Gelatin Coating Aids and Compositions (1962)
U.S. Patent No. 3,038,804, granted on June 12, 1962, to researchers at the Eastman Kodak Company, introduces a series of synthetic chemical compounds—specifically urethane salts—designed to improve the application of gelatin coatings. This invention was a major step forward in the photographic industry, where applying perfectly uniform layers of light-sensitive emulsions is critical to film quality.
The Problem: Coating Defects and Unreliable Aids
In the production of photographic film and paper, gelatin layers must be applied to a support base (like cellulose acetate) at high speeds. Without a “coating aid” (a surfactant), the gelatin tends to pull away from the surface or form “repellencies.”
- Physical Defects: Untreated coatings often showed crescent-shaped uncoated areas (the size of a pinhead) and streaks.
- The Saponin Issue: For decades, the industry used Saponin (a natural plant extract). However, saponin was notoriously inconsistent. One batch might work perfectly, while the next might contain impurities that “fogged” the photographic emulsion or failed to prevent defects.
The Innovation: Synthetic Urethane Salts
The inventors developed a way to synthesize reproducible, high-quality coating aids. These compounds are technically alkali metal salts of substituted amino acids (specifically urethanes).
1. Chemical Structure
The patent defines these compounds with the general formula:
R-X[CONH-CH(Y)-(CH_2)_m-COOM]_n
- R: A long-chain “tail” (8 to 18 carbon atoms) that provides surface activity.
- X: A linking group (like an ether or amide).
- Y & m: Define the specific amino acid backbone (often derived from Aspartic or Glutamic acid).
- M: An alkali metal (Sodium, Potassium, or Lithium) that makes the molecule water-soluble.
2. Why They Work
These molecules act as high-performance surfactants. When added to a gelatin mixture (even in tiny amounts like 0.01% to 0.1%), they lower the surface tension of the aqueous gelatin. This allows the liquid to “wet” the surface of the film base instantly and uniformly, even at high production speeds.
Key Advantages over Saponin
- Reproducibility: Being synthetic, every batch of the urethane salt is chemically identical, ensuring consistent results in the factory.
- Photographic Inertia: These compounds do not adversely affect the light sensitivity or “fog” levels of silver halide emulsions.
- Calcium Tolerance: Unlike many soaps, these aids remain effective even when used with “hard” water containing calcium.
- Wet-on-Wet Coating: They are exceptionally good for applying a second layer of gelatin over a first layer that is still wet—a common requirement in multi-layer film manufacturing.
Manufacturing the Coating Aid
The process involves a two-step reaction:
- Esterification: An alcohol derivative is reacted with an isocyanato ester (derived from amino acids like leucine or glutamic acid).
- Selective Hydrolysis: The resulting ester is treated with an alkali (like Sodium Hydroxide) to turn it into a water-soluble salt.
Practical Application (Example 1)
In a typical test, the inventors used disodium N-(carbo-p-tert.-octylphenoxypentaethoxy) aspartate. When added to an X-ray film emulsion:
- Amount: 1 gram per pound of emulsion.
- Result: A perfectly smooth coating, free of “repellencies” or “crescents,” outperforming the natural saponin standard.
