ASTM D570 Water Absorption Testing: How It Works and What the Results Mean
ASTM D570 is the standard test method for water absorption of plastics. It measures the relative rate at which a plastic absorbs water when immersed, and it reports the result as percent weight gain.
What ASTM D570 Covers
ASTM D570 (the current edition is D570-22) determines the relative rate of absorption of water by plastics when immersed. It applies to cast, hot-molded, and cold-molded resinous products, and to homogeneous and laminated plastics in rod, tube, and sheet form. Sheets must be 0.13 mm (0.005 in.) or greater in thickness.
The method works as a guide to water absorption and as a uniformity control test. A result describes how a specific material, at a specific thickness, behaves under defined immersion conditions, so the same procedure and specimen geometry have to be used on both sides of any comparison. ISO 62 addresses the same property, and both standards appear on our accredited scope. Confirm which standard, edition, and procedure your specification calls out before samples ship.
How the Procedures Work
The ASTM D570 procedure starts with conditioning. For most plastics, specimens dry in an oven for 24 hours at about 50 °C, cool in a desiccator, and are weighed immediately to set the dry weight. Materials that are not appreciably affected by higher temperatures, such as phenolic laminates, can dry for 1 hour at 105 to 110 °C instead.
The most common procedure is the 24-hour immersion. Conditioned specimens go fully under distilled water held at about 23 °C. After 24 hours, the lab removes each specimen, wipes off the surface water, and weighs it promptly. The standard also includes a long-term immersion that continues until the specimen reaches saturation, and a boiling water procedure that speeds up the water intake by including a 2 hour boil. The 24 hour result gives a short-term comparison, while the saturation result shows the equilibrium value.
Specimen geometry depends on the material form. Molded plastics use a disk 50.8 mm (2 in.) in diameter and 3.2 mm thick. Sheet material uses a bar 76.2 mm (3 in.) long and 25.4 mm (1 in.) wide, at the thickness of the sheet. If your part cannot supply those shapes, tell the lab early, and use our testing readiness checklist to confirm what to send.
How Percent Weight Gain Is Calculated
Percent water absorption equals the wet weight minus the dry weight, divided by the dry weight, multiplied by 100. The dry weight is the weight after drying the specimens in the oven, and the wet weight comes from weighing the specimens after immersion.
Some materials also lose soluble matter into the water during immersion. That loss lowers the measured weight and can hide part of the water uptake. D570 addresses this by reconditioning the specimen after immersion, which shows whether weight was lost, and by reporting soluble matter loss alongside the absorption value. If your material may leach soluble matter, ask for both numbers on the report, because a reviewer may want to see them together.
Why Thickness Matters
Water moves into a plastic by diffusion, and uptake tracks the square root of immersion time. The time a specimen needs to reach saturation depends strongly on thickness. A thin section approaches equilibrium far sooner than a thick one, so a 24 hour result on a 1 mm film and a 24 hour result on a 3.2 mm disk are not directly comparable.
This matters when a datasheet value and an OEM requirement list different thicknesses. State the thickness on the request, report it with the result, and compare values only between specimens of the same thickness. When the production wall is much thicker than the test disk, the long-term immersion gives a fuller picture of the equilibrium value than the 24 hour result alone.
Why Water Absorption Matters for Automotive Plastics
Hygroscopic polymers such as nylon absorb water, and that water changes the material. Absorbed water swells the part, which can push dimensions outside precision tolerances, and in nylon it acts as a plasticizer that shifts mechanical properties. In insulating polymers, absorbed water also reduces electrical resistivity and dielectric strength. Those effects matter for any component where fit, stiffness, or insulation performance is specified.
Several related methods sit next to D570 on our scope. Under Water Absorption, the scope lists ASTM D570, D1815, and D2842, FLTM BO 129-02, GMW14777, ISO 62, and SAE J315. Dimensional change and humidity exposure appear in separate scope categories, Shrinkage / Dimensional Stability and Humidity / Autoclave Exposures. Check which method your drawing note or material specification cites, and browse the full list on our tests page.
If your specification cites ASTM D570 or a related water absorption method, request a quote or book a consultation, and we will confirm the procedure and specimen requirements before testing starts.
Frequently Asked Questions
What does ASTM D570 measure?
ASTM D570 measures the relative rate of absorption of water by plastics when immersed. The result is reported as percent weight gain, calculated from the specimen weight before and after immersion. It also provides a way to check the uniformity of a material from lot to lot.
How long is the standard ASTM D570 immersion?
The most common procedure is a 24-hour immersion in distilled water at about 23 °C. Specimens are dried and weighed first, then weighed again promptly after removal. Other procedures in the standard run longer, until saturation, or use boiling water.
How is percent water absorption calculated?
Subtract the dry weight from the wet weight, divide by the dry weight, and multiply by 100. The dry weight is taken after conditioning, and the wet weight is taken after immersion. Soluble matter loss is determined separately when the material loses weight during reconditioning.
Why does specimen thickness matter in water absorption testing?
Water enters a plastic by diffusion, and the time to reach saturation depends strongly on thickness. A thin specimen and a thick specimen of the same material give different results after the same immersion time. Compare results only between specimens of the same thickness.
Does GPTesting test to ISO 62 as well as ASTM D570?
Yes. ASTM D570 and ISO 62 both appear under Water Absorption on our A2LA scope, along with ASTM D1815, ASTM D2842, FLTM BO 129-02, GMW14777, and SAE J315. Check your specification to see which standard it requires.
Ghesquiere Plastic Testing, Inc. is an A2LA accredited testing laboratory in Harper Woods, MI, specializing in weathering and flammability testing for automotive materials. Request a quote at quotes@gptesting.com.
