Vapour Resistance

Vapour resistance testing is a method used to measure the resistance of a material to the passage of water vapour. It is commonly performed on various materials such as textiles, films, membranes, and other porous or non-porous materials. The test helps evaluate these materials' breathability and moisture permeability, which is crucial for protective clothing, building materials, and packaging applications.

ASTM E96 is a widely recognized standard developed by the American Society for Testing and Materials (ASTM) that provides guidelines for measuring water vapour transmission through materials using different test methods. One commonly used method within ASTM E96 is the "wet cup" method.

The ASTM E96 wet cup method, also known as the desiccant method, is used to determine materials' water vapour transmission rate (WVTR). In this test, a sample of the material is placed over the mouth of a cup that contains a desiccant, such as anhydrous calcium chloride. The cup is then sealed to create a closed system.

The desiccant within the cup absorbs moisture from the sample, creating a humidity gradient across the material. This causes water vapour to pass through the material from higher humidity to lower humidity, simulating real-world conditions. The weight gain of the cup due to the absorbed moisture is measured over a specified time.

The water vapour transmission rate is calculated based on the cup's weight gain and the sample's surface area. It is typically reported in grams of water vapour per square meter per 24 hours (g/m²/day) or as a permeability coefficient such as perm (US customary units) or ng/Pa·s·m² (SI units).

The ASTM E96 standard provides guidelines on test conditions, sample preparation, and calculations to ensure consistency and accuracy in measuring the water vapour transmission rate. It also includes variations of the wet cup method, such as the inverted cup method and the calcium chloride method, which are suitable for different types of materials.

It's important to note that ASTM E96 is just one of several standard and test methods for measuring vapour resistance and moisture permeability. Other standards, such as ISO 15496 and JIS L 1099, also provide methods for determining water vapour transmission properties of materials.

When conducting vapour resistance testing or using the ASTM E96 wet cup method, referring to the specific standard and following the prescribed procedures is recommended to ensure accurate and reliable results.