3.2 Temperature
3.2.1 Physical influences.
3.2.1.1 Dimensional influences. Temperature influences the size of materials: generally, carriers expand with rising and shrink with falling temperatures. This is also true for tapes. Polyester tapes have the lowest thermal expansion coefficient, while that of CA is quoted to be three times higher.
With CA and PVC tapes, this leads to looser tape packs if temperatures rise and to tighter packs with lower temperatures. With PET base films, however, there is one anomaly: these are pre-tensilised which causes their thermal expansion coefficients to be significantly higher in the dimension of thickness than of length. Thus, wound-up PET tapes “swell” with rising temperatures — each layer of tape becomes thicker — which is not compensated by elongation. This increases the tension within the tape pack. Conversely, PET tapes become 16 FIAF 1986, 11.1.1.2. looser with falling temperatures. This calls for different relaxation measures for CA and PVC tapes to those for PET tapes when changing climatic storage conditions (3.2.4).
Dimensional changes are particularly dangerous for lacquer discs. Because of the different thermal expansion coefficients of the metal or glass substrate and the brittle lacquer coating, a temperature change may trigger the cracking of the latter.
Because of these disadvantages and potential threats, specifically for tape materials and lacquer discs, the stability of temperature is of greater importance than the chosen absolute value (see 3.3).
3.2.1.2 Irreversible influences on polymers. For some polymers used as components of audiovisual carriers, elevated temperatures have an irreversible influence. If heated beyond certain temperatures, their properties are changed and are not restored after lowering the temperature again. Temperature thresholds differ widely for various materials, but it can be stated that temperatures up to 35°C do not cause any immediate irreversible effects or lead to any deterioration of any current audiovisual carrier.
3.2.1.3 Thermoplastic materials. This group of polymers softens at higher temperatures. These polymers are used for the production of containers, cassette shells, etc. The inadvertent exposure of such materials to elevated temperature, even to sunlight, may lead to an irreversible deformation. This is also a specific threat to vinyl discs.
3.2.1.4 Print-through. Temperature influences print-through on magnetic tape: the rise of print through versus time is steeper with higher, and flatter with lower temperatures (see 3.7.2.5).
3.2.1.5 Curie point. Magnetic stability (coercivity) depends on temperature. At and beyond the Curie point, magnetic stability is lost. The lowest Curie point of widely used magnetic pigment is 128°C for CrO2, while the point for iron and iron oxide is above 300°C. This phenomenon, however, is positively employed in magneto-optical recording (2.3.1.4).
3.2.1.6 Temperature range. In order to extend life expectancy, photographic materials are often stored at temperatures below the freezing point. For magnetic tape, cold storage is discouraged because some, not all, lubricants exudate at a temperature below 8°C (2.2.1.1.1.4). At the higher end, 35°C should not be exceeded (3.2.1.2). Within this range, temperature only affects the physical dimensions of carriers and the speed of chemical processes.
3.2.2 Indirect chemical influence. Temperature determines the speed of chemical processes and, therefore, ageing or deterioration. Subject to the limits set out in 3.2.1.6, it can be stated as a rule of thumb that the speed of chemical processes is doubled by an increase of 10°C, or, on the other hand, the speed of ageing is slowed by 50% by lowering temperature by 10°C — a doubling of life expectancy.
3.2.3 Humidity and temperature interrelation. Temperature determines the absolute amount of water air can hold in gaseous form (vapour). Higher temperatures hold higher amounts of vapour, while lower temperatures hold less. If a given room is cooled without simultaneously dehumidifying the air, the relative humidity rises until 100% RH is reached. At this temperature, called the dew point, superfluous vapour condenses in the form of water on the coldest surfaces (see figure 30). Any air conditioning must, therefore, control both parameters simultaneously (see 4.3). Most conventional air conditioning equipment does not dehumidify sufficiently, thereby inadvertently raising relative humidity and increasing the threat to carriers, counter-acting the benefits of the lowered temperature
3.2.4 Temperature/humidity changes, as already mentioned in 3.2.1.1 above, can be more dangerous to carriers than stable, but sub-optimal, absolute values. Changes in temperature, but also in humidity, cause dimension changes, resulting in unnecessary stress to carriers. At highest risk are discs composed of different materials, e.g. lacquer discs, but magnetic tapes are also affected, specifically high density helical scan formats. The other basic danger is the possibility of condensation when bringing cold carriers into a warm environment.
Consequently, permanent storage conditions should be designed to have minimal temperature and humidity changes. During transportation, carriers must be protected by adequate logistics and containers (see 4.8). Changes of climatic parameters of longer duration require acclimatisation (staging) periods. For all materials, except lacquer discs, the temperature gradient should not exceed 3°C and the relative humidity gradient 5 percent points over a period of 24 hours. Additionally, to compensate for the different tensions within tape winds due to temperature changes (3.2.1.1), tapes must be relaxed by re-winding when bringing CA and PVC tapes to cooler storage conditions and PET and PEN tapes to warmer storage conditions. Lacquer discs, because of the threat of crackling due to different expansion coefficients between coating and substrate, are threatened by moves in both directions. Such transports should, therefore, be kept to the absolute minimum and be accompanied by long acclimatisation (staging) periods over several days.
The danger of condensation when bringing cold carriers into warmer areas must not be underestimated. Sufficient airing must be allowed until the carrier temperature is equalised.
