Air permeability testing measures how easily air passes through toilet paper and other tissue products under a controlled pressure differential, providing a key indicator of softness, absorbency behavior, and converting quality. This article explains the testing principle, applicable international and national standards such as ISO 5636, TAPPI T 460, GB/T 458, typical sample types, instrument methods, and practical applications in quality control and product development. Testing is routinely performed by accredited third-party laboratories using calibrated permeability instruments.
Technical Overview
Air permeability, also referred to as air permeance, quantifies the volume of air that passes through a defined area of paper per unit time under a specified pressure differential. For toilet paper, this property is exceptionally high compared with printing or packaging grades, because tissue is a lightweight, creped, low-density porous structure with a basis weight typically ranging from about 13 g/m² to 45 g/m² per ply. The measurement is usually expressed either as a flow-based permeability value, for example in mL per square centimeter per second or in mm/s, or as an air resistance value such as Gurley seconds, which is the time required for a fixed volume of air to pass through a fixed area.
Although toilet paper is not used as an air barrier, permeability is a highly informative quality attribute. The creping process, embossing pattern, ply bonding, and refining level of the pulp all influence the openness of the sheet structure. Higher permeability generally correlates with a bulkier, softer hand feel and faster liquid uptake, while excessively low permeability may indicate over-refined fiber, poor creping, or overly dense lamination. Manufacturers therefore use permeability data to control creping conditions on the Yankee dryer, to evaluate embossing designs, and to benchmark products against market references.
Because tissue structures are very open, laboratories must select instruments and pressure differentials appropriate to the high-flow range. Conventional paper testers operating at approximately 1.2 to 1.5 kPa may reach their measurement limits, so low-pressure-drop flow methods, typically at 100 Pa or 200 Pa, are frequently applied to ensure accurate and repeatable results.
Testing Samples
Finished Bath Tissue Products
Market-ready toilet paper in roll form, including two-ply, three-ply, and four-ply constructions, with or without embossing, printing, or lotion treatment. Specimens are cut from multiple positions across the roll width to capture cross-direction variability and to reflect the embossing pattern representative of the product as sold.
Parent Rolls and Base Tissue
Jumbo parent rolls sampled directly from the tissue machine before converting. Testing base tissue allows mills to verify creping and drying performance independently of downstream embossing and rewinding effects, and to establish incoming specifications for converters.
Multi-Ply Laminated Constructions
Laminated and embossed products are tested in the as-sold configuration, with all plies clamped together as a single structure. Where a decomposition analysis is required, individual plies can also be tested separately to quantify the contribution of each layer and of the bonding points to overall air flow resistance.
Related Hygiene Papers
The same test principle applies to the wider tissue family, including facial tissue, kitchen towels, handkerchief tissue, and napkin paper, enabling consistent comparison across product lines produced on similar furnish and forming conditions.
Testing Items
Core and optional test items for a toilet paper air permeability evaluation typically include:
- Air permeability (air permeance) reported as flow velocity under a stated pressure drop, commonly in mm/s or mL per square centimeter per second.
- Gurley-type air resistance, expressed as seconds required for 100 mL of air to pass through a 6.45 cm² test area at approximately 1.23 kPa.
- Bendtsen-type air permeance, expressed in mL/min through a 10 cm² measuring head at approximately 1.47 kPa, where the instrument range permits.
- Supplementary structural measurements such as basis weight and thickness, which allow permeability to be normalized and interpreted against sheet bulk and density.
Testing Methods
Specimens are conditioned to standard atmosphere, 23 °C ± 1 °C and 50 % ± 2 % relative humidity, in accordance with ISO 187, before measurement. At least ten undamaged specimens free of folds, watermark marks, and edge defects are prepared from representative areas of the sample, and the tested face is recorded. Instruments are verified before use with traceable calibration orifices and a leak check of the clamping system.
The principal standard methods applied include:
- ISO 5636-1, which defines the general medium-range method for determining air permeance of paper and board, with results expressed in μm/(Pa·s).
- ISO 5636-5 and TAPPI T 460, specifying the Gurley air resistance procedure.
- ISO 5636-3 and TAPPI T 536, specifying the Bendtsen method.
- GB/T 458, the Schopper-type method referenced in Chinese national testing practice.
- ISO 9237-based high-flow procedures, originally developed for textiles and widely adapted for very open tissue structures, reporting air permeability in mm/s at a controlled pressure drop of 100 Pa or 200 Pa.
In a typical low-pressure flow measurement, the specimen is clamped over a circular test head, the selected pressure differential is established across the sheet, and the steady-state volumetric flow rate is recorded. Permeability is calculated as the flow rate divided by the test area, and permeance is further normalized by the pressure differential. The mean, standard deviation, and coefficient of variation are reported together with the test configuration, including number of plies, tested side, pressure drop, and the specific standard applied.
For multi-ply products, the primary result is obtained on the complete laminate as sold, since ply-to-ply bonding and embossing deformation dominate the true in-service air path. Results obtained under different standards or pressure differentials are not directly interchangeable, because inertial flow effects cause apparent resistance to increase disproportionately at higher pressure drops; comparisons should therefore always reference the test conditions.
Applications
Air permeability data for toilet paper support decision-making across the entire product lifecycle:
- Mill process control, where trends in permeability are used to optimize crepe ratio, Yankee coating, refining energy, and drying strategy.
- Converting quality management, where the effect of embossing depth, ply lamination adhesives, and rewinding tension on sheet openness can be quantified.
- Product development and reverse engineering, supporting softness and absorbency trade-off studies and benchmarking against competitor products.
- Specification and acceptance testing in supply contracts between pulp suppliers, tissue mills, brand owners, and retail chains.
- Compliance verification and market surveillance programs carried out by accredited third-party laboratories.
- Research applications, including correlation of permeability with panel-evaluated softness within the broader ISO 12625 test framework for tissue products.
FAQ
Why is air permeability tested for toilet paper when it is not a barrier product?
Permeability is a sensitive indicator of sheet structure. It reflects creping quality, embossing intensity, ply bonding, and fiber refining, and it correlates meaningfully with softness perception and the speed of liquid uptake, making it a valuable quality and development metric.
What are typical air permeability values for toilet paper?
Toilet paper is among the most open of all paper grades. Gurley air resistance is often only a few seconds, and Bendtsen values can exceed the normal instrument range, which is why low-pressure-drop flow methods reporting high values in mm/s are frequently preferred. Exact values depend on basis weight, crepe level, embossing, and ply count.
How are multi-ply products handled?
The product is tested as sold, with all plies clamped together, because this represents real product performance. Individual plies may be tested additionally when a layer-by-layer analysis is required, and the configuration is always stated in the report.
What is the difference between air permeability and air resistance?
Air permeability describes flow per unit area under a pressure differential, so higher values mean a more open sheet. Air resistance, such as Gurley seconds, expresses the time for a fixed air volume to pass, so higher values mean a denser sheet. The two concepts are inversely related but are not simple reciprocals across different instruments and pressures.
How many samples are required and how long does testing take?
A minimum of ten specimens per sample is recommended by the relevant standards. Routine permeability testing is rapid, and combined with conditioning, results are typically available within a few working days from an accredited laboratory.