Overview
ISO 8226-2:1990 specifies a laboratory method for measuring hygroexpansivity (dimensional change with humidity) of paper and board when equilibrium relative humidity is changed from (33 ± 2) % to (84 ± 2) % (title: up to a maximum relative humidity of 86 %). The standard defines conditioning, measurement procedures and reporting requirements to quantify length change caused by moisture absorption, addressing applications where materials are exposed to high-humidity environments (for example, tropical climates or brief high‑moisture exposure during printing).
Key topics and technical requirements
- Scope and applicability
- Applicable generally to paper and board; not suitable for crepe papers or corrugated fibreboard.
- Intended for high-humidity conditions and short-term moisture effects (e.g., some printing processes).
- Test atmosphere and conditioning
- Preliminary conditioning at a very low RH (22 ± 3) %; measurements involve equilibrating samples at 33 ± 2 % and at 84 ± 2 % RH.
- Temperature control recommended (preferably 23 °C ± 1 °C).
- Use of saturated salt solutions or controlled cabinets to achieve specified relative humidities.
- Apparatus
- Controlled humidity cabinet with air circulation, accurate RH sensors (precision ±1% reading), temperature measurement, clamps and load application, and length-measuring device (resolution 0.01 mm).
- Measurement procedure
- Samples conditioned and measured under specified loads (load selection linked to grammage).
- Readings taken at an intermediate RH (50 ± 5) % and then at the lower and upper RH points; hygroexpansivity X is expressed as a percentage and reported to nearest 0.05 %.
- Important considerations
- Hygroexpansivity above ~68% RH is non-linear due to stress relaxation effects; do not interpolate results outside the specified humidity range by simple arithmetic.
- Repeat measurements and report standard deviations for machine and cross directions.
Applications and users
- Primary users
- Paper and board manufacturers, quality control laboratories, testing services.
- Printers, converters and packaging engineers concerned with dimensional stability in humid environments.
- Practical uses
- Assess dimensional change risks in tropical climates or high-humidity storage.
- Evaluate effects of brief moisture exposure during printing (offset) or finishing.
- Support material selection and process controls to reduce registration errors, web tension problems and packaging misfits.
Related standards
- ISO 8226-1: Hygroexpansivity up to a maximum relative humidity of 68 % (two-point method for lower RH range)
- ISO 186: Sampling to determine average quality of paper and board
- ISO 187: Conditioning of paper, board and pulps
Keywords: ISO 8226-2, hygroexpansivity, paper and board, relative humidity testing, dimensional stability, humidity conditioning, printing quality, tropical humidity testing.
Open preview (PDF)Paper and board — Measurement of hygroexpansivity — Part 2: Hygroexpansivity up to a maximum relative humidity of 86 %