Recycled PA Material Water Absorption Testing: Standards, Methods and Technical Guide

Recycled PA (polyamide/nylon) is a strongly hygroscopic engineering thermoplastic, and its water absorption rate directly affects dimensional stability, mechanical strength, electrical performance and molding quality. Water absorption testing performed to ISO 62, ASTM D570 and GB/T 1034 by accredited third-party laboratories helps manufacturers qualify recycled PA batches, verify consistency with virgin resin, and control moisture before processing. This guide covers the testing principle, sample requirements, procedures, calculation methods and typical applications for recycled PA water absorption testing.

Technical Overview

Polyamide (PA), commonly known as nylon, contains polar amide groups in its molecular chain that form hydrogen bonds with water molecules. As a result, both virgin and recycled PA grades—such as PA6, PA66 and PA12—absorb significant amounts of moisture from the surrounding environment. Water acts as an internal plasticizer: it lowers the glass transition temperature, reduces tensile strength and stiffness, increases impact toughness, and causes dimensional swelling of finished parts.

Recycled PA (rPA) is typically produced from post-industrial sprues and runners, off-spec pellets, or post-consumer streams such as fishing nets, carpets, automotive components and cable sheathing. Reprocessing history, thermal degradation, contamination, residual additives and blending with other polymers can all alter the moisture uptake behavior of recycled PA compared with virgin resin. Measuring the water absorption rate therefore serves as a sensitive indicator of material identity, purity and overall quality for recycled polyamide.

Water absorption testing determines the relative increase in mass of a conditioned specimen after exposure to liquid water or a humid atmosphere under strictly controlled temperature and time conditions. The result is expressed as a percentage of the original dry mass and is widely used for material qualification, incoming inspection, batch quality control and product design.

Testing Samples

Molded Specimens (Preferred)

The preferred specimen forms defined by the major standards are as follows:

• ISO 62: square plates of 60 mm × 60 mm with a thickness of 1.0 mm or 3.0 mm, or bars of specified dimensions, compression or injection molded from the recycled PA compound.
• ASTM D570: circular discs of 50.8 mm (2 in) diameter and 3.2 mm (1/8 in) thickness.
• Specimens cut from finished parts or plaques may also be tested to evaluate the actual product condition, in which case the specimen thickness must be reported together with the result.

Granules and Pellets

For incoming moisture control before extrusion or injection molding, recycled PA granules are tested for water content (as-received or after drying) rather than full saturation absorption, typically by Karl Fischer titration or a rapid moisture analyzer. This verifies that the material has been dried into the processing window, commonly below 0.1–0.2 %, to prevent hydrolytic degradation, silver streaks and voids in molded parts.

Specimen Conditioning

• All specimens must be dried to constant mass before testing—typically 24 h at 50 °C under vacuum, or at 80–100 °C where the material specification permits—so that the dry reference mass is accurately established.
• Surfaces must be smooth and free of cracks, and edges must be deburred to avoid trapping water or air bubbles.
• A minimum of three specimens per test condition is recommended, and the results are averaged.

Testing Items

Typical test items for recycled PA water absorption evaluation include:

• 24-hour water immersion absorption at 23 °C (short-term absorption)
• Long-term immersion at 23 °C until saturation (equilibrium absorption)
• Boiling water immersion absorption (accelerated test)
• Absorption in a 50 % relative humidity atmosphere at 23 °C (service humidity equilibrium)
• Water content of granules as received or after drying (Karl Fischer method)
• Water-soluble matter loss (optional, determined by re-drying after immersion)
• Dimensional change (swelling) associated with moisture uptake (optional)

Reported values normally include the percentage mass gain, specimen geometry and thickness, exposure condition and duration, and whether saturation equilibrium was reached.

Testing Methods

Applicable Standards

• ISO 62 – Plastics: Determination of water absorption (Methods 1 to 4)
• ASTM D570 – Standard Test Method for Water Absorption of Plastics
• GB/T 1034 – Chinese national standard for determination of water absorption of plastics, technically aligned with ISO 62
• ISO 15512 / ASTM D6869 – Determination of water content by Karl Fischer titration (for granules)

General Procedure

• Dry the specimens to constant mass and weigh each one (m1) on an analytical balance with a resolution of 0.1 mg.
• Immerse the specimens in distilled or deionized water at the specified temperature (23 °C for the immersion methods; boiling water for the accelerated method), or place them in a 50 % RH chamber at 23 °C for humidity conditioning.
• At the end of the exposure period, remove each specimen, quickly wipe off surface water with a dry lint-free cloth within a defined short time, and weigh immediately (m2).
• For saturation tests, repeat weighing at fixed intervals, for example every 24 h, until the mass change between consecutive readings falls below the specified limit.
• Calculate the water absorption: c = (m2 − m1) / m1 × 100 %.

Influencing Factors and Precautions

• Specimen thickness strongly affects diffusion time; PA6 and PA66 may take several weeks to saturate in 23 °C water at 3 mm thickness, so equilibrium values must never be confused with 24-hour values.
• Incomplete drying before testing underestimates absorption; residual moisture in recycled PA is often higher than in virgin resin.
• Contaminants, fillers such as glass fiber or minerals, and polymer blends in recycled PA change both the absorption rate and the saturation level—glass-filled grades show lower percentage uptake.
• Consistent wiping time and technique are essential for repeatability, because residual surface water creates a positive bias.

Typical reference values: PA6 and PA66 absorb roughly 1–2 % after 24 h immersion at 23 °C, reach about 8–10 % at saturation in water, and equilibrate near 2.5–3 % at 50 % RH. Significant deviations in a recycled batch often indicate contamination, excessive degradation or misidentification of the polymer grade.

Applications

• Incoming quality control of recycled PA pellets and regrind before compounding or molding
• Equivalence assessment between recycled and virgin PA for automotive interior and under-hood components, cable ties, connectors, housings and gears
• Batch release inspection and certification by accredited third-party laboratories for sustainability and recycled-content programs
• Design input for dimensional tolerances, moisture-conditioning processes and property derating in humid service environments
• Verification of drying efficiency in production and troubleshooting of molding defects such as silver streaks, hydrolytic degradation and weak weld lines

FAQ

Why is water absorption testing so important for recycled PA?

Recycled PA often carries an unknown contamination profile, degradation history and blend content. Absorption behavior is highly sensitive to these factors, so the test quickly reveals whether a recycled batch behaves like the virgin grade it is intended to replace.

What is the difference between moisture content and water absorption?

Moisture content measures the water present in the material as received or after drying, usually by Karl Fischer titration, and controls processability. Water absorption measures how much water a dry specimen takes up under defined exposure conditions and characterizes the hygroscopic nature of the material itself.

Which standard should I choose, ISO 62 or ASTM D570?

Both are widely accepted. ISO 62 is commonly requested in European and international supply chains, ASTM D570 prevails in North American specifications, and GB/T 1034 is the equivalent Chinese national standard. Always test to the standard cited in the purchase specification or on the part drawing.

Can recycled PA granules be tested directly?

Granules can be used for water content determination and comparative absorption screening, but molded specimens of defined geometry provide reproducible absorption values suitable for specifications and material-to-material comparison.

How long does saturation take?

For 3 mm PA specimens in 23 °C water, full saturation can take several weeks; therefore, many specifications use the 24-hour immersion value or the boiling water accelerated value for routine quality control.