Background:
The Preservation Research and Testing Division has been conducting research to better characterize long term stability and playability concerns related to sound and moving image recordings on magnetic tapes, with particular focus on “sticky shed syndrome”. With sticky shed degradation, tapes deposit a residue on playback equipment which can both damage equipment and impact the sound or video signal layer. The phenomenon appears most common in analog audio and video tapes manufactured using polyester-urethane (PEU) binders beginning in the 1970s. Unanswered questions remain regarding the causes, detection, and treatment of the problem.
Contributing Studies:
Breitung, E.M., Skelton, S., Rodriguez, J., Zhang, L., Cassidy, B., Alyea, P., Morgan, S.L. “Non-destructive Identification of Polymeric Binder Degradation in Audio and Video Tapes”, "Research and Technical Studies Specialty Group Post-prints ", American Institute of Conservation, 41st Annual Meeting, Indianapolis, IN, May 29, 2013, pg. 159.
Davis, Andrew. “Rapid Prediction of Polyester Magnetic Tape Playability Using Water Contact Angles” J. Audio Eng. Soc., 67 (12), pp. 953-960. http://www.aes.org/e-lib/browse.cfm?elib=20706
Davis, A., Shetzline, J. “Towards understanding the thermal remediation of degraded archival reel-to-reel audio tapes”, American Chemical Society Spring 2020 national meeting, SciMeetings Virtual Platform. https://doi.org/10.1021/scimeetings.0c01250
Project Description:
PRTD is developing rapid, non-invasive, methods for identifying degraded magnetic tapes prior to playback. Degraded tapes are often sticky, allowing magnetic material to clog play heads and guides (examples of tape playability problems show in “Supporting Info” files here), which can stop equipment during use, and can damage a sound or video signal layer. The phenomenon appears most common in analog audio and video tapes manufactured using polyester-urethane (PEU) binders beginning in the 1970s.
Assessing the conditions across a collection can pose an intimidating necessity of resources, both for large and small collections, where the combined needs of equipment, personnel, and expertise can be stretched thin. For this reason PRTD has been researching methods to rapidly identify and predict whether any individual tape would pose problems associated with sticky shed prior to any playback attempts.
Infrared (IR) spectroscopy was used in conjunction with multivariate statistical analysis to classify tapes as either degraded or non-degraded. Both audio and video formats were investigated using principal component and linear discriminate analysis (PCA and LDA) to categorize tapes using statistical classification methods. Tapes were assessed by sound or video engineers using traditional playback methods as the independent condition assessment.
Water contact angle is now also being studied as a rapid, intuitive, and low-tech method for predicting the suitability of a given tape for playback. In this technique, a small microliter droplet of water is placed on the surface of the tape. The droplet’s shape can then be observed in order to infer the current condition of the tape and its surface. After application, the droplet is immediately wiped off. Given its temporary and highly localized application, this droplet does not pose meaningful physical concerns for the tape condition.
Additional tests are ongoing to investigate the fundamental nature of magnetic tape degradation and popular remedial treatments, with the hope of providing more confident predictions of when common treatments may or may not work. Thermal analysis tools such as differential scanning calorimetry (DSC) and heat stage microscopy are being used to better understand the thermal transitions which occur during popular remedial “baking” treatments at elevated temperatures. Other characterization tools such as optical microscopy, tensile testing, and atomic force microscopy (AFM) are being used to provide additional data.
Outcomes/Findings:
Ongoing studies resulted in the following key findings regarding assessment of tape playability.
Assessment by IR spectroscopy:
- Quarter-inch audio tapes can be accurately classified more than 90% of the time using IR spectroscopy and PCA/LDA analysis.
- All tape formats will require independent datasets for proper classification.
- Infrared bands associated with polyester-urethane degradation were identified as the primary IR signals differentiating degraded and non-degraded tapes.
- Recent studies of well-sourced collections spanning decades of recording have also begun to identify changes in particular tape formulations which occurred across their production history.
Example Infrared Spectra for a degraded and non-degraded ¼”
magnetic tape with peak assignments.
Assessment by Contact Angle:
- Low contact angle (flattened droplet) was highly indicative of tapes in unplayable condition, and high contact angle (rounded droplet) was indicative of cleanly playable tapes.
- Predictions from contact angle measurements have provided individual tape-by-tape assessment of playability.
- This method appears broadly applicable across most common historic tape models.
Studies on Tape Degradation Phenomena:
Thermal testing using DSC and heat stage microscopy has been undertaken to better understand the common “baking” treatment, where elevated temperature can successfully enable the playback of many tapes showing sticky shed syndrome. The frequently cited baking temperature of 54 °C (about 130 °F) was found to be measurably and quantifiably important in sticky tapes, related to a physical state change of some component within these tapes. This transition was not observed at any temperatures lower than 50 °C and did not match those expected for dehydration or repolymerization phenomena. It also suggests that a well-controlled, well-circulating treatment oven is necessary for successful treatment for users who want to treat sticky shed syndrome by baking. Ongoing measurements are investigating the phenomenon of how and why sticky residues return to a tape after thermal treatment.
Support: IMLS Grant LG-06-12-056912
Acknowledgments: Eric Breitung, Samantha Skelton, Juan Rodriguez, Eric Bringley, Brianna Cassidy, Lu Zhenyu, Linhchi Nguyen.
Motion Picture, Broadcast and Recorded Sound Division staff members: Brad McCoy, Rob Friedrich, Caitlin Hunter, Bryan Hoffa, William Haley, Eugene DeAnna, Mike Mashon, Larry Miller, Rebecca Jones.
Updates:
PRTD Researchers completed a collaborative research project with Fujifilm, investigating the effects of different environmental aging conditions applied over the course of one year. Video here shows a tape aged for one year at 40 C and 80% relative humidity being cleanly wound on a winding deck without signs of physical degradation impeding its winding.
Publications and Presentations
Davis, Andrew. “Rapid Prediction of Polyester Magnetic Tape Playability Using Water Contact Angles” J. Audio Eng. Soc., 67 (12), pp. 953-960. http://www.aes.org/e-lib/browse.cfm?elib=20706
Oct. 2019, Andrew Davis presented “Sticky, squealing, and shedding: applying preservation science to archival audio tapes” to the LC Archives Forum, Library of Congress, Washington, DC.
June 2018, Andrew Davis contributed to panel on physical preservation of magnetic media, “Understanding Magnetic Tape Degradation by Polymeric and Material Testing”. AES Conference on Audio Archiving, Preservation & Restoration, June 28–30, US Library of Congress National Audiovisual Conservation Center, Culpeper, Virginia.
Portable Instrument Testing
The National Audio Visual Conservation Center (NAVCC) Packard Campus is home to a range of magnetic media collections, including hundreds of thousands of audio tapes. Magnetic media has changed in formulations over the years to address different concerns, like adding a backcoat to reduce static build up experienced during play back. Each manufacture had a unique recipe they followed, resulting in a number of different ways the tapes could fail. Within NAVCC's collection, the NPR collection has over 20,000 tapes ranging from 1978-1991 with a fairly well known storage history and provenance. The major advantage of this collection is that its tapes are particularly well-tracked with a known provenance, originating from a recording organization which most likely bought tapes in bulk and made daily recordings spanning decades. PRTD took portable, non-destructive equipment to analyze these tapes with our potable IR spectroscopy (FTIR-ATR), UV-vis-NIR spectroscopy (FORS), optical microscopy, and contact angle instruments. Using FTIR-ATR to measure chemical signatures, we were able to identify changes in binder chemistry from 1970s to the newer stock in the 1990s. IR measurements of the same "model" of tape across many years of recordings were able to reveal chemical changes most likely originating in the tapes' subtle year-to-year formulation changes during manufacture. Microscopy also proved useful, as we could see surface defects that often corresponded to tapes labeled in the archive as "sticky tapes", but this was not always 100% reliable method in determining if a tape was sticky or not.
A. Davis and J. Shetzline in the NPR tape archive at the Library of Congress, using portable FTIR, UV-vis-NIR reflectance spectroscopy, optical microscopy, and contact angle instruments. 2019.
Example of portable optical microscopy of a catalogued tape previously labeled as sticky. Microscopy, IR and contact angle measurements all confirmed the magnetic layer is likely sticky. Visual defects that look like tiny craters or spots are often present when the surface of the oxide layer has degraded to a sticky point.
Interestingly, a pair of tapes recorded as reel 1 and 2 of the same program on the same date were seen, with only one tape labeled sticky while he other was not. Analytical testing with portable instrumentation indicated that the labeled sticky tape was indeed sticky and the one without the sticker was not sticky, which prompts interesting and ongoing thoughts about the long-term balance between historic labeling, minute differences in tape formulations, and long term storage conditions.
