The Hardness (DH) of a material is the measurement of its resistance to permanent indentation. The device used for testing rubber materials is known as a durometer. There are twelve scales that define the hardness of various materials: A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. The different scales apply to materials with different properties, i.e. the A scale is for softer plastics. The scales range in value from 0 to 100, with 100 durometers indicating the hardest. The standard durometer testing device was developed by Albert Ferdinand Shore, and the results from this device are called Shore Hardness.
Shore Hardness becomes an important factor when considering which rubber material should be chosen when dealing with pressure. That is, the correct hardness must be chosen based on the pressure that is planned in the system.
A durometer measures the depth of an indentation in the material created by a given force on a standardized presser foot. This depth is dependent on the hardness of the material, its viscoelastic properties, the shape of the presser foot, and the duration of the test. ASTM D2240 durometers allow for a measurement of the initial hardness, or the indentation hardness after a given period of time. The basic test requires applying the force in a consistent manner, without shock, and measuring the hardness (depth of the indentation). If a timed hardness is desired, force is applied for the required time prior to taking the measurement.
The hardness (ASTM D2240) of a material is determined using one of twelve different durometer scales. The test method is based on the penetration of a specific type of indentor when forced into the material under specified conditions. The indentation hardness is inversely related to the penetration and is dependent on the elastic modulus and viscoelastic behavior of the material. The geometry of the indentor and the applied force influence the measurements, such that no simple relationship exists between the measurements obtained with various types of durometers or other instruments used for measuring hardness. This test method is an empirical test intended primarily for control purposes. The determined indentation hardness is not related to any fundamental property of the material tested.
ASTM D2240 Hardness and Elastic Modulus
There have been a few attempts to relate ASTM D2240 and elastic modulus. The following is an example of such an attempt:
where SA is the ASTM D2240 type A hardness, SD is the ASTM D2240 type D hardness, and E is the Young’s modulus in MPa.
Typical Usage and Hardness
Type (Scale) Typical Examples of Material Tested Durometer Hardness A Soft vulcanized rubber, natural rubber, nitriles, thermoplastic elastomers, flexible polyacrylics and thermosets, wax, felt, and leathers 20–90 A B Moderately hard rubber, thermoplastic elastomers, paper products, and fibrous materials Above 90 A
Below 20 D
C Medium-hard rubber, thermoplastic elastomers, medium-hard plastics and thermoplastics Above 90 B
Below 20 D
D Hard rubber, thermoplastic elastomers, harder plastics, and rigid thermoplastics Above 90 A DO Moderately hard rubber, thermoplastic elastomers, and very dense textile windings Above 90 C
Below 20 D
M Thin, irregularly shaped rubber, thermoplastic elastomer, and plastic specimens 20–85 A O Soft rubber, thermoplastic elastomers, very soft plastics and thermoplastics, medium-density textile windings Below 20 DO OO Extremely soft rubber, thermoplastic elastomers, sponge, extremely soft plastics and thermoplastics, foams, low-density textile windings, human and animal tissue Below 20 O CF Composite foam materials such as amusement ride safety cushions, vehicle seats, dashboards, headrests, armrests, and door panels See Annual Book of ASTM Standards, Vol 15.07.