Page 313 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.105

170°C. Melting of the hard phase causes morphological changes, which are reversible,
while oxidative degradation is slow and irreversible. Both processes become progressively
more rapid with increasing temperature. Polyether soft blocks give TPUs having greater
resistance to thermal and oxidative attack than do polyester blocks.
TPUs are polar materials and are therefore resistant to nonpolar organic fluids such as
oils, fuels, and greases, but they are readily attacked and even dissolved by polar organic
fluids such as dimethylformamide and dimethylsulfoxide. TPUs behave like copolyester
TPEs toward water and aqueous solutions, being resistant to these media except at very
high or low pH. Polyether TPUs are more resistant to such hydrolytic degradation than are
the polyester TPUs.
The premium cost of TPUs can be justified for those applications requiring high levels
of abrasion resistance and toughness or low coefficients of friction. These applications in-
clude caster wheels, shoe soles, automotive fascia, and heavy-duty hose and tubing.

4.6.3.4 Polyamide Thermoplastic Elastomers. The newest and highest-performance
class of TPEs are segmented block copolymeric polyamides. The soft segments may have
either aliphatic polyester or polyether structures.

The morphology is that of typical block copolymers, as shown in the upper right quad-
rant of Fig. 4.38. The polyamide TPEs typically have higher temperature and chemical re-
sistance than do TPUs or copolyesters, and their cost is greater.
The soft segments may consist of polyester, polyether, or polyetherester chains. Poly-
ether chains give better low-temperature properties and resistance to hydrolysis, while
polyester chains in the soft segment give better fluid resistance and resistance to oxidation
at elevated temperatures. As in other block copolymer TPEs, the nature of the hard seg-
ments determines the melting point of a polyamide TPE and its performance at elevated
temperatures.
These TPEs have a wide hardness range, from a high of 65 Shore D down to 60 Shore
A. They have useful tensile properties at ambient temperatures and excellent retention of
these properties at higher temperatures. For example, a 90 Shore A polyamide TPE can re-
tain more than 50 percent of its tensile strength and modulus at 100°C. Annealing at a can
result in significant increases in tensile strength, modulus, and ultimate elongation. The
polyamide TPEs are second only to TPUs in abrasion resistance and show excellent fa-
tigue resistance and tear strength.
With service temperatures ranging from –40 up to 170°C, these materials give the
highest performance of any generic class of TPE. They are also the most expensive. Poly-
ester-based polyamide block copolymers give excellent properties retention after aging at
175°C for five days. These TPEs have good resistance toward hydrocarbon fuels, oils, and
greases. Their good resistance to water and aqueous solutions decreases as the temperature
is raised. Polyester block copolyamides are more sensitive to hydrolysis in humid air at
higher temperatures than are polyether block copolyamides.
Processing temperatures (220 to 290°C) are higher than those for other TPEs because
of their higher melting points. As with the TPUs, the polyamides require thorough drying
before processing.

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