Binder Degradation
The degradation of binder polymers in magnetic tape occurs through a chemical process known as hydrolysis. Polyester links in the commonly used polyester polyurethane-based binder systems are broken up by water molecules. Think of a woolen sweater: If enough individual yarns are cut, the sweater will eventually fall apart. Binder hydrolysis causes long molecules to break after exposure to water. The reaction produces shorter molecules, which do not impart the same degree of integrity to the binder system as do the longer molecules. On tapes exposed to air, the binder polymer will also absorb water from the increasing relative humidity and temperature.

Binder hydrolysis can lead to a phenomenon commonly known as "sticky shed syndrome," which is characterized by softer-than normal binder coating, higher friction, and/or gummy tape surface residues. One result of sticky shed syndrome is high friction between the tape and the equipment surfaces, which produces tape transport problems. Broken binder particles are another problem. They can build up on the magnetic head, resulting in head clogs.

This will cause magnetic dropouts, and in extreme cases the friction will be so high that transport of the tape is stopped. Sticky shed syndrome will also cause audio tapes to seem to "squeal," as the tape very rapidly sticks to and releases from the playback head.

Lubricant Loss
Lubricants are normally added to the binder to reduce the friction of the magnetic top coat of the tape. Lower friction facilitates tape transport through the recorder, and reduces tape wear. The surface of magnetic tape is porous. A liquid lubricant is added to the binder on many tapes, and remains in the tape's "pores," similar to water absorbed by a sponge. When the tape passes over the head or a tape guide, lubricant is squeezed out onto the tape surface. This provides a slippery interface between the tape and the guide pin. The excess lubricant on the surface of the tape is then reabsorbed into the surface of the tape, after it passes the guide pin.

Like binder polymers, some lubricants are subject to degradation by hydrolysis. They will lose their essential lubrication properties in time. Also, lubricants are consumed every time the tape is played. Some of the lubricant will migrate to the guide pins and heads of the recorder on each playback. Over time, the level of lubrication in the tape decreases. Evaporation and degradation will also decrease lubricant levels-even in unplayed, archived tape. The lubricants used in some tapes are oily, volatile liquids that slowly evaporate.

Low temperatures can cause the lubricant to separate from the binder, promoting further tape transport problems with possible overheating, stretching and/or distortion. A videotape stretched by just 1 percent will be unusable.

Magnetic Particle Instabilities The pigment-a term borrowed from paint and coating technology-stores recorded information on magnetic particles. The magnetic remnants characterize the pigment's ability to retain a magnetic field. The strength of the signal magnetically recorded on a tape is directly related to the magnetic remnants of the pigments. If there are any changes in the direction of the magnetism of local particles in the pigment, or if there is a decrease in the magnetic remnants over time, recorded signals can be lost.

Magnetic pigments differ in their stability-some particles retain their magnetic properties longer than others: That is, some tapes will retain information longer than others. Iron-oxide and cobalt-modified iron-oxide pigments are the most stable pigment types used in audio and video tapes. These pigments are generally used in lower-grade audio and low-to-high grade video tape formulations. Metal-particle and chromium-dioxide pigments provide a higher tape signal output and permit higher recording frequencies than do iron-oxide pigments. They are not, however, as stable as iron-oxide pigments. A decrease in signal output of two decibels may be observed over the lifetime of metal-particle and chromium dioxide-based tapes. Even with these losses, the output signal will still be better than a comparable iron oxide-based tape. Loss of signal manifests itself as a reduction in the clarity and volume of a sound recording. Loss of hue and reduction in saturation occurs in a video recording. Chromium dioxide is used in medium-to-high grade audio tape and some high-grade video tape. Metal particulate is used in high-grade audio and low-to high-grade video tape. Metal particles are also used in most digital audio and video tape formulations. Production literature that comes with the tape indicates the type of pigment used in the formulation.

Not much can be done to prevent the magnetic deterioration inherent in metal-particulate and chromium-dioxide pigment types. The rate of deterioration can be slowed, however, by storing the tapes in cooler temperatures. Humidity has little direct effect on the deterioration, but by-products of binder deterioration can accelerate the rate of pigment deterioration. During storage, lower humidity levels help minimize the pigment's degradation.
 

Metal-evaporated video tape is prevalent in some formats. These tapes require no binder polymer, as the entire magnetic layer comprises a single, homogeneous metal alloy layer that is evaporated onto the substrate. Their chemical stabilities are similar to those of metal particle tapes. However, because the magnetic coating on a metal-evaporated tape is much thinner than the corresponding layer on a metal-particle tape, they are also generally not as durable. Repeated play or freeze-frame video applications present problems with wear.

Substrate Deformation
The substrate (tape backing) supports the magnetic layer for transportation through the recorder. Since the early 1960s, audio and video tapes have used an oriented polyester film as a tape substrate material. Polyester is chemically stable, and polyester films are highly resistant to hydrolysis. In archival situations, the polyester tape backing will chemically outlast the binder polymer.

The problem with polyester-backed videotape is that excessive tape pack stresses, aging, and poor wind-quality can result in distortion and subsequent mistracking when the tapes are played. Once the polyester backing is distorted, it cannot be corrected by subsequent treatment.

The best way to reduce the degree of tape-backing distortion is to store magnetic media in an environment that does not vary much in temperature and humidity. Each time the temperature or humidity changes, the tape pack expands or contracts. These dimensional changes can increase the stresses in the tape pack, causing permanent tape-backing distortion.

Tape backing deformation can also arise if the tape experiences nonlinear deformation as a result of uneven tape-pack stress. This normally results if the tape-pack wind-quality is poor. It is recognizable as popped strands of tape protruding from the edge of a wound roll.

Older tapes used different backing materials. Acetate film-the same material used in some older movie film-was used in the 1940s and 1950s as an audio tape backing. In general, if light can be seen coming through the tape winding when the reel is held up to light, it is an acetate-based magnetic tape. This substrate is subject to hydrolysis and is not as stable as polyester film. However, more-stable vinyl binder systems were used during this time period. Thus, the life of tapes produced during this period can be limited by the degradation of the backing rather than the binder. Backing degradation of these tapes is indicated by the presence of the vinegar syndrome, where the faint odor of acetic acid can be detected. As with motion picture film in advanced stages of degradation, the magnetic tape will become brittle and break easily if bent too sharply or tugged. The backing also shrinks as it decomposes, which changes the length of the recording. Any tape on an acetate backing should be stored in a low-temperature, low-humidity archive to reduce the rate of deterioration of the acetate backing.

Tapes with vinegar syndrome should be stored separately to prevent them from contaminating other archive materials with acetic acid. After the onset of the vinegar syndrome, acetate films degrade at an accelerated rate. Tapes that have been stable for 50 years may degrade in just a few years to the point of being unplayable. Any valuable tape showing vinegar syndrome should be transcribed as soon as possible.

Format Issues
The susceptibility of the recording to loss as a result of dimensional changes in the backing depends on recording format. Video tape uses a helical-scan recording format. It is more sensitive to disproportionate dimensional changes in the backing than analog audio tape is. Analog audio tape uses longitudinal recording.

Helical tracks are recorded diagonally on a helical scan tape at small scan angles. When the dimensions of the backing change disproportionately, the track angle will change for a helical scan recording. The scan angle for the record/playback head is fixed. If the angle that the record tracks make to the edge of the tape do not correspond with the scan angle of the head, mistracking and information loss can occur.

Helical-scan distortion can result in two types of mistracking, trapezoidal and curvature mistracking. In trapezoidal mistracking the tracks remain linear, but the track angle changes so that the fixed-angle playback head cannot follow them. Curvature mistracking can be a more serious type of deformation: The recorded tracks become curved as a result of nonlinear deformation of the tape backing. Mistracking results in a video image that looks like snow on part or all of the screen. The appearance on the screen will be similar to the playback of a good tape where the tracking-adjustment control has been purposely mistracked.

Distortion of a longitudinal audio recording tape will appear as temporary muffling, change in pitch, or loss of the audio track. Tape-backing distortion can impart a slight curve to the normal linear tape. When the distorted portion of the tape passes over the playback head, the recorded tracks can move out of alignment with the head gap, causing a temporary reduction in sound volume and quality.

Disaster and Contaminants
Collections are always at risk from unexpected accidents or deliberate damage. Safety measures can be taken to best minimize the damage within an archive but they may not be enough. Despite precautions, if a disaster strikes the results may be catastrophic. Such situations are less likely a problem though than longer-term deterioration caused by poor storage over months and years. The end result has the potential to be just as catastrophic to a collection as any fire or flood. Environmental contaminants may go largely unnoticed since their effects are due to long-term exposure.
 

Machine Obsolesce
Recording machines rapidly become obsolete and unavailable. If there is not a working machine available the information recorded on the tape will be irretrievable. Those machines that still exist are often poorly maintained and seldom able to give optimal playback. In addition, the expertise needed to maintain and operate these machines is hard to find.