Hygroscopicity Testing Standard Methods: Procedures, Conditions, and Technical Requirements

Hygroscopicity testing quantifies the ability of a material to absorb and retain moisture from its environment, a property that directly governs the dimensional stability, processing behavior, and long-term performance of plastics, textiles, pharmaceuticals, and electronic materials. This technical guide summarizes the internationally recognized standard methods for moisture absorption testing, including ISO 62, ASTM D570, GB/T 1034, and related textile standards, covering specimen preparation, conditioning atmospheres, test procedures, result calculation, and typical industrial applications.

Technical Overview

Hygroscopicity describes the tendency of a solid material to take up water molecules from the surrounding atmosphere until a state of moisture equilibrium is reached. In laboratory practice, two related but distinct phenomena are measured: the absorption of water vapor from humid air under controlled temperature and relative humidity, and the absorption of liquid water by full immersion. Both are quantified gravimetrically by comparing the mass of a specimen after exposure against its oven-dry reference mass.

The principal standards framework for polymer materials is built around ISO 62 (Plastics — Determination of water absorption) and ASTM D570 (Standard Test Method for Water Absorption of Plastics). In China, GB/T 1034 is harmonized with ISO 62, while JIS K 7209 covers the same scope in the Japanese system. For fibers and textiles, moisture regain and moisture content are determined under ISO 6741, GB/T 9995, and ASTM D1576. Standard conditioning atmospheres are defined in ISO 291 (23 °C / 50% RH for plastics) and ISO 139 (20 °C / 65% RH for textiles). For fine powders and pharmaceutical solids, dynamic vapor sorption (DVS) gravimetry is widely used to construct full sorption isotherms.

• Key variables that influence results include ambient temperature and relative humidity.
• Specimen thickness and exposed surface area control the diffusion path length and therefore the time to equilibrium.
• Material morphology, crystallinity, fillers, and additives strongly affect equilibrium uptake.
• Materials containing water-soluble matter require a corrected calculation based on re-drying after exposure.

Testing Samples

Plastic and Composite Specimens

The reference geometry specified in ISO 62 is a disc of 50 mm diameter and 3 mm thickness, molded directly from the compound being evaluated. ASTM D570 commonly uses bars of approximately 76 mm × 25 mm × 3.2 mm cut or molded from the material. Films are tested as multiple 50 mm squares, and laminates are tested with edges sealed or exposed depending on the intent of the study. All specimens must be free of cracks, voids, and surface contamination.

Fibers, Yarns, and Fabrics

Textile testing uses conditioned skeins of yarn, fiber tufts of typically 1–5 g, or fabric swatches. Specimens are first brought to equilibrium in the standard textile atmosphere (20 °C / 65% RH) and then oven-dried to establish the dry mass used in moisture regain calculations.

Powders, Granules, and Pharmaceutical Solids

Powdered samples are spread in a layer not exceeding 3 mm thickness in open weighing bottles so that the entire surface is exposed to the controlled atmosphere. For dynamic vapor sorption analysis, sample quantities of 10–50 mg are loaded into the microbalance pan.

Electronic and Packaging Materials

Molded epoxy encapsulant coupons, barrier film laminates, desiccant materials, and moisture-sensitive components are tested as flat coupons or in their functional geometry, depending on whether the study targets material characterization or component-level reliability.

Testing Items

Depending on the material and the engineering question, a hygroscopicity test program may include the following measurable items:

• Water absorption after 24 h immersion in water at 23 °C (comparative index value).
• Saturation water absorption after long-term immersion to full equilibrium.
• Equilibrium moisture content at a defined relative humidity, typically 50% RH or 90–95% RH at 23 °C.
• Moisture absorption and desorption kinetics, expressed as uptake versus time curves.
• Complete moisture sorption isotherms covering stepped relative humidity from dry to near-saturation conditions.
• Moisture regain and moisture content for fibers, yarns, and textiles.
• Corrected absorption values accounting for the loss of water-soluble matter during immersion.

Testing Methods

Oven-Dry Reference Mass Determination

Every method begins by establishing the dry reference mass m1. Plastic specimens are dried at 50 °C for 24 h, or at 105 °C where the material tolerates it, until constant mass is reached. Textile specimens are oven-dried at 105–110 °C. After drying, specimens are cooled in a desiccator and weighed immediately to prevent re-absorption during handling.

Immersion Method at 23 °C (ISO 62 Method 1 / ASTM D570)

The dried and weighed specimen is fully immersed in distilled or deionized water maintained at 23 °C for 24 h. It is then removed, surface water is blotted from all faces with a dry lint-free cloth, and the specimen is weighed within one minute to obtain the wet mass m2. Water absorption is calculated as c = (m2 − m1) / m1 × 100%. If the material loses water-soluble matter, the specimen is re-dried to constant mass and the corrected dry mass is used as the basis for calculation. At least three specimens should be tested and the mean reported.

Humid Atmosphere Equilibrium Method (ISO 62 Method 4)

For materials that dissolve, hydrolyze, or disintegrate in liquid water, uptake is measured instead by exposure to a conditioned atmosphere, typically 23 °C / 50% RH. Specimens are weighed at defined intervals until the mass change between successive weighings falls below the specified equilibrium criterion. More severe screening is performed at 90–95% RH to evaluate performance in tropical or high-humidity service environments.

Boiling Water Immersion

Short accelerated comparison is achieved by immersion in boiling water, typically 30 minutes under ISO 62 or a 2-hour boiling test under ASTM D570. Because diffusion and surface effects are strongly temperature-dependent, boiling-water results are not interchangeable with 23 °C immersion data and must be reported as a separate condition.

Textile Moisture Regain Determination

Skeins or fabric swatches are conditioned to equilibrium at 20 °C / 65% RH per ISO 139, weighed to obtain the conditioned mass, and then oven-dried at 105–110 °C to constant mass. Moisture regain is calculated as the mass of absorbed moisture expressed as a percentage of the oven-dry mass, and it serves both quality control and commercial invoicing purposes in the fiber trade.

Dynamic Vapor Sorption (DVS)

In DVS analysis, a small sample rests on a highly sensitive microbalance inside a chamber where relative humidity is stepped, for example from 0% to 90% and back to 0% at a constant temperature of 25 °C. At each humidity step the mass is recorded continuously, and equilibrium is assumed when the rate of mass change falls below a preset threshold. The output is a complete sorption–desorption isotherm with kinetic information, suitable for classifying materials from non-hygroscopic to deliquescent.

Test reports must always state the governing standard, specimen geometry and thickness, conditioning and exposure atmospheres, duration, and the calculation basis, because identical materials can yield substantially different values under different humidity and temperature schedules.

Applications

Hygroscopicity data generated by these standard methods support a wide range of engineering and commercial decisions:

• Polymer processing: establishing drying targets before extrusion or injection molding, since excess moisture in resins such as polyamides causes hydrolytic degradation, silver streaking, and voids.
• Material selection: comparing moisture uptake across candidate polymers, for example highly absorptive polyamides versus essentially non-absorptive polyolefins, for dimensional stability in humid service.
• Electronics reliability: assigning moisture sensitivity behavior to molded packages and defining bake-out and floor-life controls that prevent internal cracking during solder reflow.
• Packaging design: sizing barrier films and desiccants for moisture-sensitive goods and validating shelf-life assumptions.
• Pharmaceutical development: classifying drug substances and excipients by hygroscopicity to define handling, storage, and protective-packaging requirements.
• Textile trade and comfort: determining commercial conditioned mass for fiber transactions and evaluating wearing comfort related to moisture buffering.
• Building products: assessing moisture buffering capacity and dimensional movement of flooring, panels, and coatings under fluctuating humidity.

FAQ

What is the difference between hygroscopicity and water absorption?

Hygroscopicity refers to the uptake of water vapor from air at a controlled relative humidity, whereas water absorption refers to the uptake of liquid water by immersion. Both are expressed as percentage mass gain, but the test conditions and physical mechanisms differ, so the results are not interchangeable.

Which standard should be specified for plastic materials?

ISO 62 and ASTM D570 are the primary international references; GB/T 1034 is aligned with ISO 62 and JIS K 7209 covers the equivalent scope. The methods are technically similar but differ in specimen geometry and detail, so the exact standard and condition, for example 24 h immersion at 23 °C, should always be stated on datasheets and purchase specifications.

How long does a specimen take to reach moisture equilibrium?

Equilibrium time depends on thickness and the diffusion coefficient of the material. A 3 mm plastic disc may require several days to weeks at 50% RH, while thin films and powders equilibrate within hours. The widely reported 24 h immersion value is a comparative index, not a true equilibrium value.

What standard atmospheres are used for conditioning?

ISO 291 defines 23 °C / 50% RH as the standard laboratory atmosphere for plastics, with 27 °C / 65% RH available for tropical conditions. Textile testing uses 20 °C / 65% RH per ISO 139. Deviations must be documented because both temperature and humidity directly shift equilibrium uptake.

How are results calculated and reported?

Uptake is calculated as c = (m2 − m1) / m1 × 100%, where m1 is the oven-dry mass and m2 is the mass after exposure. Reports should include the mean of at least three specimens, the governing standard, exposure conditions, and any corrections applied for soluble matter loss.

Can hygroscopicity testing be outsourced?

Yes. Accredited third-party laboratories routinely perform ISO 62, ASTM D570, GB/T 1034, and textile moisture regain testing under ISO/IEC 17025 accreditation, providing independent data for supplier qualification, incoming inspection, and regulatory submissions.