The hardness of an FRP composite is a direct result of the resin matrix and how it is cured. The more rigid the resin the higher the level of hardness, where as the more flexible laminate will have a lower hardness level. As a resin matrix cures and crosslinks, the hardness value continues to rise. When the resin fully cures, the hardness properties will be at the highest value.
FRP hardness is measured by a simple hand-held device called an "Impressor". An Impressor uses a needle and a spring gauge a reading on a dial or digital display. The three most common measurements of hardness are:
- Shore D
- Barcol 935
- Barcol 934
For most FRP thermoset composites, the Barcol 934 Impressor will likely have a reading between 35 and 40 once the resin matrix has fully cured.
Although the hardness of an FRP laminate is commonly measured with these mentioned Impressors, this is not possible when measuring the hardness of a thermosetting gel coat. If a Barcol 934 Impressor was used on the gel coat, the needle would penetrate through the coat and actually measure the FRP substrate.