Overview
ISO 289-2:2020 specifies a laboratory method to determine the pre-vulcanization characteristics (scorch or pre-vulcanization time) of compounded, unvulcanized rubber using a shearing-disc (Mooney) viscometer. The measured pre-vulcanization time estimates how long a rubber compound can be held at elevated temperature and remain processable before significant crosslinking begins. This third edition updates the precision statement based on an interlaboratory test programme (ITP) performed in 2018.
Key topics and technical requirements
- Test principle: monitor the change in Mooney viscosity with running time at a chosen test temperature relevant to processing; record the time required for the viscosity to rise by a specified amount above the minimum.
- Apparatus: use the shearing-disc viscometer and accessories as specified in ISO 289-1 (Mooney viscosity method). Small or large rotors may be used as permitted.
- Test parameters:
- Pre-heating time: 1 minute.
- Viscosity increase to define scorch: 5 Mooney units for the large rotor (designated t5) and 3 Mooney units for the small rotor (designated t3).
- Test temperature: chosen to match the relevant processing conditions.
- Die-closing force: report if other than the standard 11.5 kN.
- Specimen preparation: two discs prepared from compound sheet as described in ISO 289-1.
- Calibration and traceability: verification/calibration of the test apparatus is mandatory; laboratories should follow the schedule guidance (Annex A) and ISO 18899 for calibration practices.
- Precision: Annex B reports precision data from a 2018 ITP (16 labs) using SBR, NBR and EPDM compounds; repeatability and reproducibility figures are provided for typical test conditions.
- Test report requirements: include sample description, compound details, apparatus model and rotor size, test temperature, minimum viscosity, pre-vulcanization time (t5 or t3), and date(s) of test.
Practical applications and users
ISO 289-2:2020 is used by:
- Rubber compounders and formulators to evaluate scorch safety and optimize accelerator/sulfur systems.
- Quality control laboratories to monitor batch-to-batch consistency and processing window.
- R&D and process engineers to select processing temperatures and estimate safe hold times for mixing, extrusion, calendaring and molding.
- Tyre and rubber-product manufacturers to reduce processing defects from premature vulcanization (scorch).
Benefits include improved process planning, reduced scrap, and more reliable product performance through standardized measurement of scorch behavior.
Related standards
- ISO 289-1 - Determination of Mooney viscosity (required apparatus and specimen prep).
- ISO 18899 - Guide to calibration of test equipment.
- ISO 19983 - Determination of precision of test methods (precision analysis used in Annex B).
Keywords: ISO 289-2:2020, pre-vulcanization characteristics, shearing-disc viscometer, Mooney viscosity, scorch time, compounded rubber testing, rubber test method, calibration, precision.