Tear strength is significantly more important from a design perspective than tensile strength at break. Polyurethanes can be formulated to achieve considerably higher tear strength than conventional rubber types. Tear strength is generally superior in polyester urethanes compared to polyether urethanes. The reason is that the ester groups in polyester urethane, together with urethane groups and urea groups, create more hydrogen bonds than is possible in ether urethanes. If polyester urethanes are excessively cross-linked, the formation of hydrogen bonds is inhibited and tear strength decreases.
Tear strength generally increases with increasing hardness
This is particularly marked for ether urethanes, and with the choice of a suitable polyol and high hardness, they become almost as tear-resistant as polyester urethanes. In this case as well, the increasing number of hydrogen bonds in the polyurethanes is a contributing factor. However, the tear strength for both polyether urethanes and polyester urethanes can vary greatly at the same hardness. For example, polyether urethanes based on propylene glycol have significantly lower tear strength than those based on polytetramethylene glycol.
The effect of temperature on tear strength.
More or less all polymeric materials experience decreased tear strength with increasing temperature. At room temperature, polyurethanes have significantly better tear strength than natural rubber. At approx. 100 degrees, standard cast polyurethanes and natural rubber have roughly the same tear strength, and above 100 degrees, natural rubber may even have better tear strength. However, there are both cast and vulcanizable polyurethanes that have better temperature properties than the standard types. The tear strength of thermoplastic polyurethanes is even more sensitive to temperature increases than that of cast types. Exceptions include those that are cross-linked by irradiation after injection molding. Tear strength also decreases with aging.
Figure 14. Relative tear strength for some materials as a function of temperature.
A = Polyester urethane
B = Natural rubber
C = Chloroprene rubber
D = Nitrile rubber
The high shear strength of polyurethanes combined with compressive load capacity makes the material interesting for applications such as punching and shape cutting of sheet metal blanks, by using steel rules working against a polyurethane punching base. This involves relatively simple and inexpensive tools for punching sheet metal parts in small and medium-sized series.