Softness is one of the most decisive quality attributes of sanitary napkins, directly influencing skin comfort, dryness perception, and consumer purchase preference. This article explains how sanitary napkin softness is measured in accredited third-party laboratories, covering blade-and-slot softness testing, bending stiffness, compression analysis, and surface friction methods with reference to standards such as GB/T 8942, ASTM D1388, ASTM D4032, and ISO 9073-7. It also outlines sample preparation, typical test items, reporting conventions, and practical applications in product development and quality control.
Technical Overview
Sanitary napkins are multi-layer composite products typically consisting of a nonwoven or apertured-film top sheet, an acquisition/distribution layer, an absorbent core (fluff pulp and/or superabsorbent polymer), and a polyethylene back sheet. Because "softness" as perceived by the user is a combined sensory response to bending flexibility, compressibility, surface smoothness, and resilience, no single physical property can fully represent it. Laboratory softness testing therefore decomposes this attribute into measurable quantities and expresses them in objective units such as millinewtons (mN) for slit softness, bending length and flexural rigidity for stiffness, and dimensionless coefficients for surface friction.
The most widely used approach for the top sheet and tissue-type layers is the blade-and-slot (slit) softness method described in GB/T 8942, in which a specimen is forced through a narrow slit and the peak resistance force is recorded as the softness value. For whole-pad flexibility, cantilever bending methods (ASTM D1388, ISO 9073-7) and the circular bend procedure (ASTM D4032) are commonly applied. These instrumental methods are frequently complemented by compression/indentation analysis and trained sensory panel evaluation to validate correlation with actual hand feel. Product-level requirements for sanitary napkins in China are governed by GB/T 8939 and the hygienic requirements of GB 15979, while softness itself is typically specified as an internal or contractual quality indicator rather than a mandatory regulatory limit.
Testing Samples
Top Sheet Materials
Nonwoven top sheets (spunbond, thermal-bonded, hot-air through-air bonded, or hydrophilic-treated polypropylene/polyester) and apertured polyethylene films are the primary substrates for slit softness and surface friction testing. Specimens are typically cut to approximately 100 mm × 100 mm from positions that exclude embossed logos, side seals, and edge welds, and are tested with a fixed face orientation as defined in the test protocol.
Finished Sanitary Napkins and Panty Liners
Complete pads, including ultra-thin, regular, overnight, and winged formats, are tested as delivered to evaluate whole-product flexibility. Specimens are measured in both the machine direction (length) and cross direction (width), since anisotropic core structures and embossing patterns produce direction-dependent results.
Core and Secondary Components
Absorbent cores, tissue wrapping layers, acquisition layers, and back sheet films may be tested individually to identify which layer governs the overall hand feel of the finished product, supporting material selection and formulation studies.
Sample Conditioning
Before testing, specimens are conditioned to equilibrium in a standard atmosphere, commonly 23 ± 1 °C and 50 ± 2% relative humidity for paper-based materials (ISO 187) or 20 ± 2 °C and 65 ± 4% relative humidity for textile materials (ISO 139, GB/T 6529), for at least 24 hours. Proper conditioning is essential because moisture content significantly affects the stiffness of cellulose-based layers.
Testing Items
Typical softness-related test items reported by accredited third-party laboratories include:
- Slit (blade-and-slot) softness value in mN, per GB/T 8942, for top sheet nonwovens and tissue layers
- Bending length in mm and flexural rigidity in mg·cm, per ASTM D1388 / ISO 9073-7, for strip specimens
- Circular bend stiffness in N or mN, per ASTM D4032, for multi-directional flexibility
- Compression softness parameters (peak force, plateau force, recovery ratio) from force–displacement curves
- Surface coefficient of friction (static and kinetic) of the top sheet against a skin-simulating probe
- Supporting measurements such as thickness, basis weight, and compressibility, which normalize softness comparisons
- Trained sensory panel scores for overall hand feel, cushioning, and silkiness, used for correlation analysis
Testing Methods
Blade-and-Slot (Slit) Softness Test — GB/T 8942
A conditioned specimen is placed over a parallel slit of defined width (commonly 5, 10, or 20 mm, selected according to specimen thickness). A blade of specified width presses the specimen into the slit at a constant speed, and the maximum force recorded during insertion is reported as the softness value in mN. Lower values indicate a softer, more pliable material. Face orientation (embossed side up or down), slot width, and blade speed must be held constant for valid comparisons. Usually at least 10 valid specimens are tested and the average and coefficient of variation are reported.
Cantilever Bending Test — ASTM D1388 / ISO 9073-7
A rectangular strip (typically 25 mm × 200 mm) is slid horizontally over a platform edge until its free end deflects under its own weight to a defined angle of 41.5°. The overhanging length at that moment is the bending length, from which flexural rigidity is calculated using the specimen mass per unit area. This method quantifies whole-pad and nonwoven drape; longer bending length and higher rigidity indicate a stiffer, less conformable product.
Circular Bend Test — ASTM D4032
A flat specimen is placed over an orifice, and a plunger forces it through, simultaneously bending it in all directions. The maximum force required is reported as the circular bend stiffness. Because the deformation is multi-axial, this method better reflects the complex bending that occurs when a pad conforms to body contours, and it is useful for comparing finished pads with similar geometry.
Compression and Indentation Analysis
A texture analyzer fitted with a cylindrical or hemispherical probe compresses the pad surface at a controlled speed to a defined strain (for example, 50–80% of initial thickness). The force–displacement curve yields a hardness/softness index, and the recovery behavior after load removal indicates resilience and cushioning capacity. Softer pads exhibit lower peak forces and higher elastic recovery, properties closely linked to the sensation of comfort during use.
Surface Friction Testing
The static and kinetic coefficients of friction between the top sheet and a standard slider are measured, following principles analogous to ASTM D1894. Lower friction values correspond to a silkier, smoother surface that feels gentler against skin and reduces drag during movement. Friction results are particularly sensitive to embossing geometry, bonding pattern, and surface treatment chemistry.
Sensory Panel Validation
Trained panelists rank coded samples for overall softness, smoothness, and cushioning using standardized scales. Panel data are statistically correlated with instrumental values to verify that laboratory numbers predict real hand feel, and the resulting correlation models are used to set internal acceptance limits for production control.
Applications
Softness test data serve multiple functions across the product life cycle:
- Material development: screening nonwoven grades, aperture patterns, thermal-bonding parameters, and surface finishes to maximize perceived softness
- Incoming quality control: verifying that purchased top sheet, core, and back sheet rolls conform to agreed softness specifications
- Product benchmarking: objective comparison of a new design against competitor products in the same market segment
- Process optimization: evaluating the effect of embossing pressure, glue application, and core compression on finished-pad flexibility
- Claims substantiation: generating accredited third-party laboratory reports that support advertising statements such as "ultra-soft surface" or "feather-flexible core"
- Stability studies: confirming that softness is retained after transport, storage, humidity cycling, and shelf-life aging
- Supplier qualification and contractual specification setting between converters and raw material producers
FAQ
What does a softness value in mN actually mean?
In the slit method, the value is the peak force needed to push a specimen through a narrow slot. Lower millinewton values mean the material yields more easily and is judged softer. Values are only comparable when slot width, blade geometry, speed, and face orientation are identical, so full test conditions must always accompany the result.
Is softness a mandatory regulatory requirement for sanitary napkins?
No. Product standards such as GB/T 8939 and hygienic standards like GB 15979 focus on absorption performance, physical dimensions, and microbiological safety. Softness is a competitive quality attribute, normally defined through internal specifications, customer agreements, or marketing claims, and it is tested using the published material-level methods described above.
Why test both the top sheet and the finished pad?
The top sheet dominates the first skin-contact impression and is best assessed by the slit softness and friction methods, while the whole pad governs conformability and is assessed by bending and compression methods. A pad may combine a very soft top sheet with a stiff core; testing both levels identifies the true source of any comfort problem.
How many specimens are required and how are results reported?
Typical protocols require at least 10 valid specimens per direction (machine and cross direction), with the report stating the arithmetic mean, standard deviation or coefficient of variation, test direction, face orientation, slot width or probe geometry, and conditioning atmosphere.
Do instrumental softness results correlate with consumer perception?
Single parameters correlate only moderately, but combined models that integrate slit softness, bending rigidity, compression force, and surface friction typically achieve strong correlation with panel ratings. This is why laboratories recommend a test package rather than a single method when the objective is to predict real-world comfort.
Can these methods be applied to related products?
Yes. The same method set is routinely extended to panty liners, baby diapers, adult incontinence products, disposable underpads, and facial tissue, with only minor adjustments to specimen dimensions and slot or probe selection.