What is Material Hardness?
The makers of the coatings and paints we use at PennCoat publish extensive technical information about the ingredients, properties and safety of each product. We use this information when selecting the best product for each application. For example, when we apply a cementitious urethane surface, we are looking for a material that will withstand heavy traffic — in other words, a hard surface.
How Hard Is It?
Hardness is a physical characteristic of matter that describes its resistance to indentation, scratches or compressive forces. For the most part, the strength of a material is a function of intermolecular bonds holding it together. Various methods and devices have been developed to quantify a material’s hardness. When dealing with polymers and elastomers, the test of choice is known as the Shore durometer test, where the word durometer refers to the measuring device and to the unit of measure. The instrument is named after Albert F Shore, who defined the durometer scale.
A Shore durometer measures how much a material indents when a standardized amount of pressure is applied. The indentation hardness is inversely related to the indentor’s penetration and is dependent on the material’s viscoelastic behavior and elastic modulus. ASTM International, formerly known as the American Society for Testing and Material, publishes a variety of industry standards, including D2240 for testing the durometer hardness of substances classified as:
- thermoplastic elastomers
- vulcanized (thermoset) rubber
- elastomeric materials
- cellular materials
- gel-like materials
- some plastics
There are in fact 12 different types of hardness durometers, and Type D is often used for hard elastomeric coatings, surfaces and films. The indentor used for Type D durometer testing is hardened steel rod 1.40 mm diameter, with a 30° conical point and a 0.1 mm radius rounded tip. It applies 44.64 newtons of force. When one of the cementitious urethanes we use, Poly-Crete MD, is subjected to the ASTM D 2240 durometer test, it registers 75 to 80 on a scale from 1 (softest) to 100 (hardest). That’s reasonably hard, similar to the hardness you’d find for a hard hat or the soft wheels of a skateboard.
The Barcol hardness test is used on composite materials like reinforced thermosetting resins and rigid plastics. The indentor has a sharp point with a flat tip. A reading of 60 Barcol units equals about 80 Shore units.
Other hardness tests are used for materials such as metals, minerals and wood, and include: