ISO 9026:2021

Overview

ISO 9026:2021 specifies a standardized tensile stress–strain method for determining the green strength of raw rubber and unvulcanized rubber compounds. Green strength describes the resistance of uncured rubber to tensile deformation or fracture and is a key property for processing operations (for example, tyre building) where the material must resist distortion before vulcanization. The standard covers test-piece preparation, apparatus, test conditions, calculation of stresses, and reporting requirements.

Key Topics and Requirements

• Test principle: Tensile stress–strain test on dumbbell or beaded-end test pieces, recorded with a constant-rate tensile-testing machine.
• Apparatus: Class 2 tensile tester per ISO 5893:2019, capable of a preferred jaw separation rate of 100 mm/min; non‑contact automatic extensometers recommended if used.
• Test pieces:
  • Preferred: dumbbell with beaded ends (moulded from ~2.2 mm sheet or die-cut from calendered sheets 2–4 mm thick).
  • Alternative: ISO 37 dumbbells (types 1, 1A or 2) with protective end pads to avoid grip damage.
  • Preparation notes: moulding example - compress 5 min at 100 °C under 2.5 MPa (use release film such as 0.25 mm PTFE or polyester when needed).
• Number of tests: At least five test pieces per determination.
• Conditioning and temperature:
  • Condition at the chosen standard laboratory temperature for between 24 h and 72 h (consistent period across comparable tests).
  • Test normally at the standard laboratory temperature (see ISO 23529).
• Measurements and calculations:
  • Thickness measured using ISO 23529 method A with a pressure of 10 kPa ± 2 kPa; width taken from die dimensions.
  • Express results as yield stress or maximum stress (MPa); stresses calculated from initial cross-sectional area using ISO 37 procedures.
• Reporting: Must include sample description, test-piece preparation, test temperature and rate, conditioning time, number of specimens, medians and ranges for yield/maximum stress, and date.

Applications

ISO 9026:2021 is used to:

• Characterize processing behaviour of raw rubbers and unvulcanized mixes (green tyre expansion, calendering, extrusion).
• Compare formulations (polymer type, filler/plasticizer effects, peptizers) on tensile resistance before cure.
• Provide quality-control data for rubber compound development and manufacturing process optimization.

Who Would Use This Standard

• Rubber compounders and formulation scientists
• Quality control and testing laboratories in the rubber industry
• Tire manufacturers and converters performing green strength evaluations
• R&D teams assessing polymer and filler effects on uncured mechanical behaviour

Related Standards

• ISO 37 - Tensile stress–strain properties of vulcanized or thermoplastic rubber
• ISO 1795 - Sampling and preparative procedures for raw rubber
• ISO 2393 - Rubber test mixes: preparation and moulding equipment/procedures
• ISO 5893:2019 - Tensile test equipment specification
• ISO 23529:2016 - Preparation and conditioning of rubber test pieces

Keywords: ISO 9026:2021, green strength, raw rubber, unvulcanized compounds, tensile stress-strain test, dumbbell test piece, tyre-building, tensile tester.