Moisture Absorption (Hygroscopicity) Testing: Chinese National Standard (GB) Methods Explained

Moisture absorption (hygroscopicity) testing quantifies how much water a material takes up from liquid immersion or humid air, and it is a critical quality parameter for plastics, composites, textiles, paper, and electronic components. In China, the authoritative procedures are defined by GB national standards such as GB/T 1034, GB/T 9995, and GB/T 2423.3, which are largely harmonized with ISO methods. This guide explains the applicable GB standards, specimen requirements, test conditions, calculations, and typical applications so that engineers and quality professionals can select the right test method with confidence.

Technical Overview

Hygroscopicity, or moisture absorption, describes the tendency of a solid material to absorb and retain water molecules from a liquid environment (immersion) or from the surrounding atmosphere (humidity). For engineering materials, absorbed water acts as a plasticizer: it lowers mechanical strength and modulus, changes dielectric properties, alters dimensions, and in thermoplastics can cause voids or blistering during subsequent high-temperature processing such as soldering or injection overmolding. Quantifying moisture uptake is therefore essential for material qualification, process control, and lifetime prediction.

Chinese national standards (GB) provide the recognized test framework for hygroscopicity evaluation. The principal standard for plastics is GB/T 1034-2008, which is technically aligned with ISO 62 and defines immersion and humid-atmosphere methods. Complementary GB methods cover fibers and textiles, paper and board, wood-based panels, rigid cellular plastics, and damp-heat environmental exposure. Key standards include:

• GB/T 1034-2008 — Determination of water absorption of plastics (harmonized with ISO 62)
• GB/T 1462-2005 — Test method for water absorption of fiber-reinforced plastics
• GB/T 8810-2005 — Determination of water absorption of rigid cellular plastics
• GB/T 9995-1997 — Determination of moisture content in textile materials (oven-drying method)
• GB/T 6503-2017 — Test method for moisture regain of synthetic filaments
• GB/T 1540-2002 — Determination of water absorptiveness of paper and board (Cobb method)
• GB/T 17657-2013 — Test methods for wood-based panels, including 24 h water absorption and thickness swelling
• GB/T 2423.3-2016 — Environmental testing, damp heat steady-state test (aligned with IEC 60068-2-78)
• GB/T 2918-2018 — Standard atmospheres for conditioning and testing of plastics

All tests described below can be performed by accredited third-party laboratories or in-house laboratories equipped with calibrated balances, temperature-controlled baths, and humidity chambers.

Testing Samples

Plastics and Polymer Materials

Tested to GB/T 1034, specimens are typically molded discs of approximately 50 mm diameter and 3 mm thickness, or square plates cut from sheets. At least three specimens per material are recommended, and all surfaces must be clean, free of mold release agents, and undamaged. Film and sheet materials are tested with larger surface areas to ensure measurable weight gain.

Fiber-Reinforced Composites

Tested to GB/T 1462, specimens are cut from laminates or molded plates with defined fiber orientation recorded in the report. Cut edges are often sealed or deburred as specified, because exposed fibers at the edge dominate water ingress and must be treated consistently between comparative samples.

Textile Fibers, Yarns, and Fabrics

For moisture content and moisture regain testing to GB/T 9995 and GB/T 6503, samples are taken from bales, packages, or fabric rolls using sealed containers to prevent moisture exchange before weighing. Sample sizes of tens of grams are typical, and pre-conditioning in the standard atmosphere (for example, 20 °C / 65 % RH or 23 °C / 50 % RH depending on the product standard) is often required.

Paper, Board, and Packaging Materials

For Cobb testing to GB/T 1540, specimens of at least 100 mm square are conditioned to equilibrium in the standard paper atmosphere defined in GB/T 10739. One specimen per test area is used, and the surface to be tested (top or wire side) is clearly identified.

Building and Insulation Materials

Wood-based panels are tested to GB/T 17657 using square specimens of defined thickness for water absorption and thickness swelling. Rigid cellular insulation materials are tested to GB/T 8810 using blocks with controlled cut geometry. Solid wood may be assessed using the immersion method of GB/T 1934.1.

Electronic Components and Assemblies

Complete components, connectors, encapsulated parts, or packaging materials are exposed to damp heat conditions per GB/T 2423.3 to evaluate moisture-related robustness, mass change, and functional drift after humidity stress.

Testing Items

Depending on the material and the governing standard, hygroscopicity evaluations typically report the following measurable quantities:

• Water absorption percentage relative to dry mass (dry-basis absorption)
• Water uptake per unit surface area, expressed in mg/cm²
• Absolute water uptake per specimen, expressed in mg
• Saturation (equilibrium) water absorption at 23 °C immersion or 50 % RH
• Moisture content (wet basis) and moisture regain (dry basis) for fibers and textiles
• Cobb value in g/m² for paper and board surfaces
• Thickness swelling after 24 h immersion for wood-based panels
• Water-soluble matter lost during immersion, for materials that leach
• Absorption kinetics curves and diffusion characteristics for research applications
• Mass change and dimensional change after steady-state damp heat exposure

Testing Methods

Specimen Preparation and Conditioning

Before testing, plastic specimens are dried to constant mass, typically in a ventilated oven at approximately 50 °C or over fresh desiccant in a desiccator, and then cooled in a dry environment before the initial weighing. All weighings are performed on an analytical balance with 0.1 mg resolution. Temperature-controlled water baths maintain immersion media at 23 ± 1 °C, and humidity chambers maintain the specified relative humidity, commonly 50 % RH at 23 °C for the hygroscopic methods.

Water Immersion Testing to GB/T 1034

GB/T 1034-2008 defines several immersion conditions, consistent with the four core methods of ISO 62:

• Method 1 — immersion in distilled or deionized water at 23 ± 1 °C for 24 ± 1 h, the most widely specified production-control condition
• Method 2 — immersion in boiling water for 30 ± 1 min, an accelerated condition for comparative screening
• Method 3 — immersion at 23 °C until equilibrium, giving the saturation water absorption; specimens are weighed at intervals until the mass change falls below the specified criterion
• Method 4 — exposure at 23 °C and 50 % RH until equilibrium, giving the saturation moisture absorption from humid air, which is the direct measure of hygroscopicity

In addition, GB/T 1034-2008 introduces short-duration screening variants covering 24 h humidity exposure and 48 h water immersion, which are useful for rapid incoming-material inspection. After each exposure period, the specimen is removed, surface water is quickly blotted with a dry lint-free cloth, and the specimen is weighed within the short defined time window to avoid evaporative losses.

Oven-Drying Methods for Textiles

To GB/T 9995 and GB/T 6503, textile samples are weighed wet and then dried in a ventilated oven, typically at approximately 105 °C, until constant mass. Moisture content is calculated on the wet basis and moisture regain on the dry basis, with regain referenced to the conventional commercial values given in GB/T 9994 for trade settlement purposes.

Cobb Absorption Test for Paper

To GB/T 1540, a defined area of the paper surface is exposed to water for a set contact time, commonly 60 seconds. The absorbed water mass per unit area (g/m²) is reported as the Cobb value, which characterizes surface sizing quality and printability.

Damp Heat Steady-State Testing

To GB/T 2423.3, specimens are placed in a humidity chamber at steady-state conditions such as 40 °C / 93 % RH or 85 °C / 85 % RH for defined durations (for example, 48 h, 96 h, or longer). Mass gain, dimensional change, and functional performance are evaluated before and after exposure, making this a combined hygroscopic and environmental stress test widely used for electronics.

Calculation of Results

The principal result for plastics is the absorption percentage c = (m2 − m1) / m1 × 100, where m1 is the conditioned dry mass and m2 is the mass after exposure. Results may alternatively be expressed as mg/cm² of specimen surface or as mg per specimen. For materials that lose water-soluble matter, the specimen is re-dried after immersion to determine the residual dry mass m3, and a corrected absorption value of (m2 − m3) / m1 × 100 is calculated. Test reports state the method used, exposure conditions, specimen geometry, and individual and mean results.

Applications

Hygroscopicity testing to GB methods supports a broad range of industrial needs:

• Automotive — moisture conditioning of polyamide connectors, gears, and under-hood components; evaluation of moisture content before welding or overmolding
• Electrical and electronics — moisture robustness of encapsulants, insulating parts, connectors, and packaging materials; mass-change screening after damp heat exposure
• Textiles and apparel — moisture regain control for spinning, weaving, and commercial weight determination; comfort-related fabric evaluation including water vapor permeability to GB/T 12704
• Packaging — surface absorbency of paper and board for printing and converting; selection of desiccants and barrier materials
• Construction — thickness swelling and water uptake of wood-based panels, flooring substrates, and insulation products for dimensional stability qualification
• Material research and development — comparing resin grades, filler loadings, and surface treatments; deriving diffusion parameters from absorption kinetic curves
• Quality control and compliance — incoming inspection, supplier qualification, and conformance demonstration for products sold in the Chinese market

FAQ

What is the difference between water absorption and hygroscopicity?

Water absorption generally refers to uptake by full immersion in liquid water, while hygroscopicity refers to uptake from humid air at a defined relative humidity, such as 50 % RH at 23 °C. GB/T 1034 covers both: immersion methods quantify water absorption, and the humidity-equilibrium method quantifies hygroscopic behavior, which better reflects real storage conditions.

Which GB standard applies to my material?

Use GB/T 1034 for plastics, GB/T 1462 for fiber-reinforced composites, GB/T 8810 for rigid foams, GB/T 9995 or GB/T 6503 for fibers and textiles, GB/T 1540 for paper and board, GB/T 17657 for wood-based panels, and GB/T 2423.3 when a combined humidity-environment stress evaluation is required. Product specifications often name the exact method and condition.

How long does equilibrium moisture absorption take?

It depends strongly on thickness and diffusivity. Thin sections of a highly hygroscopic polymer may equilibrate within days, while thick engineering parts can take weeks at 50 % RH. For this reason, the 24 h immersion method is the default routine condition, while saturation methods are reserved for qualification and research testing.

How should samples be handled before testing?

Specimens should be dried or conditioned as the standard requires and weighed immediately after removal from the conditioning environment. Textile samples must be sealed in airtight containers between sampling and weighing, and plastic specimens should be cooled in a desiccator after oven drying to prevent re-adsorption of atmospheric moisture.

Are GB results comparable with ISO and ASTM results?

GB/T 1034 is harmonized with ISO 62, so results from the corresponding conditions are generally comparable when specimen geometry and drying history are matched. Comparisons with ASTM D570 require care, because specimen dimensions, drying procedures, and default exposure conditions differ. Inter-laboratory comparisons should always state the exact method, temperature, humidity, and duration.

What should a hygroscopicity test report contain?

A complete report identifies the standard and method number, specimen description and dimensions, conditioning and drying procedure, exposure conditions and duration, individual and mean absorption values with the chosen expression (percent, mg/cm², or mg per specimen), any soluble-matter correction applied, and the identification of the testing laboratory and its accreditation scope.