Tear Strength of Elastomeric Coatings

Tear strength is the resistance of a flexible material to the growth of a cut or nick when it is put under tension. For elastomeric coatings, membranes and linings, it indicates how well the film will resist ripping at a damaged spot, at a sharp edge, or where it is stretched across a crack or joint. The standard most often cited on polyurea and polyurethane data sheets is ASTM D624, “Standard Test Method for Tear Strength of Conventional Vulcanized Rubber and Thermoplastic Elastomers.”

Why tear strength matters

An elastomeric coating can have high tensile strength and still tear easily once a nick starts. In service, tears begin at:

• Impact damage, gouges and scratches
• Fasteners, edges and penetrations
• Cracks and joints in the substrate that move
• Areas of thin film or defects such as pinholes

Good tear resistance helps a membrane survive these stresses without splitting. It matters for truck bed liners, roof coatings, secondary containment coatings, geotextile liners, and waterproofing membranes.

ASTM D624 test method

The method cuts specimens from a free film or sheet using a standardized die, then pulls them apart in a tensile testing machine at a constant rate. The maximum force is divided by the specimen thickness.

Die types

ASTM D624 describes several specimen shapes:

• Die A. A crescent with a nick cut into the inner curve. Measures tear propagation from the nick.
• Die B. A crescent with tab ends and a nick. Similar to Die A.
• Die C. A 90-degree angle specimen with no nick. Measures tear initiation and propagation at the angle. Die C is the most common type quoted on coating data sheets.
• Type T (trouser). A rectangle slit lengthwise, with the two “legs” pulled apart. Measures propagation along the slit.
• Type CP. A constrained path specimen.

Results from different dies are not interchangeable. A data sheet value should always name the die.

Units

Tear strength is reported as force per unit thickness, usually pounds per linear inch (pli) or kilonewtons per meter (kN/m). One kN/m is about 5.71 pli.

Specimen preparation

• Free films are usually cast or sprayed onto release surfaces and peeled off after cure.
• Thickness is measured at several points and must be within the method’s limits.
• Specimens are conditioned at standard temperature and humidity before testing.
• The cure time before testing should match the data sheet or specification, because properties develop over days for many elastomers.

Test speed and temperature

The standard specifies the crosshead speed, commonly 500 mm per minute, and the test temperature. Elastomers are viscoelastic, so changes in speed or temperature change the result.

ISO and other methods

• ISO 34-1 covers tear strength of rubber with trouser, angle and crescent test pieces.
• ASTM D1004 is a tear test for plastic film and sheeting.
• ASTM D1938 is a trouser tear method for thin plastic film.

Specifications may cite one of these for particular product types.

Interpreting results

Tear strength values help compare products of similar type. When reviewing data:

1. Confirm the test method and die. A Die C result cannot be compared with a trouser tear result.
2. Check the thickness and cure. Specimens tested after a short cure may show different values from fully cured specimens.
3. Consider temperature. Data at room temperature may not reflect behavior in cold climates.
4. Look at the full property set. Tear strength should be considered with tensile strength and elongation, hardness, and adhesion.

Never extrapolate a field performance claim from a single laboratory number. Coating suitability should be judged against the manufacturer’s data sheet and, for critical work, project-specific testing.

Factors that affect tear strength

• Formulation. Polymer type, crosslink density and fillers all play a role (see crosslink density).
• Mix ratio and mixing quality. Off-ratio or poorly mixed material often has lower tear strength.
• Application conditions. Moisture contamination in isocyanate systems can create bubbles that act as tear initiation points.
• Film thickness. Thicker films resist tearing at higher total force, though force per unit thickness may be similar.
• Reinforcement. Fabric or geotextile reinforcement can increase overall tear resistance of a system (see coating geotextile fabric).

Field indications of low tear resistance

• Splits that grow from fastener holes or edges
• Tears along substrate joints
• Easily propagated cuts during inspection
• Peeling and tearing at terminations

These can also result from poor adhesion or design details, so a failure investigation should consider all causes (see coating failure investigation).

Key terms

• Die C specimen. A right-angle specimen shape in ASTM D624 used for tear initiation testing.
• Trouser tear. A specimen cut along its length and pulled apart, used to measure tear propagation.
• Tear initiation. The force needed to start a tear from an uncut or sharp corner.
• Tear propagation. The force needed to extend an existing cut.
• Notch sensitivity. How much a material’s strength falls once a cut or nick is present.

Frequently asked questions

Which die is best?

There is no best die. Use the one named in the specification. Die C is common for comparing spray elastomers.

Is tear strength the same as puncture resistance?

No. Puncture resistance is measured by different tests, such as ASTM D4833 for geomembranes.

Can a field test measure tear strength?

Not accurately. Tear strength requires prepared specimens and a calibrated tensile tester in a laboratory.

Related entries

• Tensile strength and elongation of coating free films
• Elastomeric coatings
• Polyurea coatings
• ASTM standards for coatings
• Shore durometer hardness (new)
• Coating geotextile fabric (new)