Sticky-shed syndrome is a condition created by the deterioration of the binders in a magnetic tape

Magnetic tape

Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...

, which hold the iron oxide

Iron oxide

Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...

 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

Moisture

Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

 (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 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.

Solutions

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

Humidity

Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

. 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

Tape baking

Tape baking is a process that is used to restore magnetic tapes: audio cassettes and video tapes that have begun to go through a chemical breakdown due to age, known informally as shedding...

 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 procedures

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

National Recording Preservation Board

The United States National Recording Preservation Board selects recorded sounds for preservation in the Library of Congress' National Recording Registry. The National Recording Registry was initiated to maintain and preserve "sound recordings that are culturally, historically or aesthetically...

 of the Library of Congress

Library of Congress

The Library of Congress is the research library of the United States Congress, de facto national library of the United States, and the oldest federal cultural institution in the United States. Located in three buildings in Washington, D.C., it is the largest library in the world by shelf space and...

 concurs that baking is a temporary remedy and that “more research needs to be done on alternative ways of alleviating hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 for polyester tapes.”

Various scholars and engineers have noted specific problems associated with high temperatures. A report for the National Endowment for the Humanities

National Endowment for the Humanities

The National Endowment for the Humanities is an independent federal agency of the United States established by the National Foundation on the Arts and the Humanities Act of 1965 dedicated to supporting research, education, preservation, and public programs in the humanities. The NEH is located at...

, 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.”

Alternatives to baking

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.

Environmental control

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

Isopropyl alcohol

Isopropyl alcohol is a common name for a chemical compound with the molecular formula C3H8O. It is a colorless, flammable chemical compound with a strong odor...

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.