Determination of Volume Shrinkage Rate of Plastics After Water Absorption: Principles, Standards, an

The volume shrinkage rate of plastics after water absorption is a key indicator of dimensional stability, revealing how molded parts and specimens change in volume when exposed to moisture and subsequently dried. This test is essential for hygroscopic engineering materials such as polyamide, polycarbonate, and polyester, where moisture-induced dimensional changes can compromise tolerances, fit, and long-term performance. This article outlines the testing principles, applicable international standards, sample requirements, measurement procedures, and typical industrial applications.

Technical Overview

Water absorption testing combined with dimensional analysis is one of the most widely used methods for evaluating the hygrothermal stability of plastic materials. When plastics are exposed to liquid water or humid environments, moisture penetrates the polymer matrix through diffusion, causing plasticization, swelling, and measurable changes in mass, dimensions, and volume. For many hygroscopic resins, subsequent drying causes the material to contract, and the resulting dimensional change — expressed as a volume shrinkage rate — provides critical insight into the reversible and irreversible dimensional behavior of the material.

The volume shrinkage rate after water absorption is defined as the relative change in specimen volume between an initial reference state and a specified state after immersion, humidity exposure, or re-drying. It is typically expressed as a percentage. Polymers differ significantly in their affinity for water: hygroscopic materials such as polyamide (PA6, PA66), polycarbonate (PC), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), ABS, PMMA, and polyurethane absorb substantial moisture, while non-hygroscopic materials such as polyethylene (PE), polypropylene (PP), and polystyrene (PS) absorb only trace amounts at their surfaces.

Accurate determination of volume change is important because moisture-induced dimensional instability can lead to part warpage, loss of press-fit interference, seal leakage, connector misalignment, and cracking caused by internal swelling stresses. Test data support material selection, mold design compensation, tolerance stack-up analysis, and qualification of components intended for humid or submerged service environments.

Testing Samples

Thermoplastic Resins and Compounds

Compression- or injection-molded specimens of PA6, PA66, PA46, PC, ABS, POM, PBT, PET, PMMA, TPU, and their glass-fiber or mineral-filled grades. Standard geometries include square plates, discs of approximately 50 mm to 60 mm diameter and 3 mm thickness, and rectangular bars such as 120 mm × 10 mm × 4 mm, consistent with ISO 62 and ASTM D570 specimen options.

Thermosetting Materials

Phenolic, epoxy, unsaturated polyester, and melamine molding compounds, which may exhibit both water uptake and irreversible volumetric change due to hydrolysis or microcracking.

Fiber-Reinforced Composites

Glass-fiber or carbon-fiber reinforced laminates and molding compounds, where moisture ingress at the fiber-matrix interface can produce anisotropic dimensional change dependent on fiber orientation.

Finished Parts and Components

Connectors, housings, gears, seals, pipes, and medical device components tested in their as-molded condition to evaluate real-world dimensional stability.

Specimen preparation requirements include:

• Sampling from a consistent molding location and direction to account for flow-induced anisotropy
• Removal of surface contamination and edge burrs that could affect dimensional measurement
• Pre-conditioning to a defined dry state (drying to constant mass in a desiccator or oven) or to standard laboratory atmosphere of 23 °C and 50% relative humidity
• Adequate replicate quantity, typically no fewer than three specimens per condition, to ensure statistical reliability

Testing Items

Common test items evaluated within a volume shrinkage assessment program include:

• Water absorption percentage after immersion in water at 23 °C, elevated temperature, or boiling water for a specified duration
• Water absorption percentage after exposure to controlled humidity atmospheres (e.g., 50% RH or 90% RH at 23 °C or 40 °C)
• Linear dimensional change in the length, width, and thickness directions after saturation
• Volume change rate and volume shrinkage rate after immersion and after subsequent re-drying
• Density change before and after water absorption, determined by hydrostatic weighing
• Reversibility assessment, distinguishing recoverable swelling from permanent set or irreversible structural change
• Saturation kinetics, including diffusion rate and time to equilibrium moisture content

Testing Methods

Water Immersion Conditioning

Specimens are dried to constant mass, weighed, and dimensionally characterized, then fully immersed in distilled or deionized water at a controlled temperature — commonly 23 °C per ISO 62 method 1 or ASTM D570, or 100 °C boiling water for accelerated conditioning. Specimens are removed at defined intervals, blotted free of surface water, and re-weighed and re-measured until the mass change between intervals falls below the specified equilibrium criterion, indicating saturation.

Humid Atmosphere Conditioning

For applications involving atmospheric exposure rather than direct water contact, specimens are placed in humidity chambers at controlled temperature and relative humidity. This approach reflects realistic service moisture levels and is particularly relevant for electrical and electronic components evaluated under damp-heat preconditioning sequences.

Dimensional Measurement Method

Length, width, and thickness are measured at marked reference points using calibrated micrometers, digital calipers, dial gauges, optical comparators, or coordinate measuring machines (CMM). The specimen volume is calculated from the geometric dimensions, and the volume change rate is determined as the difference between the volume after conditioning and the initial volume divided by the initial volume, multiplied by 100%. Measuring at identical reference positions minimizes the influence of specimen non-uniformity.

Hydrostatic Weighing (Buoyancy) Method

For complex geometries where dimensional calculation is impractical, specimen volume is determined directly by the Archimedes principle: the specimen is weighed in air and in a liquid of known density, and the volume is computed from the buoyant force. Measurements before immersion, after saturation, and after re-drying allow direct determination of volume swelling and volume shrinkage, consistent with the density determination procedures of ISO 1183-1 and ASTM D792.

Re-Drying and Shrinkage Determination

To quantify shrinkage specifically, saturated specimens are re-dried in an oven or desiccator to constant mass, and the final volume is compared with the original dry volume. The volume shrinkage rate is calculated as: volume shrinkage rate = (V0 − V1) / V0 × 100%, where V0 is the initial dry volume and V1 is the volume after re-drying. A positive value indicates net shrinkage, a value near zero indicates fully reversible behavior, and a negative value indicates permanent swelling or structural change.

Reference Standards

• ISO 62 — Plastics: Determination of water absorption, defining immersion and humidity exposure procedures
• ASTM D570 — Standard Test Method for Water Absorption of Plastics
• GB/T 1034 — Chinese national standard for the determination of water absorption of plastics, technically aligned with ISO 62
• ISO 1183-1 and ASTM D792 — Determination of density by immersion methods, supporting volume calculation by hydrostatic weighing
• ISO 294-4 and ASTM D955 — Determination of molding shrinkage, providing context for distinguishing molding shrinkage from moisture-induced dimensional change

Applications

Volume shrinkage testing after water absorption supports quality control and material qualification across a wide range of industries:

• Automotive: under-hood components, cooling system parts, fuel system modules, sensors, and interior trim where humidity and temperature cycling affect fit and clearance
• Electrical and electronics: connectors, insulators, relay housings, and PCB-mounted components requiring tight dimensional tolerances after solder-reflow humidity exposure and damp-heat aging
• Medical devices: instrument housings, fluid-handling components, and devices subjected to washing, sterilization, or autoclave moisture cycles
• Consumer appliances: washing machine and dishwasher components, kettles, and bathroom fixtures in continuous contact with water
• Pipes, fittings, and civil engineering: water distribution systems and buried installations where long-term hygrothermal stability governs service life
• Material development: comparing resin grades, filler loadings, and processing conditions during formulation optimization, and generating input data for hygro-mechanical finite element simulation

FAQ

What is the difference between water absorption and volume shrinkage?

Water absorption quantifies the mass of moisture taken up by a specimen, expressed as a percentage of its dry mass. Volume shrinkage quantifies the dimensional change in volume — measured after saturation or after subsequent drying — and reflects how the absorbed moisture alters the geometric stability of the material.

Why do some plastics shrink after absorbing water?

During absorption, moisture typically causes swelling as polymer chains are plasticized and free volume increases. Upon drying, the material contracts; if moisture has also promoted relaxation of molded-in residual stresses, hydrolysis, or microvoid formation, the net volume after the wet-dry cycle may be smaller than the original volume, producing measurable shrinkage.

How long does the test take?

Test duration depends on specimen thickness, polymer diffusivity, and conditioning temperature. Thin specimens of highly hygroscopic polyamide may reach equilibrium in a few days at 23 °C, while thick sections or low-diffusivity materials may require several weeks. Boiling water immersion and elevated-temperature humidity conditioning are frequently used to accelerate saturation.

Which method is more accurate for volume determination?

For simple prismatic specimens, direct dimensional measurement at marked reference points offers high repeatability. For complex or finished parts, hydrostatic weighing provides a direct volume measurement independent of geometry and is generally preferred for accuracy.

Can finished parts be tested instead of molded specimens?

Yes. Accredited third-party laboratories routinely test actual components using the buoyancy method and multi-point dimensional measurement, which provides the most relevant data for tolerance verification and field-performance prediction, provided that the part can be dried and re-saturated reproducibly.