Sanitary Pad Absorbency Testing: Standards, Methods, and Laboratory Evaluation

Sanitary pad absorbency testing is a critical quality control process that evaluates how effectively feminine hygiene products acquire, distribute, and retain menstrual fluid under simulated use conditions. Performed by accredited third-party laboratories in accordance with standards such as GB/T 8939, GB 15979, and the ISO 11948 series, these tests measure key parameters including water absorption multiple, strike-through time, rewet performance, and retention capacity. This article provides a comprehensive technical overview of sample requirements, test items, methodologies, and practical applications for manufacturers, brand owners, and regulatory professionals.

Technical Overview

Absorbency is the single most important performance attribute of a sanitary pad, directly determining user comfort, skin dryness, leakage protection, and overall product safety. The fluid handling performance of a sanitary pad can be broken down into four functional stages: acquisition (how quickly liquid passes through the topsheet), distribution (how liquid spreads within the core), storage (how much liquid the core can hold), and retention under pressure (how little liquid returns to the surface when the product is compressed, known as rewet).

These performance characteristics are governed by the core materials, primarily fluff pulp and superabsorbent polymer (SAP, typically sodium polyacrylate), supported by a nonwoven topsheet and, in many designs, an acquisition-distribution layer (ADL). Because material variability, converting processes, and storage conditions can all degrade absorbent performance, systematic laboratory testing is required at multiple stages of the product lifecycle.

In the Chinese regulatory framework, absorbency performance of sanitary napkins and panty liners is addressed by GB/T 8939, while the overall hygienic and microbiological requirements for disposable sanitary products are defined by GB 15979. At the international level, the ISO 11948 series provides whole-product testing methods for absorbent hygiene aids, including capacity determination and leakage under light load. Accredited third-party laboratories routinely apply these standards, together with internally validated methods, to deliver reproducible, comparable results.

Testing Samples

Finished Products

Complete sanitary pads and panty liners in their final retail form constitute the primary test samples. Testing is typically performed on products drawn randomly from intact retail packages to reflect the condition experienced by consumers.

• Day-use and night-use sanitary napkins (regular, ultra-thin, and extra-long formats)
• Winged and wingless sanitary pads
• Panty liners and light-flow products
• Organic cotton, pure cotton, and dry-net surface variants
• Maternity pads and other heavy-absorbency variants

Raw Materials and Components

Component-level testing is essential for incoming quality control and new product development, since finished-product failures are frequently traceable to material variation.

• Superabsorbent polymer (SAP): free swell capacity, centrifuge retention capacity, and absorbency under load
• Fluff pulp: absorption capacity, sinking time, and moisture content
• Nonwoven topsheets and ADL materials: liquid penetration and rewet contribution
• Tissue and airlaid wrapping layers: liquid transfer behavior

Benchmark and Comparative Samples

Manufacturers and brand owners frequently submit competitor or legacy products alongside new formulations for side-by-side comparative evaluation, supporting claims substantiation, cost optimization, and design benchmarking. Retained batch samples are also tested for stability and shelf-life studies.

Testing Items

A comprehensive absorbency evaluation typically encompasses the following test items, selected according to the applicable standard, product positioning, and client specification.

• Water absorption multiple: the ratio of absorbed liquid mass to the original product mass, a mandatory index under GB/T 8939, with a minimum requirement of 7.0 times for sanitary napkins
• Liquid strike-through time: the time required for a defined volume of test liquid to penetrate the topsheet, reflecting acquisition speed and surface dryness
• Rewet under load (wetback): the mass of liquid forced back to the surface when a defined pressure is applied, simulating body weight during wear
• Total absorption capacity: the maximum liquid volume the product can absorb, determined gravimetrically after saturation
• Retention capacity (centrifuge retention capacity): the fraction of absorbed liquid retained after centrifugation, primarily applied to SAP
• Absorbency under load (AUL): the capacity of SAP to absorb liquid while compressed under a defined pressure, such as 2.1 kPa
• Leakage under light load: whole-product leakage resistance evaluated per the ISO 11948-2 methodology
• pH value: measured on an aqueous extract to confirm compatibility with skin physiology
• Moisture content at delivery: oven-drying determination ensuring the product is delivered in a dry, stable state

Results are assessed against the acceptance criteria of the applicable standard, the manufacturer's declared specifications, or the comparative benchmark agreed with the client.

Testing Methods

Conditioning and Test Liquid Preparation

Samples are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity prior to testing to ensure reproducibility. The standard test liquid for most absorbency tests is a 0.9% (9 g/L) sodium chloride saline solution, which approximates the ionic strength of body fluids. For surface penetration evaluations, some protocols employ a synthetic menstrual fluid or a blood-based simulant with defined viscosity and surface tension to better represent real use conditions.

Water Absorption Multiple Test

The specimen is weighed to determine its initial mass, then fully immersed in the standard test liquid for a defined period. After removal, the sample is allowed to drain under controlled conditions, and the saturated mass is recorded. The absorption multiple is calculated as the mass of liquid absorbed divided by the original specimen mass. Multiple specimens from different retail units are tested, and the average result is compared with the minimum multiple required by GB/T 8939.

Liquid Strike-Through Test

The specimen is placed on a standardized absorbent base, and successive doses of test liquid are introduced onto the topsheet surface from a fixed height. Electrodes positioned on the surface detect the moment each dose is fully absorbed, and the strike-through time in seconds is recorded for successive doses. The third-dose value is commonly reported, since it reflects sustained acquisition performance after the surface layer has become partially wetted. Shorter times indicate faster acquisition and a drier feel against the skin.

Rewet Under Load Test

A defined volume of test liquid is dosed onto the specimen and allowed to be absorbed for a set equilibration time. A stack of pre-weighed dry filter paper is then placed on the topsheet and a standardized pressure applied through a rigid plate or weighted plunger for a defined period. The rewet value is the mass gain of the filter paper expressed in grams. Low rewet values indicate effective liquid locking by the SAP core and a dry surface during wear.

SAP Capacity Tests (Free Swell, CRC, and AUL)

Free swell capacity is determined by saturating a defined mass of SAP in a permeable bag, draining, and calculating absorbed grams per gram of polymer. Centrifuge retention capacity subjects the saturated polymer to a defined centrifugal force to remove interstitial liquid, quantifying truly bound liquid. Absorbency under load confines the polymer beneath a porous plate under a specified pressure during absorption, reflecting real core conditions where the material is compressed during use.

pH and Moisture Content

pH is measured potentiometrically on an aqueous extract of the product or material using methodology consistent with established textile and paper extraction practice. Moisture content is determined by drying a weighed specimen to constant mass in a ventilated oven at approximately 105 °C and calculating the percentage mass loss, confirming that the product leaves the factory in a microbiologically stable, dry state.

Applications

Absorbency test reports from accredited third-party laboratories support a wide range of commercial and regulatory purposes across the feminine hygiene value chain.

• Manufacturer quality control: batch release testing, incoming material verification, and process capability monitoring
• Product development: formulation optimization of pulp and SAP ratios, topsheet selection, and performance validation of new structures
• Regulatory compliance: demonstrating conformity with GB/T 8939 and the hygienic requirements of GB 15979 for products placed on the Chinese market
• Market supervision: supporting sampling inspections conducted by market regulatory authorities
• Import and export verification: providing internationally recognized evidence of performance for cross-border trade
• E-commerce platform listing: supplying objective third-party documentation of absorbency claims required by major online marketplaces
• Marketing claim substantiation: validating statements such as rapid absorption, long-lasting dryness, or high capacity with quantitative laboratory data

FAQ

Q: Which standards apply to sanitary pad absorbency testing?

A: In China, GB/T 8939 specifies the absorbency and physical indices for sanitary napkins and panty liners, while GB 15979 governs the hygienic requirements for disposable sanitary products. Internationally, the ISO 11948 series provides whole-product capacity and leakage test methods for absorbent hygiene aids. Accredited third-party laboratories select the applicable methods based on the target market and client specification.

Q: What test liquid is used for absorbency testing?

A: Most absorbency tests use a 0.9% sodium chloride saline solution as the standard liquid. Surface penetration and rewet evaluations may additionally employ a synthetic menstrual fluid or blood-based simulant with defined viscosity to better represent real-use conditions.

Q: How many samples are required for testing?

A: Typical test plans require 10 to 20 intact retail units per test item, drawn randomly from the same production batch and submitted in original unopened packaging together with batch identification details. The laboratory confirms exact quantities when the test plan is confirmed.

Q: How long does absorbency testing take?

A: Routine absorbency test items are generally completed within 5 to 10 working days after sample receipt, depending on the number of test items, conditioning requirements, and reporting scope.

Q: What is the difference between absorption multiple and rewet?

A: Absorption multiple quantifies how much liquid the product can hold in total, which reflects capacity. Rewet quantifies how much absorbed liquid returns to the surface under pressure, which reflects dryness and comfort during wear. A high-capacity product with poor rewet performance can still feel wet, so both parameters must be evaluated together.

Q: Is third-party absorbency testing mandatory?

A: While GB/T 8939 is a recommended national standard, conformity assessment is widely expected during market supervision in China, by major distribution channels, and for substantiating performance claims. Independent testing by accredited third-party laboratories provides objective evidence that protects both brands and consumers.