Recent Examination of Some Irradiated Mail

David von Endt, David Erhardt, Abdel-Salam El-Esseily, Walter Hopwood, Marion F. Mecklenburg, and Charles S. Tumosa 
Smithsonian's Museum Conservation Institute, February 2002

The response of the Post Office to the anthrax problem has been the electron irradiation of select portions of the mail. The mail is packaged in sealed plastic to a thickness of over 3 inches and irradiated in two passes. Sufficient examples of the irradiated mail have been examined to permit some observations.

Color Changes

The irradiated mail exhibits definite yellowing, and this has been quantified by L*a*b* measurements of irradiated material. The color shift is not necessarily immediate and can occur over time. This indicates that the initial irradiation induces the formation of unstable intermediates, such as peroxides, that continue to breakdown after the irradiation process is over. Common inks present on irradiated papers do not seem to change color although the study is still ongoing.

Mechanical Damage

According to postal sources, temperatures of the irradiated materials can reach 130 C. High temperatures have been confirmed, for example, by the extreme distortion of polystyrene slide mounts. The softening temperature of polystyrene is about 110 C. The clear windows of some envelopes are polystyrene and these too have been found to have exhibited softening and distortion, in some cases adhering to the printed matter beneath. Certain printed materials have become stuck together, possibly due to the softening of the resins in the printing inks or photocopying toner. Moisture driven from the paper by high temperature can also condense and cause "blocking". Images on slides have been found to crack due to the high temperatures.

Tensile measurements on irradiated paper show that there is a substantial loss in the ability of the paper to be deformed. This loss of extensibility has been as high as 80% and the resulting brittleness is severe. At this point the paper will not sustain being folded over. Analyses of the soluble material in irradiated and unirradiated samples of the same paper show an increase in the amounts of degradation products. The distribution of products is very different from that seen in naturally aged materials. The amount of glucose, specifically, is not greatly increased. This shows that the damage is due to reactions other than hydrolysis, which is the primary reaction during the natural aging of cellulose. The relatively small amounts of soluble degradation products probably do not account for the large loss of strength observed, indicating that the changes are most likely due to radiation induced crosslinking. Such reactions have serious implications for the effects of irradiation on biological specimens. For example, crosslinking would severely hinder any DNA analyses.

Stress-strain curves for Whatman #1 paper irradiated to 257 kGy, a Smithsonian Institution envelope irradiated to 257 kGy, and a business envelope irradiated at the postal facility along with their control curves are presented here. The common elements to be seen are that the irradiated samples have become damaged to the point of losing strength, have not become stiffer, but have lost a significant portion of their plastic region. The irradiated specimens, notably the envelopes, have become embrittled to where they could not be folded over.

For more information, contact:
Ann N’Gadi
Technical Information Officer
MCI
tel. (301) 238-1240 option 2
fax (301) 238-3709
e-mail NGadiA@si.edu