Shore Durometer Hardness

Shore durometer hardness is a measure of a material’s resistance to indentation by a spring-loaded indenter. It is reported on a scale from 0 to 100, with the scale letter, for example “Shore A 85” or “Shore D 50.” The test is standardized in ASTM D2240 and ISO 868 (and ISO 48-4 for rubber). Durometer hardness is one of the most common properties listed on data sheets for elastomeric coatings such as polyurea, polyurethane, and flexible membranes.

How a durometer works

A durometer is a handheld or stand-mounted gauge with a presser foot and a protruding indenter. When the foot is pressed flat against the material, the indenter is pushed back into the instrument against a calibrated spring. The depth of indentation is converted to a hardness number:

• 0 means full penetration (very soft).
• 100 means no penetration (very hard for that scale).

The reading depends on the indenter shape and spring force, which differ by scale.

Common scales

The scales overlap. As a rough rule in ASTM D2240, if a Shore A reading exceeds about 90, the Shore D scale should be used; if Shore D falls below about 20, Shore A is preferred. There is no exact conversion between scales because the indenters differ in shape.

Test procedure (summary)

ASTM D2240 describes the requirements in detail. In outline:

1. Specimen. The specimen should be at least 6 mm (about 0.25 in) thick unless the standard allows stacking or a thinner specimen with known effect. It should be flat and large enough to keep the indenter away from edges.
2. Conditioning. Condition specimens at the specified temperature and humidity, commonly 23 °C and 50% relative humidity, for a set period.
3. Applying the durometer. Press the foot onto the specimen firmly and parallel to the surface, without impact. Stand-mounted operation gives better repeatability.
4. Reading time. Read the value at the specified time. The standard reading is taken at 1 second for many materials, but specifications may require readings at other intervals, such as 15 seconds. Some elastomers creep, so the reading drops over time.
5. Multiple readings. Take several readings at different locations and report the median or mean as the method requires.

Testing coatings

Applied coatings present challenges:

• Thin films. Many coatings are thinner than the minimum specimen thickness. The hard substrate under a thin film raises the reading. Free films stacked to the required thickness, or thick cast samples, give more reliable data.
• Surface texture. Rough or textured surfaces give low and inconsistent readings.
• Temperature. Elastomers soften when warm and harden when cold. Field readings should note the surface temperature.
• Cure state. Hardness often increases over the first days after application. Data sheets usually specify a cure period before testing (see cure testing).

Using hardness in quality control

Durometer readings are quick and non-destructive, which makes them useful for:

• Checking cure of polyurea and polyurethane by comparing to the data sheet value after the specified cure time.
• Spotting off-ratio material. Areas that are markedly softer or harder than the rest of the job may indicate a mix ratio problem at the proportioner (see proportioner troubleshooting).
• Batch comparison during manufacturing.

Hardness alone does not describe toughness, flexibility or chemical resistance. A coating’s suitability depends on several properties together, such as tensile strength and elongation, tear strength, and abrasion resistance.

Durometer versus other hardness tests

• Pencil hardness (ASTM D3363) is used for rigid, thin coatings such as enamels.
• Pendulum hardness (König or Persoz, ISO 1522) measures damping on thin, hard films.
• Barcol hardness (ASTM D2583) is used for rigid plastics and fiberglass.
• Microindentation methods are used in research.

Durometers are best suited to thick, resilient materials.

Calibration

Durometers should be checked regularly with calibrated test blocks and verified by an accredited laboratory at intervals defined in the quality system. The indenter tip must be undamaged, and the presser foot must be flat.

Key terms

• Shore A. The durometer scale for softer elastomers, such as rubber and many flexible polyureas.
• Shore D. The scale for harder plastics and rigid elastomers, using a sharper indenter and higher spring force.
• Instantaneous reading. The value read immediately on contact; some standards also record a value after a set dwell time.
• Durometer stand. A fixture that applies the gauge with consistent force and alignment, improving repeatability.
• Minimum thickness. Test standards require a minimum specimen thickness, often achieved by stacking layers, so the substrate does not affect results.

Frequently asked questions

Is a higher Shore number always better?

No. Higher hardness may improve abrasion and indentation resistance but can reduce flexibility and crack bridging. The right value depends on the service.

Can I convert Shore A to Shore D?

Charts exist but are approximate. Use the scale named in the specification.

Why do readings drop when I hold the durometer longer?

Viscoelastic materials creep under load. That is why the standard specifies the reading time.

Related entries

• Abrasion and hardness testing
• Polyurea coatings
• Elastomeric coatings
• Glass transition temperature
• Tear strength of elastomeric coatings (new)
• Hybrid polyurea coatings (new)