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Effect of moisture on the state of stress and dimensional stability of photographic gelatin-latex coatings
Gelatin has been used as a binder or dispersing agent for light-sensitive and non-light-sensitive photographic layers. The ability to keep the silver halide crystals finely dispersed and to protect the silver halide crystals and other additives from abrasion and other mechanical and chemical influences make gelatin desirable in photographic applications. However, gelatin is very sensitive to changes in humidity. Although this sensitivity to moisture is favorable when the film must be processed, it is also a drawback to the use of gelatin in an emulsion layer. The absorption of moisture can induce swelling stresses, causing dimensional instability commonly observed as bending or curling in the photographic films. This dissertation focuses on the effects of moisture on the state of stress and dimensional stability of gelatin coatings. The hygroscopic effects on the thermal, mechanical, and transport properties were also investigated. Two types of polymer latices, poly(ethyl acrylate) and poly(ethyl methacrylate), were studied as additives to gelatin. The effects of latex concentration, latex particle size, drying condition at vitrification, and gelatin concentration at set point were examined as a function of relative humidity. The goal is to develop an understanding of these properties and assist in controlling or selecting conditions which will minimize the dimensional instability of photographic films over a wide range of use conditions. A vibrational holographic interferometry method and a thermomechanical analyzer were adopted to measure the stresses and dimensional changes as a function of relative humidity. The incorporation of a polymer latex can reduce the moisture sensitivity, and hence increases the dimensional stability of the emulsion layer exposed to the moisture. Composite theories for an isotropic composite filled with spherical particles were applied to determine the humidity expansion coefficient and elastic moduli of the gelatin-latex films. The experimental data were in excellent agreement with the theories. The decrease in swelling stress with an addition of the polymer latex was explained by the incremental linear elasticity theory. Based on this theory, the best material (i.e., minimum swelling stress, lowest Eβ value) was found to be the gelatin film with 40 parts PEA and 15% gelatin concentration at set point, and was dried at the LMERH condition (130F/5.5% RH).
Aht-Ong, Duangdao, "Effect of moisture on the state of stress and dimensional stability of photographic gelatin-latex coatings" (1999). Doctoral Dissertations Available from Proquest. AAI9932282.