WVTR

Water vapor transmission rate (WVTR) is the rate at which water vapor passes through a material under defined temperature and humidity conditions. It is a key indicator of a material's barrier properties against moisture and is commonly expressed in grams per square meter per day (g/m^2·d). WVTR helps assess suitability of films, coatings, and laminates for moisture-sensitive applications.

Measurement and standards: WVTR testing uses standardized methods, typically the desiccant (dry-cup) or the cup (wet-cup) method. The most widely used standard is ASTM E96, which specifies test conditions and calculation procedures for polymers, coatings and packaging. Results are influenced by temperature, relative humidity, film thickness, defects, and the presence of coatings or multilayer structures.

Conceptual notes: WVTR relates to material permeability and permeance; WVTR is proportional to the water vapor pressure difference across the material and inversely proportional to thickness. In practice, designers report WVTR for the specific test conditions, since values depend on temperature and RH.

Applications: WVTR is crucial in packaging to extend shelf life, in electronics and display packaging to protect moisture-sensitive components, in pharmaceuticals and foods, and in barrier coatings and textiles. Low WVTR materials are used where moisture ingress must be minimized; higher WVTR indicates more permeable materials.

Limitations: WVTR is a condition-dependent metric and does not capture all moisture-related performance aspects, such as liquid-water ingress or gas permeability in different environmental gradients. Determining WVTR early in development guides material selection and process controls.