This method is for the determination of water vapor permeability of paper, paperboard, and flexiblepackaging materials under storage conditions at -18°C (0°F). It is generally suitable for sheet materials up to 125 µm (5 mils) thick. The method has been developed because of increased emphasis on frozen-food production, and because no existing procedure gives results which correlate with those at -18°C. [TAPPI T 448 “Water Vapor Transmission Rate of Sheet Materials at Standard Temperature and Humidity” is suitable at temperatures of 23°C (73°F) and 50% relative humidity, and TAPPI T 464 “Gravimetric Determination of Water Vapor Transmission Rate of Sheet Materials at High Temperature and Humidity” applies at 38°C (100°F) and 90% RH.]
In general, packaging materials used for storage at -18°C should be capable of protection against anappreciable loss of moisture for periods up to 1 or 2 years. The final selection of materials for such storage isprobably best done by actual package storage tests for the full length of time required. This method is a simplifiedand rapid method which has been shown to give results indicative of behavior during long-term storage. Many of thepackaging materials are based on cellulose, including paper, paperboard, cellophane, and glassine. The time required for these materials to reach moisture equilibrium when exposed at -18°C may be several weeks or months. The assumption is made in this procedure that equilibrium is reached, for practical purposes, in the 7-day initial conditioning period.
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