Water absorption testing measures how much moisture a material takes up under controlled conditions, and it is a decisive quality indicator for plastics, ceramics, paper, textiles, foams, and building stone. This article provides a professional comparison of the major water absorption testing standards, including ISO 62, ASTM D570, ISO 10545-3, EN 13755, GB/T equivalents, and related methods, covering specimen requirements, conditioning procedures, calculation bases, and typical applications. Use this guide to select the appropriate method and to interpret laboratory reports from accredited third-party laboratories correctly.
Technical Overview
Water absorption refers to the uptake of water by a material when it is immersed in liquid water or exposed to humid air under specified temperature and time conditions. The property directly influences dimensional stability, mechanical strength, electrical insulation performance, surface appearance, bonding behavior, and long-term durability. Because raw data obtained under different test conditions are not directly interchangeable, engineers and quality managers must understand how the leading standard systems define specimens, conditioning, exposure, and reporting.
The most widely applied water absorption standards and their scopes are summarized below:
- ISO 62 — Plastics; determination of water absorption by immersion and humidity exposure, adopted in China as GB/T 1034.1 and aligned with JIS K 7209.
- ASTM D570 — Standard test method for water absorption of plastics, widely specified in North American supply chains and referenced by UL 746A for polymeric material evaluations.
- ISO 10545-3 — Ceramic tiles; determination of water absorption, apparent porosity, apparent relative density, and bulk density, technically identical to GB/T 3810.3 and consistent with the classification logic of JIS A 5209 and ISO 13006.
- ASTM C373 — Determination of water absorption and associated properties of ceramic whiteware products by direct weight change.
- ISO 535 — Paper and board; determination of water absorptiveness by the Cobb method, adopted as GB/T 1540 and JIS P 8140.
- EN 13755 and EN 1925 — Natural stone; water absorption at atmospheric pressure and capillary absorption coefficient, paralleled by ASTM C97 and GB/T 9966.3.
- ISO 2896, ASTM D2842, and GB/T 8810 — Water absorption of rigid cellular plastics (foams).
- ISO 9073-6 and GB/T 24218.6 — Liquid absorbency time and related properties of nonwoven materials.
Although the physical principle is the same in every system, the standards differ in specimen geometry, drying and saturation procedures, exposure durations, and the units used for reporting. These differences can shift results by several percentage points for the same material, so the applicable standard must always be stated in test reports issued by accredited third-party laboratories.
Testing Samples
Plastic Specimens
ISO 62 and GB/T 1034.1 prefer a disc of approximately 50 mm diameter and 3 mm thickness, with a square specimen of about 60 mm × 60 mm × 1 mm permitted for thin materials. ASTM D570 specifies a bar of approximately 76.2 mm × 25.4 mm × 3.18 mm, using the natural thickness for sheet products. Specimens must be representative of the material, free from cracks and edge damage, and dried to constant mass before exposure. JIS K 7209 follows the ISO specimen logic, which makes ISO/GB and JIS data sets broadly comparable, whereas direct comparison with ASTM D570 values requires caution because of the different surface-to-volume ratio.
Ceramic Tile Specimens
ISO 10545-3 and GB/T 3810.3 require a minimum number of complete tiles or cut pieces providing sufficient test area, dried to constant mass before saturation. ASTM C373 accepts fewer specimens, typically whole tiles or cut sections of defined minimum size. Because tile edges absorb more rapidly than fired surfaces, the standards control how cut specimens are prepared and how surface water is removed before weighing, usually with a damp cloth or chamois.
Paper and Board Specimens
For the Cobb method in ISO 535, GB/T 1540, and JIS P 8140, specimens are cut larger than the 100 cm² test ring of the Cobb apparatus, and at least five replicates are recommended for each surface. Each specimen may be tested only once because the structure of the sheet is permanently changed by wetting.
Natural Stone Specimens
EN 13755 uses cubes or cylinders of approximately 50 mm, with a minimum of six specimens, while ASTM C97 and GB/T 9966.3 accept similar cubes with fewer replicates. Specimens are dried to constant mass, typically at about 105 °C, and cooled in a desiccator before immersion.
Rigid Foam Specimens
ISO 2896, ASTM D2842, and GB/T 8810 use cut prisms of specified dimensions. Because cutting opens surface cells, these standards include volume correction procedures or sealed-edge options so that the reported absorption reflects the true material rather than the cutting artifact.
Textile and Nonwoven Specimens
ISO 9073-6 and GB/T 24218.6 use strips or small swatches conditioned in a standard atmosphere. The test area and specimen count are defined so that the absorbency time measured by sinking or strip wicking is statistically representative.
Testing Items
Depending on the standard and product family, the following quantities are typically determined and reported:
- Water absorption as a percentage of dry mass, calculated as the mass increase after exposure divided by the initial dry mass and multiplied by 100.
- Water absorption per unit surface area, expressed in mg/cm², which is especially useful when comparing specimens of different thickness and is explicitly provided in ISO 62.
- Absolute water uptake in milligrams for thin films and precision components.
- Apparent porosity, apparent relative density, and bulk density for ceramic tiles, obtained in ISO 10545-3 by combining saturated mass in air with hydrostatic (immersed) mass.
- Cobb value in g/m² for paper and board, representing water absorbed per unit area of one surface within a fixed contact time.
- Capillary absorption coefficient in kg/(m²·min^0.5) for natural stone determined under EN 1925.
- Liquid absorbency time or sink time in seconds for nonwovens and textiles.
- Equilibrium moisture content after humidity conditioning at 23 °C and 50% relative humidity, relevant for electrical and electronic applications.
Testing Methods
Immersion Method for Plastics (23 °C)
ISO 62 defines immersion in water at 23 °C as its principal procedure, with weighings commonly taken after 24 hours and, if required, periodically until saturation. ASTM D570 provides an equivalent 24-hour room-temperature immersion plus a long-term immersion schedule extending to equilibrium. Both require drying to constant mass before exposure and careful wiping of surface water with a lint-free cloth before each weighing.
Boiling Water and Humidity Methods for Plastics
ISO 62 also specifies immersion in boiling water, typically for 30 minutes followed by a defined cooling period, and exposure to humid air at 23 °C and 50% relative humidity for equilibrium moisture content. ASTM D570 includes a 2-hour boiling immersion option. The humidity method is particularly important for polymeric insulating materials because dielectric parameters evaluated under IEC test programs are strongly affected by equilibrium moisture.
Boiling and Vacuum Saturation for Ceramic Tiles
ISO 10545-3 and GB/T 3810.3 saturate tiles either by boiling for 2 hours followed by cooling in water for 4 hours, or by vacuum saturation, which forces water into all open pores and is used where full open porosity must be measured. ASTM C373 employs higher drying temperature and a 5-hour boil in its typical procedure. Results feed the ISO 13006 classification bands of up to 0.5%, 0.5% to 3%, 3% to 6%, 6% to 10%, and above 10%, which determine the tile group and its permitted applications.
Cobb Method for Paper and Board
In ISO 535, GB/T 1540, and JIS P 8140, a water column is held in contact with a 100 cm² area of one surface of the specimen for a defined time, commonly 60 seconds for papers and longer for boards. The water is poured off, surface water is removed by rolling with a standardized roller, and the mass gain is converted to g/m².
Immersion and Capillary Methods for Stone
EN 13755 and ASTM C97 determine total absorption by full immersion at atmospheric pressure, generally for 48 hours, while EN 1925 measures the mass gain of a specimen standing in shallow water over time to derive the capillary absorption coefficient, a key input for frost durability and facade performance assessment.
Key Points of Standard Comparison
- GB/T 1034.1 is technically aligned with ISO 62, and GB/T 3810.3 with ISO 10545-3, so Chinese national data can generally be compared with ISO data directly.
- ASTM D570 and ISO 62 differ mainly in specimen geometry and reporting options; a material may show different percentage values simply because of the different surface-to-volume ratio, not a real change in material behavior.
- ASTM C373 and ISO 10545-3 both yield water absorption, but the different drying and boiling schedules can produce slightly different apparent porosity values for low-porosity porcelain tiles.
- UL 746A incorporates water absorption testing of polymeric materials as part of short-term property evaluation, so data generated per ASTM D570 procedures also support UL material recognition programs.
- Results from immersion, boiling, vacuum, and humidity methods are not interchangeable; procurement documents should state the standard, method variant, exposure time, and reporting unit.
Applications
Water absorption data are used across many industries for material qualification, process control, and regulatory compliance:
- Automotive and appliance engineering: predicting dimensional stability and strength retention of nylon, polyesters, and glass-filled compounds exposed to humidity and wash processes.
- Electrical and electronics: supporting material datasheets, moisture sensitivity classification, and dielectric performance evaluation, where ISO 62 humidity data complement IEC electrical test programs and UL 746A material evaluations.
- Ceramic tile production and trade: assigning tiles to absorption classes that govern frost resistance, breaking strength requirements, and suitability for floors, walls, and external paving.
- Construction and building materials: evaluating natural stone weathering resistance, mortar and render behavior, and foam insulation water uptake for facade and roofing systems.
- Packaging and printing: controlling Cobb values that influence printability, glueability, and moisture protection of paper and corrugated board.
- Hygiene and medical textiles: verifying absorbency time and capacity of nonwovens used in wipes, wound dressings, and disposable hygiene products.
- Quality control and R&D: comparing formulations, evaluating additives and surface treatments, and validating supplier consistency through accredited third-party laboratories.
FAQ
Q: What is the main difference between ISO 62 and ASTM D570?
A: Both determine water absorption of plastics by immersion, but ISO 62 (with GB/T 1034.1 and JIS K 7209) prefers a 50 mm disc and offers immersion, boiling, and 50% RH humidity procedures with results reportable as percentage, mg, or mg/cm², while ASTM D570 uses a 76.2 mm × 25.4 mm × 3.18 mm bar and focuses on immersion and boiling schedules. Values from the two systems should be compared only with an understanding of these geometric and procedural differences.
Q: Are GB/T standards equivalent to the corresponding ISO methods?
A: In most cases yes. GB/T 1034.1 is aligned with ISO 62, GB/T 3810.3 with ISO 10545-3, GB/T 1540 with ISO 535, and GB/T 24218.6 with ISO 9073-6, so test data are generally mutually acceptable between ISO-based and GB-based specifications.
Q: Which standard applies to ceramic tile water absorption classification?
A: ISO 10545-3 (or GB/T 3810.3) is the reference method, using a 2-hour boil with 4-hour cooling or vacuum saturation, and the results are classified into the absorption bands of ISO 13006. ASTM C373 is the common North American alternative and additionally reports apparent porosity and bulk density.
Q: How long does water absorption testing take?
A: Routine immersion tests are typically completed within a few days including conditioning, but saturation immersion, humidity equilibrium, and stone capillary tests can take several days to weeks because weighings continue until equilibrium is demonstrated. Accredited third-party laboratories can confirm lead times based on the specified method variant.
Q: Can results from different standards be converted to each other?
A: There is no universal conversion factor. Differences in specimen size, drying temperature, water temperature, saturation technique, and surface drying all influence the result. The correct approach is to test against the standard named in the purchase specification or to run a correlation study covering both methods.
Q: Why do some reports express absorption in mg/cm² instead of percentage?
A: Surface-area-based reporting, provided in ISO 62, better describes moisture uptake relevant to surface phenomena such as adhesion and electrical tracking, and it allows fair comparison between specimens of different thickness. Percentage-based reporting remains the default for bulk material comparison.