Accelerated Aging of Paper: Can it Really Foretell the Permanence of Paper
Aging of Paper Sealed within Polyester Film
Further evidence of the significance of acidic degradation products for the aging process came from a parallel study from our laboratories that investigated the effects of "encapsulation" of paper within two sheets of clear polyester film. Such an encapsulation treatment is frequently used in paper conservation as a protective measure to shield precious and fragile documents from physical stress, as well as from dust, etc. Some of the findings of this work are shown in Figure 2. These data were also obtained with Springhill™ Offset paper at 90°C and 50% RH for the following paper samples:
- Sheets encapsulated inside sheets of 4-mil thick polyethylene terephthalate (PET) film by sealing off all four edges completely,
- sheets sealed inside half-open PET envelopes, sealed along two adjacent edges,
- sheets encapsulated along with an alkaline paper,
- sheets deacidified with aqueous magnesium bicarbonate and then encapsulated by sealing along all four edges, and
- control sheets which were not treated in any way or encapsulated.
Paper encapsulated without deacidification became brittle by the tenth day of aging, as compared with the control which took twice as long to lose most of its strength. Even half-sealed paper aged just as rapidly as paper sealed completely. This is a clear indication of the affinity of the degradation products. They are probably hydrogen-bonded to the cellulose matrix.
Another possible explanation for the strong tendency of paper to retain these degradation products could be that as the paper ages and its morphology changes, the degradation products get trapped inside folding chains as the degree of crystallinity increases. However, other experiments not described here, have clearly demonstrated to us the tendency of paper within books to retain polar compounds such as ammonia, almost indefinitely at room temperature. To some of you this will suggest immediately an opportunity for affordable mass deacidification of paper, which is indeed another direction in which we are pursuing research in our laboratories.
Returning to our discussion of the data in Figure 2, the retention of much of its physical strength by deacidified paper and the stabilization of paper gained through mere contact with alkaline paper within the PET envelope, add further credence to the assumption that the degradation products are indeed acidic in nature. The autocatalytic effect observed in Figure 1, for paper in stacks is again evident for encapsulated sheets of paper.
Only one logical scenario can explain all of the observed facts in Figures 1 and 2: Acidic compounds produced as a result of the aging of acidic paper are retained by a paper matrix that is isolated from free interaction with the environment. As the concentration of these acidic species increases with aging time, the rate of acid hydrolysis of cellulose accelerates. This process is autocatalytic, that is, it feeds upon itself. An alkaline buffer incorporated in paper, or even in contact with an alkaline paper stabilizes acidic paper by neutralizing the acidic degradation products.
The fact that acidic degradation products are produced in the accelerated aging of acidic paper in a humid environment, was clearly established at NBS in the seventies (Parks, 1971-1972). However, these data did not point to the accumulation of these acids within a paper mass, such as a book, and the subsequent autocatalytic nature of the degradative process.
Indeed, it was not too long ago that an expert panel assembled by the National Academy of Science for the National Archives and Records Administration suggested that the rate of aging of acidic paper decreases with time (National Academy of Science, 1986). The inference was that acidic paper, even if it is in a brittle state, will last indefinitely if it is not misused or mishandled. This view enjoys wide support among the library and archival community simply because it makes intuitive sense. After all, few of us have ever observed a book self-destruct by crumbling to dust.
However, this view that the rate of degradation of paper decreases constantly and reaches a plateau is based not on any specific observation or experiment, but rather on the general shape of accelerated aging curves obtained from accelerated aging of single sheets, particularly those showing the decreasing slope of fold endurance loss with progression of time.
What is often overlooked in such plots, is that the fold endurance value cannot fall below zero, even though the paper keeps degrading in every other manner that is measurable (as shown later in Figure 4 ). However, laboratory data presented here suggest that the decline in physical properties of acidic paper does not slow down with increasing deterioration; it does not even continue to decline constantly at the same rate as for single sheets in Figure 1, but instead it continually gathers momentum as acidic degradation products continue to accumulate. Like a rock rolling down a slope, the decline in physical, as well as chemical properties of acidic paper within books or other isolated micro-environments, continues to accelerate until the paper becomes unusable.
Several other workers have stressed the important role that acidic degradation products play in the aging of paper. Work reported by Klaus Hendriks recently also showed a faster rate of aging of paper within paper stacks as compared with aging of single sheets (Hendriks, 1994). Marianne Bjorklund Jansson at the Swedish Pulp and Paper Research Institute is engaged in the identification of degradation products formed in the aging of paper and their comparison in naturally and artificially aged paper. She has already identified a few low molecular weight acids, namely, acetic, propionic and levulinic acids.
It is reported that the correlation between these and other cellulose and hemicellulose degradation products and paper strength parameters is being tested using multivariate data analyses (Frojd, 1992). In the US, the National Institute for Standards and Technology completed a preliminary study for the identification of degradation products in the presence and absence of sulfur dioxide (Parks, 1991).
Table of Contents - Introduction - Status of Accelerated Aging of Paper - Research in Accelerated Aging of Paper - Comparison of Accelerated Aging of Paper in Stacks and Sheets - Aging of Paper Sealed within Polyester Film - Inadequacy of Single Sheet Accelerated Aging Methods - Accelerated Aging within Sealed Enclosures - Comparison of Accelerated Aging Methods - Accelerated Aging under Light -Measurement of Rates of Degradation - Conclusion - References - Supporting Documents