Page 314 - Handbook of Plastics Technologies

ELASTOMERS

4.106 CHAPTER 4

Their physical properties, chemical stability, and resistance to temperatures above
135°C qualify these materials as competitors for silicone rubber and fluoroelastomers. Ap-
plications include hose, tubing, gaskets, and protective covers for use in high-temperature
environments.

4.6.3.5 Thermoplastic Elastomeric Olefins (TPOs). EPDM rubber and PP are the con-
stituents of the most common TPOs. An EPDM/PP TPO has a T near that of the hard PP
m
phase and a rubber-phase T close to that for the soft EPDM (plus additives) phase. These
g
TPOs thus melt in the range of 150 to 165°C and can be processed above these tempera-
tures. They show excellent low-temperature performance with brittle points often below
–60°C. The T clearly determines the upper theoretical service temperature limit of these
m
TPOs. The maximum long-term service temperature is usually 25 to 50°C below the T ,
m
depending in part on the resistance of the polymers to oxidative attack.
EPDM/PP TPOs compete directly with styrenic TPEs as low-cost, low-specific-gravity
(0.9 to 1.0) materials with fair to good mechanical performance and environmental resis-
tance. They range in hardness (Table 4.15) from 60 Shore A up to 65 Shore D, with the
harder products being more commonly found in commercial applications. The harder
TPOs are essentially impact-modified thermoplastics and not true rubbers. The softer
TPOs are rubbery at room temperature, but these characteristics are rapidly lost at elevated
temperatures. EPDM/PP TPOs are therefore generally useful only below 70 to 80°C.
At ambient temperatures (0 to 40°C), TPOs have quite rubber-like properties. As the
temperature is raised, however, these properties deteriorate quite sharply. On the other
hand, the absence of unsaturation in the polymer backbones of both PP and EPDM makes
these polymers and the TPOs derived from them very resistant to degradation by oxidation
or ozone attack. The nonpolar nature of EPDM/PP TPOs makes them highly resistant to
water, aqueous solutions, and other polar fluids such as alcohols and glycols, but they
swell extensively with loss of properties when exposed to halocarbons and oils and fuels.
EPDM/PP TPOs have good electrical properties such as high resistivity, dielectric
strength, and low power factor, allowing their use as primary electrical insulation where
temperature and fluid resistance are not critical.
TPOs (olefinic blends) comprise a lower-performance, lower-cost class of TPEs
(Fig. 4.39). Their performance and properties are generally inferior to those of thermoset
rubbers. Yet, they are suitable for uses where (1) the maximum service temperature is
modest (below 80°C), (2) nonpolar fluid resistance is not needed, and (3) a high level of
creep and set can be tolerated. Thus, TPOs are marketed more on the basis of cost rather
than performance, competing directly with the lower-cost general-purpose rubbers (NR,
SBR, and the like). TPOs are associated with the traditional practice of rubber compound-
ing and mixing. They can be prepared by the same techniques and equipment as for ther-
moset rubber; however, they need to be processed at temperatures above the T of the
m
thermoplastic hard phase. The amounts of elastomer, rigid thermoplastic, plasticizer, and
other ingredients can be varied to achieve specific properties in much the same manner as
with rubber compounds.
Having been first commercialized in 1972, EPDM/PP TPOs are used mainly in exter-
nal automotive and electrical applications up to 80°C. Automotive uses include exterior
trim such as bumpers, fascia, and nonsealing moldings; however, under-the-hood uses are
generally excluded because of temperature and fluid-resistance requirements in the engine
compartment.
4.6.3.6 Thermoplastic Vulcanizates (TPVs). TPVs differ from TPOs in that the rubber
phase is highly vulcanized (cross-linked). This phase of a TPO has little or no cross-link-
ing. As a result, the properties and performance of a TPV are much closer to those of a

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