KODAK Recording 2000 Film

There are 4 factors which may cause size changes in photographic films:

• Relative Humidity
• Temperature
• Processing
• Aging

Relative humidity will affect both the gelatin containing layers and the support material, usually PET (polyethylene terephthalate). Loss of moisture causes contraction of both these materials, however they do so to different degrees and dynamics. Size changes are generally referred to using the humidity coefficient, which is defined as the percentage change in length per percent change in relative humidity. For PET the figure is around 0.0008% change in length/ % change in RH. For a sheet having a length of 500mm, this is equivalent to a 4micron change in length for each percentage point change in relative humidity. The contribution of the gelatin containing layers may be equal to or slightly more than the support, depending on the film type. Assuming about the same, we are now looking at a total of 8 microns change in length over a 500mm long sheet for every percentage change in relative humidity. If conditions in the work place are controlled to plus or minus 5% in terms of relative humidity, we are looking at a potential change of 40microns, possibly 80microns, if the conditions are oscillating between the control limits.

Some thought also needs to be given to the dynamics of moisture uptake and release. Water vapour will exchange relatively rapidly with the gelatin-containing layers over a time scale of minutes to hours. On the other hand the base material will take hours and even days to fully equilibrate with new conditions. For most practical purposes in terms of film registration, it is the rapidly responding gelatin layers that are of most significance.

Most films are manufactured and packed at around 40 to 50% relative humidity, and ideally working conditions prior to exposure, during exposure and post-processing should be close to 50%. But perhaps even more important is that there are not abrupt changes during the workflow or between separations. If one separation has to be done at a different time, try to match the conditions under which the other separations were produced as closely as possible.

Temperature may also cause size changes in the film. These changes are rapid and fully reversible provided extreme high temperatures are avoided. The thermal coefficient is around 0.001% per degree Fahrenheit (0.0018% per degree Celsius). For our example of a 500mm long sheet this is a size change of 5 microns per degree (F) change in temperature. The same comments made above for relative humidity apply. The customer should avoid abrupt changes in temperature and following processing adequate time should be allowed for all the separations to cool down before attempting registration. Probably, registration problems due to temperature changes are less frequent than relative humidity.

Processing can have a significant effect on the size of the film and this is almost entirely due to changes in the coated layers. One of the main reasons for any change is the difference in response to relative humidity between the raw and processed film. There is usually a relative humidity value at which there is zero processing dimensional change for both raw stock and processed film – however this may not correspond to the working condition. Thus there will usually be a finite dimensional change associated with processing. This can be minimised and should be the same for all separations if all films are processed equivalently and at an appropriate temperature. The latter can be found by trial and error, but the general maxim is that over drying leads to oversize and under drying to undersize. So, if you are having problems, try adjusting the dryer temperature.

Finally, a summary of most of the above, plus some practical points.