Water Absorption Testing of Materials: Principles, Standards, and Test Methods

Water absorption testing quantifies the amount of water a material takes up under controlled immersion conditions, providing essential data on dimensional stability, durability, and long-term performance. This guide explains the test principle, applicable international standards such as ISO 62, ASTM D570, and EN 13755, specimen requirements, calculation methods, and typical applications across plastics, ceramics, paper, and building materials.

Technical Overview

Water absorption determination is a fundamental physical test that measures the mass of water taken up by a solid material when it is immersed in water, or otherwise exposed to moisture, for a defined period under controlled temperature. The result is normally expressed as a percentage of the dry mass of the specimen, or in specific cases as mass per unit area or as water volume relative to specimen volume.

The absorbed water content directly influences many engineering properties. For plastics, absorbed moisture causes dimensional change, plasticization, and reduced mechanical and electrical performance. For ceramics and mineral products, water absorption is an indicator of open porosity, vitrification degree, and frost resistance. For paper, board, wood-based panels, and thermal insulation, it reflects sizing quality, swelling tendency, and durability in humid service environments.

The most widely applied standardized methods include:

• ISO 62 and GB/T 1034, together with ASTM D570 and JIS K 7209, for plastics
• ISO 10545-3, GB/T 3810.3, and ASTM C373 for ceramic tiles and fired whiteware
• ISO 535 and JIS P 8140 (Cobb method) for paper and board
• EN 317 and EN 12087, together with ASTM D2842 and ISO 2896, for wood-based panels, thermal insulation, and rigid cellular plastics
• EN 13755, ASTM C97, and GB/T 9966.3 for natural stone, and ASTM C642 for hardened concrete

Testing Samples

Plastic Materials

Specimens are typically molded discs of approximately 50 mm diameter and 3 mm thickness, or rectangular bars cut from sheets. ASTM D570 commonly specifies specimens of 76.2 mm by 25.4 mm with the product thickness, preferably 3.2 mm. For anisotropic or filled plastics, sampling direction should be recorded because absorption can differ between machine and transverse directions.

Ceramic Tiles and Fired Ceramics

Whole tiles or representative cut pieces are tested. Standards generally require a minimum number of specimens per lot so that both surface and cross-section absorption behavior is captured. Edges should be clean-cut and free from visible cracks or chipping that could distort water uptake.

Paper and Paperboard

For the Cobb method, circular specimens of 100 cm² test area are cut cleanly without frayed edges. Multiple specimens from different positions of the sheet are recommended to average out formation variations.

Wood-Based Panels and Thermal Insulation

For thickness swelling tests such as EN 317, squares of 50 mm by 50 mm are cut from the panel at defined positions. Rigid foam and mineral wool products require specimens of specified dimensions with accurate geometric measurement, since results are expressed per unit volume or per cross-sectional area.

Natural Stone, Concrete, and Mineral Products

Prisms, cubes, or cores of standardized size, commonly around 50 mm, are used. The specimens must be sound, representative of the bulk material, and free from weathered surfaces unless such surfaces are specifically under evaluation.

Testing Items

Depending on the material and specification, the following quantities are commonly determined:

• Short-term water absorption after immersion, typically 24 h at 23 °C
• Long-term or saturation water absorption after extended immersion
• Water absorption after boiling immersion
• Thickness swelling of panels after immersion
• Cobb water absorptiveness expressed in g/m² for paper and board
• Apparent porosity, apparent relative density, and bulk density derived from saturated and suspended mass measurements
• Water absorption by partial immersion or capillary uptake for insulation products

Testing Methods

Specimen Conditioning

Specimens are first dried to constant mass in an oven at a temperature appropriate to the material, for example 50 °C for many plastics and higher temperatures for ceramics, stone, and concrete. After drying, specimens are cooled to room temperature in a desiccator and weighed to obtain the initial dry mass. Distilled or deionized water is used for immersion, maintained at 23 °C unless the standard specifies otherwise.

Immersion Method

The dried and weighed specimen is fully immersed in water for the specified period, most commonly 24 hours for plastics, 48 hours for stone, and up to 28 days for long-term insulation tests. After immersion, the specimen is removed, and all surface water is promptly removed by blotting or wiping with a damp-free cloth in a standardized manner. The specimen is then immediately reweighed to obtain the wet mass. Consistent wiping technique is critical, since retained surface water is one of the main sources of variability.

Boiling Immersion Method

For ceramics and fired clay products, specimens are dried, weighed, then boiled in water for a defined duration, for example up to 5 hours in ASTM C373, followed by soaking in cooling water. ISO 10545-3 also permits saturation by vacuum impregnation for low-porosity products. The saturated mass, together with the immersed (suspended) mass determined hydrostatically, allows calculation of water absorption, apparent porosity, and bulk density.

Cobb Method for Paper and Board

A defined area of one side of the specimen is exposed to water for a set contact time, commonly 60 seconds, under a sealed cylinder. The excess water is poured off, the surface is blotted with a standardized roller, and the mass gain is expressed in grams per square meter. This one-sided method reflects the sizing performance of printing papers and boards.

Calculation and Expression of Results

The standard expression is the percentage of water absorbed relative to the dry mass: water absorption equals the wet mass minus the dry mass, divided by the dry mass, multiplied by 100. For highly absorbent materials, some standards express absorption relative to the wet mass instead. For foams, a cut-cell correction factor is applied, and results may be reported as absorbed volume percentage. For paper, the Cobb value is reported in g/m² at the stated exposure time. The mean, range, and individual values of all specimens are reported together with specimen dimensions and conditioning details.

Applications

Water absorption data are used across many industries for material selection, quality control, and specification compliance:

• Plastics and composites: evaluation of nylon and other hygroscopic polymers, prediction of dimensional change and retention of mechanical and electrical properties in humid environments, and incoming material control
• Ceramic tiles: classification of tiles by water absorption groups, from compact porcelain tiles at or below 0.5 percent up to highly porous products above 10 percent, which determines permitted applications
• Construction products: frost-resistance assessment of stone, brick, and roof tiles; thickness swelling limits for flooring-grade panels; water uptake verification for facade and thermal insulation systems
• Paper and packaging: sizing degree control, printability assessment, and performance verification of barrier coatings
• Electrical engineering: qualification of insulation materials where moisture ingress reduces dielectric strength
• Research and development: comparative material screening, accelerated aging studies, and validation of formulation changes

Accredited third-party laboratories routinely perform these tests to support product certification, regulatory compliance, and export documentation.

FAQ

What does the water absorption value indicate?

It indicates the open porosity accessible to water. Higher values generally mean lower density, greater dimensional change in humid conditions, reduced mechanical or electrical performance after wetting, and for construction materials, potentially lower frost resistance.

Which standard should be selected for my material?

Plastics are usually tested to ISO 62 or ASTM D570, ceramic tiles to ISO 10545-3 or ASTM C373, paper and board to ISO 535, panels to EN 317, insulation to EN 12087 or ISO 2896, and stone to EN 13755 or ASTM C97. The choice should follow the product specification or the target market requirements.

How long does a typical test take?

A standard 24-hour immersion test, including drying to constant mass and cooling, typically requires two to three working days. Boiling methods and long-term immersion methods, such as the 28-day insulation test, take longer. Exact scheduling should be confirmed with the testing laboratory.

What specimen quantity is required?

Most standards require a minimum of three to five specimens per material or lot, prepared to the dimensional requirements of the applicable method. Additional specimens may be needed when both machine-direction and cross-direction behavior must be evaluated.

What factors can influence the results?

Specimen thickness, whether surfaces are molded or machined, water temperature, immersion duration, the consistency of surface wiping before weighing, and cut-cell effects in foams all significantly affect results. Strict adherence to the standard procedure is essential for reproducibility.

Can water absorption results predict outdoor durability?

They provide a strong indication, particularly when combined with related properties such as porosity, swelling, and freeze-thaw performance, but a full durability assessment usually requires additional testing appropriate to the actual service conditions.