low) at ambient temperatures but becomes progressively poorer with increasing tem-
perature. Above 70°C, their compression set is poor (high). The common styrenic TPEs
with butadiene- or isoprene-derived segments are among the lowest-cost, lowest-perfor-
mance classes of TPEs. Their performance, however, is adequate for a wide variety of non-
demanding rubber applications.
Uses of Styrenic TPEs. The huge market for styrenic TPEs includes applications as
fully compounded materials that can function as replacements for thermoset rubber, lead-
ing to uses in such articles as shoe soles, sporting goods, and a variety of other applica-
tions not requiring service above 70°C, and not requiring hydrocarbon fluid resistance or
exceptionally good physical properties. The styrenics are the generic class of TPEs most
widely used as compounding ingredients in other useful compositions. They are major
constituents in adhesives, resins, sealants, caulking, motor vehicle lubricants, and thermo-
set automotive body parts.
4.6.3.2 Ester-Ether Block Copolymers [Copolyester TPEs (COPs)]. Ester-ether block
copolymer TPEs, also called copolyester TPEs, are segmented block copolymers with the
-A-B-A-B- structure, where A and B are alternating hard and soft polymeric segments
connected by ester linkages. A general chemical structure can be expressed as follows:

These block copolymers have an excellent combination of properties and are priced higher
than styrenics, TPOs, or TPVs.
The morphology of COPs is that shown in the upper right-hand quadrant of Fig. 4.38.
These materials perform as TPEs if the structures of A and B are chosen to give rubbery
properties to the copolymer over a useful temperature range. The glass transition point of
the soft segment should be low enough to prevent brittleness at the lowest temperature to
which the working material will be exposed. The melting point of the hard segment should
be high enough to allow the material to maintain a fabricated shape at its highest service
temperature.
The ester-ether copolymers have a material cost higher than that of most thermoset
rubber compounds. This higher cost can be more than offset by the high strength and mod-

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