Volume 8, Number 2
Apr 1984

Accelerated Aging and the Effects of Enclosure

The long-term effects of enclosing books and paper in plastic film have been simulated with accelerated aging at the Library of Congress and perhaps elsewhere, but published reports, if any, are scarce, and there have been no large-scale studies. Natural aging studies, which would skirt some of the problems posed by accelerated aging, have not been made either. This is understandable, since encapsulation only became popular about ten years ago and shrink-wrapping has just begun its popularity.

When the LC chemists tested the effect of shrink- wrapping, they found that the shrink-wrapped sample deteriorated two to three times as fast as the control sample at 90°C and 50% RH. (This was with the intact film--no holes poked in the envelope.)

Similar results have been found in heat-aging of other sealed materials, by these and other investigators. Paper sealed in glass tubes for heat-aging at the Library of Congress deteriorated five to ten times as fast as paper surrounded by circulating air in a dry oven. David Roberson, in a 1976 survey article (1), discussed this phenomenon in connection with a review of efforts to maintain normal moisture content in test samples of paper. Browning and Wink, he says, found in 1968 no difference in deterioration of paper aged in sealed tubes and paper aged in a moist oven; however, sealed-tube aging was 11.6 times faster than aging in a dry, unventilated oven. This suggests that the increased rate for sealed materials is partly due to the higher moisture content. Cellulose is known to be very sensitive to moisture during aging. But one could not generalize from this speeding-up effect of moisture to the natural aging of sealed materials at room temperature, because the moisture of both the sealed and the unsealed materials at room temperature would be virtually the same. The only difference would be that changes in humidity of the surrounding air would affect the sealed materials less. Their moisture content would be more stable. In itself, this stability would be a big advantage, because cycling of humidity also speeds up aging, whether the paper is sealed or not. Roberson cites evidence that this effect is obtained for RH cycling in natural as well as in accelerated aging.

All polymer films, by the way, let moisture through. Eventually, humidity is equalized on both sides of the film, if conditions are stable. If the humidity outside the enclosure is not stable, the humidity inside will also vary, but not as much.

Another factor in the higher aging rate for sealed materials is autocatalysis--the speeding up of degradation by the presence of volatile degradation products, trapped and concentrated within the enclosure. Although we always wonder how closely the conditions of accelerated aging can duplicate the results of natural aging, in the case of polymer film enclosures, there is at present no reason to expect a radically different process to be taking place, if the temperature is not too high. The heat makes the polymer more permeable to gases, so more molecules and bigger ones are able to escape the enclosure in the oven; but degradation products build up faster in a heated enclosure than in real life, and because of the shortness of the test, have less time to get out. These opposite effects may more or less balance one another to make the aging tests give real-life results. However, we don't know. Since natural and accelerated aging of paper in sealed enclosures have not been systematically compared, we still don't know how serious the autocatalysis problem is at room temperature. Until we do, it is best to take precautions. The Library of Congress recommends leaving a big gap in encapsulations, not just I/B", and making holes in shrink-wrap film. (It also recommends deacidifying paper before encapsulating it, so as to minimize the rate at which degradation products are generated.)

The way these products (which are really molecules and parts of molecules) escape, we have to remember, is not by riding on a current of air, but by bouncing around by themselves at a terrific speed. Sooner or later their bouncing will carry them through any hole they can fit through. So it is not necessary to create a draft within the enclosure, only an opportunity for diffusion.

Shrink-wrapping of deacidified books, a process comparable to encapsulation of deacidified documents, was recommended by Richard D. Smith in 1968 (2). He said then:

Once the books are protected from self-destruction, it is possible to further the archival function of libraries by storing the stabilized books in an inert atmosphere. A transparent and impervious plastic film can be wrapped around a book and sealed, possibly in a partial vacuum. The small quantity of oxygen and other harmful gases remaining inside the wrapper can be expected to react with-- but cause negligible harm--to the book components. The book itself removes the harmful gases and produces its very own inert atmosphere. The transparent film allows the interested reader to verify the bibliographic data on the binding before breaking the seal, and a new wrapper could easily be applied after use. The impervious wrapper would also protect the book from dust and dirt, accidental water damage, and the mechanical wear and tear caused by fluctuations in relative humidity. Chemicals could be incorporated into the plastic film during its manufacture to provide protection against biological attack and photocatalyzed degradation.

Smith's suggestion was apparently never taken up, probably because lesser-used books, which are ideal candidates for shrink-wrapping, would rarely be deacidified because of the expense. Nevertheless, the part about the book itself removing harmful gases and producing its own inert atmosphere after deacidification is interesting. It recalls the 1982 paper by Padfield, Erhardt and Hopwood on degradation products within showcases (3). The authors conclude their survey of the problem by recommending basically the same mechanism recommended by Smith for producing an inert atmosphere within the case:

A leakage rate that gives about one air change a day is about the fastest that is compatible with effective damping of the daily cycle in relative humidity but [unfortunately] it is also slow enough to allow chemical damage to objects by internal pollution. Exhibition designers should therefore limit themselves to a short and inevitably constricting list of techniques and materials which are probably safe

Until a general purpose, wide range, showcase pollution absorber is developed we recommend, as an acid gas absorber, carbonate-buffered paper, laid in cases but not in contact with objects. Such paper is, of course, not only commercially available but made in the conservation departments of museums and libraries!

(The method of deacidification almost universally used and recommended today also involves buffering the paper with carbonates to provide an alkaline reserve. Smith's method results in deposition of magnesium carbonate, and is useful both for individual and for mass treatment.)

If further research indicates that enclosure accelerates deterioration at room temperature as well as during heat aging, perhaps we could slow deterioration down to normal--or below normal--by including buffered paper or board within the enclosure.

The effect of a pollution absorber could also be tested by accelerated aging to find out whether paper enclosed with it deteriorated faster than paper not enclosed. If the pollution absorber nullified the rapid- aging effect, then we would know that the degradation products had been responsible for it, as presumed. If it did not, then we would have a new phenomenon to speculate about.

References

1. David D. Roberson, 'The Evaluation of Paper Permanence and Durability." Tappi 59 (12): 63-69, Dec. 1976.

2. Richard D. Smith, "Guidelines for Preservation." Special Libraries, May-June 1968, p. 346-352.

3. Tim Padfield, David Erhardt and Walter Hopwood, "Trouble in Store." In Science and Technology in the Service of Conservation, Preprints of the Contributions to the (TIC) Washington Congress, 3-9 September 1982. London: TIC, 1982. Pages 24-27.