ASTM D570 Water Absorption of Plastics
Water absorption, in accordance with ASTM D570, measures the amount of water absorbed by the specimen’s weight. This parameter matters because water increases specimen size, alters dimensions, and reduces electrical conductivity and mechanical properties. Polyamide 6 can absorb more than 2% of its weight and swell accordingly. MatX Lab water absorption testing includes molded plastics, compounds, laminates, machined plastics, and components, in accordance with ASTM D570 and ISO 62.
What Is ASTM D570?
ASTM D570 specifies the procedure for water absorption testing of plastics. The specimens are dried to constant weight, cooled to room temperature in a desiccator, and then weighed. After that, the specimens are immersed in water and reweighed after a set time.
There are different exposure times provided. 24-hour immersion at 23 °C is the most common one. 2-hour immersion in boiling water accelerates absorption. Prolonged exposure continues until the specimens reach constant weight.
ASTM D570 Scope and Applications
The method applies to plastics in sheet, rod, tube, and molded form. Materials that dissolve, soften badly, or delaminate in water are poor candidates.
Applications:
- Dimensional stability prediction. Estimating how much a part will grow in a humid or wet application, which is a design-critical number for polyamide gears and housings.
- Electrical property assessment. Supporting insulation resistance and dielectric performance, since absorbed water degrades both.
- Material selection. Comparing candidate resins for applications involving immersion, washdown, or high humidity.
- Filler and laminate assessment. Detecting absorption along fiber interfaces in composites and filled compounds, which frequently exceeds the resin’s own uptake.
- Incoming verification. Checking a compound against its specification, since absorption tracks formulation changes.
Water vapor moving through a film rather than into a solid is a different measurement, covered by ASTM F1249 and ASTM E96. MatX Lab reports measured values and does not certify products or act as a standards body.
ASTM D570 Test Procedure
The weighing is the test. Every handling step between the desiccator and the balance is an opportunity to change the number.
| Step | What happens |
| Specimen preparation | Specimens are cut or molded to the standard geometry with edges finished, since a rough-sawn edge absorbs faster than a molded surface. |
| Drying | Specimens dry at the specified temperature, commonly 50 °C for 24 hours, or to a schedule the material requires. |
| Desiccator cooling | Specimens cool in a desiccator to room temperature. Weighing a warm specimen introduces a buoyancy and convection error that is larger than the effect being measured. |
| Conditioned mass | Specimens are weighed immediately on removal, and this mass becomes the baseline. |
| Immersion | Immerse specimens in distilled water at the specified temperature, supported so all surfaces contact water and specimens do not touch. |
| Removal and blotting | Remove specimens at the specified time, blot with a dry cloth, and weigh within one minute. |
| Calculation | Percentage mass increase is calculated against the conditioned mass. |
| Soluble matter check | Where required, specimens are redried and reweighed, and any mass deficit is added back as soluble matter lost. |
Limitations: Absorption depends on specimen thickness and surface area, so results from different geometries are not comparable, and a thin specimen reaches equilibrium far faster than a thick one. Even after 24 hours, the test is unlikely to reach saturation for most engineering plastics; it provides a ranking of materials, not an absolute value.
Machined and sawed surfaces absorb more than molded surfaces, which affects the measured value. The blotting method also influences the result. Materials that lose soluble matter show low apparent uptake unless the correction is applied, and heavily plasticized compounds are a common case. Temperature matters, so boiling water results and 23 °C results are entirely different measurements.
ASTM D570 Specimen Requirements and Test Conditions
| Parameter | Typical requirement |
| Disc specimen | 50.8 mm diameter, 3.2 mm thick |
| Bar specimen | 76.2 mm x 25.4 mm x 3.2 mm |
| Replicates | Three per material minimum |
| Drying | 50 °C for 24 hours, or per material requirement |
| Immersion temperature | 23 °C standard, or boiling water for the accelerated option |
| Duration | 24 hours, 2 hours boiling, or long-term to saturation |
| Balance resolution | Sufficient to resolve the expected uptake on your specimen mass |
| Material required | Enough to produce the specimen count plus spares |
Match specimen geometry to your specification exactly. Absorption scales with the surface-to-volume ratio, and a specimen 1 mm thinner than specified will read higher at 24 hours on the same material.
State your need for long-term saturation data. A 24-hour number is useful for screening material, while design calculations in a wet environment require saturation data, which the program takes several weeks to obtain.
ASTM D570 Test Results and Reporting
Results are presented in terms of the weight gain of the specimens after exposure, together with the percentage gain noted for each specimen, as well as the average value among the tested specimens. The exposure conditions will also be stated to contextualize the results of this experiment. All specimen measurements are reported individually rather than as a single overall measurement.
These include the dimensions and mass of the specimens, as these factors may affect how much of the material can absorb the liquid. The report also includes information about specimen preparation and exposure, including the drying process, immersion temperature, exposure time, and removal of liquid before weighing.
Some materials may release soluble components during immersion. When this occurs, the amount of material lost is determined independently and used to calculate a corrected absorption result. The report shows how that adjustment was made, allowing the original and corrected values to be traced.
For studies involving extended exposure, results can also be presented as an uptake-versus-time curve. This makes it possible to see how absorption changes throughout the test and whether the specimen is moving toward equilibrium or saturation.
ASTM D570 FAQs
Is 24-hour absorption the same as saturation?
No, and for most plastics it is well below that level. Polyamide, when over 2% saturation, can give about a tenth of a percent at 24 hours. Compare materials using the 24-hour value and use the saturation value for design purposes.
Why does my part absorb more than the datasheet says?
Geometry and surface condition are the usual reasons. Machined faces, thin sections, and a higher surface-to-volume ratio all increase uptake compared with a standard test specimen.