ISO 2921:2019

Overview

ISO 2921:2019 - Rubber, vulcanized - Determination of low‑temperature characteristics - Temperature‑retraction procedure (TR test) - defines a standardized laboratory method to measure the low‑temperature behaviour of stretched vulcanized rubber. The procedure quantifies the temperatures at which specified percentages of retraction occur as the frozen, elongated specimen is warmed. Thermoplastic rubbers are excluded from this standard.

Key topics and technical requirements

• Principle: Stretch a test piece at standard laboratory temperature, cool it until retraction is suppressed, remove the load and increase temperature at a controlled rate while recording retraction vs. temperature.
• Apparatus essentials:
  • Retraction rack with holders and locking device capable of up to 350% elongation and maintaining a slight tension (10–20 kPa).
  • Heat‑transfer medium (liquid or gas) that remains fluid and inert at test temperatures.
  • Temperature sensor accurate to ±0.5 °C and temperature control to ±1 °C.
  • Agitation (stirrer or fan), stopwatch and means to read retraction with ±0.25 mm or ±0.5% accuracy.
• Heat‑transfer fluids (examples):
  • Silicone fluids to about −60 °C
  • Ethanol to about −73 °C
  • Methylcyclohexane cooled by liquid nitrogen to about −120 °C
• Test pieces:
  • Standard strips with enlarged ends, 2 mm ±0.2 mm thick; reference length 100 mm or 50 mm.
  • At least three specimens per test; alternative product‑cut samples (e.g., O‑rings) possible but not directly comparable to standard pieces.
• Procedure parameters:
  • Cool bath to below −70 °C, immerse stretched specimens, hold 10 ±2 min at −70 to −73 °C.
  • Unlock holders, allow free retraction while raising temperature at 1 °C/min (tolerance: 10 °C per 10 min ±2 °C).
  • Continue readings at ≤2 min intervals until ≥71% retraction is reached.
• Conditioning & decrystallization: Samples conditioned per ISO 23529 (minimum 16 h). If crystallization is present, decrystallize at 70 °C for 30 min and re‑condition 30–60 min.
• Calibration & precision: Apparatus calibration schedule in Annex A and precision data in Annex B; normative reference to ISO 18899 for equipment calibration.

Applications and users

ISO 2921:2019 is used to assess the low‑temperature performance, freezing/retraction behaviour and crystallization effects of vulcanized rubber compounds. Typical users:

• Rubber R&D and compound development teams
• Quality control and materials testing laboratories
• Automotive, aerospace and industrial OEMs specifying low‑temperature elastomer behavior
• Suppliers validating O‑rings, seals, hoses and cold‑climate rubber components

Related standards

• ISO 23529 - sample preparation and conditioning for rubber tests
• ISO 18899 - guide to calibration of test equipment

Keywords: ISO 2921:2019, TR test, temperature‑retraction procedure, low‑temperature characteristics, vulcanized rubber, retraction temperature, test pieces, heat‑transfer medium, rubber testing.