IMPACT RESISTANCE

Polyurethanes can be compounded to give high impact resistance at very low temperatures.
However, there are great variations between different types of polyurethanes, which become harder faster during
cooling than does, for example, natural rubber. Ordinary polyesterurethanes may have obtained significant rigidity
at a temperature of minus 20 Centigrade. Part of this rigidity may remain during subsequent warming, which makes
these materials less suitable for use as sealants in low temperature applications.
However, it is possible to make polyurethanes which do not become rigid so rapidly during a cooling cycle.
Examples of these are diphenylmetandiisocyanate-based poly-E-caprolactoneurethanes and polyetherurethanes.
These become rigid faster than natural rubber when refrigerated, however less so than chloroprene rubber. In spite of
their increased rigidity, polyurethanes only become brittle  at very low temperatures, see table.  

Their low brittle point means that soft polyurethanes will not break easily up to temperatures
of ca. -50 Centigrade. At ca. -50 Centigrade even the harder polyurethanes (>70 Shore A)
the have the same impact resistance as acetal resins, and significantly better impact resistance than
amide resin 6 and amide resin 66. At room temperatures, hard polyurethanes may have up
to 5-8 times higher impact resistance than amide resin 66 and amide resin 6;
as well as approx. 10 times better impact resistance than acetal resins.
However, this is dependent upon the formulation and the compound of the polyurethanes,
which can be made to give a very high or a very low impact resistance.

Elastomer types	Hardness Shore A scale	Brittle point Centigrade
Natural rubber	71	-56
SBR rubber	72	-50
Chloroprene rubber	62	-42
Polyethyleneadipaturethane	80	-50
Adiprene L100	88	<-62
Poly-E-caprolactoneurethane	60	<-75