Rubber Hardness and Durometer Testing and Measurement — Black Art or Science?

Scarcely a day passes without someone posing a question to our technical department regarding rubber hardness and durometer testing. The nature and frequency of these questions indicate that the term “rubber hardness” is largely misunderstood. The following is being offered in hopes of clarifying these misunderstandings.

A common misconception is the term durometer. Durometer literally means “hardness measure”. It is from the Latin word, durus, meaning hard and the Greek word meter, meaning measure. The term originally pertained to the measuring device itself, which was invented by Albert Shore in the 1920’s. Presently, it describes the actual reading obtained from a rubber specimen using the measuring device.

Rubber hardness is based on the amount of penetration of a specific indenter when forced into a sample of the rubber under specific conditions. There are at least seven scales of hardness measurement available. In rubber lining the A scale is most commonly used but in hard rubber products the D scale may be referenced. In the U.S., ASTM D2240 is the accepted standard for determining rubber hardness.

In ASTM D2240, specific steps and requirements are listed to ensure that laboratory determined hardness measurements are consistent and reproducible. Even at that, a +/- 5 point tolerance with an overall range of 10 points is acceptable.

The steps described in ASTM 2240 must be followed to the letter if hardness values are to be considered valid. A summary of the “correct way” to determine hardness follows –

1- The durometer to be used must be in good working condition and be identified as being properly calibrated for use.

2- The sample must be a minimum of 6 mm (0.24 in.) thick. Plying of thinner samples to achieve this thickness is acceptable.

3- The sample must have a completely flat surface and the durometer presser foot must fit uniformly on this surface.

4- The temperature of the testing area must be 23.0 +/- 2.0° C. (73.4 +/- 3.6° F.) in addition to other conditions per ASTM D618.

5- The sample must be conditioned for 24 hours within this temperature range prior to measurement.

6- The presser foot must be applied uniformly in order to obtain valid data. One may use either a hand held durometer, which will add a certain level of operator variability to the result, or an operating stand, which will minimize operator variability but will add additional expense.

It may be seen that true hardness determination is strictly limited to laboratory testing. In a lined vessel, the gauge of the lining may not always be 6.0 mm or greater and the temperature may not always be 23.0 +/- 2.0° C. (73.4 +/- 3.6° F.). Other conditions, as in ASTM D618, may not be maintainable, a 24 hour wait for conditioning may not always be feasible and the available surfaces may not be flat enough to ensure a uniform fit for the presser foot. The biggest hindrance to obtaining a true hardness measurement in a lined vessel is the need to use a hand held durometer, which will add operator variability.

In summary, only laboratory generated hardness measurements obtained by following the steps listed in ASTM D2240 may be considered valid. Any measurements determined on the walls of a lined vessel are meant for reference purposes only. The lining materials provided by Blair Rubber Co. are tested under laboratory conditions and are certified for shipping only when their hardness requirements are met. Therefore, if the required curing conditions are met, the lining will attain the proper hardness in spite of the fact that hardness measurements determined inside a vessel may indicate otherwise.