Assessment of Zeolite Sorbents in Archival Housing Materials
Background: It has long been recognized that low quality housing materials can release volatile organic acids that hasten the deterioration of collection items stored in them. By contrast, mat boards, boxes, folders and like materials containing alkaline buffers can absorb volatiles released by the collection items as they degrade. In recent years, housing materials have appeared on the commercial market that incorporate additional sorbents for volatile organic acids and other indoor air pollutants. These purport to offer an enhanced level of stabilization for collections stored in them. However, little objective information has been available on their effectiveness.
To determine whether these materials offer concrete benefits for collections storage, in the Research and Testing Division has studied of the effectiveness of sorbent-loaded housing materials to characterize and define optimal sorbent properties for use in libraries and archives.
Project Description: Two major producers of sorbent-loaded rag boards use the same sorbent, a custom-formulated, proprietary zeolite. A sample of this zeolite was analyzed to determine its elemental composition, affinity to water, alkaline buffering capacity, and other properties. Further information was obtained from relevant patents and technical specifications.
Previous research on sorbent performance used high pollution concentrations, and a physical model that forces contaminated air to pass through a plug of sorbent before reaching the detector. While useful, this method is not ideal for studying adsorption at low concentrations, as would be encountered in a library collection. A new exposure method was developed, in which the changes in vapor concentration are measured in the air surrounding a board sample in a small test chamber. After a contaminant is added to the chamber, timed measurements reveal the rate and limiting equilibrium value of its uptake by the board sample.
Acetic acid was selected as a model contaminant for the study, as it is one of the major organic acids associated with deterioration of archival materials (particularly acetate film). An ion chromatography method was optimized for measurement of acetic acid levels, and was demonstrated to give linear results over the range of 0.5 – 100 parts per million.
Rag board samples containing a calcium carbonate buffer, calcium carbonate plus zeolite, or only rag fiber were exposed to varied acetic acid vapor concentrations, focusing on the 2-20ppm range. Levels of 2-25 ppm. have been previously observed in archives, and are considered to represent a range of low to high risk to collection items. Absorption rates and total uptake were compared for the three types of board. Samples which had adsorbed large amounts of the pollutant were transferred to uncontaminated chambers, and the rate and total amount of acetic acid vapor subsequently released from the boards were monitored.
Outcomes/Findings:
- Rag board fibers have some ability to adsorb pollutant vapors, even when no additives are present.
- The zeolite powder studied clearly adsorbs greater amounts of acidic vapor than equivalent weights and volumes of calcium carbonate and rag fiber.
- At moderate concentrations (2-20 ppm), zeolite-loaded boards adsorbed acetic acid vapor faster than plain or buffered boards.
- Although all the boards tested will subsequently release organic acid vapors if they are moved to a less polluted environment, the release rate was lower for the zeolite loaded boards than the others.
- A laboratory protocol was developed suitable for studying sorption at low concentrations of pollutant vapor.
Support:
Library of Congress Library Services
Nielsen-Bainbridge Corporation
Acknowledgements: Jack Forbes and Norman Boris of Nielsen-Bainbridge provided intellectual and institutional support, including board samples and manufacturing data. Jim Druzik of the Getty Conservation Institute shared information from his own research on sorbents.
Update and Images:
June 2006: A report on the findings was presented at the American Institute for Conservation (AIC) 32nd annual meeting in Providence, RI, and will appear in the associated postprints.