Post-water-absorption density testing measures the density of plastic test specimens (sample bars) after they have been conditioned in water, providing critical data on moisture uptake and its effect on material properties. This test is widely applied to hygroscopic polymers such as polyamide (PA), PBT, PET, and PC in accordance with ISO 62, ASTM D570, ISO 1183, and ASTM D792. This article explains the test principle, sample requirements, measurement methods, calculation procedures, and typical industrial applications.
Technical Overview
Post-water-absorption density testing evaluates how immersion in water changes the density of standardized plastic test specimens, commonly referred to as sample bars or splines. Many engineering thermoplastics—particularly polyamide (PA6, PA66), PBT, PET, polycarbonate (PC), ABS, and various glass- or mineral-filled compounds—are hygroscopic materials that absorb moisture from their service environment or from direct water contact. As water penetrates the polymer matrix, it increases the specimen mass, swells the part dimensions, and shifts the measured density in a way that directly reflects the extent and uniformity of moisture uptake.
Determining density after water absorption serves several engineering purposes. It allows manufacturers to quantify the degree of saturation achieved during conditioning, verify that moisture-conditioning processes (used before mechanical or electrical testing) are complete, and assess whether absorbed moisture creates micro-voids or free volume changes in molded parts. For filled compounds, comparing the measured density with the theoretical density derived from component volume fractions also enables calculation of void content in the conditioned state.
The test procedure combines two families of standards: water absorption conditioning methods such as ISO 62, ASTM D570, and GB/T 1034, and density determination methods such as ISO 1183-1 (immersion method), ISO 1183-2 (density gradient column), ASTM D792 (displacement method), and ASTM D1505 (density gradient technique). Testing is typically performed by accredited third-party laboratories under controlled temperature conditions, usually 23 ± 2 °C.
Testing Samples
Injection-Molded Multipurpose Specimens
The most common sample format is the injection-molded multipurpose test bar conforming to ISO 3167 or the tensile bar geometry defined in ASTM D638 (Type I). These bars provide a standardized geometry with reproducible wall thickness, ensuring that water diffusion during conditioning follows a predictable path. Specimens are normally conditioned in the dry-as-molded state before immersion so that the initial density and mass can be accurately recorded.
Plate and Bar Specimens
Square or rectangular plates with a nominal thickness of 1.0 mm to 4.0 mm, as specified in ISO 62 and ASTM D570, are also used when the objective is to characterize through-thickness saturation behavior. Thinner specimens reach saturation faster, making them suitable for short-cycle quality control, while thicker bars better represent the diffusion behavior of actual structural parts.
Finished Components and Machined Sections
Sections cut from production parts—connector housings, gears, clips, or pipe fittings—may be tested when validation of the actual article is required. In these cases, the sampling location and wall thickness must be documented because residual stress, flow orientation, and crystallinity gradients in the molded part influence both absorption rate and conditioned density.
Testing Items
A complete post-water-absorption density evaluation typically includes the following measured and calculated parameters:
- Dry-state density of the specimen before immersion (reference baseline)
- Mass of the specimen in the dry-as-molded condition
- Mass after immersion for a defined period or until saturation
- Water absorption percentage, calculated as the mass gain relative to the dry mass
- Density of the specimen after water absorption, measured immediately after standardized surface drying
- Absolute and relative change in density caused by moisture uptake
- Optional: saturated density after long-term immersion, dimensional swelling, and void content estimation for filled compounds
Testing Methods
Step 1: Dry Conditioning and Baseline Measurement
Specimens are first dried in a circulating air oven (typically 50 °C to 80 °C for polyamides, or 105 °C where permitted by the material standard) until constant mass is reached, usually defined as a mass change below 0.1% between successive weighings. After cooling in a desiccator, the specimen mass and dry density are measured. The dry density is determined using the hydrostatic immersion method: the specimen is weighed in air and then in a liquid of known density (distilled water or a suitable auxiliary liquid), and the density is calculated from the buoyancy relationship.
Step 2: Water Absorption Conditioning
The specimen is immersed in distilled or deionized water under one of the standard conditioning regimes:
- Cold water immersion at 23 °C for a fixed duration (24 h, 48 h, or 7 days) or until saturation, per ISO 62 Method 1 and ASTM D570
- Boiling water immersion (typically 2 h or 30 min) to accelerate absorption, per ISO 62 Method 3 / ASTM D570 procedures
- Humid atmosphere conditioning at 50% RH or higher for equilibrium moisture studies
- Immersion at elevated temperature (e.g., 50 °C or higher) for simulated service exposure
Step 3: Surface Water Removal
Immediately after removal from the water bath, the specimen surface is blotted with a dry lint-free cloth to remove all adhering droplets. This step must be rapid and consistent—typically completed within one minute—because excess surface liquid artificially inflates both the wet mass and the apparent density, while excessive delay allows evaporation from the surface layer and introduces a negative bias.
Step 4: Wet Mass and Post-Absorption Density Measurement
The conditioned specimen is weighed in air to determine water absorption percentage, then transferred directly to the hydrostatic weighing setup for density determination. The immersion weighing should be completed quickly to prevent leaching or further water exchange with the immersion liquid. Where diffusion of the immersion liquid into the wet specimen cannot be neglected, a non-miscible auxiliary liquid or a density gradient column (ISO 1183-2 / ASTM D1505) may be used instead. The density gradient column method is particularly valuable for small or irregular specimens, since it yields density from the equilibrium suspension position without weighing in the liquid.
Calculation of Results
Water absorption is calculated as the percentage mass gain: W = (m2 − m1) / m1 × 100%, where m1 is the dry mass and m2 is the mass after immersion and surface drying. Density by the hydrostatic method is calculated as ρ = ma × ρL / (ma − mL), where ma is the mass in air, mL is the apparent mass in the liquid, and ρL is the liquid density at the test temperature. All results are corrected for temperature-dependent liquid density and, where required, for air buoyancy.
Applications
Post-water-absorption density testing supports material development, quality assurance, and design validation across multiple industries:
- Automotive: validation of PA66 and glass-filled polyamide components such as intake manifolds, cooling system parts, and structural brackets that experience long-term exposure to coolant, rain, or humid air
- Electrical and electronics: evaluation of connector housings, coil bobbins, and PBT/PET components where moisture uptake affects dimensions, insulation resistance, and long-term reliability
- Compounding quality control: verification of filler content, batch consistency, and void levels in glass- or mineral-reinforced compounds by comparing measured density with theoretical density
- Pre-conditioning verification: confirmation that specimens used for tensile, impact, or dielectric testing have reached the specified moisture state, since mechanical properties of polyamides vary strongly with moisture content
- Research and development: studies of water diffusion kinetics, saturation limits, plasticization effects, and hydrothermal aging of new polymer grades and formulations
FAQ
Why does the density of a plastic specimen change after water absorption?
Absorbed water adds mass while simultaneously swelling the polymer volume. Because polyamides and similar hygroscopic resins gain mass faster than they expand in volume in the early stage of absorption, density typically rises toward saturation; the exact trend depends on filler content, crystallinity, and whether micro-voids are present.
Which standards govern this test?
Water absorption conditioning is performed per ISO 62, ASTM D570, or GB/T 1034, while density determination follows ISO 1183-1, ISO 1183-2, ISO 1183-3, ASTM D792, ASTM D1505, or GB/T 1033. Client specifications may select specific combinations of conditioning regime and density method.
How is surface water prevented from distorting the result?
Specimens are blotted with a dry lint-free cloth using a standardized, timed procedure immediately after removal from the bath, and weighing is completed within one minute. Repeated trials and consistent operator technique keep the surface-moisture bias below typical measurement uncertainty.
How long does the test take?
Short-cycle tests with 24-hour immersion and same-day density measurement can be completed within two working days, including drying. Saturation studies on thick bars of high-absorption resins may require several weeks of immersion until constant mass is reached.
Can the same specimen be used for both water absorption and density measurement?
Yes. In routine practice the wet mass is recorded first, and the specimen is then transferred directly to hydrostatic weighing, so a single conditioned specimen yields absorption percentage and post-absorption density in one sequence.