Typically defined as resistance to indentation under specific conditions, the hardness of an elastomer is more accurately thought of as two related properties: inherent hardness and processed hardness. As a result of chemical structure, each elastomer has its own inherent hardness. This inherent hardness can be modified (and is typically supplemented) via compounding and vulcanization. Hardness in molded rubber articles (processed hardness) is a factor of cross-link density (and the amount of fillers). The more cross-linking a given material undergoes during vulcanization, the harder the final molded part will be. When judging the potential effectiveness of a molded seal, processed hardness is one of the most common criteria in the rubber industry.

Unfortunately, hardness is also one of the least consistent concepts in that the most-used measurement scales have only limited comparability. There is no single “universal hardness” unit, so it is often impossible to draw a clear and easy correlation between readings on two different scales, even when the samples being measured are absolutely identical. There are currently two hardness tests that predominate in the rubber industry: Shore durometer and International Rubber Hardness Degrees (IRHD).

Because Shore Instruments led the way in the marketing of durometer gauges, the words “Shore” and “durometer” have become virtually synonymous within the rubber industry. Now a division of Instron Corporation, Shore Instruments offers a wide range of durometer scales conforming to the ASTM D 2240 standard. These scales are designed to gauge hardness in everything from textile windings to plastics to foam. Rubber hardness is most often measured via a Shore Type A or Type D durometer. Since there is more than one scale, you should always be specific as to which scale is being applied in a given situation, e.g. “95 Shore A” or “46 Shore D.” The full range of durometer scales and the materials they are most commonly used on are listed in Table 3.

The Shore A durometer is a portable and adaptable device which uses a frustum (truncated) cone indenter point and a calibrated steel spring to gauge the resistance of rubber to indentation. When the durometer is pressed against a flat rubber sample, the indenter point is forced back toward the durometer body. This force is resisted by the spring. Once firm contact between the durometer point and the sample has been made, a reading is taken within one second unless a longer time interval is desired. Five readings are typically taken, then an average value calculated. The amount of force the rubber exerts on the indenter point is reflected on a gauge with an arbitrary scale of 0 to 100. Harder substances generate higher durometer numbers. A reading of 0 would be indicative of a liquid, whereas 100 would indicate a hard plane surface (e.g. steel or glass).

That said, it’s important to note that readings of less than 10 or more than 90 Shore A are not really considered reliable. Materials harder than 90 Shore A (e.g. some polyurethanes and plastics) are more accurately measured on a Shore Type D durometer, which utilizes a stiffer spring and a sharp 30° angle indenter point. The majority of O-ring materials have readings between 40 and 90 Shore A. Table 4 includes approximate conversions for several of the most-used durometer scales.

Though most standard O-rings are either 70 or 90 Shore A, the application will always govern the necessary hardness. Softer compounds offering less resistance may be perfectly fine for low-pressure seals, but high-pressure seals will likely require a harder, more extrusion-resistant material. Making decisions about a property such as hardness often entails compromise in order to ensure the long-term usefulness of the seal. For example, a relatively hard compound may resist being extruded under high pressure, but its use can also lead to increased frictional buildup in dynamic seals. Increased friction leads to increased heat, which can, in turn, degrade the seal and decrease its useful life span.

It is also important to realize that measuring the hardness of a rubber sample is an imprecise art (see Figure 16). Depending on both the specific gauge in use and the expertise of its operator, it is possible (even probable) that the same sample will yield two or more different readings. The rate at which the durometer is applied to the sample, the force used, the amount of time that elapses before taking the reading, and the temperature of the specimen at the time of testing can all impact a test result. For this reason, all durometer readings normally include a tolerance of ± 5 points, but sometimes even this may not be enough to fully anticipate all of the variances to be seen in testing. Technological advances have reduced many of the discrepancies, but sometimes at the expense of the simplicity and portability that initially made durometers popular. It is generally a good idea to test a given specimen several times and average the results to ensure accuracy.

Despite the long-standing close association between “Shore” and “durometer,” be aware that there are other companies which market high-quality durometers. These include Rex Gauge Company and PTC Instruments. Microhardness testers have also been developed for use on samples that are too small or too irregularly shaped to be accurately gauged by standard durometers.

The other widely-used test, International Rubber Hardness, utilizes a spherical indenter and a dial gauge calibrated in International Rubber Hardness Degrees (IRHD). Though not as common in the United States as abroad, all IRHD testers are designed to conform to the ASTM D 1415 standard.

Figure 15 shows one of the hardness testers used at R.L. Hudson & Company.

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