The breathability of PET to water vapor: Thickness effects

A model study of the permeation of water vapor through archival polyethylene terephthalate (PET) in the context of encapsulation is reported. Using intentionally wetted paper, the rate of mass loss of a wet, encapsulated paper was measured and found to be 1000 times slower than the rate of mass loss from wetted unencapsulated paper. The results show that water vapor can permeate through the PET film and the measured rate of vapor flux increases with decreasing PET thickness. The measured mass loss from the encapsulate was used to calculate the relative humidity (RH) inside the encapsulate and the moisture content of the encapsulated paper. The moisture content of encapsulated paper was calculated to be slighter higher than the moisture content of unencapsulated paper, which might be due to a modified cellulose network structure caused by the slower mass loss or variability in the paper, or due to condensation of water vapor on the PET within the encapsulate. The results of this paper quantify the movement of water vapor through PET film and provide conservators insight on the microenvironment in their PET enclosures.