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Olefins: Thermoplastic olefin (TPO) elastomers are
available in several grades, having room-temperature hardnesses ranging
from 60 Shore A to 60 Shore D. These materials, being based on
polyolefins, have the lowest specific gravities of all thermoplastic
elastomers. They are uncured or have low levels of crosslinking.
Material cost is mid-range among the elastoplastics.
These elastomers remain flexible down to -60°F and are not brittle at
-90°F. They are autoclavable and can be used at service temperatures as
high as 275°F in air. The TPOs have good resistance to some acids, most
bases, many organic materials, butyl alcohol, ethyl acetate,
formaldehyde, and nitrobenzene. They are attacked by chlorinated
hydrocarbon solvents. Compounds rated V-0 by UL 94 methods are available.
The four oldest thermoplastic elastomer types are
polyurethanes, polyester copolymers,
styrene copolymers, and the
olefinics. Mechanical properties of the first two types are
generally higher than those of the last two. Dynamic properties, such as
flex life are also generally better. Newest TPEs are three classes of
high-performance materials. One is based on polyamide (nylon) chemistry;
another, called
elastomeric alloys, consists of polymer alloys of an olefinic resin
and rubber. The third group, melt-processible rubbers, are sold by Du
Pont under the Alcryn tradename. |
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