Sanitary Pad Absorbency Testing: Experimental Methods, Standards, and Applications

Absorbency is the single most important performance attribute of a sanitary pad, and laboratory absorbency testing experiments provide objective, reproducible data on how fast, how much, and how securely a product absorbs and locks in liquid. This article explains the testing principles, sample requirements, core test items, standard methods (GB/T 8939, ISO 11948 series, and related nonwoven test methods), instrumentation, and practical applications of sanitary pad absorbency testing in product development and quality control.

Technical Overview

Sanitary pad absorbency testing is a controlled laboratory experiment designed to quantify the liquid-handling performance of disposable feminine hygiene products. Because menstrual fluid is difficult to standardize, tests are performed with a defined surrogate fluid, most commonly a 0.9% sodium chloride (saline) solution or a formulated synthetic menses, whose ionic strength and viscosity approximate real body fluids. Performance is evaluated through a set of gravimetric and chronometric measurements that describe the complete liquid journey through the product: how quickly the liquid penetrates the topsheet, how much total liquid the structure can take in, how much it retains under pressure or drainage, and how much liquid returns to the skin-facing surface (rewet).

The experimental framework is anchored in recognized standards, including the Chinese national standard GB/T 8939 for sanitary towels (pads), the mandatory hygiene standard GB 15979 for disposable sanitary products, the ISO 11948 series for urine-absorbing aids (whole-product testing and short-time liquid release under light load), and ISO 9073-13 for repeated liquid strike-through time of nonwovens. Comparable ASTM, EN, and JIS nonwoven and absorbent-product methods may also be applied. All tests are conducted in an accredited third-party laboratory or an in-house laboratory under controlled temperature and humidity, using calibrated balances, timers, and dedicated absorbency test apparatus, so that results are repeatable and comparable between batches, suppliers, and brands.

Testing Samples

Finished Products

The primary test objects are intact, unused sanitary pads taken from sealed retail or production-line packaging. Typical sample groups include regular day-use pads, ultra-thin pads, overnight extended-length pads, winged pads, and panty liners. Samples must be free of mechanical damage, contamination, and prior moisture exposure, and each unit is identified by brand, model, size designation, production lot, and expiry date so results can be traced to a specific manufacturing batch.

Materials and Components

When the objective is failure analysis or formulation optimization, testing may be extended to individual components: the superabsorbent polymer (SAP), fluff pulp or airlaid core, the acquisition and distribution layer, and the nonwoven topsheet. Component-level testing isolates which layer limits overall absorbency, for example whether slow strike-through is caused by the topsheet or whether poor retention is caused by insufficient SAP content.

Sample Preparation and Conditioning

Before testing, samples are conditioned in a standard laboratory atmosphere, typically 23 ± 2 °C with the relative humidity specified by the applicable method, for a minimum of 24 hours to stabilize their moisture content. Conditioning in the original sealed wrapper is preferred to prevent ambient humidity from altering the dry mass. For statistically meaningful results, a minimum of five to ten units per lot per test item is normally tested, and results are reported as the mean value with standard deviation. Sampling plans may follow GB/T 2828.1 (equivalent to ISO 2859-1) where batch acceptance decisions are required.

Testing Items

A complete absorbency testing experiment evaluates the following performance parameters:

• Absorption speed — the time required for the pad to absorb a fixed dose of test fluid.
• Free absorption capacity — the total mass of liquid absorbed by the freely swollen product, expressed in grams per piece or as an absorbency multiple relative to dry mass.
• Retention capacity — the liquid mass retained after drainage or centrifugation, indicating how strongly the liquid is locked inside the core.
• Rewet under load — the mass of liquid pressed back to the surface under a defined compression, reflecting dryness against the skin.
• Strike-through time — the time for liquid to penetrate the topsheet, including performance under repeated liquid insults.
• Absorbency under load (AUL) — the capacity measured while a compressive pressure is applied, simulating body weight.
• Liquid distribution — the uniformity of liquid spread across the core, evaluated for channeling, pooling, and gel blocking.
• Leakage resistance — the mass of liquid released from the product under light load, evaluated for the whole product.

Testing Methods

Absorption Speed Test

The sample is placed flat on the stage of an absorbency tester with the topsheet facing upward. A fixed volume of 0.9% saline, for example 5 mL, is delivered from a standardized funnel or burette onto the center of the pad. A stopwatch or the automated timer of the instrument records the time from liquid contact until the surface liquid is fully absorbed and the specular reflection disappears. The measurement is repeated at the front, middle, and rear zones of the pad, and the average absorption time is reported in seconds. Faster absorption reduces the risk of surface pooling and side leakage during a sudden gush.

Absorption Capacity Test

The pad is weighed in its dry condition (m1), then fully immersed in saline at the specified temperature for the defined immersion time. It is then lifted out, allowed to drain in the specified orientation, for example hanging vertically for a defined period, and weighed again (m2). The absorption capacity is calculated as m2 minus m1 in grams, and is frequently normalized as an absorbency multiple, (m2 minus m1) divided by m1. This gravimetric method is the core capacity procedure referenced by GB/T 8939 and allows direct comparison between ultra-thin and overnight products of different masses.

Retention Capacity Test

To distinguish liquid that is chemically locked by the superabsorbent polymer from liquid merely held by capillary forces, a freely swollen sample or core specimen is placed in a centrifuge and spun at a defined relative centrifugal force, commonly on the order of 250 × g for several minutes, in accordance with recognized SAP test procedures. The retention capacity is the ratio of the liquid mass remaining after centrifugation to the dry mass of the specimen, expressed in grams per gram. High retention capacity indicates that liquid will not be squeezed out during normal body movement.

Rewet Under Load Test

A defined dose of saline is dosed onto the center of the pad and allowed to absorb for a set equilibration time. A stack of pre-weighed dry filter papers is placed over the wetted area, and a cylindrical anvil weight, for example 2 kg or another load defined by the method, is applied for a specified period. The filter papers are reweighed, and the mass gain is reported as the rewet value. A low rewet result corresponds to a dry surface feel and reduced skin maceration risk, which is one of the strongest indicators of real-world comfort.

Strike-Through Time Test

A strike-through tester uses an electrode-equipped weight plate resting on the topsheet. When a dose of conductive saline penetrates the topsheet and bridges the electrodes, the integrated timer stops automatically, recording the strike-through time. Testing per ISO 9073-13 adds repeated liquid insults to the same spot, simulating successive gushes and revealing whether the acquisition layer remains functional after the first insult.

Leakage and Multiple-Insult Testing

Whole-product leakage behavior is assessed by methods such as ISO 11948-2, in which defined liquid doses are applied to the product positioned on an inclined support under a light compressive load. The liquid mass escaping through the barrier edges or backsheet is collected and weighed. Repeating the insult cycle several times, with defined rest intervals, approximates overnight usage conditions and identifies the failure point of the product.

Liquid Distribution Assessment

A dyed test fluid is used so that the absorbed liquid is visually traceable. After dosing, the spread area, symmetry, and uniformity of the stain are photographed and measured, and the pad is dissected to inspect the core for gel blocking, channeling, or unwetted regions. Even distribution prevents localized saturation and premature leakage at the pad edges.

Instrumentation and Test Fluid

Essential equipment includes an analytical balance with 0.01 g resolution, an absorbency speed tester, a strike-through timer, a laboratory centrifuge, calibrated anvil weights, standardized filter paper, glassware for fluid dosing, and a conditioned test chamber. The 0.9% sodium chloride solution must be freshly prepared or verified, since the ion concentration of the fluid directly affects the swelling behavior of the superabsorbent polymer and therefore the reproducibility of every measurement.

Applications

Absorbency testing experiments serve stakeholders across the entire product lifecycle:

• Product research and development — comparing SAP grades, core formulas, topsheet materials, and acquisition layer designs during development iterations.
• Manufacturing quality control — verifying lot-to-lot consistency, validating process changes, and supporting incoming material inspection.
• Regulatory compliance and market access — demonstrating conformity with GB/T 8939 performance requirements and the mandatory GB 15979 hygiene standard for products sold in China, supported by accredited third-party laboratory reports.
• Competitive benchmarking — generating objective data to position a product against market alternatives of the same size class.
• Claims substantiation — providing laboratory evidence for marketing statements about absorption speed, capacity, and dryness.
• Consumer protection and e-commerce quality surveys — verifying that declared specifications match actual performance for products sold online.
• Failure analysis — diagnosing the root cause of consumer leakage complaints by combining capacity, rewet, and distribution data.

Together, these applications make absorbency testing the central experimental tool for translating subjective user perception of dryness and leak protection into measurable, auditable engineering data.

FAQ

Why is 0.9% saline used instead of pure water or real menstrual fluid?

The ionic strength of saline closely matches that of body fluids and produces swelling behavior of superabsorbent polymers representative of real use. Pure water would overstate SAP absorption because polymer swelling is strongly suppressed by dissolved ions, while real menstrual fluid varies between donors and cannot be standardized or supplied reproducibly. Synthetic menses formulations with controlled viscosity are used in research settings when surface flow behavior must be simulated more closely.

How many pads must be tested to obtain a reliable result?

A minimum of five to ten intact units per production lot per test item is standard practice, with results reported as the mean plus or minus the standard deviation. Larger sample sizes are recommended for acceptance testing of full production batches, using statistical sampling plans such as GB/T 2828.1 (ISO 2859-1).

What is the difference between absorption capacity and rewet?

Absorption capacity measures how much liquid the pad can take in under free conditions, whereas rewet measures how much of that absorbed liquid is pressed back to the skin surface under load. A pad can have high capacity but poor rewet performance, meaning it feels wet and uncomfortable despite technically absorbing a large volume. Both parameters must be evaluated together to predict real comfort.

Is compliance with GB/T 8939 mandatory?

The designation GB/T indicates a recommended technical standard rather than a compulsory one; however, its methods are almost universally adopted in procurement contracts, product marking, and third-party conformity assessment. In contrast, GB 15979, which governs hygiene and microbial safety of disposable sanitary products, is mandatory for market access in China.

Can results from different laboratories be compared directly?

Comparison is valid only when the same standard edition, test fluid, conditioning regime, dose volume, load, and timing parameters are used. Differences in any of these variables, particularly the saline concentration and the drainage or centrifugation conditions, can shift capacity results by a significant margin, so test reports must always state the complete experimental conditions.

How are test results judged as pass or fail?

There is no universal threshold applicable to all products. Acceptance criteria are defined by the product standard, the manufacturer specification, or the buyer's procurement document, often differentiated by product grade, such as ultra-thin day use versus extended overnight use. The laboratory reports measured values against the declared specification, and conformity is determined by the client or certification body.