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.[1] 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.[2] 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.[3]
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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 either of two 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. 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. 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. Many pre-recorded music cassette tapes of the late 70s and early 80s suffer from this problem, because of the unstable binder formulation that was used by the music companies suppliers of their tape stock.
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 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.[4] 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.[5]
| Engineer | Preferred heat source | Temperature °F (°C) | Time |
|---|---|---|---|
| Eddie Ciletti[2] | food dehydrator | 130 (54) | 1 to 8 hours |
| Rich Rarey[3] | convection oven or food dehydrator | 121 (49) | 8 to 48 hours |
| Mike Rivers[5] | convection oven | 130 (54) | 4 to 6 hours |
| Ampex[6] | None stated | 120 to 130 (49 to 54) | about a day |
| Jim Wheeler[7] | 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.
Charles Richardson makes several claims about possible damages done to tapes during baking:[8]
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.”[9]
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.”[10] Brown, Lowry and Smith found that tapes exposed to higher temperatures and humidity suffered from a loss of adhesion. [11] Additionally, David Luepke claims that “repeated playing [and presumably bakings] may still result in unacceptable signal loss and deterioration.”[12]
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.[13] 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.[14] 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.
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.[15] 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.[3]
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.