Noted in kilonewtons per meter (kN/m) or pound force per inch (lbf/in), tear resistance (or tear strength) is resistance to the growth of a cut or nick in a vulcanized (cured) rubber specimen when tension is applied. Tear resistance is an important consideration, both as the finished article is being removed from the mold and as it performs in actual service.

Tear resistance can be gauged via the same ASTM D 412 apparatus used in the testing of tensile strength, modulus, and elongation. As described in ASTM D 624, different specimen types can be used to measure both tear initiation (resistance to the start of a tear, see Figure 19) and tear propagation (resistance to the spread of a tear, see Figure 20). Either way, the sample is placed in the tester’s grips, which then exert a uniform pulling force until the point of rupture. This force may then be divided by the specimen’s thickness to arrive at the tear resistance for that particular sample. Three separate samples are typically tested and an average calculated.

Though epichlorohydrin, natural rubber, and polyurethane all have excellent tear resistance, many materials are not very strong in this area. Silicone and fluorosilicone have notably poor tear resistance. Though it might seem logical, it is in fact a common misconception that hardness automatically equals good tear resistance. Compounds whose tear resistance is less than 100 lbf/in are most at risk for installation damage, especially in designs featuring non-smooth areas (as with burrs, slots, threads, etc.) and/or sharp, non-radiused (non-rounded) corners. Once damaged, materials with poor tear resistance will quickly fail in service. This is especially true for dynamic seals. Poor tear resistance is linked to poor abrasion resistance.

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