Source: Wikipedia
Sticky-shed syndrome is a condition created by the deterioration of the binders in a magnetic tape, which hold the iron oxide magnetizable coating to its plastic carrier. This deterioration renders the tape unusable. Some kinds of binder are known to break down over time, due to the absorption of moisture (hydrolysis).
The symptoms of this breakdown are immediately obvious even when rewinding the tape: tearing sounds and sluggish behavior. If a tape with sticky-shed syndrome is played, the reels will make screeching or squeaking sounds, and the tape will leave dusty, rusty particles on the guides and heads.
Cause
Some tapes may deteriorate because of a breakdown in the binder (the glue) that holds the oxide particles on the tape if the tape was from any of the tape manufacturers who had inadvertently used an unstable binder formulation. That binder contained polyurethane that soaks up water and causes the urethane to rise to the tape’s surface. This problem became known as the ‘sticky-shed syndrome’. Short strands of urethane were most commonly used in tapes – until it was discovered that middle-sized strands are better and were good at absorbing moisture.[2] Baking the tape temporarily restores the tape by driving the water molecules from the binder so that it can be safely copied to another tape or a different format. After baking, the tape usually remains in good condition for approximately a month. If the tape re-deteriorates, it may be possible to bake the tape again.
Affected stock
Most tapes affected by Sticky Shed are those that were made by Ampex such as 406/407, 456/457, 499, and consumer/audiophile grade back coated tapes such as Grand Master and 20-20+.
Many tapes made by Scotch/3M are also affected and the most common offenders here are the “pro” tapes such as 206/207, 226/227, 808, and 986 as well as audiophile tapes such as “Classic” and “Master-XS”.
Though less common, many Sony branded tapes such as PR-150, SLH, ULH, and FeCr have also been reported to suffer from Sticky Shed.
Blank cassettes from the 70’s-90’s are unaffected because the hygroscopic binder was not used in cassette formulations. However, some cassette tape formulations do suffer from a similar problem caused by fatty acids working to the surface of the tape that can cause sticking to heads and guides and severe modulation of signals through the playback head until it is cleaned.
As of 2012, no documented or proven examples of Sticky Shed from Maxell, TDK, or Quantegy are known to exist. There have been a few reports of some tape from the current manufacturers ATR and RMGI exhibiting symptoms of Sticky Shed. But these may be isolated incidents relating to prototype or single bad batches and not necessarily indicative of the overall product line integrity.
Neither BASF nor 3M tape production used the unstable formulation, and their tape production rarely shows this type of coating instability although BASF LH Super SM cassettes manufactured in the mid-70s are prone to the problem.
Solutions
The binder on this tape has deteriorated to the extent that the oxide (brown), which holds the magnetic information, has come away from the polyester base (clear). It is unrecoverable.
Current solutions to sticky-shed syndrome seek to safely remove the unwanted moisture from the tape binder. Two different strategies are commonly employed: applying heat to the tape (commonly called ‘baking’), and changing the environment to lower the humidity. Even though baking is widely practiced, it can be destructive to the tapes. While modification of humidity by safely controlling the environment may take significantly longer, its major benefit is that it does not irreparably damage the tape. Alternate practices have also been developed by engineers to fix affected tapes.
Baking
Baking is a common practice for temporarily repairing sticky-shed syndrome. There is no standard equipment or practice for baking, so each engineer is left to create their own methods and materials. Generally, tapes are baked at low temperatures for relatively long periods of time, such as 130°F to 140°F (54 to 60 °C) for 1 to 8 hours. It is commonly thought that baking a tape will temporarily remove the moisture that has accumulated in the binder. A treated tape will reportedly function like new for a few weeks to a few months before it will reabsorb moisture and be unplayable again.
Baking cannot be used with acetate tapes.
When to bake
Tape baking is a somewhat risky procedure, since there is chance of damaging the tape from the heat. However, there are some important signs that show when a tape needs baking. The typical symptom is squealing when the tape passes the playback head or other fixed parts of a tape player. The squealing is audible directly from the tape and also transmitted electronically through the output of the tape recorder. Continuous use of a squealing tape risks permanently damaging the tape, as oxide is sometimes torn off the tape. This flaking residue can be seen and can feel gummy while still on the tape’s surface. There is also a risk of damage to the player. Another symptom is the tape sounding dull and distorted. In a video recording, degradation can be represented by audio-visual dropouts.
Procedures
There is no clear method or procedure for tape baking. Baking between 50 and 60 degrees C (120 and 140 degrees F) for approximately eight hours is one method. Normal domestic ovens are generally not used because of their temperature instability and significant humidity. More commonly, convection ovens or food dehydrators are used. Tom Neuman, Senior Staff Engineer at Ampex, knew of one extraordinary case where “a record company constructed their oven from a cardboard box, a hair dryer and a candy thermometer.” In 2002, Eddie Ciletti, Mix Magazine’s technology columnist, confessed to building a hair-dryer-powered Easybake oven to rescue his tapes. Most experts recommend not baking reel tapes wound on plastic reels, because the plastic reels can warp and become deformed; metal reels generally hold up well.
Engineer Preferred heat source Temperature °F (°C) Time
Eddie Ciletti food dehydrator 130 (54) 1 to 8 hours
Rich Rarey convection oven or food dehydrator 121 (49) 8 to 48 hours
Mike Rivers convection oven 130 (54) 4 to 6 hours
Ampex None stated 120 to 130 (49 to 54) about a day
Jim Wheeler convection oven 130 (54) 8 to 24 hours
These methods most likely work because the relative humidity lowers as the temperature rises, allowing the tape to release some of its excess moisture.
Problems with baking
Charles Richardson makes several claims about possible damages done to tapes during baking:
Baking is not meant to restore; it is meant to temporarily allow the tape to be played.
Baking melts the debris, which allows the tapes to be played but does not cure the chemical causes.
Because of certain heat related chemical reactions, baking makes the tape brittle.
Because heat energy destroys substances, it causes progressive and permanent damage and, thus, it is not a safely repeatable solution.
Allowing a tape to encounter temperatures higher than 90°F (32°C) causes it to have a much shorter lifespan.
The process allows debris to collect at the tape head gap, which worsens high frequency losses.
Magnetic particle attached to light-weight polymers is driven off by evaporation.
Print-through is increased as a result of baking.
Baking weakens magnetic fields and, therefore, lowers output signals.
It should be noted that Richardson has developed and represents a commercial process called “Rezerex” which may impact the veracity of his statements.
A report commissioned by the National Recording Preservation Board of the Library of Congress concurs that baking is a temporary remedy and that “more research needs to be done on alternative ways of alleviating hydrolysis for polyester tapes.”
Various scholars and engineers have noted specific problems associated with high temperatures. A report for the National Endowment for the Humanities, Division of Preservation and Access notes that “lower temperatures and drier conditions lead to longer life span.” Brown, Lowry and Smith found that tapes exposed to higher temperatures and humidity suffered from a loss of adhesion. Additionally, David Luepke claims that “repeated playing [and presumably bakings] may still result in unacceptable signal loss and deterioration.”
Dr. John Van Bogart at the National Media Laboratory has recommended the process, as well as the tape manufacturer Ampex, the sound recording industry magazine, Mix, the Association of Moving Image Archivists and the American Folklife Center and the Motion Picture, Broadcasting & Recorded Sound Division of the Library of Congress. However, The National Recording Preservation Board has put forth their desire to find a better remedy for sticky shed syndrome. It appears the roundtable meeting of March 2006 acknowledged that there is a destructive element to tape baking.” One alternative was presented in 1994. Marie O’Connell was preserving and digitizing at an archive of Radio New Zealand. She began the steady application of isopropyl alcohol to tapes with sticky shed and loss of lubricant after baking had not worked. This method appears to control friction by simultaneously keeping the tape cool and lubricated as they play. O’Connell has patented her Isopropyl Drip Machine and was the Audio Archivist at the Oral History Unit at the University of Southern Mississippi & the Chief Audio Archivist of Cups N’ Strings Studios of California. No information has been revealed about the long-term condition of the tapes once this process has taken place. Marie O’Connell is now back at Radio New Zealand Sound Archives/Nga Taonga Korero as the Analogue Tape Specialist – www.soundarchives.co.nz
Environmental control
Most preservationists and engineers would agree that any loss or deterioration should be avoided. Several engineers, therefore, have sought to show that careful and deliberate environmental control can lead to the reversal of sticky-shed syndrome.
Bertram and Cuddihy argue that sticky-shed syndrome is permanently and completely reversible through environmental control, something that has not been achieved with any baking method. Through their research, they discovered that an equilibrium of hydrolysis could be maintained at 65°F (18°C) and 40% relative humidity with slight fluctuations of only 3°F (1.5°C) and 5% relative humidity. The Committee on Preservation of Historical Records promote a similar approach. They state that to attain equilibrium at a satisfactory low level of hydrolysis, tapes should be stored at 20°C (68°F) and 40% relative humidity. Charles Richardson claims that he preserves tapes by placing them in a similarly stable environment and then safely cleaning debris from both sides of the tape.
Use of alcohol
Marie O’Connell uses a steady supply of isopropyl alcohol to solve the problem. She created a machine that steadily applies and wipes off the excess liquid. This process may work because the alcohol lubricates the tape as O’Connell suggests. Another possible reason it may work is that as the alcohol mixes with the water in the binder, it causes the water to evaporate more quickly. This procedure, then, would have to be repeated before each playing (much like baking) and may have potential negative effects such as chemical reactions, and removal of oxide from the backing.
Conclusions
Engineers may favor baking over controlling hydrolysis environmentally because of time limitations or because they are more focused on digitizing or creating duplicates of the original and are not as interested in the preservation of the original tape. They may feel that some amount of permanent damage is acceptable if they are able to create a suitable duplicate.
Research has shown that hydrolysis is reversible within very strict temperature and humidity constraints. Further research is needed, however, to determine what levels of temperature and humidity are allowable in practice and what lengths of time are needed to adequately reverse hydrolysis. It may be that certain combinations of temperature and humidity have more or less positive results. It could be found that most institutions could successfully achieve the needed levels with common and inexpensive equipment.