Affects of Relative Humidity
The polyester base itself absorbs and desorbs moisture from the ambient environment. As the plastic absorbs moisture, it expands. As the plastic releases moisture, it contracts. The process is slow. It takes one hour per 0.0254 mm (0.001 inch, or 1 mil) of thickness to reach 99% equilibrium with the ambient environment. The humidity coefficient of expansion for polyester base is 0.0008%/% RH. The humidity coefficient for the base is a constant, regardless of the thickness of the polyester base.
Gelatin in the emulsion and backing layers also absorb and desorb moisture. Both layers are tightly bonded to the plastic base. As the gelatin layers expand and contract, they exert tremendous forces on the tightly bonded plastic base, causing expansion and contraction. Films with high gelatin concentration (thick layers) will exert greater forces than films with lower gelatin concentration (thin layers). The gelatin layers coated on polyester base therefore will increase the humidity coefficient from 0.0008%/% RH to a higher value, depending on the concentration of gelatin. The process is very fast. Gelatin absorbs and desorbs moisture very quickly. It takes less than 2 or 3 minutes for gelatin layers to reach 99% equilibrium with the ambient environment.
The process of absorption and desorption by both the base and the gelatin layers is considered to be completely reversible.
Humidity Expansion Coefficient (HEC) is expressed as % Size Change per % Change in Relative Humidity (% / % RH).
HEC for Kodak films can be located in the respective Technical Information Document. Consult the manufacturer for other films.