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The Library of Congress holds nearly 200,000 reels of audio tape, spanning recordings of popular musicians, the American Folklife Center, and NPR broadcast archives. These tapes are generally comprised of a multilayer polyester-urethane material containing various additives and lubricants, and over the past few decades, many archival tapes have been observed to show signs of surface degradation resulting in unplayability. The most common treatment for an unplayable audio tape is a thermal remediation “baking” treatment where the tape is exposed to temperatures near 50 °C for multiple hours. While this thermal treatment usually succeeds in allowing for clean playback of the degraded tapes, the mechanisms underlying the thermal treatment have remained unclear.
The Library’s Preservation Research and Testing Division has used modulated differential scanning calorimetry, heat-stage microscopy, and artificial aging to study the thermal transitions which occur during remedial tape baking. In most cases of tape degradation, crystalline polyester-like residues were observed on the surface of the tapes’ polymeric binder layer, and these residues were observed to melt back into the bulk polymer binder. However, as noted by seasoned audio technicians and archivists, thermal baking treatments are not permanent. Baked tapes are known to slowly revert back to unplayable condition. Additional time-based experiments identified phase separation in these treated tapes over time, with melted residues eventually blooming and re-appearing at the tape surface. Thermal transitions were also identified in both the magnetic layer and the PET support film, indicating that one single component cannot be attributed for tape breakdown and that the detrimental residues were not isolated to the tape binder layer.
Learnings from these lab-scale experiments, undertaken on small samples of audio tape, are now being used on full-sized audio tape reels to understand how these thermal processed may proceed in tightly-wound real-world tapes to better inform preservation processes.
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