VAPOR PHASE CONSOLIDATION OF BOOKS WITH THE PARYLENE POLYMERS
Bruce J. Humphrey
10 FOLD ENDURANCE
IT IS THE AUTHOR'S OPINION that fold endurance tests currently used for paper bear little relation to stresses incurred during normal use. Parylene coatings may be progressively applied to achieve any level of fold endurance desired. The paper will fail at the same number of folds as an uncoated control, but the parylene web will hold the two sections together indefinitely. Fold endurance tests at the Library of Congress have demonstrated this phenomenon. It must be noted here that parylene does not chemically interact with paper, or any other substrate for that matter. The material is very inert and the strengthening is purely mechanical in nature. Parylene surrounds and strengthens the individual paper fibers as well as welding the fibers together where they contact one another.
The decision that has to be made concerns how much one is willing to spend to achieve this “pseudo fold endurance” created by progressive coating. Some of the books treated by the author were printed on very thick stock (17-18 microns). Treatment with parylene resulted in a significant increase in tensile strength but no increase in fold endurance was noted. The paper still broke after one or two folds. The reason this occurs is due to the fact that the ratio of paper thickness to coating thickness is so great (approx 18 $ ) that the paper breaks and simply takes the coating with it. In these specific instances the application of additional parylene will not improve this situation. It must be remembered that parylene is polycrystalline in structure and as such will gradually lose flexibility as thickness increases, thus making the paper more brittle than it was originally.
It is the author's feeling that protection against simple disintegration should be the objective for most books. This kind of protection is usually attained at thinner coating levels than those required to attain higher levels of fold endurance. In an actual case in point, a brittle book was brought to the author by a local resident who had heard of my research into this problem. The book in question was a farm implement catalog from the 1880s. The pages of the book were very brittle and many had crumbled and eroded edges. He very much wanted something to be done to save the book from complete disintegration, and the author agreed to process the book as part of the research program. After processing it was noted that fold endurance had increased from one fold to break to 5-6 folds to break. The tensile strength appeared to have increased but could not be quantified without destructive testing. The “handleability” of the book was greatly enhanced, to the point where pages could be handled and turned with little danger of further damage. The owner of the book was extremely pleased with the result of this effort.
One of the great advantages of this material is the precise degree of control which can be exercised over the deposition of the polymer. While one cannot reverse the process, it is possible to go forward by very minute increments, stopping at each stage to remove the book or object from the chamber to check for the desired result. The reactive sites for “polymerization continuation” will remain open for several hours. Materials returned to the chamber for additional coating after 8-12 hours will still produce continuous film properties on the finished item.