ISO 2535:2001

Overview

ISO 2535:2001 specifies standardized methods for measuring the gel time of unsaturated‑polyester (UP) resins at ambient temperature (18 °C to 30 °C, referee temperature 25 °C). Gel time is defined as the elapsed time from addition of an initiator to the moment the resin reaches a viscosity of 50 Pa·s. The standard provides four validated measurement methods, calibration procedures, required reagents and apparatus, precision data and reporting requirements.

Key topics and technical requirements

• Scope and definition
  • Gel time at ambient temperature (18–30 °C); referee temperature 25 °C.
  • Gel state defined as viscosity = 50 Pa·s.
• Methods
  • A1 / A2 (automatic‑release): rotating 6 mm glass rod in test tube (A1) or beaker (A2); automatic stop when threshold reached.
  • B1 / B2 (continuous‑measurement): low‑frequency horizontal oscillation of a 6 mm rod in test tube (B1) or beaker (B2); force/viscosity recorded continuously.
• Reagents and sample preparation
  • Use analytical‑grade reagents. Examples given: toluene, reference accelerator (cobalt octoate solution, 0.6% Co), and reference initiator (50% methyl ethyl ketone peroxide in dimethyl phthalate, ~9% active oxygen).
  • Typical procedure: 50 g resin, preheat to test temperature, add accelerator (0.5 ml) and initiator (0.7 ml), mix, start timing and measure gel time.
• Apparatus & calibration
  • Measurement vessels: borosilicate test tubes (Ø 18 mm) or 50 ml polypropylene beaker.
  • Viscosity device must be calibrated to detect 50 Pa·s using certified standard oils (Cannon or silicone) per annexes A and B.
• Precision and repeatability
  • Round‑robin data (method B2) provide repeatability and reproducibility figures for two UP resins; tests must be repeated until two consecutive results agree within 10%.
• Reporting
  • Reports must reference ISO 2535, method used (A1/A2/B1/B2), device details, test temperature, gel time and sample identification.

Practical applications and users

ISO 2535 is used for quality control, R&D and comparative testing of unsaturated‑polyester resins in industries such as:

• composite manufacturing (boatbuilding, automotive, wind blades)
• coatings, adhesives and laminates
• resin and hardener formulation development
• third‑party testing laboratories and certification bodies

Benefits include repeatable gel‑time measurement for process control, formulation optimization and specification compliance.

Related standards

• ISO 472:1999 (Plastics - Vocabulary)
• ISO 4619:1998 (Driers - cobalt content determination)
• ISO 15038:1999 (Perester active‑oxygen determination)
• ISO 5725‑2 (Accuracy - repeatability and reproducibility)

Keywords: ISO 2535:2001, gel time measurement, unsaturated‑polyester resins, ambient temperature, viscosity 50 Pa·s, gel time test methods, resin quality control.