Technology
Kodak details chemistry of instant film Requiring no dye couplers, 19-layer film incorporates new types of high-speed direct reversal emulsion and dye release compounds The new instant color camera marketed recently by Eastman Kodak (C&EN, April 26, page 4) uses a new type of imaging chemistry that does not require dye couplers. First details of that chemistry were revealed by the director of Kodak's research laboratories, Dr. Wesley T. Hanson Jr., at the Society of Photographic Scientists & Engineers' 29th Annual Conference in New York late last month. According to Hanson, the substantially new things about Kodak's film are a new type of high-speed direct reversal emulsion and new types of dye release compounds. The film is composed of three basic parts—an integral imaging receiver, a pod containing the activating chemicals which start development of the film, and a cover sheet that is initially transparent
but that is made effectively opaque when developing of the film begins. The film consists of a sandwich of 19 layers, including the Estar polyester backing layers on each side of the sandwich. After exposure, the pod is ruptured to release activator fluid, which spreads between the cover sheet and the imaging receiver. The activator fluid consists of potassium hydroxide, an electron transfer agent (developer) that is a derivative of l-phenyl-3-pyrazolidone, a thickening agent, an antifogging agent, sodium sulfite, and carbon particles, which act as an opacifier. Upon rupture of the pod, the carbon isolates the imaging forming section of the film from the transparent cover sheet and forms an in situ "darkroom" for development of the film. The electron transfer agent (developer) supplies electrons to a developing silver halide emulsion and is regenerated as it subsequently removes electrons from the dye release compounds in the dye layers of the film. The dye releasers then are hydrolyzed by the alkali in the activator fluid to yield mobile dyes. Hanson says that the advantage of this type of chemistry is that the electron transfer agent and its oxidation product are the only photographically active mobile species
present in the system. The released dyes and the hydrolyzed by-products are, effectively, photographically inactive. The dyes are released where development occurs; hence, to obtain a positive print, a photographic reversing mechanism is necessary. This is achieved with a direct reversal silver halide emulsion that releases an amount of dye in inverse proportion to the degree of exposure. Hanson describes the dye releasers as ballasted sulfonamidophenols containing yellow, magenta, and cyan dyes attached to the sulfonamide* group. When oxidized these compounds convert to quinoneimides, which react with alkali to form sulfamoyl-solubilized, mobile dyes and immobile quinone by-products. The ballast substituents in the phenolic carrier portions of the dye releasers provide bulk to prevent diffusion of the releaser from its own layer in the film. They also can affect the ease of oxidation of the releaser and the rate of release from the oxidized species. The sulfonamidophenol imaging chemistry of the new film requires only that the released dyes contain a primary sulfonamide group. However, Hanson explains, for practical reasons the dyes must be stable over a wide pH range (from
. dyes migrate to form image
In reversal instant print film . . .
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• 1 * 1 Backing layer Support Mordant + gel T i 0 2 + gel
Integral imaging < receiver
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C&EN June 21, 1976
W^ Pll White 1 1SBSil^SSI MliSfP:
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— image-receiving layer White opaque reflective layer Black opaque layer Cyan dye releaser
Cyan dye releaser in oil dispersion + gel Red-sensitive reversal AgX + nucleating agent + gel — Oxidized developer scavenger layer — Oxidized developer scavenger + gel Magenta dye releaser in oil dispersion + gel Green-sensitive reversal AgX + nucleating agent •+- gel Green-sensitive layer Oxidized developer scavenger — Oxidized developer scavenger layer — Yellow dye releaser in oil dispersion + gel Blue-sensitive reversal AgX + nucleating agent + gel UV absorber + gel uv-aDSorDing layer Potassium hydroxide Developing agent Antifoggant Activator fluid Carbon Water Sodium sulfite Thickener Timing layer No. 1 Timing layer No. 2 + developing inhibitor precursor Acid copolymer Suppport Estar support Backing layer Backing layer Expose
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Polymeric mordant fixes dyes in position for image CH2
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Dyes have these general structures Cyan
Electron-withdrawing group
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14 to 4), and they must be capable of rapid diffusion through several gelatin-based layers before reaching the image forming section of the sandwich. After the brief period of diffusion, the dyes are immobilized in the image receiving layer by interaction with a cationic mordant. All these requirements are fulfilled with monoazo dyes derived from enolic couplers such as phenols and naphthols. Three species are involved in the dyes. Two of the species are tautomeric forms in equilibrium. However, under basic conditions, the neutral forms are ionized to form a common anionic species in which the negative charge is spread over all the atoms of the chromophoric (color causing) pi-electron system. A unique feature of the anionic 4-aryl azo-1-naphthol dye systems described by Hanson is that appropriate substitution can be employed to provide both magenta and cyan dyes from the same basic molecule. However, yellow dyes require special substitution in an azopyrazolone. The silver emulsion finally developed for the new film is a positive working emulsion that has a film speed much higher than any previously used for color films. It is substantially different from the direct reversal system developed by Kodak in the 1940*8 and which has been used ever since. The old emulsion was a halide conversion emulsion in which sensitivity was built into the emulsion grains by a catastrophic precipitation technique. In this technique, a silver chloride emulsion was precipitated and then a halide exchange with bromide and iodide salts resulted in a mixed halide emulsion with severe internal crystal disruptions. Such an emulsion has good internal sensitivity but virtually no surface sensitivity. Hence, the speed of the older emulsions is low because only the internal centers of sensitivity account for the speed. The new emulsion is based on surface nucleation of the emulsion grains, which permits a direct reversal image—that is, unexposed grains develop whereas exposed grains do not. The new emulsions are rated at an exposure index of 150, comparable to high-speed Ektachrome at 160. After exposure of the film, pod rupture
causes activator fluid to spread between the cover sheet and the imaging receiver. This, in turn, causes the emulsions to develop and the dyes to be released. During exposure, black exposes none of the light-sensitive layers, white exposes all of them, and red, green, and blue light selectively exposes only one of the sensitized layers. Beyond each sensitive layer is its associated dye release layer. In the reversal process there is black where no exposure occurs; the silver halide in each of the color-sensitive layers develops allowing the dye releasers to release all of their dyes. The dyes then diffuse to the mordant. If there was exposure to white light, no development and no dye release occurs. The color layers are selectively developed such that the appropri-
Mobile dyes result from dye release process Ag+
Ag
ETA mobile
FTA C , M
0 Y
oxidized
OH Z
Y
Z
NS0 2 -dye
NHS0 2 -dye Dye-release compound
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ate dyes are released and migrate to the image receiving layers where the mordants fix the dyes in position to form the final image. Dye scavengers tie up any excess dyes. The final result is "reasonably color fast," Hanson says. He does not recommend that prints made with the new film be exposed to direct sunlight for prolonged periods. However, experience gained so far indicates that in typical "album storage" the new color film is at least as color fast as other color films. The film packs and cameras for the new Kodak system of photography were first placed on the Canadian market in May. Introduction into the U.S. and Puerto Rican markets is scheduled for late June. Eventually Kodak plans to market elsewhere in the world but not until after 1976. A 10-print film pack has a list price of $7.45, and cameras are listed from $53.50 to $69.50 depending on features. Late in 1976 a folding camera with a coupled range finder will be listed at $140. Print format is 2% inches X 39/i6 inches in all cases. • June 21, 1976 C&EN
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