Parylene (a class of polymers with the base monomer para-xylylene), deposited via chemical vapor deposition (CVD), has been used as a practical treatment to strengthen cultural heritage papers. The deposited monomers polymerize in situ, forming a thin, conformal coating that strengthens paper. Other unique properties of this technology are its mold resistance and its ability to treat multiple pages at once. In the 1990s, Parylene treatment for cultural heritage paper strengthening was studied by several investigators including Don Etherington, David Grattan, and Bruce Humphrey, who demonstrated a significant increase in the strength of treated versus untreated papers. In the last 20 years, however, this treatment has received limited recognition within the conservation community and no further study. Stated concerns include the uncertainty about the permanence and durability of the imparted properties and the lack of reversibility. There are now several reasons, however, to revisit Parylene as a conservation treatment for cultural heritage papers. First, certain important assessments of the efficacy of Parylene treatment were not performed in the previous work. These include the optimization of the Parylene layer thickness for conservation needs, the aging of treated papers rather than the films by themselves, and the ability to conduct subsequent conservation treatments following deposition. Second, recent technological developments may now make Parylene more useful to conservation: There are now (1) CVD deposition chambers that are cheaper and more compact, (2) new analytical techniques to characterize the coating, and (3) new Parylene materials, including soluble Parylene, which addresses the concern over reversibility. Beginning summer 2012, Heritage Science for Conservation at Johns Hopkins University convened two roundtables of scientists, conservators, and industry representatives to discuss paper strengthening and Parylene technology. We reviewed both published and unpublished reports, examined numerous artifacts treated in the 1990s at CCI and Etherington Conservation, and interviewed the scientists who performed the research. After duplicating previous strengthening results, we optimized the Parylene layer thickness to maximize the strengthening effect while minimizing side effects such as stiffening using conservators' feedback. We will present the results of mechanical testing and the behavior of Parylene coated paper in conventional paper conservation treatments to show the degree to which Parylene coating improves the durability of brittle papers on its own, or in conjunction with other conservation treatment. Our results show that groundwood pulp book papers from 1951 with a ~ 400nm Parylene coating retain many of the characteristics of new wood pulp papers in terms of rattle, turn radius, and general tactile experience. The treated paper has a > 30% improvement in tear resistance and more than three times higher folding endurance. Additionally, Parylene treated paper is receptive to, and can even improve upon, conventional paper and book conservation treatments such as tear mending, guarding, washing and resizing, and book and case binding. An acknowledged limitation of Parylene treatment is that there appears to be little improvement on the folding endurance of very brittle, low porosity papers. In conclusion, Parylene treatment imparts mechanical strength, especially tear resistance, to uncoated papers, but is especially promising for groundwood pulp papers, where it also imparts folding endurance.
Non-Presenting Authors: Mark Pollei, Sonja Jordan Mowery
