Recycled Rubber Tear Strength Testing: Standards, Methods and Applications

Tear strength testing is an essential quality control procedure for recycled (reclaimed) rubber, quantifying the material's resistance to crack initiation and propagation under tearing forces. This technical article explains the internationally recognized testing standards (ISO 34-1, ASTM D624, GB/T 529), specimen preparation, test procedures, result calculation, and typical application requirements for recycled rubber compounds.

Technical Overview

Recycled rubber, also referred to as reclaimed rubber, is produced by devulcanizing and reprocessing waste vulcanized rubber products such as scrap tires, inner tubes, and industrial rubber scrap. During the reclamation process, the original three-dimensional crosslinked network is partially broken down, which changes the mechanical behavior of the material. Tear strength, defined as the force required to propagate a tear through a specimen of a given thickness, is one of the most important indicators of the structural integrity and service durability of recycled rubber compounds.

Tear strength is expressed in kilonewtons per meter (kN/m) and is calculated by dividing the maximum or median tearing force by the specimen thickness. Because recycled rubber is frequently blended into tire compounds, conveyor belts, footwear, and molded goods, applications in which crack growth resistance is safety-critical, systematic tear strength testing is indispensable for material qualification, formulation optimization, and batch-to-batch quality control.

The principal standards governing tear strength testing of vulcanized thermoset rubbers and thermoplastic elastomers include:

• ISO 34-1 and ISO 34-2: Determination of tear strength using trouser, angle and crescent test pieces
• ASTM D624: Standard Test Method for Tear Strength of Conventional Vulcanized Rubber and Thermoplastic Elastomers
• GB/T 529: Chinese national standard, technically aligned with ISO 34
• ISO 37 and ASTM D412: Tensile property testing, frequently performed in parallel to build a complete mechanical profile

Testing Samples

Tear strength is measured on vulcanized test sheets prepared from the recycled rubber compound using a standard laboratory cure, for example press-cured at a defined temperature, time and pressure. Specimens are die-cut from these sheets, normally with the grain direction identified and, where anisotropy is suspected, in both parallel and perpendicular directions. Three specimen geometries are commonly used:

Trouser-Type Specimens

Rectangular strips of nominally 100 mm length and 15 mm width, with a thickness of approximately 2 mm, featuring a central slit cut over about 40 mm of the length. The two legs are clamped in the grips and pulled apart. Trouser specimens evaluate the steady propagation of a pre-existing tear and use the median force of the tear plateau, making them well suited to materials with relatively low tear resistance.

Angle-Type Specimens

Specimens containing a 90 degree internal angle. In the nicked version, a small cut of defined depth is made at the apex of the angle to initiate tearing. The tear path is forced through the highly stressed region at the angle tip, so the result reflects both crack initiation and propagation behavior.

Crescent-Type Specimens

Specimens with a crescent profile featuring a small nick on the inner concave edge. The nick concentrates stress and directs the tear across the reduced section. Crescent specimens generally produce high peak forces and are widely used for routine comparative testing.

For all specimen types, the thickness along the intended tear path is measured at three points using a dial thickness gauge accurate to 0.01 mm, and the median value is used in the calculation. At least five valid specimens per material and direction are recommended, and all specimens must be free of surface defects, bubbles, and visible flaws.

Testing Items

A typical tear strength test program for recycled rubber includes the following reported items:

• Maximum tear force in newtons recorded during the test
• Median tear force in newtons, used for trouser-type specimens
• Tear strength in kN/m, calculated as force divided by the median specimen thickness
• Individual and median results for the specimen set, with outliers identified
• Type of failure and description of the tear path, such as clean tear, knotty tear, or deviation from the intended path
• Test conditions including specimen type, test speed, temperature, humidity, and conditioning history
• Compound details including reclaimed rubber content, blend ratio with virgin rubber, and cure conditions

Testing Methods

Testing is performed on a calibrated universal testing machine equipped with suitable grips and an appropriately sized load cell. The general procedure is as follows:

• Condition the vulcanized specimens at 23 degrees Celsius plus or minus 2 degrees, and where required at 50 percent plus or minus 5 percent relative humidity, for at least 16 hours before testing.
• Verify the initiating nick and, where required, cut it immediately before testing using a sharp cutting tool; a depth of approximately 0.5 mm is typical for crescent specimens.
• Measure the specimen thickness along the tear path at three points and record the median value.
• Mount the specimen correctly: angle and crescent specimens are gripped at both ends, while trouser specimens are gripped by each leg.
• Apply the crosshead speed specified by the standard, typically 500 mm/min for angle and crescent specimens and 100 mm/min for trouser specimens.
• Record the force-extension curve until the specimen is completely torn and note the maximum or median force.
• Calculate tear strength using the formula T equals F divided by d, where F is the maximum or median force in newtons and d is the median thickness in millimeters; report the result in kN/m.
• Calculate the median of at least five valid results and report individual values, the median, and any observed deviations.

Key influences on the measured result include specimen geometry, nick quality and depth, test speed, temperature, cure state of the compound, filler type and loading, and the proportion and quality of reclaimed rubber in the formulation. Recycled rubber compounds typically achieve tear strengths in the approximate range of 10 kN/m to 50 kN/m depending on devulcanization quality, blend ratio, and reinforcement system, which is generally lower than comparable virgin rubber compounds.

Applications

Tear strength data for recycled rubber directly support product design, material approval, and process control across multiple industries:

• Tire manufacturing: reclaimed rubber in carcass, sidewall, and retread compounds where cut-and-tear resistance affects service life
• Conveyor belting: cover compounds and impact-resistant skid layers
• Rubber flooring and matting: tiles, sports surfaces, and industrial mats subject to gouging and tearing
• Footwear: soles and heels requiring flex-crack and tear durability
• Molded and extruded goods: bushings, gaskets, seals, dock fenders, and vibration isolation elements
• Civil engineering products: bridge bearing pads, expansion joint fillers, and roofing membranes
• Formulation development: comparing devulcanization processes, blend ratios, and reinforcing filler systems

FAQ

What is the difference between tensile strength and tear strength?

Tensile strength measures the force required to break a uniformly stressed specimen, while tear strength measures the force required to propagate a cut or nick through the material. Tear strength is more sensitive to local flaws, crack growth resistance, and the integrity of the rubber network, which is why it is particularly informative for recycled rubber.

Which standard should be selected for recycled rubber tear testing?

ISO 34-1 and ASTM D624 are the most widely applied international methods, and GB/T 529 is the equivalent Chinese national standard. The choice depends on customer specification and target market. Because specimen geometry and test speed differ slightly between methods, results obtained under different standards should not be compared directly.

Why is the tear strength of recycled rubber usually lower than that of virgin rubber?

Devulcanization partially breaks the crosslinked network and shortens polymer chains, and reclaimed rubber also contains residual fillers, ash, and impurities from the waste stream. These factors reduce the energy required to propagate a tear. Blending recycled rubber with virgin rubber and selecting appropriate reinforcing fillers can restore much of the lost performance.

How many specimens are required and how are results evaluated?

A minimum of five valid specimens per sample is standard practice. The median of the individual tear strength values is reported together with the spread of the results. Specimens that tear outside the intended path or fail at a visible defect are discarded and replaced.

Can tear strength testing be combined with other tests?

Yes. Tear strength is commonly performed alongside tensile strength and elongation, hardness, density, abrasion resistance, and aging tests as part of a full quality assurance program conducted by accredited third-party laboratories or in-house testing facilities.