Menstrual Blood Simulation Rewetback Testing for Absorbent Hygiene Products: Methods, Standards, and

The menstrual blood simulation rewetback test is a key laboratory method for evaluating how effectively absorbent hygiene products such as sanitary napkins and panty liners lock away fluid and keep their surface dry under pressure. By dosing a formulated fluid that replicates the viscosity, surface tension, and composition of real menstrual flow, the test quantifies rewetback performance, wearing comfort, and skin-dryness protection in a reproducible way. This article explains the technical principles, applicable samples, test items, methods, applications, and frequently asked questions related to menstrual blood simulation rewetback testing for hygiene products.

Technical Overview

Rewetback, also referred to as rewetting or wet-back, describes the migration of already-absorbed liquid back to the wearing surface of a hygiene product when the product is compressed, for example when the user sits, walks, or changes position. A high rewetback value indicates a damp surface against the skin, which reduces comfort and may contribute to skin maceration and irritation over prolonged wear.

Menstrual fluid differs fundamentally from the saline solutions used in many general absorbent-product test methods. It is a mixture of blood and vaginal secretions characterized by higher viscosity, altered surface tension, proteins, and a large fraction of cellular components. Because superabsorbent polymers are optimized to absorb electrolyte solutions, they interact differently with menstrual fluid: cellular components cannot diffuse into the polymer gel and tend to remain near the surface. Testing with water or saline therefore tends to overstate real-world performance, which is why a simulated menstrual fluid is required for a meaningful evaluation.

In the test, a defined volume of simulated menstrual fluid is dosed onto the usage surface of the product, allowed a fixed absorption period, and then a standardized pressure is applied through pre-weighed filter papers. The mass gained by the filter papers is reported as the rewetback value in grams, with lower values indicating a drier surface. The method is closely associated with the Chinese national standard GB/T 8939 for sanitary napkins, and sample conditioning is typically performed in the standard atmospheres defined in ISO 187 and GB/T 10739.

Testing Samples

Sanitary Napkins and Menstrual Pads

Day-use, night-use, ultra-thin, and winged sanitary napkins are the primary sample type for this test. Products may be evaluated as complete retail units, with packaging materials removed and the usage surface left uncontaminated prior to dosing.

Panty Liners

Daily-use panty liners can be assessed with appropriately reduced fluid volumes, since their absorbent cores are thinner and designed for lighter flow conditions.

Other Absorbent Hygiene Products

Maternity pads, period panties and absorbent underwear, incontinence pads, medical absorbent sheets, and developmental materials such as nonwoven topsheets, acquisition-distribution layers, and absorbent core structures can all be evaluated using adapted versions of the rewetback protocol.

Testing Items

A rewetback study is typically structured as a package of related performance measurements, including:

• Single-dose rewetback mass under a standard pressure load
• Repeated-dose, or multi-gush, rewetback performance simulating heavy-flow wear
• Liquid strike-through and absorption time on the product surface
• Free-swell absorption capacity and retention capacity under load
• Surface wetness assessment, including wetted area observation after pressing
• Comparative benchmarking between product grades, designs, or material formulations

Testing Methods

Simulated Menstrual Fluid Preparation

Formulations of simulated menstrual fluid are typically prepared from distilled water blended with glycerol, sodium chloride and other electrolytes, thickening agents such as carboxymethyl cellulose, and a small quantity of tracer dye for visual tracking. The formulation is adjusted so that viscosity and surface tension match the target properties of genuine menstrual fluid. Some standard methods specify defibrinated animal blood as the test liquid, while validated artificial substitutes are widely used in accredited third-party laboratories for better reproducibility, biosafety, and batch-to-batch consistency. The fluid is conditioned to the temperature specified by the applicable method before dosing.

Sample Conditioning and Preparation

Samples are conditioned in a standard atmosphere of 23 °C ± 2 °C and 50 % ± 5 % relative humidity, consistent with ISO 187 and GB/T 10739, until moisture equilibrium is reached. Individual products are carefully removed from packaging, identified, and weighed or dimensioned as required, avoiding any contact with or contamination of the usage surface.

Single-Dose Rewetback Procedure

The single-dose procedure follows a sequence common to GB/T 8939 and related industry methods:

• Place the sample usage-side-up on a flat, hydrophobic base plate, or on a slightly inclined platform where the method specifies it
• Dose a defined volume of simulated menstrual fluid, commonly around 5 mL within a typical range of 2 mL to 10 mL, onto the center of the usage surface at a controlled rate
• Allow a fixed absorption dwell time, commonly several minutes, for the fluid to penetrate and be taken up by the core
• Position a pre-weighed stack of filter papers of a defined grade directly over the wetted zone
• Apply a standardized pressure weight, commonly on the order of 1 kg acting over a defined circular area equivalent to roughly 2 kPa, for a specified contact time typically between 10 seconds and 60 seconds
• Remove and reweigh the filter papers; the mass gain in grams is the rewetback value
• Report the mean, maximum, and individual values across the required number of replicates

Repeated-Dose, Multi-Gush Procedure

To simulate the repeated surges of a heavy-flow day, the fluid is dosed in several increments, for example three successive 5 mL doses separated by intervals of several minutes. Rewetback may be measured after the final dose or after each successive dose. This protocol stresses the acquisition-distribution layer, core channeling, and the lock-up behavior of the superabsorbent polymer, revealing failure modes such as surface pooling and gel blocking that a single-dose test can miss.

Data Interpretation and Acceptance Criteria

Lower rewetback values indicate drier, more comfortable products. Acceptance thresholds are defined by the applicable product standard, by buyer specifications, or by internal release criteria, so the governing method must be agreed before testing. As a general industry reference, well-performing sanitary napkins commonly show single-dose rewetback values below approximately 0.5 g, though limits vary with product type, dose volume, and applied pressure. Rewetback results are most informative when interpreted together with strike-through time and absorption capacity rather than in isolation.

Applications

Menstrual blood simulation rewetback testing supports the hygiene products industry across the full product lifecycle:

• Research and development optimization of topsheet, acquisition-distribution layer, fluff pulp, and superabsorbent polymer formulations
• Substantiation of marketing claims related to dry surfaces, rapid absorption, and long-lasting comfort
• Incoming material inspection, routine quality control, and batch release testing
• Competitive benchmarking against comparable market products
• Export compliance support and acceptance testing against buyer-specified methods
• Failure analysis of consumer complaints involving leakage, dampness, or skin irritation
• Generation of technical data packages for verification by accredited third-party laboratories

FAQ

What exactly is rewetback in sanitary products?

Rewetback is the amount of absorbed fluid that returns to the product surface when a pressure load is applied, mimicking body movement or sitting. It is quantified by the mass gained by filter papers pressed onto the wetted surface under a standardized weight, and it directly reflects how dry the product feels during wear.

Why is simulated menstrual fluid used instead of water or saline?

Menstrual fluid is more viscous than saline, contains proteins and cellular components, and interacts differently with superabsorbent polymers. Water and saline are absorbed too easily and systematically overestimate performance, whereas a properly formulated simulated menstrual fluid reproduces the penetration resistance and surface-hold behavior of real flow.

Which standards govern this test?

The rewetback performance of sanitary napkins is principally addressed by the Chinese national standard GB/T 8939, with sample conditioning conducted per ISO 187 and GB/T 10739. Many international absorbent-product methods rely on saline-based fluids, so when testing for a specific market or buyer, the exact method and acceptance limit must be confirmed in advance.

How much fluid and what pressure are typically applied?

Typical single-dose protocols use about 5 mL of simulated menstrual fluid, a dwell time of several minutes, and a pressure load on the order of 1 kg applied through filter papers for roughly 10 to 60 seconds. Multi-gush protocols repeat dosing two to four times at intervals to simulate heavy flow. Exact values always follow the governing method.

What result is considered a pass?

There is no universal pass limit; criteria are set by the applicable standard, the buyer, or the manufacturer's internal specification. As a practical benchmark, single-dose rewetback values below about 0.5 g are generally regarded as indicating a dry, comfortable surface, while values approaching or exceeding 1 g suggest noticeable dampness.

Can this test be performed externally?

Yes. Accredited third-party laboratories routinely conduct menstrual blood simulation rewetback testing under controlled atmospheres with validated fluid formulations and calibrated dosing and loading equipment, providing independent data for product development, quality disputes, and market claims support.