Changes in the physical dimensions of the film have a major impact on the ability of the film to be accessed. As the degree of shrinkage increases it becomes more risky to transport the film through equipment.
Over time a certain degree of shrinkage occurs due to the evaporation of residual solvents, casting and release agents left over from the manufacture of the film base. Early acetates also suffered a loss of plasticiser which led to a strong smell of naphthalene or 'mothballs’.
A much greater degree of shrinkage occurs due to decomposition reactions. The plasticiser commonly used in cellulose triacetate, triphenyl phosphate (TPP), readily migrates from within the base polymer as the acid concentration rises. Since TPP can make up as much as 12-15% of the film base the shrinkage can be very significant.
The stresses induced by shrinkage that build up in a film wound as a reel are potentially very great and will distort the shape of the reel, 'spoking’ (Fig 6.8). Other ways in which the film can relieve the stress induced by shrinkage is by 'curl’ and 'buckle and wave’.
Sometimes a decomposing film may be unwound and appear not to be suffering from curl or buckle and wave to any extent, however with more handling the film will start to shows signs of these problems.
Shrinkage prevents film from transporting through film equipment. Thebecomes narrower and the sprocket teeth will no longer match with the perforations. As little as 1.5% shrinkage can cause problems with some equipment.
It is possible to modify some equipment to handle slightly shrunken film. Changing the pitch on the sprocket teeth or modifying the film path can be successful. This compensates for the longitudinal shrinkage however lateral shrinkage, which can be greater, still remains a problem.
The emulsion will compensate for a small amount of shrinkage in the base by compressing. Eventually the stress between the emulsion and the base will become so great that the emulsion will lift away from the base, '’ or 'channelling’.
It is also possible to redimension the film base, to return the base dimensions to as close as original as possible. This is done by softening the base and making it absorb some form of filler. There are several proprietry processes that have a great deal of success with this treatment.
Simpler methods using just elevated humidity or a solution of water, acetone and glycerol can be adequate. However the effect is short lived and the film needs to be duplicated almost immediately. The later process is also unsuitable if the film is severely decomposed as the gelatin emulsion will become very soft and may disintegrate. Using this technique it is also possible for severely decomposed film base to become opaque, very soft and easily torn.