Page 306 - Handbook of Plastics Technologies

ELASTOMERS

4.98 CHAPTER 4

5. TPE parts can readily be recycled after they have given a normal, useful lifetime of
service.
6. Thermoplastics processing allows tighter control on part dimensions than thermoset
rubber processing.
7. TPEs permit thermoplastic fabrication methods not feasible for thermoset rubbers.
These methods include blow molding, coextrusion with rigid thermoplastics, thermo-
forming, heat welding, and film blowing.
It should not be surprising that there are offsetting disadvantages to the use of TPEs com-
pared to thermoset rubbers, as listed below:

1. To thermoset rubber processors, TPEs belong to a new technology requiring unfamil-
iar processing equipment and techniques. Thermoplastics processors are familiar
with this technology and have the necessary equipment, although they are generally
not familiar with the markets for rubber articles. The capital investment for thermo-
plastics equipment is often a major hurdle for a thermoset rubber processor to partic-
ipate in the market for TPE parts.
2. Many TPEs must be dried before processing. While this is a familiar step to thermo-
plastics processors, it is not necessary for thermoset rubbers. Drying equipment is
usually not available in a rubber shop.
3. A TPE becomes molten at a specific elevated temperature, above which a part will
not maintain its structural integrity. Cross-linked thermoset rubbers do not display
such melting behavior and are limited in upper service temperature only by chemical
degradation such as oxidation.
4. TPEs require moderately high production volume for good processing economics.
Thermoplastics tooling costs are generally higher than those for thermoset rubber
parts, some of which are compression molded in volumes of only a few hundred per
year.
A compounded rubber stock is often less costly on a volume basis than a competitive
TPE. However, lower processing costs can more than compensate for the material cost dif-
ference. The needed equipment investment and production volumes must be weighed
against the fabrication savings and material cost differences.

4.6.2 General Characteristics of TPEs
A TPE generally comprises two polymeric phases: a hard thermoplastic phase and a soft
elastomeric phase. The properties of the resulting TPE depend, at least in part, on the
properties of each of the two phases and their mutual interactions. The two phases may re-
sult from simply mixing two different polymers, as in a blend of a hard thermoplastic such
as polypropylene (PP) with a soft elastomer such as ethylene-propylene terpolymer
(EPDM rubber), to give a thermoplastic elastomeric olefin (TPO). Dynamic vulcanization
(under conditions of high shear and temperature) of the elastomer phase of such a blend
gives rise to a thermoplastic vulcanizate (TPV), with properties close to those of a conven-
tional thermoset rubber. The two phases of a TPE may also be present as hard and soft seg-
ments along a common polymer backbone. This is the case for block copolymers, the
basis for many commercially important TPEs. Table 4.15 compares the performance char-
acteristics of six different generic classes of TPEs.
The performance characteristics of a TPE depend on the T (or T if the hard phase is
m
g
glassy rather than crystalline) of the hard thermoplastic phase and the glass transition tem-

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