Summary
In modern ferric oxide tapes on PET substrates, the PE-U and PVC binders are likely to be the least stable component. Hydrolysis is indicated as a significant factor in the failure of PEU whilst dehydrohalogenation can pose a threat to PVCs. Moisture (airborne humidity), temperature and acidity are all contributing factors.
When properly conducted, solvent extractions and pH measurements provide a useful indication of relative binder condition and show a strong correlation with other physical characteristics.
Accelerated ageing can show up relative stability amongst similar formulations and should be used systematically by archives on new stocks for recording and dubbing. Further studies of tapes aged and treated under conditions closer to ‘real life* are needed to derive more accurate rate constants for the forward and reverse equations.
The complexities of tape composition, and the interaction between tape condition, playback environment and machinery make it difficult to exactly predict future behaviour of media, although basic housekeeping rules must still be followed. Storage at low humidity and temperature (less than 35% RH and 18°C) is indicated, although both are hard to maintain simultaneously. Because the effects of prolonged storage or treatment at very low humidities (less than 20%) are not completely understood (possible loss of lubricant, plasticiser migration, etc.), common sense would indicate a balance of low humidity and temperature; the rate of most reactions such as hydrolysis is slowed at low temperature.
Despite a modicum of success with low humidity rejuvenation, prevention is still better than cure. We cannot be sure of total reversibility of the decomposition process or a complete recoverability of playability for all formulations. The long-term costs of low humidity storage are certainly less than for treatment and dubbing of hydrolysed tapes. Indeed, low-humidity storage and treatment are part of a continuum of environmental conditions; having stored our tape under conditions which stabilise or even reverse hydrolysis, they can be played immediately after retrieval rather than having to wait even 24 hours for baking! Packaging with desiccants to provide a dry micro-climate, or hermetic sealing may be appropriate in some cases (PE-U can consume moisture from a finite micro-environment as it hydrolyses until an equilibrium is reached). Further investigations are needed, however.
