Impact Resistance

The impact or shock resistance of an elastomer is determined by striking a sample with a swinging pendulum (hammer). The sample is placed at the lowest point of the arc traveled by the pendulum head. Measuring the difference in the distance of the upswing of the pendulum after the impact, compared to the same upswing with nothing in its path, determines the energy in breaking the sample which is the measure of impact strength.

In the Izod impact method, (ASTM D-256) the test piece is gripped upright and struck with the pendulum 7/8" above the edge of the gripping point. The sample has a standardized notch at the edge of the vise on the side toward the hammer. The Izod Impact Tester and Izod Bar are shown in Figures 1, 2, and 3.


Most vulcanizates of urethane rubber flex and bend in this type of test. 75D urethane is a material which approaches structural plastics in hardness. Yet even at this hardness, urethane has significantly better impact resistance than plastics. Values for this 75D material are compared in Table I with those of several plastics. In order to approach these high values for 75D urethane, plastic materials require reinforcement with glass fibers.

TABLE I
Impact Resistance, ISOD, Ft Lb/In (ASTM D256)
Hardness, Rockwell or durometer Nylon Rockwell R108 Acetal Rockwell R120 Acrylic Rockwell M103 Urethane Durometer 75D (Rockwell R90)
Notched, 75F 2.0 1.4 0.3 15.0
Notched, -40F 0.5 1.2 0.3 1.1