MIL-PRF-131K, PERFORMANCE SPECIFICATION: BARRIER MATERIALS, WATER VAPORPROOF, GREASEPROOF, FLEXIBLE, HEAT-SEALABLE (18 AUG 2005) [SUPERSEDING MIL-B-131H]., This specification covers the requirements for heat-sealable, greaseproof, flexible barrier materials having low water vapor transmission characteristics for use in military packaging.