Gelatin Coating Compositions (1967)
U.S. Patent No. 3,306,749, granted on February 28, 1967, to William J. Knox, John F. Stenberg, and Charles V. Wilson (assigned to the Eastman Kodak Company), discloses advanced coating aids designed for high-speed photographic film and paper manufacturing. The inventors, working out of Rochester, New York, tackled a persistent challenge in mass photographic emulsion production: surface defects and uneven chemical wetting.
When manufacturing photographic film and paper at commercial speeds, applying liquid gelatin layers without introducing flaws is notoriously difficult. Without an effective surfactant, coatings suffer from repellencies, mottling, and distinctive “pencil line” streaks. Traditional natural additives like saponin varied wildly from batch to batch, often degrading the emulsion or failing to improve spreading. Knox, Stenberg, and Wilson solved this by developing synthetic, water-soluble acyl derivatives of iminodiacetic and iminodipropionic acids that ensure uniform flow, slickness, and rapid developer penetration.
The Innovation: Acyl Iminodiacetic and Iminodipropionic Acid Salts
The core breakthrough lies in incorporating alkali metal or ammonium salts of specific acyl derivatives into aqueous proteinaceous solutions (primarily gelatin or casein) at precise concentrations of 0.2 to 2.7 parts per 100 parts of protein.
Why These Specific Synthetic Surfactants?
- Streak and Mottle Elimination: They dramatically reduce surface tension without creating foaming issues, wiping out pencil line streaks and repellency spots during high-speed machine application.
- Developer Wettability: Photographic processing demands that processing chemicals penetrate the emulsion instantly and evenly. These compounds ensure rapid, uniform wetting when submerged in photographic developer baths.
- Multi-Layer Hopper Compatibility: Unlike older surfactants that failed when multiple liquid layers were laid down simultaneously (such as in slide-hopper coating methods), these compounds maintain layer integrity without interlayer mixing or streak defects.
Key Chemical Formulations
The patent details several tailored synthetic salts derived from 12- to 18-carbon acyl chains, each evaluated for photographic film and paper coatings:
| Coating Aid Compound | Salt Type | Primary Function & Coating Benefit |
| N-lauroyl-N-carboxymethyl glycine | Sodium salt | Provides high developer wettability and smooth laydown in silver halide emulsions. |
| N-oleoyl-N-carboxymethyl glycine | Disodium salt | Delivers defect-free surface uniformity across broad concentration ranges (0.3 to 1.0 g per lb emulsion). |
| N-dodecyloxybenzoyl-N-carboxymethyl glycine | Sodium salt | Offers clean surface smoothness and compatibility at concentrations as low as 0.13 g per lb emulsion. |
| N-(2,4-di-tert-amylphenoxyacetyl)-N-carboxymethyl glycine | Disodium salt | High-efficiency synthetic surfactant eliminating mottle while retaining excellent latent-image stability. |
| Gelatin / Casein Base | Aqueous matrix | The protein carrier (37 g per pound of wet emulsion) supporting the suspended silver halide crystals. |
Coating Performance and Application Methods
The inventors demonstrated the effectiveness of these additives across several practical coating regimes:
- Hopper Technique Testing: In standard gelatin-silver halide emulsions applied to paper supports via hopper applicators, additions of 0.13 g to 1.0 g of active salt per pound of wet emulsion yielded streak-free, blemish-free coatings with immediate chemical uptake.
- Simultaneous Multi-Layer Coating: The compounds proved especially effective in high-speed manufacturing where multiple liquid layers are extruded onto a running web simultaneously (under the Russell method, U.S. Patent No. 2,761,791).
- Wet-on-Wet and Wet-on-Dry Versatility: The compositions performed reliably whether coated sequentially over an already dried gelatin base or layered directly onto freshly coated, wet protein layers.
About the Inventors and Eastman Kodak
The patent represents a collaborative achievement by Eastman Kodak research chemists in Rochester, New York:
- William J. Knox, Jr.: A prominent African American chemist who earned his Ph.D. from MIT and worked on the Manhattan Project (focusing on uranium hexafluoride corrosion and gaseous diffusion) before joining Kodak’s Research Laboratories. Knox became Kodak’s foremost expert on surfactants, surface tension, and coating dynamics, earning over 20 patents on coating aids and photographic emulsion technology.
- John F. Stenberg and Charles V. Wilson: Longtime research chemists at Kodak who co-authored numerous foundational patents and publications in organic synthesis and photographic material formulation.
- Industrial Impact: Knox and his colleagues provided the chemical formulations necessary to transition color and multi-layer black-and-white film from labor-intensive individual layer drying to modern, simultaneous multi-layer slide hopper extrusion, drastically lowering production defects and manufacturing costs.
Summary of Claims
The patent explicitly claims:
- An aqueous protein coating composition containing 0.2 to 2.7 parts per 100 parts of protein of an acyl iminodiacetic or iminodipropionic acid salt represented by the general formula RCON[(CH2)xCOOM]2 (where x is 1 or 2, R is an open-chain C11-C17 radical, dodecyloxyphenyl, or di-tert-amylphenoxy radical, and M is a water-solubilizing cation).
- Photographic silver halide emulsions formulated with water-soluble salts of acyl iminodiacetic and acyl iminodipropionic acids.
- Photographic products having one or multiple proteinaceous layers applied over a support containing these compounds to ensure streak-free coating and rapid developer wettability.
