Overview
ISO 291:2008 - "Plastics - Standard atmospheres for conditioning and testing" specifies preferred constant atmospheric conditions for the conditioning and testing of all plastics and all types of test specimens. The standard defines standard atmospheres, conditioning and test atmospheres, conditioning procedures and minimum conditioning periods, and requirements for maintaining reproducible temperature and humidity. It excludes special or simulated climatic atmospheres that are specific to particular tests or materials.
Key Topics and Requirements
- Standard atmospheres: Two primary atmospheres are defined:
- 23/50 - 23 °C and 50 % RH (for non-tropical countries; recommended for comparative polymer data and databases)
- 27/65 - 27 °C and 65 % RH (for tropical countries)
- Notes: values apply at normal altitudes (86–106 kPa) with an air‑circulation velocity of 1 m/s.
- Classes and tolerances: Two classes control permitted deviations:
- Class 1: ±1 °C temperature; ±5 % RH
- Class 2: ±2 °C temperature; ±10 % RH
- Temperature-only conditions: If humidity is negligible, use “temperature 23” or “temperature 27.” If neither temperature nor humidity matters, use “ambient temperature” (typically 18–28 °C).
- Conditioning periods: Default minimums unless otherwise specified:
- 88 hours for atmospheres 23/50 or 27/65
- 4 hours for ambient temperatures 18–28 °C
- Moisture equilibrium: Annex A describes methods to attain moisture equilibrium (including scaling of conditioning time with specimen thickness per ISO 62), alternatives such as controlled drying (e.g., ~50 °C ±2 °C) and oven conditioning with defined RH, and criteria for determining equilibrium.
- Testing protocol: Specimens shall be tested in the same atmosphere in which they were conditioned and tested immediately after removal. Conditioning cabinets should be calibrated regularly (manufacturer recommendations; at least annually).
Applications and Who Uses It
- Plastics laboratories and QA/QC teams establishing reproducible test conditions for mechanical, thermal and physical testing.
- R&D and materials scientists generating comparable polymer property databases.
- Manufacturers and test houses performing standard compliance testing or inter-lab comparisons.
- Standards writers and accreditation bodies who need defined ambient conditions for test methods.
ISO 291:2008 ensures that conditioning and testing of plastics are reproducible and comparable across labs by standardizing temperature and humidity control, conditioning times, and tolerance classes.
Related Standards
- ISO 62 - Plastics - Determination of water absorption (normative reference cited by ISO 291).
