Impact strength

Polyurethanes can be formulated to achieve high impact resistance even at very low temperatures. However, there are significant differences between various polyurethanes. Polyurethanes stiffen earlier during cooling than natural rubber, for example. Common polyester urethanes may have stiffened considerably already at -20°C. A certain degree of stiffening may remain upon warming. This makes such materials less suitable for use in seals intended for low-temperature applications.
However, it is possible to produce polyurethanes that do not stiffen as rapidly during cooling. Examples of these include diphenylmethane diisocyanate-based poly-ε-caprolactone urethanes and polyether urethanes. While they stiffen faster than natural rubber during cooling, they do so less rapidly than chloroprene rubber. Despite the increase in stiffness, polyurethanes do not become brittle until very low temperatures are reached, as shown in the table.

The low brittle point means that soft polyurethanes can hardly be broken at temperatures above approx. -50°C. At approx. -50°C, even the harder polyurethanes (>70 Shore A) have the same impact strength as acetal plastics and significantly better impact strength than polyamide 6 and polyamide 66. At room temperature, hard polyurethanes can have up to 5–8 times higher impact strength than polyamide 66 and polyamide 6, and approx. 10 times better impact strength than acetal plastics.

However, it depends on how the polyurethanes are structured. It is possible to make polyurethanes that have very high impact strength or those that have very low impact strength.