5.2.1.5 Substrate Hardness

Also referred to as ‘durometer’, hardness is measured empirically by observing how far a spiked probe extends into a surface. This property is usually correlated with the flexibility of a material. In some instances, it may be better for the durometer of the adhesive to be much lower than the substrates, such as in a compressed silicone gasket on a rigid flange. The Shore scales of hardness are the most common method of quantifying this property. Table 20 shows the different Shore scales with examples of materials that fall into these ranges.

Table 20 This chart illustrates how the Shore hardness scales relate to each other. These three Shore scales are the most commonly used to measure the hardness of adhesives

Most adhesives will be measured in the Shore A or Shore D scales; however, there is overlap between the scales of hardness. For example, a high rating on the Shore A scale could be the same as a low rating on the Shore D scale. When comparing multiple materials, it is always best to use the same Shore hardness scale and the same testing equipment if possible.

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