Volume 25, Number 6
Apr 2002

Electron Beam Radiation and the Permanence of Paper

The permanence of paper is very important for archival or permanent records. A lack of permanence is typically observed over time by discoloration of the paper, a loss of strength of the paper, and a resistance to moisture penetration. The loss of strength is often manifested as a loss of fold endurance, that is, the paper becomes brittle. These changes are associated with a change in the chemical properties of the paper, including cellulose degradation. Papers made from chemical pulps that are high in cellulose content and low in lignin typically have higher permanence. Groundwood papers with high lignin content are the least stable. It is known that pure cellulose can last indefinitely¹.

The permanence of paper is directly related to its acidity or pH (hydrogen ion concentration). Acidity in paper can come from the absorption of acid gases from the atmosphere or the presence of organic acids in the pulp, coating, or sizing agents. A primary source is the use of alum for sizing. Over the past 15 years many paper producers have gone to alkaline sizing processes and the use of alkaline pigments. Such papers are expected to have greater permanence.

The measurement of permanence is difficult in that natural aging (over tens or hundreds of years) is impractical. Accelerated tests that employ high intensity light or heat are typically used to speed up the aging process. Neither approach, however, actually reproduces the aging that would occur naturally in the dark at room temperatures.

According to the U.S. Postal Service, typical business mail received in government offices (excluding magazines, etc.) is uncoated free sheet. This paper would be made from 80 to 90% bleached chemical pulp, contain 5 to 15% filler, and typically be sized with alkaline synthetic chemicals. Such papers would be expected to have very high permanence.

Recently the U.S. Postal Service has used electron beam radiation at dosages up to 50 kGy to sanitize mail going to certain zip codes. The impact of high-energy electron beam irradiation on papers, however, is known to break chemical bonds, form acids, and alter the chemical composition and thus the paper structure. There is ample evidence in the literature that among the degradation products created from the irradiation of paper are acid groups, including carboxylic acid ². In addition, the irradiation of other polymeric materials in the mail (such as polystyrene envelope windows, polyethylene coatings, or polyvinylchloride) also generates a variety of degradation products, including acid gases ^3,4. Finally, the high-energy electron beam radiation of the tightly packed mail also results in temperatures estimated to approach 180 ^oF.

The irradiated mail is discolored and brittle, and the severity of these effects seems to be related to the dosage level. It seems probable that the direct breakage of chemical bonds in the cellulose and the presence of acid gases generated during irradiation, whether from the paper or from other polymeric materials present in the mail, together with the high temperatures, create a very rapid aging of the paper. The loss of paper strength arises from both the degradation of the fibers from which the paper is made, as well as the bonds that naturally occur between fibers. The result of these changes could well be paper that is more prone to damage and dusting. In addition, it is likely that fibers recycled from irradiated papers will be of significantly inferior quality because of their strength loss and discoloration.

In summary, it appears that electron beam irradiation severely decreases paper permanence and quite likely will impact the ability to recycle fibers from these papers. These tentative conclusions, of course, will require verification through further research.

References:

1. Klemm, P., Paper Trade J. 89(6):53 (1929)

2. Takahashi, M., et. al., "Effect of Electron Beam Irradiation on Characteristics of Paper." Japan TAPPI 49(7): 68 (1995)

3. Chapiro, A., "General Consideration of the Radiation Chemistry of Polymers." Nuclear Instruments and Methods in Research B. 105: 5 (1995)

4. Buchalla, R., Boess, C., Bogl, K.W., "Characterization of Volatile Radiolysis Products in Radiation-Sterilized Plastics by Thermal Desorption-Gas Chromatography-Mass Spectrometry: Screening of Six Medical Polymers." Radiation Physics & Chemistry 56: 353 (1999)

Dr. Baum is Vice President of Institute Operations at the Institute of Paper Science and Technology.
Reprinted with permission of the author.