Aug 20, 2020
Definition
Water Vapor Permeability (WVP) refers to the ability of a material to allow water vapor to pass through it via diffusion, driven by a vapor pressure gradient. It is the rate of water vapor transmission per unit area per unit of vapor pressure differential under test conditions. It is often expressed as a rate (e.g., grams of vapor per square meter per day, g/m²/day) under standardized conditions.
The higher the value of the permeability of the material, the more rapidly vapour can pass through it. Vapour permeability is the rate at which vapour passes through a material.
Water vapor permeability (WVP) is the most studied property of biodegradable films, mainly because of the important role of water in spoilage reactions, keeping food fresh, crisp, or preventing dehydration.
The most widely used method to measure the water vapor permeability is ASTM Standard Test Method E96, also well-known as the cup method. This method is based on calculating the WVP of a sample by gravimetry.
Water Vapor Permeability vs. Water Vapor Permeance
- Permeability (intrinsic): Material-specific, independent of thickness (unit: perm·inch).
- Permeance (practical): Depends on thickness (unit: perm), often reported as Moisture Vapor Transmission Rate (MVTR).
Conclusion
Water vapor permeability is a vital material property for balancing moisture control across industries. Its measurement and optimization ensure functionality, durability, and user comfort, highlighting the interplay between material science and practical application needs.
Jinan Saicheng offers specialized Water Vapor Transmission Rate (WVTR) testers, compliant with gravimetric (weight method) and infrared detection (sensor method) standards. These instruments enable precise measurement of water vapor permeability (WVP) under controlled environmental conditions (e.g., temperature, humidity, and pressure gradients), ensuring accuracy for applications in packaging, textiles, and building materials.
