Water absorption testing is a critical evaluation method for determining how plastics interact with moisture, directly impacting dimensional stability, mechanical performance, and electrical properties. This article provides a comprehensive overview of the major plastic water absorption test standards, including ISO 62, ASTM D570, GB/T 1034, and JIS K 7209, covering sample requirements, test procedures, calculation methods, and practical applications. Whether you are involved in material selection, quality control, or product certification, this guide will help you understand the essential requirements for water absorption testing of plastics.
Technical Overview
Water absorption testing measures the amount of water that a plastic material absorbs when exposed to moisture under controlled conditions. When plastics absorb water, several material properties can be significantly affected, including dimensional stability (swelling), mechanical strength, electrical insulation performance, and thermal behavior. The degree of water absorption depends on the polymer chemistry, the presence of fillers and additives, the crystallinity of the material, and the ambient conditions.
Several international and national standards govern water absorption testing of plastics. The most widely recognized standards include ISO 62 (Plastics — Determination of water absorption), ASTM D570 (Standard Test Method for Water Absorption of Plastics), GB/T 1034 (the Chinese national standard, technically aligned with ISO 62), and JIS K 7209 (the Japanese industrial standard). Although these standards share similar principles, they differ in sample dimensions, conditioning procedures, immersion conditions, and reporting requirements. Understanding these differences is essential for laboratories, manufacturers, and engineers who need to compare data across different standards or submit test reports for regulatory compliance.
The test is typically performed by conditioning the specimen to a dry state, weighing it, immersing it in water under specified conditions for a defined period, then reweighing it after surface water is removed. The result is expressed as the percentage increase in mass relative to the dry specimen mass. For some applications, the saturation water absorption (equilibrium value) and the rate of water diffusion are also determined.
Testing Samples
Specimen Geometry and Dimensions
ISO 62 specifies preferred specimen forms, including a disc of 50 mm diameter and 3 mm thickness, or square plates of 60 mm × 60 mm with thicknesses of 1 mm or 2 mm. Specimens may also be taken directly from finished products or manufactured by molding. ASTM D570 typically uses a disc of 50.8 mm (2 inches) diameter and 3.2 mm (1/8 inch) thickness, though other geometries such as bars and strips are permitted when agreed upon.
Specimen Preparation
Specimens should be free from surface contamination, moisture, and stresses that could influence results. For molded specimens, careful control of processing conditions is necessary to ensure representativeness. Edges may need to be smoothed to avoid edge effects that accelerate water uptake. The number of specimens is typically at least three per condition to ensure statistical reliability.
Dry Conditioning
Before testing, specimens must be dried to a constant mass. ISO 62 recommends drying in a vacuum oven at 50 °C or another agreed temperature until the mass change between successive weighings is negligible (typically less than 1 mg). ASTM D570 specifies drying in an oven at 50 °C for 24 hours, followed by cooling in a desiccator before weighing. Proper drying is critical because residual moisture in the specimen will cause the measured absorption to be understated.
Testing Items
Water absorption testing evaluates several distinct parameters, depending on the standard and the end-use requirements:
- Short-term (24-hour) water absorption: the mass increase after 24 hours of immersion in water at 23 °C, the most commonly reported value for material datasheets.
- Saturation (equilibrium) water absorption: the mass increase at equilibrium, obtained by repeated weighing until the change becomes negligible.
- Water absorption after immersion in boiling water: used to assess performance under severe moisture and temperature conditions.
- Water absorption at elevated temperatures: immersion at temperatures such as 50 °C or higher to accelerate moisture uptake.
- Water absorption under humid atmosphere: exposure to 50% relative humidity at 23 °C to simulate typical service environments.
- Diffusion coefficient and solubility parameters: derived from absorption kinetics for engineering and simulation purposes, particularly relevant for electronic packaging and encapsulation materials.
Testing Methods
ISO 62 Test Methods
ISO 62 defines four principal test methods:
- Method 1: Immersion in distilled water at 23 °C. Specimens are weighed at defined intervals (typically 24 hours) to obtain both the 24-hour value and, if required, the saturation value.
- Method 2: Immersion in boiling distilled water at 100 °C. Specimens are immersed for 30 minutes or 2 hours, removed, cooled in room-temperature water for a short period, surface-dried, and weighed.
- Method 3: Immersion in water at an elevated temperature (commonly 50 °C or another agreed temperature) to accelerate the absorption process.
- Method 4: Exposure to a humid atmosphere of 50% relative humidity at 23 °C, which reflects moisture uptake under typical indoor service conditions.
ASTM D570 Test Procedures
ASTM D570 offers several conditioning options, including 24-hour immersion in water at 23 °C, 2-hour immersion at 23 °C, immersion in boiling water for 30 minutes or 2 hours, long-term immersion at 23 °C with weighings every 48 hours until equilibrium is reached (or for a maximum agreed period such as 12 weeks), and exposure to 50% relative humidity at 23 °C. After immersion, specimens are removed, surface water is blotted off with a dry lint-free cloth, and weighing is completed within one minute of removal from the water to minimize mass loss from evaporation.
GB/T 1034 and JIS K 7209
GB/T 1034.1 is the Chinese national standard for water absorption of plastics and is technically consistent with ISO 62, adopting the same four-method structure. JIS K 7209 is the corresponding Japanese standard for plastic film and sheet materials, with methodology that closely follows ISO 62 in principle. Test reports issued by accredited third-party laboratories typically specify the exact standard, method number, immersion conditions, specimen geometry, and calculated results to ensure traceability and comparability.
Calculation of Results
The water absorption percentage is calculated using the following formula: water absorption (%) = (mass after immersion − dry mass) / dry mass × 100. For humid-atmosphere exposure, the same calculation applies using the conditioned mass. When equilibrium is reached, the saturation value is reported. Some standards also allow expression of results as milligrams of absorbed water per unit surface area, which is useful when comparing specimens of different geometries, as absorption is strongly influenced by the surface-area-to-volume ratio.
Factors Affecting Test Results
- Specimen thickness: thicker specimens absorb water more slowly, so the 24-hour value depends strongly on thickness.
- Surface condition and edge effects: cut edges absorb water faster than molded surfaces.
- Water temperature: higher temperatures significantly accelerate absorption kinetics.
- Polymer type: polar polymers such as polyamides (nylon) absorb substantially more water than non-polar polymers such as polyethylene or polypropylene.
- Fillers and reinforcements: glass fiber and mineral fillers generally reduce the relative absorption percentage but may introduce interfacial pathways.
Applications
Water absorption data are used across many industries and stages of product development:
- Material selection and comparison: engineers compare datasheet water absorption values to screen materials for moisture-sensitive applications.
- Electrical and electronics: absorbed moisture degrades dielectric strength, insulation resistance, and tracking resistance, making absorption testing essential for connectors, enclosures, and encapsulants.
- Automotive: components exposed to humidity, wash water, or coolant require verified moisture stability to maintain mechanical strength and dimensional tolerances.
- Packaging: barrier performance of plastic films and containers is partly characterized by moisture interaction behavior.
- Quality control: incoming material inspection and batch-to-batch consistency verification rely on standardized absorption testing.
- Product certification and compliance: test reports from accredited third-party laboratories demonstrating compliance with ISO 62, ASTM D570, or GB/T 1034 are frequently required for market access, supplier qualification, and customer approvals.
- Dimensional stability analysis: measured absorption values feed into predictions of swelling and tolerance changes in precision parts.
FAQ
What is the difference between ISO 62 and ASTM D570?
Both standards measure water absorption using the same fundamental gravimetric principle, but they differ in details. ISO 62 defines four methods (23 °C immersion, boiling immersion, elevated-temperature immersion, and 50% RH exposure) with metric specimen dimensions, while ASTM D570 uses inch-based specimen dimensions (typically a 50.8 mm × 3.2 mm disc) and slightly different drying and blotting procedures. Results between the two standards are generally comparable in trend but may not be numerically identical, so the specific standard used should always be stated in reports.
Why is the 24-hour immersion value so widely quoted?
The 24-hour value at 23 °C is a convenient, repeatable, short-duration measurement that provides a standardized basis for comparing materials. However, it does not represent saturation. Non-polar materials such as polypropylene may already be near equilibrium after 24 hours, while polar materials such as nylon 6 continue absorbing water for weeks, so the 24-hour value alone can be misleading for such polymers.
How is surface water removed before weighing?
Immediately after removal from the water, each specimen is wiped with a dry, lint-free cloth to remove all surface droplets, and then weighed within one minute. This step is critical because residual surface water inflates the result, while delayed weighing causes evaporation losses that deflate it.
Can finished products be tested instead of molded specimens?
Yes. Both ISO 62 and ASTM D570 allow testing of specimens cut from finished articles, provided the geometry, thickness, and edge condition are documented. This is common for failure analysis and incoming inspection, although comparisons with datasheet values should account for differences in thickness and processing history.
How long does a saturation water absorption test take?
The duration depends on the material and specimen thickness. Thin specimens of rapidly absorbing polymers may reach equilibrium within days, while thick sections of slowly diffusing materials may require several weeks or months. ISO 62 practice involves reweighing at regular intervals until the mass change per interval falls below the specified threshold, confirming that equilibrium has been achieved.
What does a high water absorption value indicate?
A high value indicates that the material is hygroscopic, which typically leads to dimensional swelling, reduced mechanical strength and stiffness in the wet state, degraded electrical insulation properties, and possible processing issues such as bubbles during molding if the resin is not properly dried. Materials with high absorption should be used with appropriate design allowances or protective measures in moisture-exposed applications.