Accelerated Aging of Paper: Can it Really Foretell the Permanence of Paper
Comparison of Accelerated Aging of Paper in Stacks and Sheets
Several years ago at the Library of Congress, we undertook an investigation aimed at studying the effect of cycling relative humidity and temperature conditions on the accelerated aging of paper (Shahani, 1989). For these experiments it was obvious that a paper in a bulk, such as in a book, would absorb and desorb moisture more slowly than a single sheet of paper. Therefore, we compared the effect of cycling relative humidity conditions on the aging of the same paper samples as single sheets, as well as in 100-sheet thick stacks.
In this study two different wood pulp papers, one an unsized, unadulterated waterleaf, and the other, an alum-rosin sized 15-year old paper, were aged at 90°C and constant relative humidities of 40, 50 and 60 percent and also a relative humidity that cycled constantly between 40 and 60 percent. The result of these experiments, which is not really relevant to the present subject was that the cycling humidity conditions did not exert a measurable effect on the aging of the paper in a stack, while the same paper in single sheets deteriorated significantly faster due to the fluctuating relative humidity. But from our present perspective, a much more interesting result of these experiments was that paper within a stack aged faster than the same paper aged as single sheets that were free to interact with the environment.
These data supported an observation from a condition survey of pre-1840 paper records at the Philadelphia Center of the National Archives, that paper within bound volumes was invariably weaker than loose rolled up sheets inside ventilated cardboard boxes (Shahani, 1980). Extending our earlier work on the comparison of aging of single sheets with paper within stacks, we undertook the aging of a few selected papers over a wide range of temperatures (60 through 90°C). Some of these data obtained with Springhill™ Offset, an alum-rosin sized bleached Kraft book paper, are shown Figure 1.
All of the experimental data presented in Figure 1 and elsewhere in this work, were obtained with Springhill™ Offset paper, which was manufactured by International Paper. This paper was obtained as a 6-foot wide roll. A 3-inch width was discarded from both ends, and the rest sectioned into 1 1-inch wide rolls. Eight-inch long sheets were cut from the same 1 1-inch wide roll for use in each set of these experiments. All papers were preconditioned according to TAPPI T402, then conditioned at 73°F and 50% RH for at least 24 hours for single sheets, or for at least 4 weeks in the case of stacks of paper, before being sealed inside an enclosure or before introducing them into temperature and humidity controlled aging chambers.
Turning our attention `back Figure 1, the data at 60°C should have evidently been pursued for a longer time interval. For the rest of the data obtained at 70, 80 and 90°C, it is clear that the paper inside a stack ages appreciably faster than a single sheet hung on a rack. Unlike the single sheets, the paper inside the stack is not in immediate contact with the air and moisture flowing through the humid aging chamber. The only explanation that suggests itself is that degradation products formed in the aging process accumulated inside the stack of paper accelerate its degradation.
It is remarkable that significant concentrations of the degradation products are retained within the paper mass in spite of the high vapor pressures that must prevail at 90°C, even though these stacks of paper were not sealed or enclosed in any way. The top sheets were kept in place by a 1/8-inch thick Plexiglas™ sheet. The high affinity of the degradation products for paper suggests a high degree of polarity. Also, the acidity within the stacked paper was about a half pH unit lower than that of the single sheets. These observations, along with the fact that the stacked paper aged faster, suggest that at least some of these accumulated degradation products must be acidic.
It was also noticed that the paper aged within stacks had an odor somewhat reminiscent of old books. While one generally notices an odor in humid aging chambers where even single sheet samples are being aged, especially at the times that the chamber door is opened to remove samples periodically, we cannot recollect any persistent odor in single sheet samples, whether aged in a dry or humid oven.
An important feature of the data in Figure 1 is that while the single sheets lose strength at a constant rate, the rate of degradation of the paper within stacks increases constantly, due in all probability to the increasing acid concentration within paper.
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