Stewing in Its Own Juices? The Permeability of PET by Water and Acetic Acid

Abstract

Abstract Historic documents are frequently protected by placing them in a sealed polyethylene-terephthalate (PET) envelope. Although the paper is mechanically stabilized, the PET film may limit transmission of moisture or acidic degradation by-products of the paper. This creates a microenvironment for the encapsulated document. The permeation of water and acetic acid vapor through the PET film was measured to understand the microenvironment within an encapsulated enclosure. For encapsulation with a 102 μm (4 mil) PET film, the water vapor mass flux through the encapsulated film was measured. The water vapor was found to flow into or out of the PET film depending on the sample and environmental conditions. Because the encapsulated paper needed a longer time to dry than paper in open air, PET encapsulation provides a microenvironment that will buffer the encapsulated object from large swings in humidity. Acetic acid either did not diffuse through the PET, or it diffused at such a slow rate as to not be measured, due to the larger size of the acetic acid molecule than the water molecule. Keeping one edge of an encapsulation open allowed a drying rate that was four times faster than when all four sides were sealed. Because acetic acid vapor does not readily diffuse through the PET, these results reinforce the recommendations for deacidification and/or addition of buffering agents to the paper or enclosure. The results of this study point to a critical need for the investigation of new materials in conjunction with further evaluation of currently used materials.

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