Overview
ISO 16533:2014 specifies a laboratory test method to measure the exothermic and endothermic thermal behaviour of sheet-like textiles (fabrics, knits, nonwovens) when environmental humidity changes. The method quantifies heat released during moisture absorption (exothermic) and heat absorbed during moisture desorption/evaporation (endothermic) by recording temperature changes of a specimen transferred between controlled humidity environments.
Key topics and technical requirements
- Scope: Measurement of exothermic/endothermic properties caused by moisture absorption/desorption for clothing, bedding, upholstery and similar products.
- Definitions: includes terms such as exothermic property, endothermic property, hygroscopic and exothermic, and hygroemissive and endothermic.
- Principle: specimen is equilibrated at one humidity level, transferred to another, and the temperature response is recorded. The effect is determined as the temperature difference between measurements with and without a specimen.
- Specimen preparation:
- Cut three specimens of (50 ± 1) mm × (50 ± 1) mm.
- Dry at (105 ± 3) °C for ≥2 h and store in a desiccator until test.
- Apparatus:
- Temperature and humidity controlled chamber (≈220 L) with tolerance ±0.5 °C and ±3 % RH.
- Constant-humidity containers (2 L flasks) using chemical humidity buffers.
- Airtight silicone rubber plugs with a temperature sensor probe protruding ~10 cm into the flask.
- Temperature sensor probe: disk type (5 ± 1 mm diameter, 2.5 ± 0.5 mm thickness).
- Humidity control & reagents:
- Sulfuric acid–water solutions provide fixed RH at test temperature (example at 34 °C: ~65 wt% H2SO4 → 10 % RH; 48 wt% → 40 % RH; 20 wt% → 90 % RH; uncertainty ±3 % RH).
- Water for lab use per ISO 3696.
- Test procedure:
- Equilibrate plug+specimen in first (low or high) humidity flask for ≥3 h.
- Transfer plug to second (target) humidity flask, record temperature ≥30 min.
- Determine peak (exothermic) or bottom (endothermic) temperature and compute ΔT = T_specimen − T_blank.
Practical applications and users
- Textile manufacturers and R&D labs validating thermal performance of materials.
- Product developers of active thermal garments (outdoor, sports, innerwear), bedding, upholstery, healthcare textiles.
- Quality control teams comparing hygroscopic heat generation or cooling performance between materials.
- Researchers and standards bodies assessing thermal comfort, product claims, or enabling comparability across suppliers.
Related standards
ISO 16533 provides a repeatable, objective way to quantify humidity-driven thermal effects in textiles, aiding material selection, product development, and performance claims related to moisture-induced heating or cooling.