Preservation

Desiccants

Molecular Sieves and Other Desiccants

Typical Zeolite Structure
Fig 7.2 Typical zeolite structure

Zeolites, also known as "molecular sieves", are passive sorbents that can absorb decomposition products from within the film can microenvironment. By absorbing the decomposition products from the air inside the can diffusion of the decomposition products from within the film will increase. This has the beneficial effect of lowering the level of decomposition acids trapped in the film, which in turn has a slowing effect on the rate of the decomposition reaction. Molecular sieves are slow to absorb the decomposition products. However molecular sieves also provide a desiccant action that will quickly reduce the %RH inside the film can microenvironment reducing the amount of water available for the decomposition reaction and biological factors. Other desiccants are also valuable in the preservation of film. The main problem is determining when the desiccant is saturated. Silica gel often has an indicator built in that changes colour from blue to pink when the gel has absorbed all the water it can. Some desiccants, like silica gel, can be regenerated by heating at around 100°C.

Packaging of the desiccant is important. The packaging must be robust enough to contain the desiccant and prevent dust from the desiccant damaging the film. The packing should not harm the film, this rules out most paper materials unless they do not contain lignin or other sources of acid. Terylene fabric, which is just polyester under a commercial name, is suitable, but sometimes the effort involved in saving a few dollars can actually cost more than an off-the-shelf solution!

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