Magnetic tape degrades by known chemical processes. When the kinetics of these processes is fully understood, the degradation mechanisms can be modeled and the life expectancy (LE) of tapes can be estimated. The binder systems used in today’s audio and videotapes are generally based on polyester polyurethanes. These polymers degrade by a process known as hydrolysis – where the polyester linkage is broken by a reaction with water. One of the by-products of this degradation is organic acids. These organic acids accelerate the rate of hydrolytic decomposition. Furthermore, the acids can attack and degrade the magnetic particles.
The lifetime of a tape is defined as the length of time a tape can be archived until it will fail to perform. Tape failure in terms of a change in tape properties will be a characteristic of the particular system on which the tape is intended for play. An end-of-life criterion is a key property and a value which, if exhibited by the storage medium, would indicate a situation where significant data loss is expected. For example, the degree of hydrolysis of a tape binder system is a critical property that may determine the lifetime of a magnetic tape. Figure 10 shows the life expectancy for a Hi Grade VHS tape assuming that the tape will fail when 12% of the binder polymer has hydrolyzed.
Figure 10. Life Expectancies for a Hi Grade VHS Tape Estimated by the degree of binder hydrolysis using an end-of-life criteria of 12%. LE values are indicated as a function of storage conditions.
Note that from the above chart, humidity is more important in determining the lifetime of the VHS tape than the storage temperature. At 20° C (68° F) and 50% RH, an estimated LE value of ~30 years is indicated. If the storage temperature is raised to 25° C (76° F) at 50% RH, the LE is reduced to ~10 years. However, if the humidity is raised to 80% at 20° C (68° F), the LE is reduced to ~5 years.
The life expectancy chart above was generated solely on the basis of a specific degree of hydrolytic degradation of the binder polymer. Tapes can fail for several reasons, however. Tapes can become too sticky to play as a result of an increase in the coefficient of friction or an overabundance of hydrolysis products. They can fail due to a loss in the magnetic signal as a result of a decrease in magnetic remanence or coercivity. They can fail because the magnetic coating has failed to adhere to the tape backing. They can fail due to irreversible shrinkage of the tape substrate.
The above information was provided to show how estimates of life expectancies can be made. The LE method outlined above is a simple explanation of a much more complicated issue. Standards committees such as the ANSI IT 9-5/AES Joint Technical Commission are endeavoring to determine procedures by which the life expectancy of magnetic tape materials can be determined.
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