Typically noted in either pounds per square inch (psi) or megapascals (MPa), tensile strength is the amount of force required to break a rubber specimen. To convert from MPa to psi, simply multiply the MPa figure by 145. For example, 14 MPa converts to 2,030 psi. Converting from psi to MPa is just a matter of dividing the psi number by 145.

To better understand tensile strength, first recall that there are intermolecular forces (known as van der Waals forces) helping to hold long polymer chains in place. These forces are at their weakest when, due to structural irregularities, the molecules cannot fit closely together, resulting in a non-regimented, amorphous structure. Some polymers, however, have their constituent molecules aligned in very regular patterns. The combination of this regularity and the intermolecular forces may be enough to “fit” the chains into a rigid, crystalline pattern.

Tensile strength largely depends on an elastomer’s ability to partially strain crystallize when stretched. With greater crystallization comes increased strength and resistance to stress. Natural rubber is an example of an elastomer with a very regular chain structure that strain crystallizes. As a result, natural rubber has high tensile strength. Of course, the temporary nature of strain crystallization allows natural rubber to regain its original shape once the stress is removed. An elastomer with inherently poor tensile strength, such as styrene butadiene, can be improved through the addition of highly particulate reinforcing agents. Silica and carbon black are common reinforcing agents. Be aware that the majority of dynamic applications will require an elastomeric compound with tensile strength of at least 1000 psi (6.9 MPa).

Per ASTM D 412, a compound’s tensile strength is generally tested using a molded dumbbell (see Figure 17). The dumbbell is placed in the grips (jaws) of a tensile tester (see Figure 18). The best-known tester is Instron, but tensiTECH and others are also used. When the tester is activated, the dumbbell is pulled steadily at a rate of 20 inches per minute until it breaks. The force being exerted on the sample at the time of rupture is said to be the sample’s tensile strength. Minimum tensile strength is often used as both a qualification criterion when specifying a new material and as a control criterion (with a ± 15% production tolerance) when testing batches of mixed material.

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