Recycled Rubber Tensile Strength Testing: Standards, Sample Preparation, and Test Methods

Recycled rubber tensile strength testing is a critical quality control procedure that measures the maximum tensile stress a reclaimed rubber compound can withstand before fracture, providing essential data for material grading, product design, and trade acceptance. This testing follows established international and national standards such as ISO 37, ASTM D412, and GB/T 528, which specify specimen geometry, conditioning requirements, and test conditions to ensure reproducible results. Understanding the complete testing workflow, from vulcanized sheet preparation to data evaluation, helps manufacturers of reclaimed rubber and downstream rubber products verify material performance and optimize formulations.

Technical Overview

Recycled rubber, also known as reclaimed rubber, is produced by devulcanizing and mechanically processing waste rubber products such as scrap tires, inner tubes, and industrial rubber scraps. During the devulcanization process, the three-dimensional crosslinked network of the original vulcanizate is partially broken down, allowing the material to be re-processed, re-compounded, and re-vulcanized into new products. Because the degree of devulcanization, purity of the feedstock, and quality of the reclamation process directly determine the mechanical performance of the final compound, tensile strength serves as one of the most important indicators of recycled rubber quality.

Tensile strength is defined as the maximum tensile stress reached when a test piece is stretched to rupture. For reclaimed rubber, this property reflects the integrity of the rubber network after reclamation, the level of filler and fiber contamination, the fineness of the reclaimed particles, and the effectiveness of the subsequent compounding and vulcanization process. National and international specifications for reclaimed rubber, such as the GB/T 13460 series, classify commercial grades largely according to minimum tensile strength and elongation at break values, making standardized tensile testing essential for product classification, supplier qualification, and contractual acceptance.

Testing Samples

Standard Vulcanized Test Sheets

The primary sample form for recycled rubber tensile strength testing is a standard vulcanized sheet prepared from the reclaimed rubber compound. The reclaimed rubber is first mixed with a standard or agreed compounding recipe, typically containing sulfur, accelerators, zinc oxide, stearic acid, and reinforcing fillers, using a laboratory two-roll mill or internal mixer. The compound is then vulcanized into flat sheets, normally with a thickness of 2.0 mm ± 0.2 mm, in a hydraulic hot press at a specified temperature and time selected from rheometer data or standard cure schedules. Dumbbell-shaped specimens are subsequently die-cut from these sheets for testing.

Natural-State Compounds and Modified Reclaimed Rubber

Samples of tire reclaimed rubber, butyl reclaimed rubber, EPDM reclaimed rubber, and latex reclaimed rubber may each be evaluated using grade-specific compounding formulas. In addition, devulcanized rubber modified with reclaiming agents, activated swell agents, or thermomechanical treatments can be tested to compare process improvements against baseline material. Test programs may also include blended compounds in which reclaimed rubber is substituted for a defined percentage of virgin rubber, allowing assessment of tensile property retention at different blending ratios.

Finished and Semi-Finished Products

When testing is required for quality verification or failure analysis, specimens may be taken directly from finished goods such as molded mats, tire retread stock, hose covers, footwear components, and sealing products. Product samples are cut parallel to a defined direction, and the actual measured thickness of the product is used in the stress calculation. Results from finished products should be interpreted with care because thickness deviations, curvature, fabric layers, and processing history can differ from laboratory-prepared sheets.

Testing Items

A complete tensile evaluation of recycled rubber typically includes the following measured and derived parameters:

• Tensile strength — the maximum stress recorded during extension to break, expressed in megapascals (MPa), calculated as the maximum force divided by the original cross-sectional area of the test piece.
• Elongation at break — the extension of the gauge length at the moment of rupture, expressed as a percentage of the original gauge length.
• Tensile stress at given elongation (100% and 300% modulus) — the stress recorded when the test piece has been extended to a specified elongation, used to characterize stiffness and cure state.
• Tensile strength retention after aging — the percentage retention of tensile strength and elongation after heat aging in an air oven, which evaluates the durability of the reclaimed compound.
• Optional related properties — tear strength, hardness, and density are frequently tested alongside tensile properties to provide a fuller mechanical profile of the reclaimed rubber grade.

Testing Methods

Applicable Standards

The most widely applied test methods for rubber tensile properties are ISO 37, ASTM D412, GB/T 528, and JIS K 6251. In China, reclaimed rubber products are graded under the GB/T 13460 product standard, while test piece preparation is governed by GB/T 6038 and physical testing of vulcanizates follows the GB/T 528 procedure. ASTM D3182 and ISO 23529 provide complementary guidance on mixing and vulcanizing standard compounds. Laboratories performing these tests should operate under ISO/IEC 17025 accreditation requirements to ensure technically valid and traceable results.

Specimen Preparation and Conditioning

Dumbbell-shaped test pieces, such as ISO 37 Type 1 or Type 2 specimens, are stamped from the vulcanized sheets using a sharp die to avoid edge defects, since nicked or rough edges cause premature tearing and artificially low results. The thickness of the narrow working section is measured at three points with a calibrated thickness gauge, and the median value is used for cross-sectional area calculation. Specimens are conditioned in a controlled atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 16 hours after vulcanization and before testing. Products requiring de-molding agent removal or marking of the gauge length are prepared before clamping.

Test Procedure and Equipment

Testing is performed on a calibrated universal tensile testing machine fitted with pneumatic or mechanical grips and, where elongation must be measured precisely, a contact or non-contact extensometer. Key procedural requirements include the following:

• Set the crosshead speed at 500 mm/min ± 50 mm/min, the standard rate for most dumbbell specimens under ISO 37 and GB/T 528; narrower specimens under ASTM D412 may use 500 mm/min as specified for the relevant die.
• Align the test piece in the grips so that the major axis coincides with the direction of the applied force, and pre-load only minimally to avoid introducing initial strain.
• Record the force-extension curve continuously until rupture occurs within the narrow section; breaks outside the gauge length are discarded.
• Test at least five valid specimens and report the median value for tensile strength and, where required, the arithmetic mean for other parameters.
• Verify machine force accuracy, extensometer calibration, and die dimensions at defined intervals in accordance with the laboratory quality system.

Calculation and Result Evaluation

Tensile strength is calculated by dividing the maximum force at break by the original cross-sectional area (width multiplied by median thickness) of the test piece. Elongation at break is the increase in gauge length at rupture divided by the original gauge length, multiplied by 100. The reported results are compared against the specification limits of the applicable reclaimed rubber grade; for example, typical commercial tire reclaimed rubber grades specify minimum tensile strength values in the approximate range of 9 MPa to 16 MPa together with minimum elongation at break requirements, and the exact thresholds should be confirmed against the current edition of the governing product standard and the contractual specification between supplier and purchaser.

Applications

Recycled rubber tensile strength testing supports quality and engineering decisions across the entire rubber value chain:

• Reclaimed rubber manufacturing — process control and grade classification at reclaiming plants, optimization of devulcanization parameters, and release testing of finished bales before shipment.
• Tire and retread industry — verification of reclaimed rubber blends used in carcass compounds, inner liners, and retread materials where strength and durability requirements are demanding.
• Molded and extruded products — qualification of reclaimed rubber in mats, footwear soles, hoses, conveyor belt components, dock fenders, and playground surfaces.
• Trade and regulatory compliance — provision of accredited third-party laboratory test reports for customs inspection, buyer acceptance, and certification of exported or imported reclaimed rubber.
• Research and development — comparison of new devulcanization technologies, evaluation of sustainable formulations with higher reclaimed content, and long-term aging studies supporting circular-economy initiatives.

FAQ

Why must reclaimed rubber be vulcanized into test sheets before tensile testing?

Reclaimed rubber in its raw state is a plastic, processable material without a fully formed elastic network. Meaningful tensile properties only exist after the material is compounded and vulcanized. Testing therefore uses standardized vulcanized sheets so that results reflect both the quality of the reclaimed rubber and its behavior in a realistic cured state, enabling fair comparison between suppliers and grades.

What specimen thickness and test speed are standard?

Standard test sheets are prepared at 2.0 mm ± 0.2 mm thickness, and dumbbell specimens are tested at a crosshead speed of 500 mm/min ± 50 mm/min under ISO 37 and GB/T 528. Deviations from these conditions should be documented because speed and thickness influence the measured strength values.

How many specimens are tested and how are results reported?

A minimum of five valid test pieces is normally tested. The median tensile strength is reported as the characteristic value, and elongation results are typically reported as the median or mean as defined by the governing standard. Specimens that break outside the gauge section or at edge defects are excluded and replaced.

What causes unusually low tensile strength in reclaimed rubber?

Common causes include excessive devulcanization that degrades the polymer chains, contamination by fiber, metal, or foreign polymers, coarse particle size or poor dispersion, insufficient or over-vulcanization of the test compound, moisture in the material, and poor specimen preparation with nicked edges. A structured root-cause investigation covering feedstock, process, and laboratory variables is recommended.

Can tensile results from different test standards be compared directly?

ISO 37, ASTM D412, GB/T 528, and JIS K 6251 are technically harmonized in many respects, but differences in die geometry, speed tolerances, and calculation conventions can produce small systematic deviations. For contractual purposes, the test standard should be agreed in advance and stated on the test report, and comparisons across methods should be treated as indicative rather than equivalent.