Film as a Composite Material
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through the camera and processor. Many of the major advances in photography over the past 120 years, such as the rollfilm camera, Thomas Edison's invention of the motion-picture camera, and the miniaturization of modern still cameras, have been made possible by advances in film-support technology. While glass is still used for special applications requiring the ultimate in dimensional stability, such as astronomy and photolithographic masks for solid-state circuitry, most silver-halide imaging is done using film having a support made of cellulose triacetate (CTA) or poly(ethylene terephthalate) (PET). The support provides many functions critical to the performance of the final photographic film. In addition to dimensional stability and optical clarity, the polymer support must have good tear strength, be resistant to curling (both dry and wet), provide antistatic protection, and be abrasion-resistant. Critical
for use, the gelatin-based emulsion layer must adhere well to the support through all stages of use and processing. The processing environment is quite harsh, including a development step in a high pH (12) 38°C aqueous bath for 20 min. Coatings applied directly to the support are necessary to instill these required properties. The first support coating illustrated in Figure 1 is a thin (typically 0.1-0.5-/nmthick) subbing—or adhesion-promotion—layer/layers that can be used on one or both sides of the support. The purpose of the subbing layer is to provide a surface to which the gejatin-based silver-halide emulsion layer will adhere. Gelatin has been the binder used in photographic emulsions since its discovery in 1871.' Gelatin is a water-soluble protein that acts as a dispersing aid for the silver-halide grains, couplers, sensitizing dyes, and other addenda to form the photographic emulsion.7 In a typical color film, the top layers of the multilayer emulsion shown in Figure 1 are bluesensitive, with lower layers sensitive to green and red light. The emulsion layer is overcoated with an abrasion-resistant layer to prevent scratching of negatives before, after, and during processing. The emulsion side of a typical PET-based film is illustrated in the transmission electron-microscopy (TEM) cross section in Figure 2. A variety of different layers are used on the back side of the support (backing layers) as illustrated in Figure 1. A thin conductive layer is used for antistatic protection. This is required to provide protection against static electrical dis-
Overcoat Emulsion Layers
mm
Subbing C" Layer
ft^
Support
Conductive • c Layer
Overcoat (not to scale) Figure 1. Typical multilayer photographic film structure. The support is 100-200-ixm-thick, the emulsions are 5-7-^m-thick, and other layers are 0.5-3-fim-thick.
MRS BULLETIN/JULY 1996
Film as a Composite Material
charge during conveyance in high-speed manufacturing processes, which could expose the sensitive silver-halide emulsion and pose a fire hazard. A processsurviving antistatic layer is desirable to reduce dirt on the negatives, which c
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