What Is Absorbency Testing?
Absorbency testing is the laboratory measurement of a material’s ability to take in, distribute, and retain liquid — one of the most commercially important performance characteristics across a vast range of industries. From personal hygiene products and medical textiles to paper packaging, industrial oil sorbents, building materials, and agriculture, the capacity to manage liquids effectively determines whether a product functions as intended.
The scope of absorbency testing is broader than it may first appear. It encompasses not just how much liquid a material can hold (capacity), but also how quickly it absorbs (rate and acquisition time), how effectively it distributes liquid through its structure (wicking), how much it retains under pressure (absorption under load, or AUL), whether retained liquid re-emerges under subsequent pressure (rewet), and — for superabsorbent polymer-based products — how gel formation under saturation affects ongoing absorption. Different applications require different measurements, and different materials require different standardized test methods.
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Key Absorbency Parameters Measured in Laboratory Testing
A complete absorbency characterization of a material requires measuring several interrelated properties:
- Absorption capacity: The total amount of liquid a material can hold, expressed as grams of liquid per gram of dry material (g/g) or as a percentage of the material’s dry weight. This is the most fundamental absorbency metric.
- Absorption rate: How quickly the material takes up liquid — measured as time for a fixed volume to be absorbed (e.g., seconds for a drop to disappear in AATCC TM79) or as uptake rate in g/g/sec.
- Retention capacity: The amount of liquid held after excess is removed by gravitation, blotting, or centrifugation — quantifies how effectively a material retains absorbed liquid against moderate drainage.
- Absorption Under Load (AUL): How much liquid a material absorbs while under a specified compressive load (typically 0.3 psi or 0.7 psi for hygiene applications) — the most functionally relevant parameter for products used under body weight or compression.
- Wicking/transport rate: How rapidly and how far liquid spreads through the material’s structure — critical for products designed to draw moisture away from a contact surface (topsheets in diapers, athletic moisture-wicking fabrics).
- Rewet / strike-through: For multi-layer products (diapers, wound dressings), the amount of previously absorbed liquid that returns to the outer surface under subsequent pressure directly determines skin wetness and comfort.
- Wettability: Whether a surface initially resists or accepts liquid contact — measured as the contact angle of a droplet or the time to initial wetting.
Absorbency Testing: Applications, Methods, and Standards
Different material categories are governed by different standardized test methods. The table below maps the most common applications to their applicable test standards.
| Material category | Key test method(s) | Standard(s) | What is measured |
| Textiles & nonwovens (general) | Drop absorption test; wicking evaluation; gravimetric absorption | AATCC TM79 (drop test); ISO 9073-6 (nonwoven absorption); ASTM D5802 (geosynthetics wicking); NWSP 010.1 (nonwoven wicking) | Time for drop to disappear (wettability); wicking distance and rate; total absorption capacity and retention |
| Absorbent hygiene products (diapers, pads, incontinence) | Acquisition/rewet test; Centrifuge Retention Capacity (CRC); Absorption Under Load (AUL); strike-through time | NWSP 070.7 R2 (rewet/acquisition); NWSP 070.8 R1 (strike-through); ISO 17190-6 (CRC for SAP); NWSP 241.0 (SAP free swell); NWSP 242.0 (SAP CRC) | Strike-through time; rewet (liquid returned to topsheet under pressure); SAP free swell capacity and centrifuge retention; AUL at 0.3/0.7 psi |
| Superabsorbent polymers (SAPs) | Free Swell Capacity (FSC); Centrifuge Retention Capacity (CRC); Absorption Under Load (AUL); Permeability (Saline Flow Conductivity / SFC) | NWSP 241.0 R2 (FSC); NWSP 242.0 R2 (CRC); NWSP 243.0 R2 (AUL/permeability); ISO 17190-4 (CRC); ISO 17190-5 (AUL) | Maximum absorption in free-swelling state; absorption retained after centrifugation; absorption under 0.3 or 0.7 psi load; gel-bed permeability to continued fluid flow |
| Paper and tissue (bibulous / uncoated) | Gravimetric absorption; wicking rate | TAPPI T432 cm-09 (Water absorptiveness of bibulous papers); TAPPI T437 (Dirt in paper, absorbency-related); ISO 12625-8 (Tissue products, water absorption) | Mass of water absorbed per unit area (g/m2) or time to absorb a fixed volume; Klemm wicking rate for tissue |
| Paper and board (sized / packaging) | Cobb test (contact water absorption) | TAPPI T441 om-20 (Cobb test); ISO 535 (Cobb test — paper and board); DIN EN ISO 535 | Grams of water absorbed per m2 in a 60-second (or 30/120s) test — standard QC test for corrugated board, linerboard, packaging |
| Medical textiles (wound care, surgical) | Vertical absorbency; fluid retention; horizontal strike-through | EN 13726-1 (wound dressings fluid handling); ISO 9073-6 (nonwoven absorption); NWSP 010.1 (wicking) | Free swell absorbent capacity; fluid retention capacity at specified pressure; wound dressing absorption and retention |
| Industrial oil sorbents (spill containment) | Oil sorption capacity; water retention; buoyancy after saturation | ASTM F726 (Sorbent Performance — crude oil and hydrocarbons); ASTM F1402 (sorbent booms); ASTM F1860 (sorbent pads/rolls) | Grams of oil sorbed per gram of dry sorbent; water retention (hydrophobic sorbents should reject water); buoyancy retention after oil absorption |
| Plastics and polymers | Immersion water absorption | ASTM D570 (Water Absorption of Plastics); ISO 62 (Water absorption of plastics) | % weight gain after 24-hour immersion; equilibrium water uptake — used for engineering plastics, composites, medical device polymers |
| Geotextiles and construction materials | Wicking height; mass per unit area absorbency | ASTM D5199 (geotextile thickness under load); ISO 9864 (geotextile mass per unit area); ASTM D7367 (wicking of geosynthetics) | Wicking height over time; moisture retention in construction and drainage applications; soil interaction absorbency |
Core Laboratory Methods in Detail
1. Gravimetric Method — Fundamental Capacity Measurement
The gravimetric method is the foundation of quantitative absorbency testing. The sample is weighed dry, immersed in the test liquid for a defined period, removed and drained of free surface liquid (by blotting, drip draining, or centrifugation, depending on the protocol), then weighed wet. Absorbency is calculated as the ratio of liquid gain to dry sample weight.
The specific procedure varies significantly by standard — the immersion time, draining method, test liquid temperature, and calculation basis all differ between TAPPI T432 (for bibulous papers), ISO 9073-6 (for nonwovens), and NWSP methods (for hygiene applications). For superabsorbent polymers, free swell capacity (FSC) is the gravimetric capacity at complete equilibrium without applied pressure — a value that can exceed 200 g/g for high-performance SAPs. Test liquid is typically 0.9% saline solution (physiological saline) for hygiene applications rather than distilled water, as electrolyte concentration affects SAP gel formation.
2. Drop Test (Wettability / Initial Absorption Rate) — AATCC TM79
AATCC Test Method 79 (Absorbency of Textiles) is the definitive US standard for assessing the wettability of textile fabrics. A 0.1 mL drop of distilled water is applied to the fabric surface from a burette held 10 mm above, and the time until the water drop’s surface specular reflection disappears is measured in seconds. The result is reported as the absorption time for a single drop — providing a direct measure of initial wettability and surface absorption rate.
Classification under AATCC TM79: <1 second = very fast; 1-5 seconds = fast; 5-30 seconds = average; 30-200 seconds = slow; >200 seconds = very slow/resistant. This method is widely used for towels, washcloths, athletic performance fabrics, medical linens, and any textile where rapid liquid uptake matters. ISO 9073-6 Section 2 provides a similar drop absorption test for nonwoven fabrics.
3. Centrifuge Retention Capacity (CRC) — The Core SAP Test
Centrifuge Retention Capacity (CRC) is the fundamental characterization test for superabsorbent polymers and advanced hygiene absorbents. In the NWSP 242.0 procedure: a known mass of SAP is sealed in a tea-bag type pouch; the pouch is submerged in 0.9% saline for 30 minutes under free-swelling conditions; the pouch is then centrifuged at 250×g for 3 minutes to remove unretained fluid; and the retained fluid is calculated as g/g SAP by difference.
CRC values for commercial diaper SAPs typically range from 25–40 g/g in 0.9% saline. This parameter governs how much liquid the SAP core can hold once it has swollen — directly determining diaper capacity between changes. ISO 17190-6 covers the same measurement under the formal ISO framework for incontinence product materials.
4. Absorption Under Load (AUL) — Functional Performance Under Pressure
Absorption Under Load (AUL), also called absorption against pressure (AAP), measures how much liquid a SAP or absorbent composite absorbs while under a defined compressive stress — typically 0.3 psi (2.07 kPa) for baby diapers and 0.7 psi (4.83 kPa) for adult incontinence applications. The measurement is critical because SAP gels swell against body weight pressure in real-world use conditions, and gel blocking (where swollen SAP particles obstruct fluid distribution) can prevent full capacity utilization.
NWSP 243.0 (Permeability-Related Absorption Under Pressure) uses a piston-and-cylinder apparatus: SAP is spread in a petri dish or cylinder, a weight applies the test load, and the apparatus is placed on a saline-saturated filter paper. Absorbed mass is determined gravimetrically after a defined period. High-performing SAPs show AUL values of 20–30 g/g — materials that maintain functional absorption even under compression are essential for consistently performing hygiene products.
5. Wicking Test — Moisture Transport and Distribution
Wicking tests measure how liquid travels through a material by capillary action — important for products that must transport moisture from a contact surface (topsheet) through to an absorbent core (diaper), or wick sweat from skin across a fabric surface (sportswear). The simplest protocol places one end of the sample in contact with a liquid reservoir and measures wicking height or distance over time. More sophisticated methods measure transport rate in multiple directions.
NWSP 010.1 covers wicking of nonwoven fabrics. For textiles, AATCC TM197 (Vertical Wicking of Textiles) is the standard vertical wicking test. The Moisture Management Test (AATCC TM195 / ISO 9073-12:2002) is an advanced method for sportswear and technical fabrics that simultaneously measures liquid absorption rate, spreading speed, and moisture transport efficiency across the fabric thickness — producing a moisture management index (MMI) that characterizes the full moisture management performance of two-layer systems.
6. Rewet / Strike-Through — The Comfort Test
For multi-layer absorbent products like diapers and incontinence pads, rewet and strike-through testing assesses product comfort performance — specifically, whether the product keeps the skin contact layer dry after absorbing liquid. These tests are among the most important commercial performance differentiators for hygiene products.
NWSP 070.7 (Repeated Purge Time/Simulated Urine) measures the acquisition time for repeated liquid insults — how quickly the product absorbs successive doses of simulated urine. NWSP 070.8 (Face/Back After Repeated Impact Time) measures strike-through time — the time for simulated urine to penetrate through the topsheet during repeated insults — and rewet — the mass of liquid that transfers back to a dry filter paper stack placed on the topsheet under a defined load after the last insult. Rewet values <0.3 g are generally considered high performance for a baby diaper; values above 1 g indicate poor skin-dryness performance.
7. Cobb Test — Water Absorptiveness of Paper and Board (TAPPI T441 / ISO 535)
The Cobb test is one of the most widely run tests in the paper and packaging industry, measuring how much water a sized (surface-treated) paper or board absorbs in direct contact with water under defined conditions. A metal ring containing 100 mL of distilled water at 23°C is placed on the test specimen for a fixed time (30 seconds, 60 seconds, or 120 seconds, depending on the standard version); the specimen is removed, lightly blotted with a roller to remove surface water, and weighed. The result is expressed as grams of water absorbed per m2.
The 60-second Cobb60 is the most common version. For linerboard and corrugated medium, Cobb60 values of 80–200 g/m2 are typical for standard grades; water-resistant treated grades may achieve <25 g/m2. This test is fundamental for packaging materials that must resist moisture during transport and storage. The test is defined by TAPPI T441 (US paper industry standard) and ISO 535 (international equivalent).
8. ASTM F726 — Oil Sorbent Performance
Industrial oil sorbents — used for marine oil spills, industrial floor containment, and hazardous material cleanup — are evaluated by a fundamentally different set of tests from hydrophilic absorbents. ASTM F726 (Standard Test Method for Sorbent Performance for use on Crude Oil and Related Spills) measures: oil sorption capacity (grams of oil per gram of dry sorbent) after a defined immersion period; drainage loss (oil released on removal from the water surface); and retention (oil held after vertical drainage for 30 seconds).
Unlike hydrophilic absorbents, effective oil sorbents are typically oleophilic and hydrophobic — they attract oil while repelling water. Materials tested under ASTM F726 include polypropylene melt-blown nonwovens, natural fiber sorbents (cotton, peat, cellulose), and mineral sorbents. Oil sorption capacities range from 2–5 g/g for mineral sorbents to 10–30 g/g for polypropylene melt-blown pads. An important complementary metric is water retention — a sorbent that absorbs water alongside oil is less effective for marine spill applications.
Superabsorbent Polymers (SAPs): Testing the Materials Behind Modern Hygiene Products
Superabsorbent polymers (SAPs) — predominantly cross-linked sodium polyacrylate — are the high-capacity absorbent materials inside virtually every modern disposable diaper, incontinence pad, and feminine hygiene product. SAPs can absorb 200–300 times their own weight in aqueous fluids, locking liquid into a stable gel that resists release even under mechanical pressure. Understanding SAP testing is essential for any laboratory working with absorbent hygiene products.
SAP testing is governed primarily by the NWSP (Nonwovens Standard Procedures) framework developed jointly by EDANA (European Disposables and Nonwovens Association) and INDA (Association of the Nonwoven Fabrics Industry), and by the ISO 17190 series. The core SAP test suite covers:
- Free Swell Capacity (FSC) — NWSP 241.0: Maximum absorption in free-swelling conditions without applied load. Measures the SAP’s theoretical capacity ceiling. Values of 40–60 g/g in 0.9% saline are common for commercial diaper SAPs.
- Centrifuge Retention Capacity (CRC) — NWSP 242.0 / ISO 17190-6: Absorption retained after centrifugation at 250×g — the most commercially relevant free-absorption metric. CRC is lower than FSC because centrifugation removes weakly held fluid. Typical CRC: 25–40 g/g.
- Absorption Under Load (AUL) — NWSP 243.0 / ISO 17190-5: Absorption under compressive stress at 0.3 psi or 0.7 psi. Tests real-world performance under body weight. AUL at 0.3 psi is typically 20–30 g/g for high-performance SAP; significantly lower AUL indicates gel blocking issues.
- Saline Flow Conductivity (SFC) — NWSP 244.0: Measures gel-bed permeability — how freely liquid can flow through a compressed, swollen SAP bed. Critical for products requiring multiple liquid insults; low SFC means gel blocking prevents subsequent liquid absorption.
- Particle size distribution — NWSP 250.0: SAP particle size affects both absorption kinetics and processability in hygiene product manufacturing. Measured by sieve analysis.
These SAP tests are performed routinely by raw material suppliers qualifying SAP grades for diaper manufacturing and by hygiene product manufacturers validating incoming SAP batches and new product formulations.
Absorbency Testing Standards Bodies, and Frameworks
- AATCC (American Association of Textile Chemists and Colorists): Publishes TM79 (Absorbency of Textiles), TM195 (Moisture Management), TM197 (Vertical Wicking). Primary US textile absorbency authority.
- EDANA / INDA (via NWSP — Nonwovens Standard Procedures): The global harmonized framework for hygiene nonwovens and SAP testing. NWSP methods (formerly WSP) cover all key hygiene product performance tests. Published at edana.org and indana.org.
- ISO (International Organization for Standardization): ISO 9073-6 (Nonwoven Absorption); ISO 17190 series (incontinence product materials); ISO 535 (Cobb test); ISO 12625-8 (tissue water absorption). ISO methods are the reference for EU regulatory submissions.
- TAPPI (Technical Association of the Pulp and Paper Industry): T432 (Bibulous Papers), T441 (Cobb test). Definitive US paper and packaging absorbency standards. Members include major paper mills, packaging manufacturers, and testing labs.
- ASTM International: F726 (oil sorbents); F716 (chemical sorbents); D570 (water absorption of plastics). F726 is the primary US environmental sorbent performance standard.
- ISO 10993 (Biocompatibility): For medical textile applications (wound dressings, surgical drapes), absorbency testing is performed alongside biocompatibility assessments per ISO 10993 to ensure materials are safe for tissue contact.
Finding Accredited Absorbency Testing Laboratories
The required laboratory capabilities for absorbency testing depend heavily on the material and application. A laboratory serving the hygiene products industry needs centrifuge testing capability, SAP test apparatus (AUL test stands, SFC measurement), and familiarity with NWSP and ISO 17190 protocols. A laboratory serving the paper and packaging industry needs Cobb test apparatus and TAPPI-calibrated procedures. An oil sorbent laboratory needs ASTM F726 testing capability and appropriate hydrocarbon handling infrastructure.
ContractLaboratory.com connects manufacturers, importers, and quality managers with accredited materials evaluation and testing laboratories, consumer goods testing labs, and textile testing specialists equipped for absorbency testing across all material types and standards frameworks. See also our guides to textile testing and medical textile testing.
Frequently Asked Questions About Absorbency Testing
AATCC Test Method 79 (Absorbency of Textiles) is the primary US standard for measuring the wettability and absorption rate of textile fabrics. A 0.1 mL drop of distilled water is applied to the flat fabric surface, and the time for the drop’s surface reflection to disappear is measured in seconds. Results of less than 1 second indicate very fast absorption; times above 200 seconds indicate very slow or resistant fabrics. ISO 9073-6 provides a similar test for nonwoven fabrics. Both are routinely performed by contract textile testing laboratories for towels, wipes, medical linens, athletic wear, and industrial fabrics.
The Cobb test (TAPPI T441 / ISO 535) measures how much water a sized paper or board absorbs when in direct contact with water for a fixed time — typically 60 seconds (Cobb60). A metal ring is placed on the specimen, filled with 100 mL of water, and held for the test period. After removal of the ring and light blotting, the specimen is weighed, and the water absorbed is expressed as grams per square meter. The Cobb test is one of the most widely used quality control tests in the paper and corrugated packaging industry, used to verify that surface sizing treatments are providing adequate water resistance. Typical Cobb60 values range from about 80 g/m2 for standard liner to below 25 g/m2 for moisture-resistant grades.
Superabsorbent polymers (SAPs) are cross-linked polymers — predominantly sodium polyacrylate — capable of absorbing 200–300 times their own weight in aqueous fluids by forming a stable hydrogel. They are the primary absorbent ingredient in disposable diapers, incontinence pads, and feminine hygiene products. SAP testing uses the NWSP (Nonwovens Standard Procedures) framework developed by EDANA and INDA. The core test suite includes: Free Swell Capacity (NWSP 241.0) — maximum absorption in free-swelling conditions; Centrifuge Retention Capacity (NWSP 242.0) — absorption retained after centrifugation; Absorption Under Load at 0.3/0.7 psi (NWSP 243.0) — absorption under compressive pressure simulating body weight; and Saline Flow Conductivity (NWSP 244.0) — gel-bed permeability for multi-insult performance.
Absorption Under Load (AUL), also called absorption against pressure (AAP), measures how much liquid a superabsorbent material or composite absorbs while under a specified compressive stress — typically 0.3 psi for baby diapers and 0.7 psi for adult incontinence products. In real use, hygiene products must absorb liquid against the weight of a sitting or lying person. If a SAP has a high Free Swell Capacity but poor AUL, its gel may be squeezed shut under compression — a phenomenon called gel blocking — preventing full capacity utilization. AUL values of 20–30 g/g at 0.3 psi are typical for high-performance SAPs. AUL is measured by the NWSP 243.0 protocol using a piston-and-cylinder apparatus with a weighted piston above the SAP sample drawing saline by capillary absorption.
Absorption testing measures how much liquid a material takes in; rewet testing measures how much previously absorbed liquid returns to the product surface under subsequent pressure. Rewet is the key determinant of skin dryness in absorbent hygiene products. After absorbing one or more insults of simulated urine, a dry filter paper stack is placed on the product’s topsheet surface, and a defined weight is applied. The mass of liquid that transfers to the filter paper is the rewet value. Values below 0.3 g are considered excellent for baby diapers. Rewet is tested per NWSP 070.8 (Face/Back After Repeated Impact Time) for hygiene products. The companion metric, strike-through time, measures how quickly liquid penetrates through the topsheet on initial insult — also per NWSP 070.8.
Oil sorbent testing uses ASTM F726 (Standard Test Method for Sorbent Performance for use on Crude Oil and Related Spills). Unlike hydrophilic absorbency tests that use water or saline, ASTM F726 tests performance with crude oil or refined petroleum products floating on a water surface — mimicking actual spill conditions. The key measurements are: oil sorption capacity (g oil / g dry sorbent) after immersion; drainage (oil released on removal from the water surface); and retention (oil held after 30-second vertical draining). Effective oil sorbents are oleophilic (oil-attracting) and hydrophobic (water-repelling) — high water retention alongside oil absorption is undesirable. Commercial polypropylene melt-blown pads typically achieve 10–30 g/g oil sorption capacity.
Absorption refers to the total uptake of liquid into a material’s bulk — quantified as mass of liquid per mass of material. Wicking refers specifically to the spontaneous lateral or vertical transport of liquid through a material’s porous structure by capillary action — quantified as distance traveled per unit time or wicking rate. A material can be highly absorbent but wick poorly (holding liquid where it contacts, not distributing it) or wick rapidly but have low total capacity. For many applications — athletic fabrics, diaper topsheets, wound dressings — wicking is more important than raw absorption capacity, because effective moisture management requires transporting liquid away from the contact surface quickly, not just holding it locally. The AATCC TM195 Moisture Management Test measures both wicking rate and transport efficiency together.
Medical wound dressings are tested for fluid handling per EN 13726-1 (European standard for primary wound dressings — fluid handling), which specifies free swell absorbent capacity and fluid retention capacity at defined pressures. ISO 9073-6 applies to nonwoven wound dressing fabrics. For medical device materials with skin or tissue contact, absorbency testing is conducted alongside biocompatibility evaluation per ISO 10993 (the international standard for biological evaluation of medical devices), which ensures materials do not cause adverse reactions despite prolonged moisture contact. Sterility testing per ISO 11135 (sterilization by ethylene oxide) or ISO 11137 (radiation sterilization) also applies to sterile wound care products.
Conclusion
Absorbency testing spans a remarkably diverse range of materials, applications, and standardized methods — from the Cobb test on corrugated board and AATCC TM79 on towels, to sophisticated SAP characterization with AUL, CRC, and SFC measurements for diaper cores, to ASTM F726 performance testing of oil sorbents for spill response. The key to selecting the right test is understanding both the material being tested and the end-use conditions it must perform under: free absorption capacity, absorption under pressure, wicking rate, rewet resistance, and oil vs. water selectivity. Each measures distinct functional properties that matter differently depending on the application.
ContractLaboratory.com connects manufacturers, product developers, and quality teams with accredited materials testing laboratories, consumer goods testing labs, and textile testing specialists experienced across all absorbency testing methods and standards frameworks. Submit a testing request or contact our team.