Shore Hardness and Soft TPEs

Thermoplastic Elastomers, or Thermoplastic Rubber compounds, as they're sometimes called, are a class of materials that behave a lot like rubber, but process like plastic. TPEs consist of hard and soft phase systems in the form of blends, alloys, or copolymers.  TPE materials are often used in applications requiring a "soft touch", like a grip on a toothbrush handle, or in applications where flexibility is desired. TPEs can even be used to absorb energy in acoustic or vibrational dampening applications. TPEs are quite unique materials, because depending on the recipe, they can take the form of a super-soft gel or a hard plastic like polypropylene.  It is no wonder why "Hardness" is an important property that is measured and specified when talking TPEs.

What is Shore Hardness and How is it Measured?
The Shore Hardness scales were created to provide a common reference point when comparing different materials.  Shore Hardness is measured using a durometer gauge, which uses a spring loaded steel rod to compress the surface of the TPE sample and provide a reading.  A reading of 0 Shore describes the maximum possible indentation of the rod into the specimen, and 100 Shore indicates almost no indentation al all or a very high resistance to indentation.

There are different Shore Hardness scales for measuring the hardness of different materials.  For each scale there is either a different probe/indenter shape as well as a specific spring force. The Shore OO Scale measures extremely soft materials like gels (think of gel insoles). The Shore A  Scale measures a wide range of material types; from very soft and flexible to semi-rigid plastics with almost no flexibility at all.  The Shore D Scale measures very hard rubbers, semi-rigid and rigid plastics (PVC pipe, for example).  As you can see from the graphic, there is overlap on the different scales. For example a material with a Shore hardness of 95A is also a Shore 50D.  TPEs can range from 10 Shore 00 up to 72 Shore D, spanning all three hardness scales, however, the most commonly used is the Shore A scale.

Shore Hardness and How it Relates to Other Properties
Shore hardness is often used as an indication of flexibility or stiffness, which is called flexural modulus.  Flex Modulus measures resistance to bending, whereas hardness measures resistance to indentation.  However, within a TPE family there exists a direct relationship between these properties; as hardness increases, so does flexural modulus.  The tensile strength of a TPE material, which measures it's resistance to breaking under stress, is also directly related to hardness within a TPE family.  One property that varies inversely with hardness is coefficient of friction.  Softer TPEs usually contain more oil, resulting in higher surface tack, in general.