Rubber can be mechanically deformed in shape when strain is applied to the material. Strain can be evident in several forms including compression (squash), tensile (stretch), shear (tensile and compression) and torsion (twist). We summarise below the major mechanical properties for rubber although for detailed advice please contact us.
Rubber hardness is the most common property to be specified and tested for in rubber. Hardness is measured in degrees on the Durometer Shore Scales or International Rubber Hardness Degrees (IRHD scale). Typically in the UK we use the Shore Scales. Hardnesses are normally based on a nominal figure e.g. 60 ±5° or as a hardness range e.g. 60-70°.
Below is a comparison table ilustrating the various shore scales and the corresponding hardnesses.
Tensile properties are the most measured rubber parameter after hardness. When specifying rubber tensiles, the following parameters should be determined: minimum tensile strength, minimum elongation at break and modulus of elasticity (i.e. minimum tensile stress at a given strain). Tensile requirements should be determined by calculations based on the requirements of the application compared to known values for similar applications or materials. Alternatively by testing a material which has been proved satisfactory by trial or experiment.
Compression set measures the ability of cured rubber to recover its original shape after the deforming force is removed. This is usually expressed as the percentage of the compression which is not recovered within a short time after release. Generally the lower the compression set percentage the better. The below table illustrates typical compression set values for compression of 24 hours at 70°C.