Overview
EN ISO 291:2008 - Plastics - Standard atmospheres for conditioning and testing (CEN adoption of ISO 291:2008) specifies preferred constant atmospheres for the conditioning and testing of all plastics and all types of test specimens. It defines standard temperature and relative-humidity sets, classes of control tolerance, conditioning and testing procedures, and guidance on attaining moisture equilibrium. Special or application-specific atmospheres are excluded.
Key topics and requirements
- Standard atmospheres (Table 1)
- 23/50: 23 °C and 50 % RH - recommended for non-tropical countries and for comparative polymer databases.
- 27/65: 27 °C and 65 % RH - recommended for tropical countries.
- Values apply at normal altitudes (86–106 kPa) and low air-circulation velocities.
- Classes of atmosphere control (Table 2)
- Class 1: tighter tolerances (±1 °C; ±5 % RH).
- Class 2: wider tolerances (±2 °C; ±10 % RH).
- Cabinets/enclosures shall be calibrated (manufacturer guidance) and at least once per year.
- Conditioning times and procedures
- Default minimum conditioning: 88 h for 23/50 or 27/65; 4 h for ambient temperature (18–28 °C) unless the test standard states otherwise.
- Specimens should be tested in the same atmosphere in which they were conditioned and tested immediately after removal.
- Moisture equilibrium (Annex A)
- Time to reach moisture equilibrium depends on material and specimen thickness. Conditioning time scales approximately with the square of thickness (t′ ∝ d′2).
- Alternatives for slow-equilibrating materials: controlled drying (e.g., ~50 °C) with cooling in a desiccator, or conditioning until a defined fraction (70 %) of saturation according to ISO 62. Reports must state any alternative procedure used.
- Definitions: clear terms for standard atmosphere, , , , and .
