Water absorption determination is a fundamental physical test that measures how much moisture plastic materials take in when exposed to water or humid environments, directly affecting their dimensional stability, mechanical strength, and electrical insulation performance. This article provides a comprehensive technical overview of plastic water absorption testing based on internationally recognized standards such as ISO 62, ASTM D570, and GB/T 1034, covering specimen preparation, testing procedures, calculation methods, and typical industrial applications.
Technical Overview
Water absorption determination quantifies the amount of water that a plastic specimen absorbs under controlled immersion or humidity exposure conditions. Because most polymers contain polar functional groups, voids, or hygroscopic fillers, they absorb moisture to varying degrees, which can cause swelling, plasticization, reduced mechanical strength, degraded dielectric properties, and dimensional changes in finished parts. The result is typically expressed as the percentage increase in mass relative to the dry specimen mass, or as the mass of absorbed water per unit surface area.
The principal standards governing this test include ISO 62 (Plastics — Determination of water absorption), ASTM D570 (Standard Test Method for Water Absorption of Plastics), and GB/T 1034, the Chinese national standard technically aligned with ISO 62. These standards define standardized specimen geometry, conditioning requirements, exposure conditions, and calculation formulas so that results are reproducible and comparable between laboratories, including accredited third-party laboratories performing material qualification and quality control testing.
Testing Samples
Standard Disk Specimens
The most common specimen for ASTM D570 is a disk of 50 mm diameter and 3 mm nominal thickness. This geometry provides a defined surface-to-volume ratio, allowing water absorption values to be compared across different materials on a consistent basis.
Square Plaques
ISO 62 and GB/T 1034 typically specify square plaques of 60 mm × 60 mm with a preferred thickness of 1 mm, although other thicknesses may be tested when required by the material specification. The specimen thickness must always be reported because diffusion-controlled absorption strongly depends on thickness.
Finished Parts and Cut Sections
Injection-molded components, machined parts, films, sheets, tubes, and granules can also be evaluated. When testing actual products, specimens should have a defined exposed surface area, and the surface-to-mass ratio must be documented. Specimens must be free of cracks, burrs, and contamination, and they are usually conditioned by drying to constant mass before testing.
Testing Items
- Water absorption after 24-hour immersion in distilled water at 23 °C
- Water absorption after immersion in boiling water (typically 30 minutes or 2 hours)
- Long-term immersion testing until saturation equilibrium is reached
- Water absorption after exposure to 50 % relative humidity atmosphere at 23 °C
- Water absorption at elevated water temperatures (e.g., 50 °C or higher) for accelerated evaluation
- Corrected water absorption accounting for water-soluble matter loss during boiling immersion
- Absorption kinetics curves showing mass gain as a function of immersion time
Testing Methods
Method 1: Immersion in Water at 23 °C
The specimen is dried to constant mass in an oven at a temperature appropriate for the material (commonly 50 °C or as specified), cooled in a desiccator, and weighed to obtain the initial mass m₁ with an accuracy of 0.1 mg. It is then fully immersed in distilled or deionized water maintained at 23 °C, with the water level at least 25 mm above the specimen surface. After the specified exposure time, usually 24 hours, the specimen is removed, all surface water is immediately removed with a dry lint-free cloth, and the specimen is reweighed to obtain m₂. The water absorption is calculated as c = (m₂ − m₁) / m₁ × 100 %.
Method 2: Immersion in Boiling Water
This accelerated procedure exposes the dried and pre-weighed specimen to boiling distilled water for a defined period, typically 30 minutes or 2 hours. After removal and wiping, the saturated mass m₂ is recorded. Because some plastics lose water-soluble constituents during boiling, the specimen may be re-dried to constant mass m₃, and a corrected absorption value is calculated to account for this mass loss, giving a truer picture of actual water uptake.
Method 3: Immersion at Elevated Temperature
For materials that equilibrate slowly at room temperature, immersion in water at an agreed elevated temperature, such as 50 °C or higher, shortens the diffusion path time and accelerates saturation. The test temperature and duration must be agreed upon between interested parties and clearly reported with the results.
Method 4: Exposure to 50 % Relative Humidity at 23 °C
Instead of liquid immersion, the dried specimen is placed in a conditioned atmosphere of 50 % relative humidity at 23 °C until equilibrium moisture content is reached. This method better simulates service conditions in normal indoor environments and is particularly relevant for engineering plastics used in electrical and electronic applications.
For all methods, long-term testing involves weighing the specimen at regular intervals until the mass change between successive weighings falls below a defined threshold, indicating that saturation equilibrium has been reached. In addition to mass gain, dimensional changes such as thickness swelling may be measured to evaluate hygroscopic expansion behavior.
Applications
- Material research and development, comparing the moisture sensitivity of different polymer grades and formulations
- Incoming material inspection and production quality control for resin producers and compounders
- Verification of material datasheet values and compliance with purchase specifications
- Electrical and electronic industries, where absorbed moisture directly degrades dielectric strength, insulation resistance, and tracking performance
- Automotive and aerospace engineering, evaluating components exposed to rain, condensation, and washing fluids
- Packaging and construction, assessing moisture barrier performance and long-term dimensional stability
- Predicting drying requirements prior to processing operations such as injection molding of hygroscopic resins like PA, PC, ABS, and PBT
FAQ
What is water absorption of plastics?
Water absorption is the percentage of mass a plastic specimen gains after exposure to water or humid air under standardized conditions, calculated from the mass difference before and after exposure relative to the dry mass.
Which standards are most commonly used?
ISO 62 and ASTM D570 are the two dominant international methods, while GB/T 1034 is the equivalent Chinese national standard. All three define immersion and humidity exposure procedures with comparable principles but slightly different specimen geometries and conditioning details.
How long does a water absorption test take?
A standard 24-hour immersion test can be completed within one to two working days including drying and conditioning. Saturation testing may require several days to several weeks depending on specimen thickness and the diffusion characteristics of the material.
Why do some plastics absorb much more water than others?
Absorption depends on polymer chemistry and structure. Polar polymers such as nylon (polyamide) absorb significant moisture because amide groups form hydrogen bonds with water, while nonpolar, semicrystalline materials such as polyethylene and polypropylene absorb very little. Fillers, glass fiber content, and crystallinity also strongly influence the result.
Can finished products be tested directly?
Yes. Complete parts or representative sections can be tested, provided the exposed surface area is defined and reported, although results are generally not directly comparable with standard specimen data due to differences in geometry and surface condition.
How should results be reported?
Reports should state the test standard, specimen type and dimensions, conditioning method, exposure condition and duration, individual and mean absorption values, and whether any correction for water-soluble matter loss was applied.