Transmission Rate
Rates at which water vapour permeates through a solid material under specified conditions of temperature and humidity indicate the effectiveness of a moisture barrier. This wvtr is measured in grams per square metre over twenty-four hours. It determines how well a package can keep its contents dry or retain necessary internal moisture.
Environmental Sensitivity
Tropical conditions with high heat and humidity put the greatest strain on the barrier properties of paperboard. Because cellulose is naturally hygroscopic, the wvtr of a coated sheet is primarily a function of the integrity and thickness of the applied polymer layer. Gravimetric methods involve sealing the material over a cup containing a desiccant and weighing it at regular intervals to track the mass gain.
More modern instruments use infrared sensors for faster and more precise detection of water molecules.
Performance Grading
Dry foods such as crackers and cereals require very low transmission rates to stay crisp and avoid staleness. If the wvtr is too high, the product will absorb moisture from the air and lose its consumer appeal. Conversely, some applications like fresh produce packaging might require a specific level of breathability to prevent the buildup of condensation and mould.
Selecting the right grade of coated board involves balancing the cost of the barrier against the sensitivity of the product being protected.
