Page 282 - Handbook of Plastics Technologies

ELASTOMERS

4.74 CHAPTER 4

The molecules are segmented block copolymers. The hardness, toughness, and abra-
sion resistance increase with increases in the length or concentration of hard polyurethane
segment.
Cross-link sites are urethane active hydrogen atoms, active methylene groups (e.g.,
from the use of a diisocyanate such as methylenebis(4-phenylisocyanate), or allylic hydro-
gen atoms from double bonds incorporated into for vulcanization by sulfur.
Isocyanates and peroxides are the usual vulcanizing agents for AU elastomers. Sulfur
vulcanization, which can be done with some grades, is far less prevalent. Antidegradants
are not generally required for AUs. Polyurethane elastomers can be degraded by the action
of water. EU grades are generally more resistant to hydrolysis than are the AU grades.
Thus, hydrolysis inhibitors are important, especially for the AU grades. Polycarbodiimides
are use for this purpose.
AU grades are frequently compounded with filler. A small amount of a reinforcing
black or fumed silica can considerably improve tear strength and increase hardness.
Properties of Polyurethane Vulcanizates. Polyurethane vulcanizates can exhibit ul-
timate tensile strengths as high as 40 MPa. Hardnesses are generally high, ranging from 70
to 99 Shore A. Vulcanizates have high degrees of elasticity, even at the higher levels of
hardness. Abrasion resistance is better than that of other rubber vulcanizates. However,
compression set is relative high when measured at elevated temperatures.
The vulcanizates have relatively good resistance to heat at temperatures up to about
100°C or a bit higher. However, they are attacked by water, steam, acids, and bases, as
well as by some lubricants. Except for in tropical climates, weathering resistance is good.
Ozone resistance of the vulcanizates is excellent.
The brittleness temperatures of polyurethane rubber vulcanizates generally range be-
tween –22 and –35°C. The vulcanizates swell very little in aliphatic solvents and motor fu-
els. Their swelling resistance to polar liquids (e.g., chlorinated hydrocarbon, esters, and
ketones), however, is not as good. The harder grades generally exhibit the better resistance
to swelling.
The gas permeability of polyurethane elastomer vulcanizates is very low, on the order
of that of butyl rubbers.
Uses of Polyurethane Vulcanizates. Polyurethane elastomers are used in the auto-
motive and engineered products industries for seals, shock absorbers, power-transmission
flexible joints, and suspension and support members. Polyurethane vulcanizates are also
used in solid tires, elastic thread, footwear, and others. Problems with hydrolytic stability
and resistance to heat must be considered when one uses these vulcanizates.

4.5.3.15 Polysiloxane or Silicone Rubber (Q). The polymer backbones of silicone rub-
bers contain no carbon atoms, but they contain alternating silicon and oxygen atoms. The
predominate structural unit is generally dimethylsiloxane, as follows:

Polydimethylsiloxane is designated MQ.
Another repeat unit is the phenylmethylsiloxane diradical (PMQ):

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