Historic documents are frequently protected by placing them in a sealed polyethylene-terephthalate (PET) envelope. Although the paper is mechanically stabilized, the PET film retards transmission of moisture or acidic degradation by-products of the paper. This creates a microenvironment for the encapsulated document. The permeation of water, acetic acid, and formic acid vapor through the PET film was measured to understand the microenvironment within an encapsulated enclosure. The vapor mass flux through 3 mil, 4 mil, and 5 mil PET film was measured. Water vapor was found to flow into or out of the PET film depending on the sample and environmental conditions. As expected, the measured rate of vapor flux through the PET film increases with decreasing PET thickness. We find that the rate of mass loss from a 3 mil PET enclosure is approximately 38% greater than from a 4 mil enclosure. The rate of mass loss from a 5 mil enclosure is approximately 20% smaller than from a 4 mil enclosure. The PET encapsulation provides a microenvironment that will buffer the encapsulated object from large swings in humidity. This attenuated fluctuation in humidity is expected to protect the paper since humidity fluctuations may expose the paper to unwanted stresses.<br/><br/>The permeation of formic acid and acetic acid, two by-products of cellulose degradation, was also measured. Formic acid permeated through the PET, but at a much slower rate than water vapor. Acetic acid either did not diffuse through the PET, or it diffused at such a slow rate as to not be measured. The difference in the rate of permeation is due to the size difference between the vapor molecules. The impact of these results on historic documents will be discussed.

Permeability of PET by water, acetic acid, and formic acid: A study of the microenvironment of encapsulated documents

256th National Meeting (2018)

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