Synthetic leather breathability testing measures the ability of polyurethane (PU), polyvinyl chloride (PVC), and microfiber synthetic leathers to transmit air and water vapor, which directly determines wearing comfort in footwear, apparel, and automotive interiors. This article explains the standard test methods, sample requirements, instrumentation, and evaluation criteria used by accredited third-party laboratories for the determination of air permeability and water vapor permeability of synthetic leather, with reference to ISO, ASTM, and GB standards.
Technical Overview
Synthetic leather is a polymer-based material—typically polyurethane (PU), polyvinyl chloride (PVC), or microfiber-reinforced PU—widely used as a substitute for natural leather in footwear uppers, garments, bags, furniture upholstery, and vehicle interiors. Unlike natural leather, which possesses an inherent porous fiber network, the surface layer of synthetic leather is essentially a continuous polymer film. Breathability must therefore be engineered through microporous coating structures, hydrophilic polyurethane chemistry, perforation, or permeable textile substrates.
Breathability determination for synthetic leather generally covers two related but distinct properties: air permeability, which quantifies the volume of air passing through a defined area under a specified pressure differential, and water vapor permeability (often expressed as the water vapor transmission rate, WVTR), which quantifies the passage of moisture vapor from the human body through the material. Both properties govern thermal comfort, sweat management, and the prevention of moisture accumulation inside shoes and garments. Accredited third-party laboratories perform these tests in accordance with recognized standards such as ISO 9237, ASTM D737, GB/T 5453, ASTM E96, ISO 2528, and GB/T 12704 to support product development, quality control, and buyer compliance verification.
Testing Samples
Samples suitable for breathability determination include the principal commercial categories of synthetic leather, provided the specimens are representative of the finished material in its final state, including any surface coating, finishing, and backing layer.
Polyurethane (PU) Synthetic Leather
Dry-process PU, wet-process microporous PU, solvent-free PU, and thermoplastic polyurethane (TPU) laminates are the most commonly submitted sample types. Wet-process PU contains a microporous coagulated layer and usually exhibits measurable air and water vapor permeability, making it a frequent target of breathability evaluation for premium footwear applications.
Polyvinyl Chloride (PVC) Synthetic Leather
PVC leather with a compact plasticized surface layer typically shows very low air permeability. For these materials, determination usually focuses on water vapor transmission, and results are often compared against minimum thresholds specified by downstream buyers or product standards.
Microfiber Synthetic Leather
Ultrafine-fiber nonwoven substrates impregnated with PU combine a dense fiber structure with a semi-permeable polymer matrix. Breathability values are typically intermediate between natural leather and coated fabrics, and testing supports grading and marketing claims such as "breathable microfiber leather."
Coated Fabrics and Laminates
Synthetic leather used for clothing, gloves, and linings, including multilayer laminates with membranes or foams, can be tested as composite specimens provided the full layer stack is intact and delamination has not occurred.
Specimen Preparation and Conditioning
- Cut specimens from at least three positions across the width of the roll, avoiding obvious defects, coating irregularities, and edge zones.
- Condition specimens prior to testing in a standard atmosphere of 20 ± 2 °C and 65 ± 4 % relative humidity for a minimum of 24 hours, in accordance with ISO 139 / GB/T 6529 or the conditioning clause of the applicable product standard.
- Prepare the number of replicate specimens required by the chosen method—typically five to ten pieces for air permeability and at least three dishes or cups for water vapor transmission.
- Mark the technical face and backing of each specimen so that the direction of testing can be documented, as some materials show directional differences in permeability.
Testing Items
The core testing items for synthetic leather breathability determination are summarized below.
- Air permeability: the rate of airflow through the specimen under a fixed pressure drop, expressed in mm/s (or L/m²·s, cm³/s/cm²) at 100 Pa, 125 Pa, or 200 Pa depending on the standard applied.
- Water vapor transmission rate (WVTR): the mass of water vapor passing through the specimen per unit area per 24 hours, expressed in g/m²·24h, determined under controlled temperature and humidity conditions such as 38 °C / 90 % RH or 23 °C / 50 % RH.
- Water vapor absorption (supporting item): the mass of moisture absorbed by the material surface over a defined period, occasionally required alongside WVTR for comfort characterization.
- Comparative and lot-release testing: verification of breathability claims, comparison of breathable versus standard grades, and conformity assessment against buyer specifications or contractual limits.
Testing Methods
Air Permeability – Pressure Differential Method (ISO 9237 / GB/T 5453 / ASTM D737)
The specimen is clamped over a calibrated test head exposing a defined circular test area, commonly 20 cm² for ISO 9237 / GB/T 5453 and approximately 38 cm² for ASTM D737. A fan or vacuum system draws air through the specimen while the instrument maintains a constant pressure drop—typically 100 Pa for clothing and footwear materials (200 Pa for technical applications under ISO 9237, or 125 Pa under ASTM D737). The steady-state volumetric airflow is measured, and air permeability is calculated by dividing the flow rate by the test area, yielding a result in mm/s. Because the polymer coating usually represents the dominant flow resistance, specimens must be mounted flat and wrinkle-free, with the edges fully sealed by the clamping ring to prevent leakage.
Water Vapor Permeability – Gravimetric Dish Method (ASTM E96 / ISO 2528 / GB/T 12704)
In this classical gravimetric method, the specimen is sealed over the mouth of a shallow dish or cup. Two variants are used for synthetic leather:
- Desiccant (dry-indicator) method: the dish contains a desiccant that maintains near-zero interior humidity, while the assembly is exposed to a controlled humid atmosphere; mass gain of the dish reflects vapor transmission through the specimen.
- Water (evaporation) method: the dish contains a specified depth of water or saline solution separated from the specimen by an air gap; mass loss of the dish reflects the vapor transmission rate. Inverted-cup variants bring the water into direct or near contact with the specimen for high-permeability materials.
The assemblies are placed in a conditioned chamber, for example 38 ± 2 °C with 90 ± 2 % relative humidity or 23 ± 2 °C with 50 ± 2 % relative humidity, and weighed at successive intervals until a constant rate of mass change is established. WVTR is calculated from the slope of mass change versus time divided by the exposed specimen area, and the result is reported in g/m²·24h.
Sweating Guarded-Hotplate Method (ISO 11092 / GB/T 11048)
For advanced comfort evaluation, especially of layered footwear and apparel assemblies, the sweating guarded-hotplate method determines the total and evaporative thermal resistance of the material system. While primarily a textile comfort test, it is increasingly applied to synthetic leather composites to obtain a more functional picture of moisture management than a static dish test can provide.
Gas Transmission – Differential Pressure Method (GB/T 1038)
When breathability of the pure polymer film or coating layer must be characterized independently of the textile substrate, the gas transmission rate can be measured manometrically on the separated film using the differential pressure method, reporting gas permeance and permeability coefficients for oxygen or other gases.
Instrumentation, Calibration, and Reporting
- Verify air permeability instruments using certified calibration orifices or reference films of known resistance before each test series.
- Check dish-method assemblies for seal integrity, since edge leakage is the principal source of falsely high WVTR results.
- Report the standard applied, test area, pressure drop or climatic conditions, conditioning regime, individual and mean results, and any deviation from the reference method.
- Handle outliers according to the statistical treatment specified in the standard; results from perforated or damaged specimens must be discarded and replaced.
Applications
- Footwear manufacturing: verifying that shoe upper and lining materials meet minimum breathability specifications to reduce sweat accumulation, odor, and discomfort during wear.
- Apparel and fashion accessories: substantiating breathable-leather marketing claims for jackets, gloves, and bags, and comparing material grades during sourcing.
- Automotive interiors: evaluating seat and door-trim synthetic leathers for long-term moisture comfort and dimensional stability in occupied vehicles.
- Furniture upholstery: balancing breathability with durability and cleanability in residential and contract seating applications.
- Research and development: optimizing microporous coating formulations, hydrophilic PU chemistry, substrate construction, and perforation patterns to raise permeability without sacrificing mechanical strength or abrasion resistance.
- Quality control and compliance: routine lot release testing, supplier qualification, benchmarking against natural leather, and third-party verification for retailers and brand owners.
FAQ
What is the difference between air permeability and water vapor permeability of synthetic leather?
Air permeability measures the convective flow of air through the material under a pressure difference and is expressed in mm/s, whereas water vapor permeability measures the diffusion of moisture vapor through the structure and is expressed in g/m²·24h. A material can be nearly airtight yet still transmit water vapor, particularly if it uses hydrophilic polyurethane, so many specifications require both parameters.
Which standards are most commonly used for synthetic leather breathability testing?
Air permeability is typically determined according to ISO 9237, GB/T 5453, or ASTM D737, while water vapor transmission is determined according to ASTM E96, ISO 2528, or GB/T 12704. The choice depends on the product sector, the buyer's specification, and the destination market.
What breathability values can be expected for different synthetic leather types?
Compact PVC leather often shows air permeability close to zero and WVTR below roughly 200 g/m²·24h. Wet-process microporous PU and microfiber leathers commonly reach WVTR values of several hundred to well over 1000 g/m²·24h, with measurable air permeability depending on the coating architecture. Exact acceptance limits, however, are set by individual product and buyer specifications.
How can the breathability of synthetic leather be improved?
Effective routes include wet-process coagulation to create a microporous coating, incorporation of hydrophilic polyether segments in the PU formulation, reducing coating thickness, engineering a more open textile substrate, and controlled perforation or laser micro-drilling of the surface layer.
How should specimens be prepared before breathability determination?
Specimens should be cut from representative positions of the finished material, kept free of creases and defects, and conditioned in a standard atmosphere of 20 ± 2 °C and 65 ± 4 % relative humidity for at least 24 hours so that results are comparable between laboratories.
Can these tests be outsourced?
Yes. Accredited third-party laboratories routinely perform air and water vapor permeability determination on synthetic leather under the standards described above, providing test reports suitable for supplier qualification, product certification support, and compliance with international buyer requirements.
