Determination of the Water Vapor Permeability Coefficient by the Electrolytic Method

The electrolytic (coulometric) method is one of the most sensitive and widely accepted techniques for determining the water vapor transmission rate and permeability coefficient of plastic films, sheets, and high-barrier materials. This article explains the measurement principle, applicable standards, sample types, test procedures, and calculation methods. It also outlines typical applications in food packaging, pharmaceutical packaging, photovoltaics, and lithium battery materials.

Technical Overview

The electrolytic method, also known as the coulometric sensor method, is an instrumental technique for measuring the rate at which water vapor permeates through a flat specimen of film or sheeting. The permeation cell is divided by the specimen into a saturated-humidity chamber (test side) and a dry chamber (detection side). A stream of dry carrier gas, typically nitrogen, continuously sweeps the detection side and carries the permeated water vapor to an electrolytic sensor whose surface is coated with phosphorus pentoxide (P2O5).

Inside the sensor, the P2O5 coating absorbs the water vapor quantitatively and, under an applied voltage, electrolyzes it into hydrogen and oxygen. According to Faraday's law, the electrolysis current is directly proportional to the mass of water reaching the sensor, so the steady-state signal provides an absolute, drift-resistant measure of the transmission rate. Modern instruments often employ modulated electrolytic sensors, in which the carrier gas alternates between a zeroing path and a measurement path, improving baseline stability, extending sensor life, and enabling detection limits at the 0.001 g/(m²·24 h) level for high-barrier materials.

From the measured water vapor transmission rate (WVTR), the water vapor permeance and the water vapor permeability coefficient can be derived once the specimen thickness and the water vapor partial pressure difference across the specimen are known. Typical reporting units are g/(m²·24 h) for WVTR and g·cm/(cm²·s·Pa) for the permeability coefficient.

Testing Samples

Flexible Plastic Films and Laminates

Monolayer and multilayer films such as PE, PP, PET, PVC, PVDC, PA (nylon), EVOH, CPP, and aluminum-metallized or AlOx/SiOx-coated laminates commonly used in flexible packaging structures.

Pharmaceutical Packaging Materials

Blister films, cold-form aluminum laminates, tropical blister laminates, composite membranes, and other primary packaging components that must satisfy pharmacopoeial moisture-barrier requirements.

Photovoltaic and Energy Materials

Solar cell backsheets, encapsulant films, lithium battery separators, and battery pouch foils, where moisture control directly affects product life, performance, and safety.

Electronic and Optical Materials

OLED and display encapsulation films, transparent high-barrier coatings, and optical films that require ultra-low water vapor transmission for long-term device reliability.

Rubber, Membranes, and Coated Sheets

Thin elastomer sheets, waterproofing membranes, and coated fabrics whose transmission falls within the measurable range of the electrolytic sensor.

Testing Items

• Water vapor transmission rate (WVTR), expressed in g/(m²·24 h)
• Water vapor permeance, expressed per unit partial pressure difference
• Water vapor permeability coefficient, expressed in g·cm/(cm²·s·Pa)
• WVTR as a function of temperature and relative humidity for barrier mapping and shelf-life modeling

Testing Methods

Reference Standards

• ISO 15106-3: Plastics — Film and sheeting — Determination of water vapor transmission rate — Electrolytic detection sensor method
• GB/T 21529: Chinese national standard specifying the electrolytic sensor method for plastic films and sheeting
• JIS K 7129: Japanese standard for instrumental determination of water vapor transmission rate of plastic film and sheeting
• Related gravimetric and infrared methods for comparison: ASTM E96/E96M, ISO 2528, GB/T 1037, ASTM F1249, and ISO 15106-2
• Pharmacopoeial standards for pharmaceutical packaging materials, which designate coulometric (electrolytic) detection as a first-method procedure

Test Conditions

Commonly applied conditions include 38 °C / 90 % RH (tropical condition), 23 °C / 85 % or 90 % RH (temperate condition), and 40 °C / 90 % RH or higher for accelerated evaluation of photovoltaic and battery materials. The condition is selected to reflect the end-use environment or to satisfy a specified product or regulatory requirement.

Specimen Preparation

• Condition specimens at 23 °C / 50 % RH, or as specified by the applicable standard, prior to testing.
• Cut defect-free specimens of the required size, ensuring they are flat, crease-free, and free of pinholes.
• Seal each specimen into the permeation cell using molten wax or a compatible gasket to eliminate edge leakage.
• Measure specimen thickness at multiple points and record the mean value for coefficient calculation.

Test Procedure

• Purge the system with dry carrier gas until a stable zero baseline is established.
• Introduce saturated water vapor to the test side and start data acquisition.
• Monitor the electrolytic sensor output until steady-state transmission is reached, as defined by the convergence criterion of the standard.
• Record the steady-state value, calculate WVTR, and convert to permeance and permeability coefficient using the test area, thickness, and water vapor partial pressure difference.

Calculation

WVTR is calculated as the transmitted water mass divided by test area and time. The water vapor permeability coefficient P is then obtained from P = WVTR × d / Δp, where d is the mean specimen thickness and Δp is the water vapor partial pressure difference across the specimen under the test conditions. Results are reported in g·cm/(cm²·s·Pa), normally as the mean of at least three replicate specimens.

Quality Control

Reference films are tested periodically to verify sensor performance and sealing integrity, and acceptance criteria for repeatability are applied in accordance with the selected standard. Any specimen showing edge leakage, wrinkling, or abnormal signal drift is rejected and retested.

Applications

• Food packaging: shelf-life prediction, barrier structure selection, and incoming inspection of laminate rolls.
• Pharmaceutical packaging: moisture protection of tablets, capsules, and effervescent or hygroscopic products, with compliance testing for regulatory submissions.
• Photovoltaic modules: qualification of backsheets and encapsulant films against damp-heat degradation.
• Lithium batteries: validation of separators, pouch foils, and moisture-sensitive cell components.
• Electronics: OLED encapsulation, moisture barrier coatings, and semiconductor packaging evaluation.
• Research and development: comparison of barrier coatings, study of temperature and humidity dependence, and support for accelerated aging programs.

FAQ

What is the detection range of the electrolytic method?

Typical instruments cover WVTR values from approximately 0.001 to 1000 g/(m²·24 h), depending on the cell configuration, making the method particularly suitable for medium- and high-barrier materials. For very high-transmission substrates such as paper and nonwovens, a gravimetric method such as ASTM E96/E96M or ISO 2528 may be more appropriate.

How does the electrolytic method differ from the infrared and gravimetric methods?

The gravimetric method measures the mass gain of a desiccant cup; it is simple but slow and comparatively insensitive. The infrared sensor method (ASTM F1249, ISO 15106-2) offers high throughput and sensitivity. The electrolytic method provides an absolute measurement based on Faraday's law, with excellent low-level sensitivity and long-term stability, and it is widely accepted for pharmaceutical and high-barrier applications.

How long does a test take?

For ordinary packaging films, steady state is usually reached within one to three hours. High-barrier laminates and coated films may require many hours to several days because of their low permeability. Total test time depends on the material, the temperature and humidity condition, and the convergence criterion of the standard.

Which temperature and humidity condition should be selected?

Conditions should reflect the intended storage environment or comply with the governing product standard. Typical choices are 38 °C / 90 % RH for tropical-climate evaluation, 23 °C / 85 % RH for temperate evaluation, and 40 °C / 90 % RH or above for accelerated assessment of photovoltaic and battery materials.

Can the electrolytic method evaluate finished packages?

Yes. With an adapted carrier-gas test chamber, finished containers such as bottles, pouches, and blister packs can be tested for overall water vapor ingress, complementing film-level measurements with a whole-package result.

What factors most strongly affect accuracy?

Sealing quality, specimen conditioning, control of temperature and humidity, carrier gas dryness, and sensor condition are the dominant factors. Regular leak checks, careful wax sealing, and periodic verification with certified reference films are essential for reliable data.